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The Effect of Metal Wires on the Fracture of a Brittle-Matrix Composite
Abstract
The behaviour of stainless steel, work-hardened nickel and annealed nickel wires bridging a crack in a brittle-matrix has been studied as a function of the length and orientation of the wire. The pull-out stress for stainless steel wire in epoxy resin increases less than linearly with wire length, following the behaviour predicted by Takaku and Arridge [6]. Wires inclined at 20 and 40 to the tensile axis gave pull-out stresses some 30% higher than wires parallel to the tensile axis, this increase being attributed mainly to enhanced friction on the bent wire near its point of exit from the matrix. Work-hardened nickel wires fractured when their length exceeded a critical value, and the critical length was significantly shorter for inclined wires than for wires parallel to the tensile axis. In contrast, annealed nickel wires, no matter how long, did not fracture but pulled out at a limiting stress which was slightly higher for inclined wires than for wires parallel to the tensile axis. The results show that, in some cases, there does not exist a critical length above which an embedded wire will fracture rather than pull out of the matrix.
... The bonding strength of fibres also affects paper toughness . It is a well-known phenomenon in fibre reinforced composites as well as in paper that there is an optimal bonding strength that maximizes the fracture work per RIF [33,34] . Too strong a bond leads to matrix rupture or RIF failure and too weak a bond leads to low energy consumption in fibre pull-out . ...
Paper properties can be controlled by mixing different furnishes . The outcome of the elastic, strength and toughness properties is analyzed in this work using results from other fields of material science . particularly from composites . We discuss the micromechanics of reinforcement fibres, their conformability to the background fibre web and the fracture processes in reinforced paper. Reinforcement fibres should have high ductility and they be similar to the mechanical furnish in their micromechanical stiffness.
... The fiber was oriented at an arbitrary angle u on the crack surface as shown in Fig. 2. For flexible fiber, the bridging force P was enhanced caused by the tilt angle of fibers. Morton [59]and Li [60]proposed an empirical equation (6) of the force P by simulating an Euler friction pulley at the fiber pull-out point. ...
Strain-hardening cementitious composites (SHCC) are a class of fiber-reinforced cementitious composites characterized by strain hardening behavior accompanied by multiple cracking, which is designed by tailoring matrix, fiber, and fiber/matrix interface based on the micromechanics theory. At present, there are several numerical simulation methods to study the multiple cracking behavior, such as discrete crack model, unity finite element method, 3D smeared crack model, multiscale constitutive models, and lattice model. However, it is difficult to provide a good balance between accuracy and computational efficiency due to the vast number of fiber and the complex relationship between fibers, matrix, and interface. This paper developed a combined fiber-interface constitutive model based on the single fiber pull-out theory, and established a combined unit. A three-dimensional two-phase finite element model was established, which had been proved to be effective on simulating the cracking behavior of SHCC. Furthermore, the effects of fiber volume fraction and matrix cracking strength on the four-point bending properties of SHCC were investigated. Further study showed that there was a critical matrix cracking strength for a specified fiber and matrix. This value can optimize the design of SHCC to achieve high strength and high toughness performance.
... In addition to the bond stress estimation, the fiber angle effects in terms of snubbing effects are also modeled using the formulation proposed by Morton and Groves [76] and Li et al. [15] (Fig. 7e). The estimated nominal fiber forces, σ f (0), were further enhanced by introducing the snubbing effects due to the orientation angle as proposed by Li et al. [15]. ...
Fiber-reinforced cementitious composites (FRCC) can effectively develop the bond ductility of deformed rebar due to the high tensile resistance and strain capacity. This study presents a numerical method to evaluate the bond behavior of deformed rebar in FRCC based on 3D rigid body spring model (3D RBSM). Beam elements are used to model the rebar and the link elements are used to connect beam elements with RBSM elements as matrix. The same local bond-slip relationship for rebar is assigned at the link elements for both the simulation cases of plain mortar and FRCC. The model could properly simulate the single fiber to mechanical behavior of FRCC and the bond performance of rebar in FRCC. The results revealed that discrete fibers are the principal factors that preventing the splitting bond crack propagation and the stress propagation suggests the presence of high local bond stress around the rebar, improving bond ductility.
... For flexible steel and polymeric fibers, Morton and Groves (1976) and Li et al. (1990) found an increase of P with the inclined angle h, of fiber to the direction of loading, which is called the 'snubbing effect' (Li et al., 1991). In addition, the pullout of an inclined steel fiber may cause stress concentration at the exit point and thus local failure of the supporting matrix, resulting in a drop of P. ...
Strain hardening behavior can be observed in steel fiber reinforced concretes under tensile loads. In this paper, a statistical micromechanical damage framework is presented for the strain hardening steel fiber reinforced concrete (SH-SFRC) considering the interfacial slip-softening and matrix spalling effects. With a linear slip-softening interface law, an analytical model is developed for the single steel fiber pullout behavior. The crack bridging effects are reached by averaging the contribution of the fibers with different inclined angles. Afterwards, the traditional snubbing factor is modified by considering the fiber snubbing and the matrix spalling effects. By adopting the Weibull distribution, a statistical micromechanical damage model is established with the fracture mechanics based cracking criteria and the stress transfer distance. The comparison with the experimental results demonstrates that the proposed framework is capable of reproducing the SH-SFRC’s uniaxial tensile behavior well. Moreover, the impact of the interfacial slip-softening and matrix spalling effects are further discussed with the presented framework.
... Based on the Naaman's single fiber shear lag model [13], Lee [15] obtained a model for an inclined straight fiber pulled out from the matrix. The Lee's model considered the snubbing effect and matrix spalling effect [15,[34][35][36][37][38] when the inclined straight fiber is pulled out from the matrix. Compared with the straight fiber, the bending point of the end-hooked fiber provides an additional interlocking force, which might enhance the pullout behavior of a single end-hooked fiber. ...
This paper proposed a new method for predicting tensile response of UHPC based on a spatial stochastic strength distribution model. The steel fibers in UHPC were aligned by a developed device and chemically treated by ZnPh. A direct tension test was conducted to obtain the tensile response of UHPC with treated fibers. Image analysis was used to obtain the frequency of overall fiber orientation distribution. The spatial stochastic distribution model was proposed for cross sectional strength of UHPC based on the Ornstein-Uhlenbeck stochastic process. The model introduced some assumptions such as the correlation coefficient between the cracking strength and fiber bridging strength. Parametric analysis was conducted to obtain the crucial stochastic parameters in the model. Combining with the single fiber pullout model, tensile behavior of a single crack, and minimum crack spacing, the model predicted the strain hardening and multiple cracking behaviors with precise accuracy, as compared to experimental results.
... To include the effect of fiber orientation (being random in the composite) and its consequent implication on frictional forces, Morton and Groves [28] and subsequently Li [29] introduced the snubbing effect f (> 0) as proposed in the following empirical equation [Eq. (7)] to account for the increase in the bridging load P at an inclination angle h by analogy to a friction pulley at the exit point. ...
