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“The effect of cell wall microstructure on the deformation and fracture of aluminum-based foams,”
Abstract
This study primarily concerns the role of cell wall microstructure in influencing the mechanical behaviour of metallic foams. Three closed-cell foams have been examined, having rather similar relative densities and cell structures but significant differences in cell wall microstructure. It is concluded that these differences can substantially affect the micro-mechanisms of deformation and failure under different types of loading and can also have an influence on the macroscopic mechanical response. Cell wall ductility and toughness are impaired by high volume fractions of coarse eutectic, fine oxide films and large brittle particles, all of which were present in one or more of the foams studied. This impairment can lead to extensive brittle fracture of cell walls, with little energy absorption, even under nominally compressive loading conditions. The influence of cell wall ductility tends to become more significant when the loading state is such that local tensile stresses are generated.
... Therefore, the scientific community is interested in further improving the mechanical properties of the foams used in such applications. Controlling foam density, cell structure, microstructure, and the solid-constituent contained within the cell wall can improve their mechanical properties [3,6,7]. ...
... Aluminum melts containing ex-situ particles, for example, 5-20 vol% of SiC [8,9,11,12] or Al 2 O 3 [8,11] of sizes up to 20 μm, can produce stable foams. A large quantity of particles in solid foams induces brittleness, which in turn degrades their compressive performance, especially energy absorption properties [6,12]. Moreover, the high amount of ceramic particles present in the foam makes machining and recycling process much more difficult. ...
... The mechanical properties of a closed-cell foams are strongly influenced by its macro- [7,41,42] and micro-structural [6,7] characteristics. The effect of microstructure on the deformation of foam is because the cell walls are embedded with reinforcing ZrB 2 particles. ...
Stabilization is an essential requirement to produce closed-cell metal foams. In the melt route of foaming, usually ceramic particles are used as foam stabilizers. For the first time, the present study introduces ZrB2 particles as foam stabilizers. We demonstrate the foaming of in-situ based Al composite containing submicron ZrB2 particles. The effect of foaming temperature and holding time on the structural and mechanical properties of the foams was studied. The composites and foams were characterized using XRD, SEM/EDS, and optical scanning techniques. The mechanical properties of the foams were determined by subjecting the foams to a quasi-static compression test. Submicron ZrB2 particles present in the cell wall and at the gas-solid interface promoted foam stability. All the foams exhibited a good cellular structure with high expansion. Among all the foams, the foams prepared at 680 °C with a holding time of 120 s exhibited the smallest cell size and the best mechanical properties. The structural and mechanical properties of the Al–5ZrB2 foams were found to be comparable to conventional foams.
... Fig. 9.-Compressive strength of the present foams and a few other foams produced using TiH 2 . Source of data: Alporas foams, [42,[50][51][52][53][54][55] Alulight foam, [50] other PM foams. [43,50,55,56] Note: error bar is not available for all the data points. ...
... Source of data: Alporas foams, [42,[50][51][52][53][54][55] Alulight foam, [50] other PM foams. [43,50,55,56] Note: error bar is not available for all the data points. ...
Mg-based blowing agents exhibit the potential to yield aluminum foams with better structure and properties than those achieved by using conventional blowing agents. However, all the studies to date used a high amount of such blowing agents (e.g., 15 wt pct Mg) for foaming aluminum. In this study, we investigate the minimum amount of Mg blowing agent required for foaming. Al-Si13-MgX (X = 2.5–15 wt pct) alloy foams were produced employing the powder metallurgy route where Mg acted as the blowing agent. The macro- and microstructure of the foams were analyzed using X-ray tomography, microscopy, and X-ray diffraction. The foams were subjected to hardness and compression tests to evaluate their mechanical properties. Deformation behavior was studied in situ by monitoring the foam surface during the compression test. For the first time to our knowledge, the present study established that 5 wt pct of Mg is sufficient to achieve foam expansion similar to that achieved by 15 wt pct Mg. Moreover, the structural and mechanical properties of the 5 wt pct Mg-containing foams were much superior to the 15 wt pct Mg-containing foams. However, the highest strength was obtained using 10 wt pct of Mg. Many cracks were observed at the early deformation stages of 10 and 15 wt pct Mg-containing foams. We correlate the Mg content with the structure, properties, and deformation behavior of the foams.
... The peak strength data of some conventional foams were extracted from Refs. [14,54,57,[59][60][61][62][63][64][65]. The comparison is presented in Fig. 14. ...
... Comparison of the peak strength of the foams produced in this study with other foams. Source of data: Alcan foams [54,57,[59][60][61], Alporas foams [54,57,[60][61][62]65], PM route foams [61,63,64,66], AlMg15Cu10 foam [14]. Note that error bar is not available for all the data points. ...
The main focus of the present study is to compare the effect of different alloying elements on Al–Mg alloy foams. Al–Mg15–X10 (X = Cu, Zn and Si) alloy foams were produced via powder metallurgy route by using Mg as a blowing agent. Macro- and microstructural characterisations of the foams were performed using X-ray tomography, X-ray diffraction and scanning electron microscope. Corrosion studies such as weight loss measurement, hydrogen evolution method and potentiodynamic test were conducted. Mechanical properties were evaluated by subjecting the samples to quasi-static compression and microhardness tests. All the alloy foams showed a comparable structure. The Cu-containing foams exhibited the highest strength, while the Zn-containing foams showed the highest expansion. However, the other properties such as brittleness, elastic modulus and burning nature were found to be better for the Si-containing foams.
... This enhanced stability leads to a lower D mean value and a slightly more uniform cell size distribution in Al-TiB 2 foam compared to Al foam (refer to Table 1). In general, the mechanical properties of foams strongly depend on their density, cell size distribution and cell wall material strength [11,29,[34][35][36][37][38]. The compressive strength and energy absorption of Al-TiB 2 foam are significantly higher than those of Al foam. ...
... Arrows in a and b indicate the MIT foams produced in this study. The data for other foams were extracted from Refs.[27,35,[38][39][40][41][42][43] ...
Aluminium and Al-TiB2 closed-cell foams were produced by a recently developed foaming method called melt injection technique where bubbles are created by air entrainment. The macro- and microstructures of the foams were characterised by X-ray tomography and scanning electron microscopy, respectively. Mechanical properties were evaluated by quasi-static compression tests. Al-TiB2 foam exhibited finer cells and better mechanical properties compared to Al foam. Also, the structural and mechanical properties of these foams were compared with closed-cell foams produced by other existing methods.
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... Unlike this, the pronounced oscillations of long deformation plateau are demonstrated by Al6Zn2.3Mg alloy foam, indicating contribution of fracture in global cell collapse [23,24]. In addition, deformation plateau for Al6Zn2.3Mg ...
The study presents mechanical performance metrics, especially, energy absorption, of aluminium foams fabricated by melt processing with CaCO 3 blowing agent without Ca additive. Relatively ductile Al1Mg0.6Si alloy and high strength Al6Zn2.3Mg alloy comprising brittle eutectic domains were employed for the foams manufacture and then examined in conditions of uniaxial quasi-static compression. It was recognized that mechanical response of the foams and energy absorption is radically defined by the mechanism of cell collapse which, in turn, depends on the nature of structural constituents of the cell wall material. In particular, the presence of brittle eutectic domains in the cell wall material of foam based on Al6Zn2.3Mg alloy results in reducing the compressive strength and energy absorption compared to those of foam processed with Al1Mg0.6Si alloy, both deviate markedly from the theoretical predictions. In spite of this experimental verification of foams cell collapse is considered to be strongly required before their engineering application.
... The microstructure, macrostructure and density of closed cell metal foam influences the mechanical behavior of foams [28,44]. In the present study, the concentration of Al (ρ = 2.74 g/cm 3 ) is added up to 9 wt % in the base alloy Mg-3Ca (ρ = 1.73 g/cm 3 ). ...
... The data for other foams were extracted from Refs. [24,27,28,34,[46][47][48][49]. ...
This study compared the foaming behavior of in-situ 6061-2MgAl2O4 composites processed by ultrasonic treatment and conventional route. Foaming of these composites was performed by melt route using TiH2 as blowing agent. Macrostructural characterization of the foams was performed by combining X-ray tomography and image analysis. The mechanical properties of the foams were extracted by conducting quasi-static compression tests. Foams produced using the ultrasonically-treated composite exhibited superior structure and microstructural strength, resulting in higher compressive strength and energy absorption than those produced using the composite processed by the conventional route.
... This is because tensile membrane stress affecting closed cell faces causes the plateau stress to rise up, making the hardening rate extraordinarily fast [2]. As opposed to the above, more or less hardening/softening sequences are observed within plateau stress, implying cell collapse by fracture [29,30]. ...
The study presents the comparative analysis of the compressive response for the experimental aluminium foams of different parent alloys fabricated by melt processing with/without Ca additive and an expensive conventional TiH2 foaming agent or a cheap alternative CaCO3. It was recognized that the response of the foams is significantly dependent on the type of foaming agent and Ca additive due to the formation of low ductile and brittle products created in the foaming process. The presence of deformation bands and brittle eutectics in material, Al3Ti particles/layers, partially decomposed TiH2, Ca containing compounds, etc. cause a reduction of the foam’s compressive strength and deviation of its mechanical profile from the theoretical predictions. In addition, the usage of an inexpensive CaCO3 foaming agent offers numerous indisputable advantages compared to TiH2, resulting, particularly, in enhancing the energy absorption ability of foams.
... 50%), progressive collapse of cell walls occurs. [56] This is characterized by a distinct plateau region having waviness and serrations, [57] as shown in the enlarged view of Figure 12. ...
The low density of the aluminium foam and high energy absorption make them favourable for automobile, aerospace, and defence industries. However, the melt route for foam fabrication has challenges in achieving homogeneous distribution of pores. Hence, we have adopted the space holder technique using the powder metallurgy methodology to overcome such a challenge. In the current study, we fabricated Al–Zn alloy foams reinforced with varying volume fractions of B4C particles using NaCl as a space holder implemented by hot pressure‐assisted sintering and dissolution. X‐ray computed tomography revealed a homogeneous distribution of pores. The quasistatic compression studies showed that the samples containing a higher volume fraction of pores exhibited higher energy absorption efficiency in the fabricated foam. The maximum energy absorption efficiency (η) achieved was ≈93% for the pristine Al alloy foam with ≈50% porosity, which is ≈11% higher than the η value of 9 vol% B4C samples with similar porosity. Additionally, B4C particles delay the sudden collapse of cell walls and stabilize the compression behaviour. Adding B4C improves the η and strength at higher relative density. This fabrication methodology would help us develop foams with a homogenous pore distribution and regular geometry, achieving highly desirable mechanical properties.
... However, when the cell diameter decreased to a certain extent, the compressive strength increased as the cell diameter decreased. The increase in compressive strength came from the increase in the polymer fraction in the cell walls [51][52][53]. Chen [39] investigated the mechanical behavior of 3D layered nanofoam, and believed that the solid surface had fewer atoms than the inside (so it had excess surface energy). They found as the cell cross-sectional diameter decreased, the influence of surface effects on modulus and strength increased. ...
Cellular media materials are used for automobiles, aircrafts, energy-efficient buildings, transportation, and other fields due to their light weight, designability, and good impact resistance. To devise a buffer structure reasonably and avoid resource and economic loss, it is necessary to completely comprehend the constitutive relationship of the buffer structure. This paper introduces the progress on research of the mechanical properties characterization, constitutive equations, and numerical simulation of porous structures. Currently, various methods can be used to construct cellular media mechanical models including simplified phenomenological constitutive models, homogenization algorithm models, single cell models, and multi-cell models. This paper reviews current key mechanical models for cellular media, attempting to track their evolution from their inception to their latest development. These models are categorized in terms of their mechanical modeling methods. This paper focuses on the importance of constitutive relationships and microstructure models in studying mechanical properties and optimizing structural design. The key issues concerning this topic and future directions for research are also discussed.
... However, for 10TiB 2 -0.1P, the obvious cracking of the cell walls can be observed in the deformation band, which is consistent with the report that the cell wall of 10TiB 2 foams is highly brittle in nature [16]. For 5TiB 2 -0.1P and 10TiB 2 -0.1P, the site of the onset of local deformation usually initiates at larger cells generating a concentration of stress in adjacent areas [21,22]. Nevertheless, 10TiB 2 -0.24P and 10TiB 2 -0.4P foams form deformation bands at 45 degrees to the compression direction. ...
The application of increased pressure was used as a strategy to investigate the effect of different cell structures on the mechanical properties of Al-TiB2 composite foams. In situ Al-xTiB2 (x = 5, 10 wt.%) composites were foamed under three different pressures (0.1 MPa, 0.24 MPa, 0.4 MPa) through the liquid melt route. The macro-structure of the composite foams was analyzed in terms of cell size distribution measured by X-ray microcomputed tomography (micro-CT). It was found that the mean cell size decreases, and the cell size distribution range narrows with increasing pressure. Uniaxial compression tests revealed that the stress fluctuation (Rsd) of 10TiB2 foams is larger than that of 5TiB2 foams under the same pressure. Moreover, cell size refinement causes the simultaneous deformation of multi-layer cells, which leads to an enhancement in the energy absorption efficiency and specific energy absorption. The comparison of experimental data with theoretical predictions (G&A model) is discussed.
... In contrast, although carbonaceous reinforcements had substantially impacted the properties of porous Al, high wettability and bonding strength remained unresolved [21]. The interfacial bonding would affect the microstructure, and the deformation and fracture mechanism of foams would depend upon the microstructure [22]. ...
Porous aluminum (Al) composites are lightweight and high-strength materials composing of Al as a matrix material with some strengthening reinforcements and pore-forming agents that result in the formation of new material with superior physical properties and energy absorption capacities. This work gives an overview of the porous Al-foams developed thus far, including the foaming agents and space holders, their properties, production techniques, and applications. First, it deliberates the foaming agents and space holders responsible for the foaming and formation of pores in the composites followed by the mechanical properties of the foams. Al has huge potential for applications that require lightweight, high-strength, and high-energy absorption capacity materials, especially in structural construction and automobile manufacturing. Although Al-foams have been successfully used in automobiles for crashworthiness, lightweight structure, and other functional applications, the development of Al foams with enhanced characteristics and properties has limitations. This review discusses various reinforcements used for improving the characteristics of Al-foams. This review also provides an overview of various commercial foams and their contribution to several applications. Finally, it attempts to reveal impediments in foam production with suggested solutions for overcoming the problems in this area.
... Cellular solids can maintain low constant stress when experiencing large nominal strain, which have been widely used as energy absorption structures in aerospace, automotive and other fields (Gibson and Ashby, 1999;Lu and Yu, 2003). Thereinto, the most systematically studied and the most frequently used cellular solids are foams (Jang and Kyriakides, 2009;Markaki and Clyne, 2001) and honeycombs (Papka and Kyriakides, 1994;Hu et al., 2013;Khan and Mirza, 2012;Liu et al., 2017).In the past two decades, three-dimensional (3D) lattice structures are identified to be the most promising cellular solids for their superior designability, including superior ability in carrying complex loads and multi-functional properties. (Evans et al., 2001;Fan et al., 2008;Fleck et al., 2010;Schaeder et al., 2011;Zok et al., 2016). ...
Three-dimensional lattice structures have the advantages of light-weight and strong designability, and have good prospects in the application for energy absorption. However, there are seldomdesign methodsof three-dimensional lattice structures forenergy absorption, especially a three-dimensional lattice structure design method for energy absorption based on energy absorption diagrams has not yet been proposed.The present study establishes a mechanical theoretical model of self-supporting lattice structures for additive manufacturing. The aluminum alloy self-supporting lattice structures are fabricated by selective laser melting (SLM). An improved characterization method on energy absorption diagram is proposed. Energy absorption diagram of self-supporting lattice structures isestablished based on finite element analysis (FEA) and uniaxial compression experiments. The results show that the envelope lines of shoulder points of three-dimensional lattice structures with various configurations are different, which variesfrom the foams. A design method of three-dimensional lattice structures for energy absorption is proposed for the first time on the basis of this energy absorption diagram, which is of great significance for the design of energy-absorbing structures in the fields of aerospace, aviation and transportation.
... The mechanical behavior of this class of materials is affected by the loading complexity and its rate [27][28][29][30] and various processing defects (non-uniform cell size and its distribution, cell-wall thickness, curved and wrinkled cell walls, cell wall misalignment, broken cell walls, missing cells, variations in density, microstructures, etc.). Such defects have to be considered when optimizing the foam's performance [31,32]. ...
The aim of this new experimental work was to understand the effect of loading biaxial combined compression-tor-sion complexity on the plastic response of three aluminum foams having porosities of 93%, 85% and 78% and nominal relative densities of 7%, 15% and 22%, respectively. An investigation was made of the biaxial plastic response of these open-cell foams, which have a highly uniform architecture with a spherical porosity. These foams were tested under quasi-static complex loading paths using a patented rig, called ACTP. Biaxial combined compression -torsion loading paths were then applied with different torsional component rates. The key responses to be examined were yield stress, stress plateau, energy absorption capacity, and densification strain. It was revealed that the greater the density of the foam, the higher the loading complexity, and the greater the yield strength and the energy absorption capacity. The highest foam strength was thus recorded under the most complicated loading path (i.e., biaxial 60°) for the densest foam (i.e., 78% porosity). However, the foam with a porosity of 93% demonstrated a lower strength under biaxial loading compared to its uniaxial response. This was due to its small cell wall thickness, which was easily damaged. The effect of the loading complexity on the pore closure mechanism of the deformed foams was studied using an image analysis that targeted the axial and the transverse sections. For the 85% and 78% foams, the loading complexities of biaxial-37°and biaxial-45°provided the highest pore closure compared to the other loading complexities. This analysis supported the interpretation of the densification strain results.
... They increased from 149% and 3.9 MPa to 191% and from 5.1 MPa to 222% and 5.5 MPa to 235% and 5.6 MPa, respectively. This result may have originated from the stress concentration around the large cells within the foamed material, which would induce failure initiation of the material [60]. For example, the TPU perforated membrane saturated under 3.0 MPa was used to separate the polystyrene microsphere so as to evaluate the filtration performance. ...
The way in which a perforated structure is formed has attracted much interest in the porous membrane research community. This novel structure gives materials an excellent antifouling property as well as a low operating pressure and other benefits. Unfortunately, the current membrane fabrication methods usually involve multi-step processes and the use of organic solvents or additives. Our study is the first to offer a way to prepare perforated membrane by using a physical foaming technique with CO2 as the blowing agent. We selected thermoplastic polyurethane (TPU) as the base material because it is a biocompatible elastomer with excellent tensility, high abrasion resistance, and good elastic resilience. Various processing parameters, which included the saturation pressure, the foaming temperature, and the membrane thickness, were applied to adjust the TPU membrane’s perforated morphology. We proposed a possible formation mechanism of the perforated membrane. The as-prepared TPU membrane had good mechanical properties with a tensile strength of about 5 MPa and an elongation at break above 100%. Such mechanical properties make this novel membrane usable as a self-standing filter device. In addition, its straight-through channel structure can separate particles and meet different separation requirements.
... Metallic stochastic foams are widely considered as an ideal material in the protective engineering field due to their low relative density, high specific strength and the superiority in effective energy absorption. There exist considerable published literatures focusing on the deformation [1,2], crushing stress [3], cell morphology [4] and energy absorption capability [5] of uniform aluminum foams through the experimental method. Other metallic foams like metal matrix syntactic foams have also been well studied due to their excellent properties [6][7][8]. ...
This paper presents the study on crushing response of functionally stepwise graded foam under quasi-static and
dynamic compression using three-dimensional (3D) Voronoi model. A series of simulations is conducted to acquire
proper computing parameters and to validate the predictability of numerical model by comparing with
experiment results. Two different strategies are proposed to achieve the gradient change in relative density by
changing equivalent radius and cell-wall thickness. The main focus of present paper is placed on the effect of
gradation configuration on the stress-strain response, deformation propagation and energy absorption. The results
show that the gradation configuration has little effect on the crushing behavior of graded foams under
quasi-static compression. However, the performance of graded foam under dynamic compression could be
promoted by tailoring its gradation configuration. Adopting a negative gradation is beneficial for reducing the
stress at the stationary side and maximizing the energy absorption under low strain level. A positive gradation
would be preferred as an energy absorber under high strain level.
... Nanoindentation was originally developed to characterise the mechanical properties of thin films [14][15][16]. Over time, the method has been increasingly used for a much wider range of bulk and porous materials [17][18][19][20][21][22][23]. Commonly, nanoindentation is J o u r n a l P r e -p r o o f 5 used under one of two measurement regimes: i) quasi-static loading, where hardness and stiffness are calculated from force-depth curves obtained from single, relatively slow indentation events; ii) nano-DMA (dynamic mechanical analysis) tests, assessing loading-rate dependent stiffness, where the loading amplitude is kept constant during frequency sweeps. ...