In the context of enhancing concrete ecoefficiency through the valorization of domestic materials into concrete design, increasing research attention is being paid to the development of strain-hardening cementitious composites (SHCC) with various supplementary cementitious materials (SCM) in replacement of the commonly used fly ash (FA). In this regard, ground-glass pozzolans [or simply glass powder (GP)] obtained by grinding post-consumer waste glass can shape a potential candidate. This study is aimed at characterizing the interface properties of polyvinyl-alcohol (PVA) fibers in SHCC incorporating high-volume GP (HVGP) at 0–100% replacement of FA. Single-fiber pull-out tests were conducted to characterize the interface properties [frictional bond (τ0), chemical bond (Gd), and slip-hardening coefficient (β)] necessary for micromechanical tailoring of SHCC. Results indicate that with higher matrix compactness obtained using GP, τ0 increased significantly, while Gd slightly decreased. Whereas higher τ0 in HVGP-SHCC was found to increase the maximum pull-out load of PVA fibers, excessive τ0 causes fiber damage, thereby adversely affecting composite ductility. Therefore, a novel approach was adopted herein to nanomodify SHCC matrix as well as fiber/matrix interface by incorporating nanoscale cellulose filaments (CF) at rates of 0.03–0.10% per cement mass. This allowed to significantly alter the pull-out behavior whereby τ0 and Gd were relatively attenuated, while a significant increase in β (~1.0–1.5) was obtained. Thus, the incorporation of CF imparted a characteristic slip-hardening effect that contributed towards enhancing the strain-hardening capacity in HVGP-SHCC as experimentally validated by uniaxial tensile tests.
... For the sake of brevity, two-dimensional fiber distribution was assumed and both the matrix micro-spalling and the Cook-Gordon effect were not taken into account. The apparent strength [43], snubbing coefficient [43] and strength reduction factor [44] [45] of the PVA fiber were assumed to be the same as those found or estimated for cement matrix. To determine the effect of fiber volume fractions (V f ), three different input values of 1.0, 1.5, and 2.0% were used. ...
Strain-hardening ductile fiber-reinforced geopolymer composite, named Engineered Geopolymer Composite (EGC), is a promising material for achieving green and durable civil infrastructure. Despite increasing attentions of the unique material, inefficient trial-and-error approaches are often employed in the material design, which slows the research and development. This paper proposed a new design methodology of EGC that integrates three design techniques: Design of Experiment (DOE), micromechanical modeling, and Material Sustainability Indices (MSI). The mix design of a preliminary version of EGC was optimized to achieve higher compressive strength, maintain high tensile ductility, and enhance the material greenness simultaneously. With the aid of the systematic design process, an optimized EGC with improved compressive strength of 43.1 MPa and high tensile ductility of 4.7% was developed, while achieving 11% less embodied energy and 55% less CO2 equivalent emissions compared with a standard Engineered Cementitious Composite (ECC). Therefore, the applicability and effectiveness of the proposed design method were successfully demonstrated.
... When a randomly oriented fiber is subjected to a pullout force due to pure opening of a crack as shown in Fig. 1(a), the fiber bridging force will increase due to the "snubbing effect" [29]. The pullout load for an inclined fiber (P(ϕ)) is then related to the pullout force of an aligned fiber (P(0)) through the following equation, assuming the fiber to change direction over a frictional pulley [4,30,31]. ...
The mechanical behavior of Engineered Cementitious Composites (ECC) is strongly dependent on the bridging of cracks by fibers. Due to the bridging action of fibers, tensile and shear stresses can be transferred through cracks in ECC members. In this study, a micromechanics based theoretical model is proposed to describe the shear transfer mechanism on the crack surface due to fiber bridging effect. The model focuses on flexible fibers and both the normal stress along the crack opening direction and the shear stress transferred across the crack surfaces are derived under the coupled effect of crack opening displacement (COD) and shear sliding. With the proposed model, the mechanism of fibers contributing to the shear transfer can be understood and the effect of various micromechanical parameters can be investigated. The simulation results can provide insight on the behavior of ECC under shear loading when cracks are propagating under mixed mode.
... In the fiber-bridging model, however, the randomness of fiber distribution must be considered by varying L e and φ. It has been reported that misaligned fiber (φ ≠ 0) is subjected to additional load due to the Euler friction pulley effect at the fiber exit point, and new P(φ) can be calculated as suggested by [25]. ...
Fiber-reinforced cementitious composites (FRCC) represent a large group of construction and building materials. While numerous experimental studies have been conducted on fatigue of FRCC, predicting FRCC fatigue performance remains difficult. This paper proposes a novel multi-scale analytical model to capture the fatigue dependency of fiber bridging constitutive law in FRCC. On the micro-scale, a new analytical model to predict the post-fatigue single-fiber pullout behavior (P-u curve) is established based on the understanding of the fatigue dependency of fiber and fiber-matrix interface. On the macro-scale, the fatigue-induced fiber strength reduction was considered and probabilistics is introduced to describe the randomness of fiber location and orientation so that the fatigue dependent fiber-bridging constitutive law can be predicted. The model proposed in this paper is the first analytical model that is able to capture the effects of fatigue cycle as well as the fatigue loading level on deterioration of fiber bridging in FRCC.
... The fiber pullout load increases when the fiber orientation is increased due to the increase of the normal force between the fiber and the matrix, which increases the frictional bond between the fiber and the matrix. This phenomenon is known as the snubbing effect, and currently, an empirical equation between the pullout load of inclined fiber and the pullout load of fiber without an inclination angle is being adopted [17,18]. On the other hand, the fiber strength in a matrix decreases when the fiber orientation is increased due to an additional stress at the exit point of the crack plane by bending [19]. ...
The distribution of fiber orientation is an important factor in determining the mechanical properties of fiber-reinforced concrete. This study proposes a new image analysis technique for improving the evaluation accuracy of fiber orientation distribution in the sectional image of fiber-reinforced concrete. A series of tests on the accuracy of fiber detection and the estimation performance of fiber orientation was performed on artificial fiber images to assess the validity of the proposed technique. The validation test results showed that the proposed technique estimates the distribution of fiber orientation more accurately than the direct measurement of fiber orientation by image analysis.
... Conversely, constant friction or slip-softening are often observed when the fiber hardness is higher than that of the surrounding matrix [17]. The effect of the fiber orientation, known as the snubbing effect, on the pullout load is expressed as Equation (3), which is an empirical relation [18,19]. This is because actual short fiber composite fibers are randomly oriented. ...
The basalt fiber is a promising reinforcing fiber because it has a relatively higher tensile strength and a density similar to that of a concrete matrix as well as no corrosion possibility. This study investigated experimentally the bonding properties of basalt fiber with cementitious material as well as the effect of fiber orientation on the tensile strength of basalt fiber for evaluating basalt fiber's suitability as a reinforcing fiber. Single fiber pullout tests were performed and then the tensile strength of fiber was measured according to fiber orientation. The test results showed that basalt fiber has a strong chemical bond with the cementitious matrix, 1.88 times higher than that of polyvinyl alcohol fibers with it. However, other properties of basalt fiber such as slip-hardening coefficient and strength reduction coefficient were worse than polyvinyl alcohol and polyethylene fibers in terms of fiber bridging capacity. Theoretical fiber-bridging curves showed that the basalt fiber reinforcing system has a higher cracking strength than the polyvinyl alcohol fiber reinforcing system, but the reinforcing system showed softening behavior after cracking.