Struts are the main load carrying elements in cyclically loaded open cell metal foams. Little is known about the local fatigue behaviour and the influence of the microstructure on nanoscale deformation mechanisms. Different to the bulk counterpart, the millimetres-sized struts in open-cell, precision-cast AlSi7Mg0.3 foams contain only 1–2 Al-dendrites, Si-Al-eutectic and intermetallic phases. We applied cyclic nanoindentation to N = 10⁵ to assess nanofatigue. The change in minimum depth per cycle and the ratio of minimum to maximum indentation depth versus the number of cycles correspond to cyclic plastic processes. These and the indent and pile-up morphologies were correlated with the microstructure and dislocation formations were revealed by phase-contrast-enhanced micro-computed tomography and transmission electron microscopy, respectively. Our results reveal that Si-particles affect deformation to a distance of 5 to 10 μm near the indent, and they favour the formation of fatigue induced dislocation cells in this affected volume. We believe that this interaction is mediated through residual stresses. Furthermore, local variations in microstructure strongly influence the cyclic deformation behaviour and the indent pile-up size and morphology. Interestingly, the results well coincide with observations during fatigue of the bulk alloy reported in the literature.
... Therefore, characterizing the solid phase of the foam is an important step in the global understanding of the foam behavior. Several methods have been used in this context, including nanoindentation [10][11][12], micro-mechanical tests on single struts [13][14][15] and X-ray tomography scanning [16][17][18][19] as regards non destructive imaging of the material structure. Standard nanoindentation experiments may allow to quantify the heterogeneity of local Young's modulus and hardness of the different internal phases of the struts, provided that the microstructure length scale is about ten times bigger than the intended indentation depth. ...
The tensile behavior of individual struts extracted from an open-cell aluminum foam is investigated here. X-ray microtomography is used to characterize the initial state of the struts in 3D, before micro-tensile testing performed under digital image correlation. A microstructure-sensitive finite element (FE) model is run afterwards, using a FE mesh conforming to the tomography volume and a constitutive model based on Gurson-TvergaardNeedleman (GTN) porous plasticity. The model is made dependent on the local measure of intermetallic particles volume fraction in order to account for their embrittlement effect. Model and experiments delineate the first order effect of structure and shape on the plastic flow and fracture of the struts. The distribution of intermetallic particles influences fracture location only where minor variations of cross-section can be found. The model performs well at predicting the fracture zones but misses additional ingredients to assess the dispersion of yield strength among the struts.
... Therefore, characterizing the solid phase of the foam is an important step in the global understanding of the foam behavior. Several methods have been used in this context, including nanoindentation [10][11][12], micro-mechanical tests on single struts [13][14][15] and X-ray tomography scanning [16][17][18][19] as regards non destructive imaging of the material structure. Standard nanoindentation experiments may allow to quantify the heterogeneity of local Young's modulus and hardness of the different internal phases of the struts, provided that the microstructure length scale is about ten times bigger than the intended indentation depth. ...
The tensile behavior of individual struts extracted from an open-cell aluminum foam is investigated here. X-ray microtomography is used to characterize the initial state of the struts in 3D, before micro-tensile testing performed under digital image correlation. A microstructure-sensitive finite element (FE) model is run af-terwards, using a FE mesh conforming to the tomography volume and a constitu-tive model based on Gurson-Tvergaard-Needleman (GTN) porous plasticity. The model is made dependent on the local measure of intermetallic particles volume fraction in order to account for their embrittlement effect. Model and experiments delineate the first order effect of structure and shape on the plastic flow and fracture of the struts. The distribution of intermetallic particles influences fracture location only where minor variations of cross-section can be found. The model performs well at predicting the fracture zones but misses additional ingredients to assess the dispersion of yield strength among the struts.
... The mechanical behaviour of closed cell foams is influenced by the base material properties [9], their relative density [10,11] and their microstructural geometrical features [12][13][14][15]. Many authors assessed microstructural effects on the compressive response of foams experimentally [12,[16][17][18][19][20]. ...
This study presents an imaged-based shell modelling strategy for closed cell metallic foams exploiting X-ray Computed Tomography scans, with its illustration on ALCORAS foams. Based on an in situ X-ray CT compression test, the 3D segmentation is complemented by a watershed method and a geodesic reconstruction technique to isolate cells and to identify missing walls. An implicit 3D geometry is reconstructed for each cell based on a distance field computation technique. An automated procedure extracts a shell geometry from this implicit 3D geometry, followed by a finite element meshing step. The resulting FE model is solved using ABAQUS/Explicit with an elasto-plastic-damage constitutive model for the cell walls behaviour, and with consideration of contacts between cell walls. A comparison of the simulation results is conducted with experimental force-displacement curves and scanned deformed configurations. The deformation and failure mechanisms under quasi-static compression are investigated numerically and compared with the result of the in-situ experimental measurement. The simulation is shown to predict the yield zones on both the macroscopic and cell levels. A combination of correlated microstructural morphological features (cell size, cell wall thickness, surrounding cells) is identified as critical in deformation and failure mechanisms.
... The mechanical properties of foams depend on their density and macrostructure [25,35,36]. The strength of the foams is also influenced by the strength of the solid constituent. ...
... A 2D characterization of the cell walls has been performed on Alporas and Alcan foam [8] indicating that the mean cell walls thickness range between 40 lm to 160 lm depending on the density: the higher the density the thicker the cell walls. At microscopic scale the microstructure of the cell walls have been studied by Simone et al. [8] and their influence on mechanical properties has been described by Benaouli et al. [10] On the 3D scale, few studies have been performed and almost no quantitative analyses have been done. Qualitative studies have been performed on IFAM and Norsk-Hydro foams, [11,12] generally to characterize defects and look at deformation mechanisms [11,12] but this was done at a resolution of 150 lm or 230 lm, and no quantitative measurements have been performed. ...
The aim of this paper is to present a methodology for 2D and 3D characterization of different foams using X‐ray microtomography with a resolution of 30 micrometer. 2D and 3D quantitative image analyses have been performed to obtain information about the cells. The main parameters of interest are the cell size, the cell size distribution, morphology of the cells, connectivity of the cells, and fraction of matter at the edges. We use morphological operations such as opening granulometry to separate cells when they are not perfectly closed. This characterization was performed on Alporas, IFAM, and Norsk‐Hydro foam.
... However, presence of inhomogeneities, or microstructural defects in the bulk material can seriously compromise the reliability and performance of these architectured solids in structural applications [3][4][5]. To ensure better design and safe use of cellular solids, it is of immense relevance to develop a deeper understanding of the factors that play a vital role in the the complex processes during failure of these solids. ...
Uni-axial compressive failure of silica-epoxy based heterogeneous honeycombs is investigated in detail for a range of volume fractions. Introduction of heterogeneity in compression of staggered-square honeycomb is seen to result in damage initiation at multiple locations and subsequent damage growth to be more stable compared to pure epoxy in which damage was observed to be localized until peak load when catastrophic failure of the honeycomb specimen occurs. The increase in stiffness and comparative stability of the response is accompanied with reduction in strength, however, between 0-5% the total work of compressive failure is comparable. From the elastic-plastic analysis it is evident that the non-linearity in the response of pure honeycombs, prior to peak load, is largely due to formation of plastic hinges near corners of cells, whereas in case of heterogeneous honeycomb the non-linearity is mostly due to debonding of hard filler particles and matrix cracking leading to damage growth in cell walls.
... Cellular solids, by virtue of their high specific mechanical properties and multifunctionality, find wide applications from cheap daily necessities to critical components in industries such as automobile and aerospace etc. Inspired by the concept of cellular architecture in biological materials such as bone and wood, cellular structures have been successfully designed to exhibit better mechanical properties, such as high specific stiffness, resistance to fracture and energy absorption, than their bulk counterparts [1,2]. However, presence of inhomogeneities, or microstructural defects in the bulk material can seriously compromise the reliability and performance of these architectured solids in structural applications [3][4][5]. To ensure better design and safe use of cellular solids, it is of immense relevance to develop a deeper understanding of the factors that play a vital role in the complex processes during failure of these solids. ...
Uni-axial compressive failure of silica-epoxy based heterogeneous honeycombs is investigated in detail for a range of volume fractions. Introduction of heterogeneity in compression of staggered-square honeycomb is seen to result in damage initiation at multiple locations and subsequent damage growth to be more stable compared to pure epoxy in which damage was observed to be localized until peak load when catastrophic failure of the honeycomb specimen occurs. The increase in stiffness and comparative stability of the response is accompanied with reduction in strength, however, between 0-5% the total work of compressive failure is comparable. From the elastic-plastic analysis it is evident that the non-linearity in the response of pure honeycombs, prior to peak load, is largely due to formation of plastic hinges near corners of cells, whereas in case of heterogeneous honeycomb the non-linearity is mostly due to debonding of hard filler particles and matrix cracking leading to damage growth in cell walls.
... The geometries/topologies of closed-cell aluminium foams are more complex with random shape, size and distribution of pores and cell walls; therefore, they undergo more complex local deformations compared to open-cell foams, honeycombs [14] and lattices [10,[15][16][17][18][19][20]. Since the geometry/ topology of a cellular structure is critical in controlling its mechanical response [21][22][23][24], it is necessary to investigate the geometrical/topological evolution of closed-cell aluminium foams during dynamic loading. ...
The mechanical properties of closed-cell aluminium foams are governed by their geometrical and topological evolution during impact. Here we non-destructively investigate the deformation mechanisms of a closed-cell aluminium foam sample at the cell scale. The sample has been compressed with 21 interrupted drop-weight impacts at a nominal strain rate of 40 s⁻¹ and the post-impacted sample has been imaged at four pertinent strain states with high-resolution X-ray micro-computed tomography (XCT). Moreover, a number of qualitative and quantitative structural analyses are carried out using advanced 3D image analyses to understand the effect of foam geometry/topology on its mechanical response. Our results show that the deforming microstructure creates strong correlations across a range of geometrical, topological and shape characteristics. Quantitative image analyses of the sample at four different strain states reveal that the regions with large void fractions predominantly undergo collapse and subsequently reduces the structural heterogeneity. Further, we show that the deformation mechanism leaves topological signatures in the evolving microstructure, which can be used to better understand the mechanical response of the sample at various stages of impact-deformation. We demonstrate here, for the first time, how the topological quantities can be used to explore the foam deformation mechanisms and to correlate the deformation with mechanical response during impact.