... Misaligned fibers are subjected to additional frictional stress due to the interaction with the matrix when the fiber exits the matrix. For flexible polymeric fiber, Morton and Groves (1976) and Li et al. (1990) suggested the following empirical relation to account for the increase of bridging force P due to an inclination angle by making analogy to an Euler friction pulley at the fiber exit point, ...
... Modeling for the bond behavior of inclined fiber is based on the pullout model for the aligned fiber (as shown in Figures 1-3). The modeling of the bond behavior for inclined fibers considers the variation of load due to the snubbing effect and matrix spalling effect assumed that the fiber inclination angle (φ) is equal to zero (in the case where the fibers are not positioned in the tensile load direction and are inclined, the bridging force will be increased, this phenomenon is called the 'snubbing effect' [6,15,45,46]. This is accomplished by introducing the apparent bond strength (τ ( ) , τ ( ) ) which is illustrated as a function of the inclination angle φ. ...
Steel fiber reinforced self-compacting concrete (SFRSCC) is a relatively new composite material which congregates the benefits of the self-compacting concrete (SCC) technology with the profits derived from the fiber addition to a brittle cementitious matrix. Steel fibers improve many of the properties of SCC elements including tensile strength, ductility, toughness, energy absorption capacity, fracture toughness and cracking. Although the available research regarding the influence of steel fibers on the properties of SFRSCC is limited, this paper investigates the bond characteristics between steel fiber and SCC firstly. Based on the available experimental results, the current analytical steel fiber pullout model (Dubey 1999) is modified by considering the different SCC properties and different fiber types (smooth, hooked) and inclination. In order to take into account the effect of fiber inclination in the pullout model, apparent shear strengths (τ
(app)) and slip coefficient (β) are incorporated to express the variation of pullout peak load and the augmentation of peak slip as the inclined angle increases. These variables are expressed as functions of the inclined angle (ϕ). Furthurmore, steel-concrete composite floors, reinforced concrete floors supported by columns or walls and floors on an elastic foundations belong to the category of structural elements in which the conventional steel reinforcement can be partially replaced by the use of steel fibers. When discussing deformation capacity of structural elements or civil engineering structures manufactured using SFRSCC, one must be able to describe thoroughly both the behavior of the concrete matrix reinforced with steel fibers and the interaction between this composite matrix and discrete steel reinforcement of the conventional type. However, even though the knowledge on bond behavior is essential for evaluating the overall behavior of structural components containing reinforcement and steel fibers, information is hardly available in this area. In this study, bond characteristics of deformed reinforcing steel bars embedded in SFRSCC is investigated secondly.
... (1) and (2) are for fibers pulled out in a direction along the fiber axis. For nonaligned fibers, which is often the case in randomly distributed discontinuous FRCC, various studies have indicated an angle effect on the pullout load P. For flexible (in bending, dependent on elastic stiffness and fiber diameter) steel and polymeric fibers, Morton and Groves (1976) and Li et al. (1990b) found an increase of P with angle 4> of inclination of fiber to the loading axis. This snubbing effect could be incorporated into the pullout force by recognizing (3) as originally suggested by Morton and Groves, where /is a snubbing coefficient. ...
The prepeak and postpeak stress-displacement relations are derived for the bridging mechanism associated with randomly oriented discontinuous flexible fibers in cement-based composites. The postcrack strength and fracture energy are examined in light of the scaling micromechanical parameters, including fiber snubbing coefficient, diameter, aspect ratio, volume fraction, and interface bond strength. Comparisons of theoretically derived postcracking stress-displacement relation and pullout fracture energy with experimental data of both steel-fiber and synthetic-fiber reinforced cementitious composites of widely varying micromechanical parametric values suggest that the simple model approximates the bridging behavior in this type of composite.
... For a non-aligned fiber (that is, when there is an inclination angle a between the fiber and the loading axis, see Fig. 5a), which is the case of randomly distributed fiber composites (see Section 4.2), various studies have indicated an increase of the peak load P P with increasing a. As was originally proposed by Morton and Groves (1976), we can assume: ...
A fibrous composite beam with an edge crack is submitted to a cyclic bending moment and the crack bridging actions due to the fibers. Assuming a general elastic-linearly hardening crack bridging model for the fibers and a linear-elastic law for the matrix, the statically indeterminate bridging actions are obtained from compatibility conditions. The elastic and plastic shake-down phenomena are examined in terms of generalised cross-sectional quantities and, by employing a fatigue crack growth law, the mechanical behaviour up to failure is captured. Within the framework of the proposed fracture mechanics-based model, the cyclic crack bridging due to debonding at fiber–matrix interface of short fibers is analysed in depth. By means of some simplifying assumptions, such a phenomenon can be described by a linear isotropic tensile softening/compressive hardening law. Finally, numerical examples are presented for fibrous composite beams with randomly distributed short fibers.
Mechanical properties of engineered cementitious composites (ECC) are highly dependent on the pore structural characteristics and fibre-matrix interaction. The relationship between them has not been extensively explored. This paper proposes a practical micromechanical analytical model accounting for pore structure characteristics and crack-bridging properties to predict the strain-hardening and multiple microcracking behaviour of ECC. Using polyethylene fibre reinforced ECC (PE-ECC) as an example, Monte Carlo simulations were undertaken to investigate the tensile behaviour in terms of crack strength, fibre bridging strength and uniaxial tensile properties against heterogeneity of material property, which were validated with experimental data. A parametric study was then conducted to estimate the effects of fibre-matrix bond and fibre properties on stress-strain relationship and microcracking features of PE-ECC. Results indicate that the tensile properties of PE-ECC can be reasonably predicted. Under constant fibre dosages, the tensile ductility of PE-ECC is dominated by interfacial bond, followed by fibre location, orientation and diameter. Such insights are helpful to the design of ECC composites for practical applications.
In light of the well-established multi-scale nature of flaws in concrete, it follows that existing diverse attempts in fiber-reinforced concrete (FRC) technology-intended to mitigate the inherent tendency of concrete to cracking-remain relatively inefficient. This is majorly attributed to the fact that the large inter-fiber spacing in conventionally used macrofibres does not promote an effective bridging of multiscale cracks. As a result, increasing research and development are currently being invested to develop concretes incorporating nanoscale particles. Thus, nanoscale fibers emerged as a promising tool for manipulating concrete nanostructure towards a controlled macrobehaviour necessary for enhanced overall performance. In this context, while carbon nanostructure (CNS) such as carbon nanofibers (CNF) and carbon nanotubes (CNT) have gained a relative popularity, it should be noted that eco-efficiency incentives would favor the currently emerging nanocellulose materials (NCM) extracted from cellulose-based systems, the most abundant and renewable resource on the planet. NCM have been demonstrated as a means to engineer superior composite properties necessary for versatile applications including optics, biomedical applications, and transparent electronics. The current study is aimed at disclosing the possibilities of re-engineering concrete properties using a new type of NCM, namely, cellulose filaments (CF) in order to achieve superior concrete performance necessary for specific applications. CF are cellulosic fibrils with a nanometric diameter (30–400 nm) and micrometric length (100–2000 µm), thereby exhibiting the highest aspect ratio (100–1000) among all currently available NCM. The study focuses on the influence of CF as a tool for nano-tailoring the properties of concrete in its three major states (fresh, hardening, and hardened). As a result, three applications in relation to the above concrete states were identified through intensive experimentation: (i) enhancing the properties of fresh concrete by using CF as a viscosity modifying admixture (VMA), (ii) enhancing the properties of hardening concrete by using CF as a shrinkage reducing admixture (SRA), and (iii) improving the performance of concrete at the hardened state by using CF as a nanoscale reinforcement. The study further aims at leveraging those different enhancements in concrete properties obtained with CF towards developing a new concrete formulation that optimizes the advantages of CF. The enhancement of concrete properties at fresh state was undertaken in the context of valorizing the hydrophilic and flexible nanoscale CF to function as a VMA in self-consolidating concrete (SCC) whereby the design of flowable (yet stable and robust) mixtures requires a delicate balance between flowability and stability. For this, CF were incorporated at concentrations ranging from 0.05 to 0.30% per weight of binder in cement pastes and SCC. CF were demonstrated as a valuable tool not only for rheology modification, but also to impart collateral positive effects on mechanical performance (strength enhancement of 12–26% in compression, splitting-tension, and flexure) when compared to commercially available VMA of Welan Gum type. CF were found to serve as a VMA due to the buildup of flexible nanoscale networks as demonstrated by a geometry-based percolation model as well as by microstructure investigations. Interestingly, this effect was found to be accompanied by a shear thinning effect attributable to the streamlining of flexible nanocellulose fibrils in the direction of flow under increasing shear rates, thereby potentially enhancing pumpability. The potential of CF to enhance the properties of hardening concrete was attempted in the context of exploiting the hydrophilic and hygroscopic characters of CF to mitigate autogenous shrinkage (AS) in ultra-high-performance concrete (UHPC). While s such, when UHPC formulation was adjusted to accommodate CF at rates of 0-0.30% per cement mass, and silica fume content was varied (from 15 to 25%), CF were found to be more beneficial in reducing AS at early-age with a reduction of up to 45% during the first 24 hours and 35% at 7 days. On the other hand, adjusting SF content from 25 to 15% had a negligible effect on AS at early-age (0–4% reduction at 1 day) but a higher effect at later-age (28% reduction at 7 days) attributable to the time-dependent pozzolanicity of silica fume. However, this alternative was found to have adverse effects on mechanical performance (32% lower flexural capacity). Finally, the potential of CF as a nanoscale reinforcement was investigated on cement pastes and on concrete. In the former, strength enhancement in engineering properties (compressive strength, flexural capacity, and elastic modulus) of up to 25% were achieved. In the latter, strength improvements of up to 16% (in compression), 34% (in splitting tension), 22% (in flexure), and 96% (in energy absorption) were obtained. To disclose the mechanisms underpinning the effect of CF on strength of cement systems, the above findings were supplemented by microstructure investigations, namely, degree of hydration and micromechanical properties (indentation modulus M, hardness H, and contact creep modulus C) of major microstructure phases using nanoindentation coupled with quantitative energy-dispersive spectroscopy (NI-QEDS). As a result, the improved macromechanical performance was found to sprout from a twofold microstructure change, i.e.: an increased degree of hydration and higher micromechanical properties of C-S-H gel matrix (~12–25%). To leverage the above different advantages offered by CF on cement and concrete composites, particularly the nanoreinforcing effect and the potential synergy between the nanoscale CF and macrofibers, a novel multi-scale fiber-reinforced strain-hardening cementitious composite (SHCC) was developed. The design of this SHCC followed a new approach that couples packing density optimization with micromechanical tailoring. Thus, high-volume ground-glass pozzolans (HVGP) were incorporated under the guidance of particle pacing optimization to replace fly ash (FA) commonly used in SHCC such that composite strength can be increased. The newly formulated SHCC was further improved in terms of ductility and strain-hardening capacity by the incorporation of CF whereby the latter was a useful tool to nanomodify SHCC matrix and interface properties towards enhanced strain-hardening behavior. In outcome, HVGP-SHCC formulations with GP replacement of fly ash of up to 100% were developed. The resulting formulations have self-consolidation ability (mini-slump dimeter in the range of 250 mm) and exhibited (at 28 days) 60-75 MPa compressive strength, 9-15 MPa flexural capacity, 3-6 MPa tensile strength, 2-5% tensile strain capacity, and a significantly increased electrical resistivity (up to 60% enhancement). Thus, the mechanical properties of the newly developed HVGP-SHCC exceed those reported in the commonly used high-volume fly ash (HVFA)-SHCC. Nevertheless, while the strength enhancement obtained with GP does not jeopardize composite ductility up to 40% GP content, a reduced ductility was noticed at GP>40%. As a result, CF were used to impart a nanoreinforcing effect to HVGP-SHCC as well as to nanomodify the interface properties of PVA fibers. In outcome, a twofold effect was obtained by nanomodifying SHCC with CF: (i) CF imparted higher elastic modulus to the bulk cementitious matrix (Em) thereby contributed to attenuating the crack tip toughness (J_tip=K_m^2/E_m) with Km being matrix fracture toughness, (ii) CF led to attenuating the excessive frictional bond encountered at higher GP content (densifying the matrix and increasing its strength, but limiting strain-hardening behavior) and imparted a characteristic slip-hardening effect (β) which contributed towards improving composite strain-hardening capacity and ductility. Thus, enhancement in ultimate strain-capacity above 200% as compared to systems without CF were obtained. Therefore, with the incorporation of CF, it was possible to produce SHCC with up to 100%GP replacement of FA while exhibiting higher strength and ductility. To scale-up the enhanced mechanical performance (particularly the high strength and ductility) demonstrated by the new SHCC, the latter was used as a topping to develop a novel type of composite deck slabs at full-scale (dimensions of up to 2400 × 900 mm). The composite deck slabs thus constructed are intended to benefit from the improved strength and ductility of nanomodified HVGP-SHCC topping such that better compatibility between the steel deck and its concrete topping can be obtained. This has the potential to increase the performance of composite deck slabs under shear bond failure, a major failure mode in composite deck slabs. Results indicated that, compared to composite deck slabs with a high-strength concrete topping having similar compressive strength as the nanomodified HVGP-SHCC, the slabs constructed with the SHCC exhibited up to 35 and 42% enhancement in ultimate load-carrying capacity and ductility, respectively. Furthermore, composite deck slabs with nanomodified HVGP-SHCC exhibited higher shear bond capacity. Considering theses results, it is perceivable that the newly developed SHCC (implemented from the material level at the nanoscale to the structural level at the macroscale) has benefited from a twofold ecoefficiency perspective. The first concerns the valorization of post-consumer recycled glass into the development of high-performance concrete, thereby contributing to relieve a significant socio-economical burden created by landfilling post-consumer glass. The second concerns exploiting the power of cellulose, the most abundant naturally occurring polymer on the planet, towards a biomimetic design of high-performance multiscale-reinforced cement composites necessary for sustainable and resilient concrete infrastructure systems.