... The base material properties [3], relative density [4,5] and geometrical features [2,6,7,1] are known to affect the mechanical behaviour of closed cell foams with different magnitudes of influence. Nevertheless, the effect of individual features of the microstructure on the macroscopic mechanical response was not yet fully quantified. ...
This contribution addresses the finite element modelling of closed cell metallic foams using Representative Volume Elements (RVEs) based on shell geometries directly extracted from implicitly defined 3D geometries. 3D RVEs of closed cell foam materials are produced by means of a generation strategy allowing a close morphological control reproducing fine scale geometrical features incorporating cell size, cell wall thickness and cell wall curvature distributions. The strategy is built on three computational ingredients: (i) a random packing algorithm based on random sequential addition assisted by neighbour distance control, (ii) a distance field-based shape tessellation (morphing) that allows incorporating cell wall curvatures and varying cell wall thicknesses and (iii) a close control on the shape of the cells. In order to decrease the computational cost of a full 3D finite element model, an original approach is proposed to produce a shell-based geometry directly from 3D information. Extracting the shell geometry from the implicitly defined 3D geometry based on the zero level of distance fields that would represent the cell walls is computationally impossible. Therefore, a novel robust procedure is proposed using careful cutting operations on distance fields for this purpose. The effect of the different microstructural geometrical features of interest on the average mechanical behaviour of the foam is investigated using shell-based finite element analyses. The computational cost and the accuracy of the proposed shell models are then assessed by comparing their results to full 3D simulations. The macroscopic behaviour of the generated shell-based model under compressive loading is then assessed up to the densification stage (including contact), and compared qualitatively with experiments from the literature. The macroscopic behaviour of the shell-based model is explained by linking it to cell/wall level deformation mechanisms.
... The data for other foams used here were extracted from Refs. [3,30,39,40] . Table 3 also shows that the amount of gas released for the former two alloys during semisolid stage is small and during solid stage is negligible when foamed by using 480-12h and 520-6h TiH 2 , respectively. ...
The requirement for an alloy-specific heat treatment of TiH2 for producing foams by the powder metallurgy route was demonstrated in this study. Three heat-treated TiH2 powders were used to produce foams from three Al–Si–Mg alloys having different liquidus temperatures. Dehydrogenation behaviour of heat-treated TiH2 was studied using mass spectroscopy and thermogravimetry analysis. Foams were characterised by means of X-ray tomography. Quasi-static compression tests were employed to test their mechanical property. Using a combination of three alloys and three heat treatments, it was demonstrated that a good foam structure can be obtained when the maximum hydrogen release from TiH2 takes place after the complete melting of the alloy. The amount of hydrogen released during solid and semisolid stages of foaming also play a role in determining the final structure of the foams. It was also observed that such foams with a good structure possess a higher strength compared to the foams produced by using blowing agent that releases maximum hydrogen before complete melting.
Energy absorbers find extensive applications in various industries, particularly in automotive and impact protection sectors. Various techniques have been employed to enhance the performance of energy absorbers, resulting in notable transformations such as the shift from asymmetric to symmetric collapse modes, increased energy absorption capacity, or reduced peak impact forces. In this novel study, the utilization of a burr-filled structure has been introduced as a means to increase the energy absorption capacity. The energy absorbers employed in this research comprised cylindrical bodies with a circular cross-section, which were filled with burr material of identical composition. The absorbers underwent testing at three distinct energy levels, namely 800, 1200, and 1600 Joules. The experimentation encompassed both unfilled samples and those filled with honeycomb structures, with four different mass configurations being considered for the burr fillers. The findings of the study demonstrate that increasing the mass of the burr filler within the absorber leads to a corresponding increase in energy absorption capacity. Specifically, the inclusion of a 40-g honeycomb burr-filler resulted in a notable 40% enhancement in energy absorption. However, the presence of the burr-filler had a minimal impact on the peak force. This is noteworthy as burr-filled structures are relatively lightweight and currently underutilized and waste materials, suggesting that the increased energy absorption achieved through their implementation can offer cost-effective solutions in industrial contexts. To ensure the reliability and accuracy of the observed improvements in absorbed energy, identical experimental samples were also simulated using the finite element software ABAQUS/Explicit, yielding consistent outcomes.
Closed-cell aluminum foams have many excellent properties, such as low density, high specific strength, great energy absorption, good sound absorption, electromagnetic shielding, heat and flame insulation, etc. As a new kind of material, closed-cell aluminum foams have been used in lightweight structures, traffic collision protections, sound absorption walls, building decorations, and many other places. In this paper, the recent progress of closed-cell aluminum foams, on fabrication techniques, including the melt foaming method, gas injection foaming method, and powder metallurgy foaming method, and on processing techniques, including powder metallurgy foaming process, two-step foaming process, cast foaming process, gas injection foaming process, mold pressing process, and integral foaming process, are summarized. Properties and applications of closed-cell aluminum foams are discussed based on the mechanical properties and physical properties separately. Special focuses are made on the newly developed cast-forming process for complex 3D parts and the improvement of mechanical properties by the development of small pore size foam fabrication and modification of cell wall microstructures.
The demand for advanced transportation in modern communities is increasing day by day. This has led to a steady increase in the number of vehicles on the roads. After traffic accidents, the death and disability rates of living things are quite high all over the world. Energy absorbers used in various transportation vehicles, especially automobiles, such as trains and light rail vehicles, are one of the most important factors that protect the living life and health from bad situations that may occur during an accident. With increasing customer demand and government regulations, more attention has recently been drawn to improving structural collision resilience to reduce passenger deaths and injuries.
In recent years, many studies have been carried out to increase the collision performance of energy absorbers. In these studies, it is aimed that energy absorbers can basically absorb the force and energy that will occur in the vehicle during the collision as much as possible, thus minimizing the damage to the living beings in the vehicle.
When designing energy absorbers, it is noted that they can protect passengers by converting most of the kinetic energy into other forms of energy in a predictable and controllable way during a collision. However, the high amount of total energy absorbed is that the resulting forces are low and close to the average impact force.
Finite element analysis was performed by using three different materials and four different wall thicknesses of six different geometric models determined as a result of literature research within the scope of the thesis study. In addition, for each of these simulation conditions, the capabilities of the geometric model with energy absorption were examined by filling aluminum foam filling. In the design phase, Catia-V5 software were used. Visual Mesh, Visual Performance Solution and Visual Viewer software were used to define finite element model, the simulation boundary conditions and to examine the results obtained. JMatPro software was used to obtain material data suitable for collision simulation.
In this study, the effects of alloy elements, oxide phases, and created intermetallic compounds of aluminum alloy foams with different percentages of silicon, zinc, and combinations of both these elements were investigated on the compressive properties and fracture behavior of these foams. The results showed that the addition of alloying elements formed intermetallic compounds in the structure of the foam, and the number of intermetallic compounds increased with increasing the percentage of these elements which causes inconsistency between these compounds and base materials. All these factors changed the failure mechanism of these foams so that the failure behavior of pure aluminum foam and zinc-containing foams was a ductile failure, whereas the failure behaviors of silicon-containing foam samples were brittle. The Al–4%Si–4%Zn sample had the highest compressive strength among all of the fabricated samples due to the uniformly distributed intermetallic compounds in the cell wall of these samples, whereas the Al–8%Si, Al–8%Si–4%Zn, and Al–8%Si–8%Zn foam samples were more brittle than the other fabricated foam samples due to the more formation of brittle compounds such as CaAl2Si2 and their accumulation in a certain area which reduced the compressive strength of these foams. Also, it was observed that the energy absorption capacity and yield strength of pure aluminum foam generally increased with the addition of alloy elements due to the increase in the amount of created intermetallic compounds in the structure of these foams.
This experimental work aimed to understand the effect of a biaxial combined compression-torsion loading complexity on the mechanical properties of two types of aluminum foams having porosities of 85% and 80% and nominal relative densities of 15% and 20%, respectively. In this investigation, open-cell aluminum foam of highly uniform architecture with a spherical porosity was designed and used to investigate the biaxial plastic response. These foams were tested under different quasi-static complex loading paths using a patented rig, called ACTP with different torsional component rates. The key responses to be examined were yield stress, stress plateau, energy absorption capacity, densification strain, micro-hardness, and microstructure. It was revealed that the greater the density of the foam with the higher the loading complexity, the greater the yield strength, the energy absorption capacity, and the micro-hardness. The highest foam strength was thus recorded under the most complicated loading path (i.e., biaxial 60°) for the densest foam (i.e., 80% porosity). This was due to the variation in the cell wall thickness. In addition, the effect of the loading complexity on the microstructure was studied using SEM. The loading complexity of biaxial-45° provides higher particle segregation at the grain boundary and a larger densification strain for the 85% porosity foams.