In this research work, the bonding characteristics of plasma treated basalt fibres were analysed by employing the fibre pull-out test. 80 samples were prepared with two different spans of basalt fibres (such as 25 mm and 50 mm) and four levels of embedded length (10, 15, 20 and 25) inside standard M20 grade concrete. Debonding and bonding characteristics of the plasma treated fibres were compared with raw basalt fibres through the fibre pull-out test. The plasma treated and raw basalt fibres were characterised through Field emission scanning electron microscope (FESEM) and Fourier transform infrared (FTIR) analysis. It was observed that confirmation of the presence of hydroxyl groups on the basalt fibre surface was realised through the FTIR test and that there was higher adsorption of concrete particles by the plasma treated basalt fibres through FESEM. The de bonding and fibre pull-out energy of the plasma treated basalt fibres were improved by about 9% and 10% compared with 25 mm and 50 mm raw basalt fibres. From the observation above, it can be stated that the surface modification of basalt fibre may lead to a change in the debonding and pull-out energy level.
Alumina (Al2O3) ceramics have today become a multibillion dollar global industry which has changed the world in the last few decades. Alumina uniquely combines low cost with extreme hardness, extreme electrical resistivity, extreme corrosion resistance, and high refractoriness, and it is the most biocompatible material in current clinical use. Of the 1.3 million hip replacements implanted annually in the global $7 Billion hip replacement industry, 55% now use alumina or zirconia-toughened alumina (ZTA) bearings. A decade ago, alumina or ZTA bearings were rare and still considered experimental. This meteoric rise has led to strong interest in ceramic hip resurfacing and a ceramic knee, a field currently solely serviced by metal bearings. This is not possible with alumina, and is right at the limit of what is possible with ZTA. A quantum leap in toughness could bring alumina ceramic hip resurfacing and an alumina ceramic knee to the mass market. As discussed in Chapter 2, metal microfiber reinforcement can provide this quantum leap in toughness as it can produce up a 600-fold toughness enhancement, and commonly 100-fold or more (two orders of magnitude) increase in work of fracture over the parent ceramic. Zirconia toughening of alumina can only give a threefold work of fracture enhancement of alumina. Alumina is the number one wear-resistant ceramic used in wear-resistant linings in the global $500 Billion mining industry. For high impact applications, such as jaw crushers and heavy-duty ore-chute liners, the tough cermet tungsten carbide (Chapter 8) is the material of choice, but it is more than four times more expensive than alumina. Metal microfiber-reinforced alumina could potentially compete with tungsten carbide in this role. However metal fiber-reinforced SiC composite technology developed by author Ruys, and discussed in Chapter 4, is a much lower cost high-impact wear-resistant ceramic technology, much better suited to competing with tungsten carbide. This chapter outlines the background to alumina bearings in orthopedics and alumina in the mining industry. This is followed by a comprehensive literature review of alumina ceramic matrix composites. The chapter then concludes with an overview of research done by the authors on metal microfiber-reinforced alumina as a biomaterial, and a synopsis of the proposed manufacturing process for such a material.
A functionally graded material (FGM) is a composite material whose properties change over a gradient across one or more axes of the composite material. This is most commonly a compositional gradient, but it can also be a microstructural gradient, such as a gradation in porosity or fiber reinforcement. It can also be graded atom order, or other property gradient. The most common type of FGM is the compositionally graded FGM, and of these, the most relevant to the present book is the metal–ceramic continuous bulk FGM. The concept of the FGM as an engineering material was first proposed in 1972, but it did not rise to prominence until it was proposed in 1984 at the National Aerospace Laboratory in Japan in response to a demand for a new material for the hypersonic space plane. The engineering objective was to develop a thermal barrier spaceplane skin capable of withstanding a surface temperature of 2000 K and a temperature gradient of 1000 K across a cross-sectional thickness of less than 10 mm. The material was also required to have corrosion and high temperature resistance on the outer face. The only material capable of this would be a metal–ceramic continuous bulk FGM. Metal–ceramic FGM thin-film coatings and thin-film interface layers are now a well-established technology. Metal–ceramic continuous bulk FGMs remain an experimental technology. They show significant potential for extreme applications for which few alternatives exist, such as spaceplane heat shields, plasma facings for nuclear reactors, ballistic armour, and load-bearing implantable medical devices. While many methods have been published for manufacturing continuous bulk metal–ceramic FGMs (mm to cm thick), few are capable of producing broad regular continuous gradients. A case study in this chapter demonstrated that thixotropic casting successfully produced regular gradients in hydroxyapatite-316L stainless steel biomaterial FGMs. An impeller dry blending (IDB) case study in this chapter showed the potential of IDB for producing excellent linear gradients in metal–ceramic FGMs. A hydrostatic shock forming case study showed its potential as a densification method for continuous bulk FGMs for which the metal and ceramic components have greatly differing melting points. Metal infiltration in combination with IDB forming of pore-graded ceramics was shown to be the most viable densification for FGMs for which the metal and ceramic components have greatly differing melting points. This chapter overviews the FGM concept, the manufacturing methods, and has three research case studies: continuous bulk FGM forming by thixotropic casing, continuous bulk FGM forming by impeller dry blending, and densification by hydrostatic shock forming.
This chapter represents the first detailed review of metal fiber-reinforced ceramics since the 1970s. In the 1960s, the space race began. This era of rapidly advancing aerospace technology brought in a new materials revolution. Lightweight, tough, heat-resistant materials suddenly became an imperative. Cemented carbides, the outstanding metal-reinforced cermet technology reviewed in Chapter 8, are extremely heavy and unsuitable for this application. As a result, in the 1960s, there was a sudden burgeoning of interest in metal fiber-reinforced aerospace ceramics. In the early 1970s, two ground-breaking papers were published on metal fiber-reinforced ceramics. The first one, from Canada in 1971, reported a 225-fold enhancement in work of fracture for molybdenum microfiber-reinforced alumina ceramics. The second one, from the United Kingdom in 1972, showed a 600-fold enhancement in work of fracture for nickel microfiber-reinforced magnesia ceramics. These achievements were extraordinary at the time, and even today they are comparable to the best achievements in metal microfiber-reinforced ceramics that followed later. They are well ahead of the best achievements in ceramic fiber-reinforced ceramics, and transformation-toughened ceramics, to date. This chapter reviews the field of metal fiber-reinforced ceramics, from their genesis in the 1960s, to the present day. Three of the chapters in this book concern metal fiber-reinforced ceramics (Chapters 4, 5, and 6456), but each has a different focus. Therefore this review is not just a standalone review on the topic, but it also lays the groundwork for Chapters 4, 5, and 6456. Cermet technologies are reviewed in their respective chapters (Chapters 7 and 878). Functionally graded materials are reviewed in Chapter 9. There are a number of other interesting metal-reinforced ceramic technologies whose scope is not large enough to justify an entire chapter. These are briefly overviewed at the end of this chapter.
Poor adhesion of synthetic macro fibers to a cementitious matrix limits their reinforcing capacity when used in fiber-reinforced cementitious composites (FRCCs). As a remedy to this problem, high concentrations of fibers must be incorporated into such mixtures, which makes their dispersion in fresh mixtures difficult to achieve. Several strategies have been adopted to improve bonding between synthetic fibers and a cementitious matrix, but most of them cause deterioration of fiber properties. In previous studies, plasma treatment of polymer fibers and strengthening of the fiber-matrix interface using recycled concrete powder (RCP) increased pull-out resistance at a scale of a single fiber. Here, it was found that these results cannot be easily scaled, and the tension-softening behavior of FRCCs can be influenced negatively, despite positive pull-out test results, due to random orientation of fibers. Treating fibers with plasma appears reasonable at any scale, but RCP matrix modification must be carefully performed. Nevertheless, RCP may contribute to more sustainable concrete mixtures and considering its use when designing FRCC elements is recommended.
Strain-hardening geopolymer composite (SHGC) lately emerged as a promising alternative to traditional strain-hardening cementitious composite with added advantages of industrial by-product utilization and enhanced sustainability. However, as the design of SHGC requires multi-factor optimization, the application of traditional trial-and-error method is inefficient and hinders the development of this material.
This paper aims at the development of a slag/fly ash-based SHGC with low slag content using a micromechanical model to guide the composite mixture design. To this end, experimentally characterized physical properties of fiber, matrix and interface are used as input for the micromechanical model, which serves as a predictive tool for the tensile performance of SHGC. Following the guidance, a slag/fly ash-based SHGC with tensile strain capacity of 4.8% and ultimate tensile strength above 3.8 MPa was systematically developed. The feasibility and effectiveness of using micromechanics as the design basis of SHGC are demonstrated and experimentally verified.
The model of multiple cracking of high toughness concrete is established by analyzing the interface stress of fiber in the cement matrix. Based on this model, the effects of friction bond strength, chemical bond strength, matrix cracking strength and elastic modulus of fiber on the strain and crack width at the end of multiple-cracking zone are analyzed. The analysis results show that the strain and the crack width decrease with the increase of the friction bond strength; the strain increases and the crack width decreases with the increase of the chemical bond strength; the strain and the crack width increase with the increase of the matrix cracking strength; the strain and the crack width decrease with the increase of the elastic modulus of fiber. The crack control ability of high toughness concrete with initial crack defects is researched through direct tensile test. The notched specimens are used in the research. The test results show that, when the initial crack-depth ratio is 0.2, the ultimate stress is 5.3MPa at the notch and is 4.3MPa at the non-cutting position, and the phenomenon of multiple cracking appears in the whole specimen, and the crack width is controlled in less than 0.1mm. When the initial crack-depth ratio is 0.4, the ultimate stress is 5.8MPa at the notch and is 3.5MPa at the non-cutting position, and the development of cracks is not controlled effectively by the fibers at the fracture plane. Therefore, only some similar arc cracks appear near the notch of the specimen.
Although spray-applied fire-resistive materials (SFRM) are the most commonly used passive fire protection for steel structures, its performance is often called into question due to their unsatisfactory durability performance. The inherent brittleness, low strength and poor bond with steel lead to delamination and partial loss of fire protection even under service loads, significantly lowering the structural fire resistance. This could be further exaggerated under multi-hazards, such as post-earthquake and post-impact fires, greatly endangering the fire safety of steel structures.
To address this critical issue, a new ductile cementitious SFRM, namely spray-applied fire-resistive engineered cementitious composite (SFR-ECC), has been developed that overcomes the aforementioned problem. Following a parallel design process, SFR-ECC has been developed to satisfy multiple performance targets in one material, including low thermal conductivity, high tensile ductility, high adhesion to steel and sprayability. The resultant material possesses tensile strength, strain capacity, interfacial adhesive energy of 1-2 orders of magnitude higher than the conventional SFRM, yet possesses similar thermal conductivity and sprayability, providing a durable alternative to conventional SFRM. SFR-ECC is also found to exhibit ductile cellular behavior under compression, further enhancing its energy absorption capacity.
The micromechanical and microstructural mechanisms underlying the unique tensile and compressive behavior of SFR-ECC are studied. The material macro- and micro-mechanical behavior under high loading rates have also been investigated. SFR-ECC is found to maintain high tensile ductility under high rate loadings, ensuring its performance under earthquake and impact loads.
This paper presents a discussion of the research challenges in fiber reinforced cementitious composites within the context of the Performance-Processing-Property-Structure framework. Specific examples on the inter-relationships between two types of quasi-brittleness and the material structure are described, and their links to pseudo-ductility and processing conditions are briefly mentioned. The need to bring the disciplines of micromechanics and materials processing closer together underlines a fundamental challenge to the research community in advancing the engineering design of high performance fiber reinforced cementitious composites.
As part of an ongoing research development at Carleton University in ceramic matrix composites (CMCs) for hightemperature gas turbine applications, it was recognized that the performance of an oxide matrix could be improved by incorporating a metal reinforcement material. For this reason, a low cost CMC was created by reinforcing a yttriastabilized zirconia (7YSZ) ceramic matrix with a Hastelloy X (HX) wire mesh. The CMC was manufactured by coating the HX mesh with a NiCrAlY bond coat, and then 7YSZ ceramic matrix, both using plasma spraying. The bond coat was employed to improve bonding and also to act as an oxygen diffusion barrier. In order to evaluate the performance of the HX/7YSZ composite at high temperatures, isothermal and cyclic oxidation tests were carried out for 1000 hours at 1050°C. The results showed that oxidation resistance was improved by vacuum heat treatment prior to testing due to the formation of stable thermally grown oxides (TGO) on the NiCrAlY bond coat. In the cyclic oxidation test, differences in thermal expansion coefficients caused cracking at interfaces between mesh/bond coat and bond coat/7YSZ. Minimizing the effect of thermal expansion by better material combination, as well as modifying manufacturing methods will allow for improved performance of metal mesh reinforced CMCs.
Paper properties can be controlled by mixing different furnishes. The outcome of the elastic, strength and toughness properties is analyzed in this work using results from other fields of material science, particularly from composites. We discuss the micromechanics of reinforcement fibres, their conformability to the background fibre web and the fracture processes in reinforced paper. Reinforcement fibres should have high ductility and they be similar to the mechanical furnish in their micromechanical stiffness.
A micromechanics-based fiber-bridging constitutive model that quantitatively takes into consideration the distribution of fiber orientation and the number of fibers, is derived and a fiber-bridging analysis program is developed. An image processing technique is applied to evaluate the fiber distribution characteristics of four different types of strain-hardening cementitious composites. Then, the fiber-bridging curves obtained from image analysis are compared with those obtained from the assumption of two- and three-dimensional fiber distributions. The calculated ultimate tensile strains are also compared with experimental results. Test results showed that the tensile behavior of strain-hardening cementitious composites can be more accurately predicted and analyzed using the fiber-bridging curve obtained from image analysis.