Aluminiumschäume mit kompakter Außenhaut zeichnen sich sowohl durch eine hohe gewichtsspezifische Biegesteifigkeit als auch ein hervorragendes Energieabsorptionsvermögen aus. Darüber hinaus weisen sie ausgeprägte akustische Dämpfungseigenschaften auf. Die auf Basis des Druckgießens entwickelte Verfahrensmodifikation des Integralschaumgießens erlaubt hierbei die direkte Herstellung geschäumter Bauteile mit kompakter Außenkontur: eine metallische Dauerform wird mit einer Aluminiumschmelze bei hoher Geschwindigkeit und unter turbulenter Einwirbelung des pulverförmigen Treibmittels Magnesiumhydrid gefüllt. Die hohen Erstarrungsgeschwindigkeiten an der temperierten Formwand unterbinden die thermische Zersetzung des Treibmittels zu Magnesium und Wasserstoff, wodurch sich eine dichte Randschicht ausbildet. Demgegenüber führt die Freisetzung des Wasserstoffs im Kernbereich zur Ausbildung von Poren zeitgleich zur hier leicht verzögert erfolgenden Erstarrung der Legierung. Zentrales Ziel dieser Arbeit stellt die Reduzierung der Porengröße unter Beibehaltung des Porositätsgrads zur prozesssicheren Herstellung sog. mikrozellularer Aluminiumintegralschäume dar. Durch Vermeidung von Inhomogenitäten mit größeren Poren, wie sie in den Strukturen von Standardaluminiumschäumen statistisch auftreten, bietet sich nicht nur das Potential einer Reduzierung der Streuung mechanischer Kennwerte, sondern auch der Fertigung dünnwandigerer Komponenten. Denn eine ausreichend hohe Anzahl an Poren über dem Bauteilquerschnitt ist Voraussetzung dafür, lokale Schwachstellen zu verhindern. Erreicht werden soll dies durch den Einsatz unterschiedlicher Treibmittelpartikelgrößen und -größenverteilungen, sowie durch die Einbringung einer größeren Pulvermenge in die Schmelze. Im Rahmen der vorliegenden Arbeit werden hierzu Erkenntnisse im Hinblick auf die Vorgänge bei der Porenkeimbildung, des Porenwachstums und der Porenkoaleszenz gewonnen. Ein besonderes Augenmerk wird dabei auf den Mechanismus der endogenen Stabilisierung der sich während der Schaumentstehung ausbildenden Zellstege gelegt. Dies geschieht durch die in der Legierungsschmelze primär erstarrenden α-Phasenkörner, welche der Vereinigung von Einzelporen entgegenwirken. Das Zusammenspiel von Schmelzeerstarrung und Treibmittelzersetzung bestimmt dabei die Porositätsverteilung im Bauteilquerschnitt wie auch die Ausprägung der Randschicht. Hierzu erfolgt der Einsatz zweier Legierungen (AlSi9Cu3(Fe) und AlMg5Si2Mn) mit unterschiedlichem Erstarrungsverhalten in Kombination mit verschiedenen, hinsichtlich des Zersetzungsverhaltens modifizierten Treibmittelvarianten. Um die Kinetik des kommerziell erhältlichen Magnesiumhydridpulvers Tego Magnan® zu verändern, wird dieses entweder zur Erhöhung der Reaktivität einem Mahlprozess oder zur Phlegmatisierung einer gezielten Oxidation unterzogen. Dies erlaubt letztendlich die Herstellung von Integralschaumstrukturen mit unterschiedlicher Deckschichtausprägung sowie Dichteverteilung über der Erzeugnisdicke und damit die gezielte Einstellung der Eigenschaften der Leichtbaustruktur, abgestimmt auf das spezifische Anwendungsgebiet. So erweist sich ein ausgeprägter Deckschichtanteil als vorteilhaft für eine erhöhte Biegesteifigkeit, wohingegen eine geringe Ausprägung der kompakten Außenhaut das Dämpfungsvermögen verbessert. Mit Hilfe der Gusssimulationssoftware Flow-3D® (Flow Science, Inc.) werden im Rahmen dieser Arbeit nicht nur Informationen zur Verteilung der Pulverpartikel in der Schmelzeströmung gewonnen, sondern auch Erkenntnisse hinsichtlich der thermischen Verhältnisse beim Integralschaumgießen in Abhängigkeit von der eingesetzten Legierung gesammelt. Letzteres in Kombination mit der ausgehend von Thermogravimetriemessungen ermittelten Zersetzungskinetik der Treibmittelmodifikationen ermöglicht schließlich die Erklärung für die Ausbildung unterschiedlicher Strukturen in Abhängigkeit von den jeweiligen Prozessbedingungen.
Aluminum alloy foams with high specific compressive strength and good energy absorption capacity are widely used as structural materials and shock absorbers. The investment casting process offers an interesting opportunity for the production of open-pore aluminum foams. However, foam casting differs from casting of solid parts, and there is a lack of practical knowledge about the microstructure tuning of the ultra-thin cast struts. Our study focuses on the influences of the casting conditions such as mold temperature on the microstructure and consequently on the mechanical properties of aluminum-silicon alloy foams. We investigated microstructural features using various metallography techniques and defined mechanical properties under uniaxial compression test. Here, we observed improvements in the strut microstructure together with decrease of the mold filling by reduction of the mold temperature. Microstructure improvements involve transformation of a single dendritic aluminum grain in each strut to globular grains along with a more homogeneous distribution of the silicon particles across the strut cross-section. These changes bring higher ductility and energy absorption efficiency to the foams, which are evident from the smoother plateau of the stress-strain curves. Decline of the mold filling, on the other hand, has negative effect on the overall mechanical properties.
Aluminum foam featured with high specific stiffness, high specific strength and efficientenergy absorption has been employed in a wide variety of applications including metallurgy,construction, traffic, chemical industry, communication, etc. The mechanical property ofaluminum foam especially under the impact condition is a focus in the related researches.Currently, the measurement of dynamic mechanical property of aluminum foam mainlyreplies on the split Hopkinson pressure bar technology. The SHPB-based analysis stems fromthe assumptions of one-dimensional wave propagation and stress equilibrium in the bars. Thestrain of specimen refers to the average values calculated according to the difference ofdisplacements between the ends of incident bar and transmission bar, simply assuming auniform deformation within the specimens. The deformation of aluminum foam may be notuniform under high strain rate for abundant pores existed in specimen, which means that itmay be not satisfied with the assumptions of SHPB technology during the SHPB test. Inaddition, the researches on the effect of strain rate of aluminum foam at home and abroad arecontradictory. In order to objectively evaluate the validity of SHPB technology on aluminumfoam and the effect of strain rate, we must know the characteristics of meso-structuraldeformation during the SHPB impact test. Focusing on this key point, we carry out ourstudies based on the observation of meso-structural and the simulation of SHPB tests withmodeling the meso-structure of aluminum foam. The details of the research include:(1) We have built a testing system combining SHPB with high-speed photography tomonitor the in-situ deformation of aluminum foam during SHPB impact test. The in-situdeformation of aluminum foam during SHPB test was successfully monitored by the designof synchronization between SHPB and high-speed photography, the lighting, the surfacetreatment of specimen and so on. The localized and global deformation of specimen wasquantitatively measured by the image-processing technology. Through the experimentalsystem, the relationship of stress and strain of aluminum foam at various strain rates weremeasured by the traditional method. The collision between aluminum foam specimen andincident bar, especially their meso-structure deformation and failure characteristics, wereobserved during SHPB test. The experimental result showed that the unapparent damage ofaluminum foam was found in the initial impact and further damage was caused by thesubsequent multiple impacts. An analysis of the strain distribution in different times ofimpacts revealed that the deformation of aluminum foam was nonuniform with the localized strain two or three times higher than the average strain.(2) For in-depth understanding of the meso-structure deformation features of aluminumfoams in the SHPB test, the finite element models including the specimen, using3D voronoisimulated the meso-structure, and SHPH system were developed. The simulation resultshowed that apparent deformation of aluminum foam occur mainly in the ends of thespecimen: when the strain rate is relatively low, it happens mainly in the specimen’s end nearto the transmission bar, whereas it changes to concentrate in the end near to the incident bar atthe high strain rate; a nearly symmetrical strain distribution between both ends of thespecimen within the strain rates. To quantitatively describe the heterogeneity of aluminumfoam, the ratio of localized deformation to average strain is difined in this paper. Within thelow-middle strain rates (41/s-585/s) studied in the paper, the maximum localized strain ofaluminum foam with relative density of0.2is two or three times of the average strain, whichindicated that the actual strain rate can reach two or three times of the nominal strain ratesduring the SHPB test. This fact can be used as reference to correct the SHPB experimentresults.(3) In order to investigate strain rate effect of aluminum foam, building a SHPB modelwith meso-structure of aluminum foam specimen and assuming that aluminum foam matrix isno strain rate sensitivity, the relationship of stress and strain of aluminum foam at differentimpact speeds were measured by simulation of the SHPB test system. The inertia effect ofaluminum foam matrix and character of meso-structure deformation of aluminum foam weremeasured quantitatively, which indicated that the strain rate sensitivity of aluminum foam wascaused by the nonuniform deformation of meso-structure and inertia effect of matrix material.As a result, the strain rate sensitivity of aluminum foam calculated by the signal on theincident bar and transmission bar was explained well. Furthermore, the relationship betweenthe heterogeneous degree R and the nominal strain rate sensitivity mof aluminum foam wasanalyzed quantitatively, which complied with the linear relationship during a certain strainrate range.
In the present work, porous Mg was fabricated via powder metallurgy technique with the aids of polymethylmethacrylate (PMMA) as the space holder material in establishing a desirable porous structure at various proportions of 10 wt%, 30 wt%, and 50 wt%, respectively. The microstructure of porous Mg revealed a complete spherical closed-cell structure with the addition of 10 wt% and 30 wt% of PMMA whereas irregular open-cell structure were observed with the addition of 50 wt% of PMMA. On the other hand, the densities of sintered porous Mg decreased from 1.209 g/cm3 to 0.923 g/cm3 when the PMMA content was increased from 10 wt% to 50 wt%, respectively. In contrast, the porosities of porous Mg increased from 9.88% to 48.40% as the content of PMMA was increased from 10 wt% to 50 wt%, respectively. Moreover, XRD analysis detected the formation of Mg and MgO phases. Finally, the compressive strength and energy absorption of the established porous Mg enhanced from 17.67 MPa and 1.18 MJ/m3 to 55.44 MPa and 2.42 MJ/m3 when the PMMA was increased from 10 wt% to 30 wt%, respectively. However, the addition of 50 wt% of PMMA reduced the compressive strength and energy absorption of established porous Mg to 8.79 MPa and 0.97 MJ/m3 due to the formation of irregular open-celled pores that easily propagate thus fractured at much lower stress. Therefore, based on the current findings, 30 wt% of PMMA content was considered as the optimum in establishing porous Mg with desirable microstructure, density, and compressive properties.KeywordsPorous MgPowder metallurgy techniquePMMAMicrostructureDensityCompressive properties
In this study, the corrosion resistance of AZ31B magnesium alloy was evaluated against time. Because of its sensitivity to corrosion, AZ31B magnesium alloy was chosen to be coated with elephoretic coating method. With the electrophoretic coating method, it is possible to coat the alloy surfaces practically with biocompatible materials in one step. The characterization of the alloy surfaces has been changed in order to eliminate the susceptibility of magnesium to corrosion. Surface characterization has been made superhydrophobic and hydrophobic by coating the alloys. Stearic acid and magnesium nitart-containing coating materials are aimed to reduce the surface energy of alloys. It has been shown that the corrosion resistance of the surface coated alloys is higher than the uncoated alloy samples. To discuss the authenticity of the thesis, it was observed for the first time that AZ31B alloys were kept in corrosive liquids for a long time and preserved their hydrophobic properties thanks to the coating. Since AZ31B alloy is used as a bio-implant raw material, corrosion has been evaluated for the first time in an environment simulating body conditions. Coated AZ31B samples have been shown to retain their stabilization in DMEM for approximately one month. It has been shown that K.10AZ31B and K.12AZ31B alloys taken as coating samples have superhydrophobic surface characterization, and K.8AZ31B and K.D.10AZ31B samples are hydrophobic coated. For the first time, AZ31B samples were coated at these concentrations. It has been shown that the coating samples were made successfully and the coatings continued for a long time both in 3.5% NaCl environment and in DMEM containing antibiotics.KeywordsAZ31BCorrosion resistanceElectrophoretic coatingStearic acidBio-implant
Metal foams possess remarkable properties, such as lightweight, high compressive strength, lower specific weight, high stiffness, and high energy absorption. These properties make them highly desirable for many engineering applications, including lightweight materials, energy-absorption devices for aerospace and automotive industries, etc. For such potential applications, it is essential to understand the mechanical behaviour of these foams. Producing metal foams is a highly challenging task due to the coexistence of solid, liquid, and gaseous phases at different temperatures. Although numerous techniques are available for producing metal foams, fabricating foamed metal still suffers from imperfections and inconsistencies. Thus, a good understanding of various processing techniques and properties of the resulting foams is essential to improve the foam quality. This review discussed the types of metal foams available in the market and their properties, providing an overview of the production techniques involved and the contribution of metal foams to various applications. This review also discussed the challenges in foam fabrications and proposed several solutions to address these problems.
In this study, aluminum foams reinforced with multi-walled carbon nanotubes (0%, 0.5%, 1%, and 2% by weight) were produced by powder metallurgy method using different proportions of spherical urea (15%, 30%, and 50% by weight) as space holder. It analyzes the pore morphology and pore distribution of the produced composite foams and examines their mechanical properties under qua-static compressive loading. The results show that the effect of multi-walled carbon nanotubes existing in the cell wall on pore morphology and pore distribution was insignificant. The highest hardness value (65 HV) was determined in the foam samples containing 2% multi-walled carbon nanotube produced with 15% urea. Composite aluminum foam samples with 30–69% porosity and 0.84–1.90 g cm−3 density were successfully produced. The compression properties of the samples decreased with the decrease in the relative densities.
The mechanical signals sensed by the alveolar cells through the changes in the local matrix stiffness of the extracellular matrix (ECM) are determinant for regulating cellular functions. Therefore, the study of the mechanical response of lung tissue becomes a fundamental aspect in order to further understand the mechanosensing signals perceived by the cells in the alveoli. This study is focused on the development of a finite element (FE) model of a decellularized rat lung tissue strip, which reproduces accurately the mechanical behaviour observed in the experiments by means of a tensile test. For simulating the complex structure of the lung parenchyma, which consists of a heterogeneous and non-uniform network of thin-walled alveoli, a 3D model based on a Voronoi tessellation is developed. This Voronoi-based model is considered very suitable for recreating the geometry of cellular materials with randomly distributed polygons like in the lung tissue. The material model used in the mechanical simulations of the lung tissue was characterized experimentally by means of AFM tests in order to evaluate the lung tissue stiffness on the micro scale. Thus, in this study, the micro (AFM test) and the macro scale (tensile test) mechanical behaviour are linked through the mechanical simulation with the 3D FE model based on Voronoi tessellation. Finally, a micro-mechanical FE-based model is generated from the Voronoi diagram for studying the stiffness sensed by the alveolar cells in function of two independent factors: the stretch level of the lung tissue and the geometrical position of the cells on the extracellular matrix (ECM), distinguishing between pneumocyte type I and type II. We conclude that the position of the cells within the alveolus has a great influence on the local stiffness perceived by the cells. Alveolar cells located at the corners of the alveolus, mainly type II pneumocytes, perceive a much higher stiffness than those located in the flat areas of the alveoli, which correspond to type I pneumocytes. However, the high stiffness, due to the macroscopic lung tissue stretch, affects both cells in a very similar form, thus no significant differences between them have been observed.
Aluminum (Al) metal is highly reactive but has excellent corrosion resistance because of the formation of a self-healing passive oxide layer on the surface. Here, we report that this native aluminum oxide shell can also stabilize and strengthen porous Al when the ligament (strut) size is decreased to the submicron or nanometer scale. The nanoporous Al with native oxide shell, which is a nanoporous Al-Al 2 O 3 core-shell composite self-organized in a galvanic replacement reaction, is nonflammable under ambient conditions and stable against coarsening near melting temperatures. This material is stronger than conventional foams of similar density consisting of pure Al or Al-based composites, and also lighter and stronger than most nanoporous metals reported previously. Its light weight, high strength, and excellent stability warrant the explorations of functional and structural applications of this material, if more efficient and scalable synthesis processes are developed in the future.
Closed-cell aluminum foams are gaining widespread acceptance, especially in automotive, aviation and defence sectors, by virtue of their high strength-to-weight ratio, good energy absorption performance and affordable manufacturing costs, which makes them a shock absorber material with enormous potential. Microstructural parameters such as cell size, cell aspect ratio, cell wall thickness variation, cell wall geometry and cell shape irregularity, have a by no means negligible influence on the compressive response of closed-cell aluminum foams. The most recent studies, both experimental and numerical, on the macroscopic properties-microstructure relationship are scrutinized and their results are discussed in this work.
The static compression of porous aluminum specimens, with free vertical surfaces fixed in a cylindrical holder, was investigated both theoretically and experimentally. Calculation were performed using the principle of 3D similarity of the stress-strain state in structural elements of constructions. Such an approach allows one to take into account the nonuniform distribution of stresses and strains caused by pores and to vary the small number of structural elements (representative volumes) in calculations, while retaining the porosity index and the characteristic dimensions of construction. Based on the calculation-experimental method, properties of the carcass material were identified using the results of compression tests of the porous specimens. The numerical results obtained are compared with experimental data, and they testify to the effectiveness of the principle of 3D similarity for this class of problems.
7075 Al-SiO2 composite foams were successfully produced by using low-cost materials, including as-received pressed blocks of recycled beverage aluminum cans and silica waste particles as a reinforcement and thickening agent. The composite foams were produced by using direct foaming of melt method. Different percentages of foaming agent and SiO2 particles were used to optimize the properties of the produced foams. The compressive behavior at different strain rates of the composite foams was investigated. At all strain rates studied in the present work, the addition of SiO2 particles into 7075 Al foam considerably enhanced the compressive properties of the composite foams, including the compressive plastic stress, plateau stress, modulus of elasticity, normalized modulus, energy absorption and energy absorbing efficiency without increasing in the relative density of the foams. The compressive properties of the composite foam produced by using low-cost materials, in the present work, are comparable or higher than those of the aluminum alloy foams reported in the literature. The composite foams exhibited higher strain hardening exponents in comparison with those of the 7075 Al foams, indicating that a strong interfacial bond between 7075 Al alloy and SiO2 particles was developed. The compressive plastic stress and plateau stress of the investigated foams exhibited a significant sensitivity to altering the strain rate, even at low and narrow range of strain rate used in the present work.
In this paper, two types of the foam-filled tubes (FFTs), including O-FFTs and T6-FFTs, were prepared by directly inserting Al foams (AFs) into 6061-O Al alloy empty tubes (O-ETs) and 6061-T6 Al alloy empty tubes (T6-ETs), respectively. Quasi-static compression tests of studied specimens were carried out using an environmental chamber under the temperatures ranging from 25 °C to 250 °C. The results show that the mechanical properties of FFTs decrease with temperature increasing. The T6-FFTs show better energy absorption capacities while the deformation modes are less stable than the O-FFTs. The T6-ETs and T6-FFTs change the deformation mode when the temperature is higher than 150 °C. Both of the number of defects and the lengths of cracks decrease for the T6-FFTs with the increment of temperature. A couple of criteria are proposed to effectively describe and predict the deformation mode of FFTs under quasi-static compression.
In this study, ternary Al-12Si-0.6Mg material was manufactured by gravity die casting method in inductionmelting furnace. Microstructure images of alloy were taken on optical microscope after T6 heat treatment.Hardness, yield and tensile strength and breaking elongation of as-cast and heat-treated materials weremeasured by universal methods. CNC lathe was used for cutting tests and dynamometer was used tomeasure cutting force. Cutting tests were performed by using different cutting speeds-CS (450-500-550 m/min), feed rates-FR (0.05–0.15-0.25 mm/rev) and constant depth of cut-DOC (1.5 mm). Uncoated (A), CVD-TiCN + TiN (B) and PVD-TiAlN+TiN (C) coated carbide inserts were selected as a cutting tool. In themicrostructural observations, it was determined that the structure of the material made up of aluminumrichα, primary and eutectic silicon,δ(Al4FeSi2)andπ(Al8Mg3FeSi6) phases. The heat treatment refined thephases in the structure of the alloy. In addition, it has been determined that it improves mechanicalproperties (hardness, yield and tensile strength) by spheroidizing silicon particles. As a result of the cuttingtests, it was detected that the cutting force (CF) reduced with T6 heat treatment at all CS and FR values. TheCF, BUE (Built up edge) and BUL (Built up layer) heightened with increasing FR, while it reduced withincreasing CS on all cutting tools. CF, BUE and BUL were formed at least in tools A, B and C, respectively.While continuous chip formation was detected in the as-cast part, brittle chip formation was observed in theheat-treated part due to the reduction in breaking elongation of the material.
Static and low velocity instrumented impact tests were carried out on Al-12Si foam / Al 2 O 3 laminates. Different support, specimen and indenter / impactor geometries were employed, yielding a two mode response of the laminate to loading. The first failure mode involved local crushing of the cross-section, and the second mode, flexural deformation. For both modes, laminates were found to exhibit similar responses and damage patterns when tested under static and dynamic loading. Energy absorption efficiency was significantly higher for the first mode. In this case, the extent of damage inflicted by static and impact testing was assessed and compared.
The mechanisms of compressive deformation that occur in both closed and open cell Al alloys have been established. This has been achieved by using X-ray computed tomography (CT) and surface strain mapping to determine the deformation modes and the cell morphologies that control the onset of yielding. The deformation is found to localize in narrow bands having widths of order of a cell diameter. Outside the bands, the material remains elastic. The cells within the bands that experience large permanent strains are primarily elliptical. A group of cells work collectively to allow large localized deformation. Size does not appear to be the initiator of the deformation bands. Equiaxed cells remain elastic. The implications for manufacturing materials with superior mechanical properties are discussed.