The fiber bridging model is the crucial factor to predict or analyze the tensile behavior of fiber reinforced cementitious composites. This paper presents the fiber bridging constitutive law considering the distribution of fiber inclined angle and the number of fibers in engineered cementitious composites. The distribution of fiber inclined angle and the number of fibers are measured and analyzed by the image processing technique. The fiber distribution are considerably different from those obtained by assuming two- or three-dimensional random distributions for the fiber inclined angle. The simulation of the uniaxial tension behavior was performed considering the distribution of fiber inclined angle and number of fibers measured by the sectional image analysis. The simulation results exhibit multiple cracking and strain hardening behavior that correspond well with test results.
Two computational models for the simulation of the cracking behaviour of fibre-reinforced brittle–matrix composites – based on a continuous finite element (FE) approach and on a lattice approach, respectively – are presented. Such a class of materials is characterized, in the case of aligned fibres, by a high level of anisotropy due to the preferred fibre orientation in the bulk material. The main mechanical aspects being involved, such as crack appearance, crack propagation, fibre bridging effects, fibre debonding and breaking can be taken into account by the presented models. The advantages and drawbacks of the above approaches are outlined through some applications related to plain and fibre-reinforced brittle materials under both Mode I and Mode I + II loading. The numerical findings emphasize that the lattice approach allows us to describe in detail the crack pattern at microscale level, whereas the continuous FE approach allows us to perform computationally economic analyses yielding overall information of the mechanical problem.
This paper describes the tensile, impact, flexural properties and aging behavior of short banana fiber reinforced polyester composites with special reference to the effect of fiber length and fiber content. Maximum tensile strength was observed at 30 mm fiber length while impact strength gave the maximum value for 40 mm fiber length. Incorporation of 40% untreated fibers gave a 20% increase in the tensile strength and a 341% increase in impact strength. On treatment with silane coupling agent, composites showed a 28% increase in tensile strength and a 13% increase in flexural strength. Aging studies showed a decrease in tensile strength of the composites. The experimental tensile strength values were compared with theoretical predictions according to Piggot equation. Scanning electron microscopy studies were carried out to understand the morphology of the fiber surface, fiber pullout and interface bonding. Water absorption studies showed an increase in water uptake with increase in fiber content. Finally, the properties of banana fiber reinforced polyester composites have been compared with other natural fiber reinforced composites.
This paper presents a probabilistic micromechanical framework for analyzing crack bridging stress-displacement in short steel hooked-end fiber-reinforced cementitious composites, featuring the fiber length/diameter aspect ratios of 45 and 80, with varying fiber volume fractions and varying snubbing coefficients. The proposed formulation is constructed based on the randomly located, randomly oriented distribution of steel fibers. This random nature accounts for the dominant features of the composite failure mechanisms. The composite fracture energy dissipation may be obtained from the area under the tension-softening curve. The fracture energy dissipations contributed by the fiber interfacial debonding and fiber pullout of both the straight-part and the hooked-end element are systematically investigated. Further, the fiber bridging micromechanical mechanism and the bridging stress-displacement are accommodated. The fracture energy dissipation attributable to the hooked-end element is shown to be smaller than that of the straight element, but still remains significant. Comprehensive comparisons between the constant shear model predictions and experimental data manifest significant improvements when the hooked-end effects are incorporated into the composite fracture energy dissipation analyses. Based on the experimental data and micromechanical predictions, we recommend the range of the fiber volume fraction to be from 0.75% to 1%.
Essais d'arrachement des fibres metalliques dans 3 systemes: fil de cuivre-matrice d'aluminium, fil de kanthal-matrice d'aluminium et fil de kanthal-matrice en alliage Al-11%Si. Differences des courbes obtenues. Conclusions sur une liaison chimique entre Cu et Al, et sur la formation d'une zone intermediaire dans les systemes comportant du fil de kanthal
In the present paper, the mechanical behaviour of fibre-reinforced brittle-matrix composites, with emphasis to cementitious composites, is examined by adopting both a discontinuous-like FE approach and a lattice model. The main phenomena involved, such as crack formation and propagation, crack fibre bridging, fibre debonding, fibre breaking, are taken into account. The basic assumptions and theoretical background of such approaches are outlined, and some experimental data related to plain and fibre-reinforced concrete specimens under Mode I and Mode I + II loading are analysed. The comparison of the numerical simulation results shows that the lattice model allows us a very detailed description of the fracture pattern, whereas the discontinuous FE approach mainly gives us only global information in terms of both crack path and stress–strain response curve. Nevertheless, the FE approach is computationally convenient and a useful tool for studying problems which do not require a detailed description of the fracture process.
The problem of a broken fiber, embedded in an infinite medium with distinct elastic properties, is studied theoretically. The composite is subjected to tensile loading parallel to the fiber. To simulate the influence of a weak fiber-matrix interface, interfucial slippage governed by a Coulomb friction law is permitted. The solution method that is employed reduces the problem to four coupled singular integral equations which are solved numerically. Results for the average axial fiber stress, for the enhancement in the tensile stress in the matrix and for the opening of the crack are presented: where relevant, comparisons are made with simplified, highly approximate methods of analysis.
Discontinuous, randomly oriented nickel fibre/glass-ceramic composites have been prepared by a hot-pressing technique and the effect of expansion mismatch on the resultant mechanical properties has been investigated. Composite strength is reduced by fibre additions, the effect being greatest at low volume fractions of fibre. However, in contrast to the unreinforced materials which are extremely notch-sensitive, the strength of the composites is not adversely affected by the presence of a notch. Work of fracture is significantly increased but the effect is greatest for matched-expansion materials. A 40 vol % matched-expansion composite exhibits a value for work of fracture of 15.7±0.5 J m−2×103, and a flexural strength of 182±8 MN m−2, and is tougher and stronger than similar systems reported in the literature.
This article presents an experimental and numerical investigation into the influence of transverse stitching on failure of composite L-joints under tensile loading. Six unstitched and six stitched L-joint specimens were manufactured and tested under quasi static tensile loading. It was observed that the average measured failure load and the associated crosshead displacement for the stitched L-joint specimens are increased significantly compared to those for the unstitched specimens. Full 3D and 2D plane strain finite element (FE) models were developed to simulate both stitched and unstitched L-joints with an implemented stitch element. The load—displacement curves and results predicted via FE models compare favorably with the experimental results. For the stitched L-joints, it is shown that the observed delamination in the elbow region of the flange can be modeled by using a softening model for epoxy layer.
This paper analyzes the pseudostrain-hardening phenomenon of brittle matrix composites reinforced with discontinuous flexible and randomly distributed fibers, based on a cohesive crack-mechanics approach. The first crack strength and strain are derived in terms of fiber, matrix, and interface micromechanical properties. Conditions for steady-state cracking and multiple cracking are found to depend on two nondimensionalized parameters that embody all relevant material micromechanical parameters. The results are therefore quite general and applicable to a variety of composite-material systems. Phrased in terms of a failure-mechanism map, various uniaxial load-deformation behaviors for discontinuous fiber composites can be predicted. The influence of a snubbing effect due to local fiber/matrix interaction for randomly oriented crack-bridging fibers on the composite properties is also studied.