The impact response of laminated composites consisting of alternate layers of Al alloy foam and Al2O3 was studied experimentally in low and intermediate velocity regimes. Low velocity impacts (1.2–2.8 m s-1) were conducted using an instrumented falling weight apparatus and were compared with static indentation tests (0.2×10-4 m s-1). Intermediate velocity impacts were carried out by means of both Hopkinson bar (60 m s -1)and gas gun (200 m s-1) tests. Postimpact damage was assessed using X-ray radiography and microscopy. It was found that there is good correlation between low velocity impact and quasistatic responses. In both cases, penetration of the layered targets resulted in the formation of a distinctive plug. Increasing impact velocity (intermediate velocity range) switched the penetration mode from plugging to fragmentation, giving rise to an increase in the absorbed energy. In this range, impacts led to localisation of damage in the region under the projectile. Furthermore, a comparison has been made between the penetration response of foam laminates and dense metal laminates of equivalent areal density. Preliminary results suggest that the dense metal laminates are superseded by the foam laminates on an energy absorption basis.
Na+ currents recorded from Xenopus oocytes expressing the Na+ channel subunit alone inactivate with two exponential components. The slow component predominates in monomeric channels, while coexpression with the 1 subunit favors the fast component. Macropatch recordings show that the relative rates of these components are much greater than previously estimated from two-electrode measurements (30-fold vs 5-fold). A re-assessment of steady-state inactivation, h
(V), shows that there is no depolarized shift of the slow component, provided a sufficiently long prepulse duration and repetition interval are used to achieve steady-state entry and recovery from inactivation, respectively. Deletion mutagenesis of the 1 subunit was used to define which regions of the subunit are required to modulate inactivation kinetics. The carboxy tail, comprising the entire predicted intracellular domain, can be deleted without a loss of activity; whereas small deletions in the extracellular amino domain or the signal peptide totally disrupt function.
This article describes investigations of the relations between mechanical properties (stiffness and strength) and structure of the cells of foam materials. A new analytical approach based on shell theory was used to study closed cell foams and the finite element method was used to study open cell and closed cell foams. Results were compared with experimentally obtained data for copper and polymethacrylimide foams and with results of prior analytical methods. Experimental results lie within the region bounded by the predicted behaviour of fully closed cell and fully open cell foams.
Foamed aluminium -an isotropic highly porous metallic material with a cellular structure has been prepared by powder metallurgical method. Its typical density lies in the range of 0,3-1 g/cm 3 . The porosity is essentially spherical and closed with the average pore size between 1-8 mm depending on matrix composition and heating rate during foaming. Foamed aluminium is very efficient in sound absorption, impact energy absorption, electromagnetic shielding and vibration damping. Mechanical and physical properties depend strongly on the foam density. Modulus of elasticity of 4-13 GPa, plastic collapse stress of 3-17 MPa, thermal conductivity of 4-15 W/mK, electric conductivity of 3-5.10 6 S.m -1 , have been obtained for the densities of 500-800 kg m -3 . The relationship between density and properties seems to obey the power-law dependence with an exponent of about 1.6. Foamed aluminium is form-stable also at elevated temperatures, even at temperatures considerably higher than the melting point of the metal matrix.
Cellular metals have ranges of thermomechanical properties that suggest their implementation in ultralight structures, as well as for impact/blast amelioration systems, for heat dissipation media and in acoustic isolation. The realization of these applications requires that the properties of cellular metals be understood in terms of their manufacturing constraints and that their thermostructural benefits over competing concepts be firmly established. This overview examines the mechanical and thermal properties of this material class, relative to other cellular and dense materials. It also provides design analyses for prototypical systems which specify implementation opportunities relative to competing concepts.
Microstructural developments in discontinuously reinforced aluminium composites have been monitored during a variety of thermo‐mechanical treatments. The material, made by powder blending or casting, followed by extrusion, contains particulate or short‐fibre alumina in a matrix of commercial purity aluminium. Attention is centred on the distribution of reinforcement, the fine alumina particles derived from the prior oxide layers on the aluminium powder, and the effects that these inclusions have on cavitation, recovery and recrystallization processes. Optical metallography, by viewing anodized sections in polarized light, is used to reveal grain and coarse subgrain structures. Scanning electron microscopy has also been employed to reveal fine oxide stringers, cavities and (in backscattered mode) fine subgrain structures. The emphasis in this paper is on the type of information that can be obtained for these materials from careful application of standard microscopy techniques.
The elastic constants of an open-cell foam model, having tetrakaidecahedral cells on a BCC lattice, were found. The Young's modulus, shear modulus and Poisson's ratio were derived, as functions of the edge cross section and the foam density, by considering the bending, twisting and extension of the cell edges. If edge bending were the only mechanism the moduli would vary with the square of the foam density. The other deformation mechanisms are predicted to reduce the power law exponent by 3–5%, and the effect of edge torsion on the modulus level is small. The foam bulk modulus is predicted to vary linearly with its relative density, so Poisson's ratio approaches 0.5 at low densities. The lattice model is elastically isotropic, whereas other lattice models of foams are highly anisotropic.
The high strain compression of open-cell foams is analysed, using a lattice model with tetrakaidecahedral cells. The buckling of elastic cell edges, under combined bending and torsional loads, is analysed, and the deformed shapes predicted. The stress-strain relation and Poisson's ratio are predicted for strains up to 70%, for compression in the [001] and [111] directions of the BCC lattice. The prediction for [111] compression is closest to experimental stress-strain curves for polyurethane foams, especially when the cell shape anisotropy is taken into account. A plateau in the compressive stress-strain curve is not predicted, whereas one is observed for compression along the foam rise direction. Reasons for this are discussed; there is a small contribution from the polymer non-linearity, but irregular cell structures need to be analysed. The deformation, both of individual cells and of the PU foam, is compared with the theory.
Cell ellipticity and non-planar membranes are thought to be dominant degradation factors of cellular metals. The mechanisms operating at the cell/membrane level in both closed and open cell Al alloys have been established by following the evolution of deformation with cell-level resolution using digital image correlation analysis.
A micromechanical analysis for the linear elastic behavior of a low-density foam with open cells is presented. The foam structure is based on the geometry of a Kelvin soap froth with flat faces: 14-sided polyhedral cells contain six squares and eight hexagons. Four struts meet at every joint in the perfectly ordered, spatially periodic, open-cell structure. All of the struts and joints have identical shape. Strut-level force-displacement relations are expressed by compliances for stretching, bending, and twisting. We consider arbitrary homogeneous deformations of the foam and present analytic results for the force, moment, and displacement at each strut midpoint and the rotation at each joint. The effective stress-strain relations for the foam, which has cubic symmetry, are represented by three elastic constants, a bulk modulus, and two shear moduli, that depend on the strut compliances. When these compliances are evaluated for specific strut geometries, the shear moduli are nearly equal and therefore the elastic response is nearly Isotropic. The variational results of Hashin and Shtrikman are used to calculate the effective Isotropic shear modulus of a polycrystal that contain grains of Kelvin foam.
The mechanical properties of three different commercially available closed cell Al alloys all made by foam casting are examined. The objective is to assess the roles of cell morphology and of imperfections in governing the basic properties: stiffness, yield strength and fracture resistance. This assessment provides goals for manufacturing strategies that enable attainment of good mechanical performance with affordable process technologies. A prevalent role of curves and wiggles in the cell walls on stiffness and strength (anticipated by models) is affirmed by the present measurements. Systematically larger stiffnesses and yield strengths found in tension than in compression are consistent with a prominent role exerted by such imperfections. Moreover, foam casting is apparently capable of cell morphologies that impart properties approaching the best achievable values for an isotropic closed cell solid, devoid of imperfections. There are associated implications for performance and affordability. Fracture measurements indicate crack growth occurring along the cell walls by a mechanism analogous to the plastic tearing of thin sheets. The crack growth resistances are in the range of 1 kJmâ»Â². This mechanism infers a toughness that scales with the cell wall thickness and its yield strength.
The plastic flow characteristics of cellular materials containing a large proportion of voids are discussed. A very low Poisson's ratio and a tendency toward orderly cellular collapse are identified as important characteristics of such materials.Compression studies reveal the appearance of slightly inclined Lüder's-like bands and upper and lower yield points resembling those observed in mild steel. A simple cellular collapse mechanism is shown to be consistent with all observed behavior.The yield criterion for a cellular material is found to be the maximum compressive stress when experiments are performed on foamed polystyrene under a variety of loading conditions.The deformation properties of cellular materials are also investigated for conditions of localized loading. The ratio of hardness to compressive flow stress that is normally close to three for ordinary metals is found to be close to one for a cellular material having a very low Poisson's ratio. The ratio of hardness to flow stress is shown to vary from one to three as the plastic Poisson's ratio goes from 0 to .
Metallic foams, a kind of cellular solid which does not exist naturally like wood, coral and sponge, can be used as new functional materials because of the unique combination of properties which can be derived from their cellular structure. Many methods, including casting, powder metallurgy and metallic deposition, are available to produce this material. Especially two casting methods, low pressure infiltration (LPI) and foaming technique, are relatively useful for commercial production of this special material for their low cost. In the authors recent report, the difficulty of open-celled cellular structure control during aluminium foams (AF) preparation by LPI process has been described, and a high pressure infiltration process has also been put forward by the authors to solve this problem. In fact, cellular structure control is also a problem to be solved for FT process to prepare AF because of the non-uniformly distribution of the bubbles during the foaming process of aluminium melt. This problem is directly related to the viscosity control of the melt. Two low viscosity leads to rapid floating of the bubbles while too high viscosity results in suppression of formation of bubbles. It is very important to control the viscosity of the melt during the foaming process. In this study, the authors simply do the real-time measurement of viscosity of the melt by measuring the voltage of paddle motor during the stirring of the melt. The effect of viscosity on the final cellular structure of AF is investigated using the data of viscosity measured by this method.
The development of microstructure during the deformation and annealing of discontinuously reinforced metal-matrix composites is discussed with particular reference to composites having a pure aluminium matrix. The deformation inhomogeneities induced by the ceramic particles are examined, and the role of the particles in the nucleation and growth of recrystallised grains during subsequent heat treatment is considered. The effect of the ceramic particles on the microstructures developed during multipass hot rolling is shown to be complex, depending on particle size and volume fraction. Superplasticity is shown to occur by several mechanisms in particulate composites. The effect of deformation processing on the ceramic is discussed, with emphasis on the fracture and realignment of ceramic particles, platelets, and whiskers.MST/1299
The indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%.
The compression properties of an aluminium foam containing a nonuniform density gradient have been examined. Specimens were taken from various locations within the foam slab, and were tested in two directions. Measured foam properties were compared to calculated values using models derived by Ashby and Gibson [1]. The effect of the density gradient on the compression properties and also the energy absorption characteristics of the foam was found to be significant.
Experiments have been carried out to investigate the mechanical behavior of foamed aluminum with different matrixes and states. It is found that the matrix composition has a significant influence over the deformation, failure and fracture of foamed aluminum. Like other cellular solid materials, Al foam shows a smooth compression stress–strain curve with three regions characteristic of plastic foams: linear elastic, plastic collapse and densification. AlMg10 foam has a serrated plateau and no densification, characteristic of brittle foams. AlMg10 foam has higher compressive and tensile strength but lower ductility than Al foam. The difference in the mechanical properties between Al foam and AlMg10 foam decreases as the relative density decreases, and when it is lower than roughly 0.15, no difference can be discerned. The mechanical properties in compression are clearly higher than those in tension, which can be explained in terms of dislocation theory and stress concentration behavior.
The mechanical response of composite foam AlSiC has been examined in compression and indentation. The foam has a closed cell structure and is made of aluminium matrix with SiC particles dispersed in it. The cell walls have a complex microstructure consisting of non-uniform distribution of particles, voids and cavities as well as micro-segregation and precipitates resulting from dendritic solidification. Consequently, the mechanical response is complex. In compression, deformation localizes in a band which extends outward with increasing strain. A similar response is observed in indentation, where localization takes place near the indenter and deformation proceeds radially outward. The mechanism of deformation in individual cell walls is identified to be a combination of processes, such as debonding at the particle/matrix interface, particle pull-out and microvoid coalescence in the ductile matrix. The growth of cracks in the cell membranes is associated with a wide damage zone, resulting in high specific energy absorption capacity.
The stiffness of open and closed cell low density cellular solids, or solid foams, is affected by “imperfections” such as non-uniform cell size (multi-dispersity), non-uniform cell wall thickness, wavy distortions of cell walls, etc. Metal foams generally have lower relative stiffnesses than, for example, expanded PVC based polymer foams, and a comparison of the morphologies suggests that the main difference between these cellular solids is wavy distortions of the cell walls of the metal foams. The influence of wavy distortions on stiffness is modeled in this paper. The concepts are introduced through application to open cell materials, for which closed form solutions are obtained, primarily to illustrate the phenomenon. Closed cell materials are analysed subsequently, and results that are considered to be in good agreement with experimental observations are obtained.
Lightweight cellular materials can be used in the construction of composite plates, shells and tubes with high structural efficiency. Metallic sandwich construction with integrally bonded face-sheet/foam core configurations offer a cost-efficient alternative to conventional skin-stringer and honeycomb core components. The potential effectiveness of such constructions is dependent on the properties and performance of the core materials. In this study, aluminum foams made by two liquid-state production methods are considered. The cellular structure and mechanical properties of these foams are investigated, and the influence of the production method on the structural performance of the materials is discussed.
The yield behaviour of two aluminium alloy foams (Alporas and Duocel) has been investigated for a range of axisymmetric compressive stress states. The initial yield surface has been measured, and the evolution of the yield surface has been explored for uniaxial and hydrostatic stress paths. It is found that the hydrostatic yield strength is of similar magnitude to the uniaxial yield strength. The yield surfaces are of quadratic shape in the stress space of mean stress versus effective stress, and evolve without corner formation. Two phenomenological isotropic constitutive models for the plastic behaviour are proposed. The first is based on a geometrically self-similar yield surface while the second is more complex and allows for a change in shape of the yield surface due to differential hardening along the hydrostatic and deviatoric axes. Good agreement is observed between the experimentally measured stress versus strain responses and the predictions of the models.
There is increasing interest in the use of metallic foams in a variety of applications, including lightweight structural sandwich panels and energy absorption devices. In such applications, the mechanical response of the foams is of critical importance. In this study, we have investigated the effect of specimen size (relative to the cell size) on selected mechanical properties of aluminum foams. Models, described in the companion paper, provide a physical basis for understanding size effects in metallic foams. The models give a good description of size effects in metallic foams.
The uniaxial compressive and tensile modulus and strength of several aluminum foams are compared with models for cellular solids. The open cell foam is well described by the model. The closed cell foams have moduli and strengths that fall well below the expected values. The reduced values are the result of defects in the cellular microstructure which cause bending rather than stretching of the cell walls. Measurement and modelling of the curvature and corrugations in the cell walls suggests that these two features account for most of the reduction in properties in closed cell foams.
The influence of each of the six different types of morphological imperfection—waviness, non-uniform cell wall thickness, cell-size variations, fractured cell walls, cell-wall misalignments, and missing cells—on the yielding of 2D cellular solids has been studied systematically for biaxial loading. Emphasis is placed on quantifying the knock-down effect of these defects on the hydrostatic yield strength and upon understanding the associated deformation mechanisms. The simulations in the present study indicate that the high hydrostatic strength, characteristic of ideal honeycombs, is reduced to a level comparable with the deviatoric strength by several types of defect. The common source of this large knock-down is a switch in deformation mode from cell wall stretching to cell wall bending under hydrostatic loading. Fractured cell edges produce the largest knock-down effect on the yield strength of 2D foams, followed in order by missing cells, wavy cell edges, cell edge misalignments, Γ Voronoi cells, δ Voronoi cells, and non-uniform wall thickness. A simple elliptical yield function with two adjustable material parameters successfully fits the numerically predicted yield surfaces for the imperfect 2D foams, and shows potential as a phenomenological constitutive law to guide the design of structural components made from metallic foams.
The utility of unit cell models that assume periodic microstructures may be limited when applied to cellular materials that have non-periodic microstructures. We analyzed the effects of non-periodic microstructure and defects on the compressive failure behavior of Voronoi honeycombs using finite element analysis. Our results indicate that the non-periodic arrangement of cell walls in random Voronoi honeycombs (with cells approximately uniform in size) results in higher strains in a small number of cell walls compared to periodic, hexagonal honeycombs. Consequently, the Voronoi honeycombs were approximately 30% weaker than periodic, hexagonal honeycombs of the same density. The strength difference between the Voronoi and periodic honeycombs depended slightly on density, due to density-dependent interactions between failure modes (i.e. plastic collapse and elastic buckling). Defects, introduced by removing cell walls at random locations, caused a sharp decrease in the effective mechanical properties of both Voronoi and periodic honeycombs (e.g. a 10% reduction in density due to defects caused a 60% reduction in the strength of Voronoi honeycombs). The sensitivity to defects was comparable for thin-walled, elastomeric honeycombs (relative density 0.015) and for thicker walled, plastic honeycombs (relative density 0.15). The properties degraded to zero when 35% of the cell walls were removed, consistent with the percolation limit for a two-dimensional network of hexagonal cells. When four or more adjacent cell walls were removed, the localized band of cell collapse passed through the defect site and the effective strength and modulus were reduced, indicating that even those defects which have a negligible effect on density can alter the failure pattern as well as the effective properties of honeycombs with cells of approximately equal size and strength.
- W E Warren
- A M Kraynik
Warren, W. E. and Kraynik, A. M., ASME J. Appl. Mech.,
1997, 64, 787.
- E Andrews
- W Sanders
- L Gibson
Andrews, E., Sanders, W. and Gibson, L. J., Mater. Sci.
Eng. A, 1999, A270, 113.
- H Fusheng
- Z Zhengang
Fusheng, H. and Zhengang, Z., J. Mater. Sci., 1999, 34,
291.
Foamed Metal and Method of Producing Same
- S Akiyama
- H Ueno
- K Imagawa
- A Kitahara
- S Nagata
- K Morimoto
- T Nishikawa
- M Itoh
Akiyama, S., Ueno, H., Imagawa, K., Kitahara, A., Nagata,
S., Morimoto, K., Nishikawa, T. and Itoh, M., Foamed
Metal and Method of Producing Same, in US Patent
4,713,277, 1987.
- E W Andrews
- G Gioux
- P Onck
- L J Gibson
Andrews, E. W., Gioux, G., Onck, P. and Gibson, L. J.,
Int. J. Mech. Sci., 2001, 48, 701.
- A G Evans
- J W Hutchinson
- M F Ashby
Evans, A. G., Hutchinson, J. W. and Ashby, M. F., Progress Mater. Sci., 1999, 43, 171.
- V S Deshpande
- N A Fleck
- J Mech
Deshpande, V. S. and Fleck, N. A., J. Mech. Phys. Sol.,
1999, 48, 1253.
- V Gergely
- T W Clyne
Gergely, V. and Clyne, T. W., Adv. Eng. Mater., 2000,
2, 175.
- J T Beals
- M S Thompson
Beals, J. T. and Thompson, M. S., J. Mater. Sci., 1997,
32, 3595.
- O Prakash
- H Sang
- J D Embury
Prakash, O., Sang, H. and Embury, J. D., Mater. Sci.
Engng., 1995, A199, 195.
- Y Sugimura
- J Meyer
- M Y He
- H Bart-Smith
- J Grenestedt
- A G Evans
Sugimura, Y., Meyer, J., He, M. Y., Bart-Smith, H., Grenestedt, J. and Evans, A. G., Acta mater., 1997, 45, 5245.
- A E Markaki
- T W Clyne
Markaki, A. E. and Clyne, T. W., Mat. Sci. & Tech., 2000,
16, 785.
- C P Chen
- R S Lakes
Chen, C. P. and Lakes, R. S., Cell. Polym., 1995, 13, 186.
- W E Warren
- A M Kraynik
Warren, W. E. and Kraynik, A. M., ASME J. Appl. Mech.,
1997, 64, 787.
- M J Silva
- L J Gibson
Silva, M. J. and Gibson, L. J., Int. J. Mech. Sci., 1997,
39, 549.
- L Chandra
- T W Clyne
Chandra, L. and Clyne, T. W., J. Mater. Sci. Letts., 1993,
12, 191.
- A F Whitehouse
- R A Shahani
- T W Clyne
Whitehouse, A. F., Shahani, R. A. and Clyne, T. W., J.
Microscopy, 1995, 178, 208.
- V S Deshpande
- N A Fleck
Deshpande, V. S. and Fleck, N. A., J. Mech. Phys. Sol.,
1999, 48, 1253.
Microstructural Development during Thermomechanical Processing of Aluminium-based Composites
- R A Shahani