End-shaped copper fibers are placed in a brittle thermoset epoxy matrix at 10vol% and tested in four-point bending to determine the fracture toughness of the composite. Results from four-point bend tests agree well with the theoretical predictions of the fracture toughness increment ‘ΔG’ of a metal fiber/brittle thermoset matrix composite based on single fiber pullout (SFP) tests. This close agreement demonstrates that SFP testing, along with the theoretical model, can be used as an effective end-shape screening tool for ductile fibers before full scale composite testing. The model predicts that the composite’s fracture toughness will be 46% higher with flat end-impacted fibers and 4% lower with rippled fibers compared to straight fibers at a 0° orientation. Four-point bend results show the actual composite’s fracture toughness is 49% higher with flat end-impacted fibers and 5% lower with rippled fibers compared to straight fibers. Further, four-point bend results show that end-shaped copper fibers improve both the flexural strength and modulus of the composite, demonstrating that end-shaped ductile fibers provide a good stress transfer to the fibers by anchoring the fibers into the matrix. Lastly, experimental validation of the model also indicates that at low fiber volume fractions, fiber–fiber interaction has only a minor influence on the fracture toughness for the tested ductile fiber/brittle matrix composite.
Improvement in composite fracture toughness can be achieved by using short ductile fibers with shaped ends which utilizes the plastic work potential of the fiber volume through anchoring of the fiber end into the matrix. Single fiber pullout tests performed on copper wire demonstrates that a 2D flat-end-impacted fiber geometry that is easier to produce improves the fracture toughness increment at least as well as a 3D “axisymmetric” end-impacted fiber. Calculations based on pull out tests and a model for predicting the fiber contribution to the fracture toughness increment “ΔG” show a 46% higher ΔG for the flat-end-impacted fiber compared to a straight fiber at 0° orientation. Results further indicate that for a given fiber geometry there is an optimum end volume; above or below this volume results in a lower ΔG. Due to the small fiber end volume, the packing density of the fibers will not be significantly affected by the end shaping of the fiber.Annealing and subsequent oxide removal of the end-impacted fiber is not necessary because there is no significant improvement in the ΔG. However, a moderate 3.5–15% decrease in the debonding force is seen in the annealed fibers, depending on the embedment depth. This indicates that annealing weakens the fiber–matrix bond. Furthermore, results indicate that a fiber that has multiple mechanical interlocks in the matrix is not as effective in fracture toughening as single anchoring of a shaped fiber end. However, the load displacement curve for the light rippled fiber shows a unique “wavy” behavior related to the geometry of the fiber end which may be useful in other applications. Round fiber ends produced with an acetylene torch had a 70% lower ΔG compared to a straight fiber because of the large fiber end volume and the poor microstructure resulting from the high torch temperature.
Pull-out tests on ductile fibers embedded in epoxy resin have shown an additional component of pull-out work from the plastic deformation of the fiber, which component is not available in traditional brittle fibers. This additional deformation energy is available to improve the toughness of composite materials without sacrificing composite stiffness. The plastic deformation energy in ductile copper and nickel fibers is exploited by anchoring the fibers in the matrix through modification of fiber ends. Shaped fiber ends were produced by end-impacting and knotting fibers to facilitate anchoring, similar to previous work with ‘bone-shaped’ short (BSS) fibers. Both shaped and straight fibers were embedded in epoxy to various depths and angles, and pull-out tests were performed to determine whether an increase in pull-out work was evident for shaped fibers. Cold-working the fibers or treatment with release agent prior to embedment served as a tool to isolate the frictional component from the plastic deformation component of pull-out work during pull-out tests. The effect of fiber yield stress and hardenability on the pull-out work was also investigated. Calculations based on the test results and a model for predicting fracture toughness increment showed a 5–109% higher toughness increment for shaped fibers compared to straight fibers, dependent on material conditions and fiber orientation.
The design, fabrication and preliminary assessment of novel metal reinforced ceramic matrix composite (CMC) materials are reported in this study. The design is based on the assumption that the metallic reinforcing structure can increase the work of fracture through the action of wire pullout, crack deflection and plastic deformation. In particular, the current CMC is composed of a molybdenum wire structure within a 7.5 wt% yttria-stabilized zirconia (7YSZ) ceramic matrix (Mo/YSZ). A unique jig is designed to assist the fabrication of the Mo mesh. Subsequently a NiCrAlY bond coat and finally a 7YSZ matrix are applied to the Mo mesh structure using a plasma spray technique. The as-fabricated and heat treated Mo/YSZ are subjected to impact and 3-point bend tests. The impact testing results show that heat treatment after spraying increases the impact energy possibly due to the improved bonding between Mo, NiCrAlY and 7YSZ. During 3-point bend testing, the incorporation of Mo mesh in 7YSZ increases the load at yield point, the peak load and the displacement to failure. After isothermal and cyclic oxidation tests at 1050°C for 330 hours, the Mo/YSZ CMC is examined under optical and scanning electron microscopes (SEM). The results show that the NiCrAlY bond coat delaminates from the Mo wire and also forms radial cracks during the spraying process. It is for this reason that the Mo wire suffers from rapid oxidization during isothermal and cyclic oxidation tests, causing separation of the reinforcement from the 7YSZ matrix. Future improvement to the current process will be to select and deposit a more effective oxidation resistant coating on the Mo wire in order to allow the metal frame-reinforced CMC concept to achieve the desired chemical and mechanical properties at high temperatures.
Both theoretical and experimental approaches have been used to examine the micromechanics of interfaces in ceramic-matrix, fiber-reinforced composites. In order to examine variables which allow for the optimization of fracture, toughness, analytical solutions first were developed to describe the stress displacement fields for a cylindrical inclusion adjoining a surface of an elastic half-space (equivalent to a single fiber intercepting a matrix crack). This same equivalent inclusion model was used to approximate multiple fibers spanning an internal crack. An energy approach was used to describe conditions of crack propagation and arrest in systems containing long fibers as a function of interface toughness, residual thermal stress, fiber size and concentration. Experimentally, the validity of fiber pullout tests was examined as means of acquiring accurate interfacial data. A new fiber pullout test was developed which best approximates matrix fracture, and fiber debonding and pullout in a full scale composite. Bridging forces by both on and off-axis fibers have been evaluated for a SiC/borosilicate composite, the latter providing the first experimental measures of bridging forces of inclined fibers. Finally, studies on fiber-reinforced cements have been conducted and compared to Mura's crack arrest theory with excellent agreement.
Crack bridging by inclined fibers has been studied in a brittle fiber–brittle matrix model ceramic composite. Results of the fiber bridging force vs the crack opening displacement have been obtained for different fiber inclination angles using a fracture mechanics approach. Localized matrix cracking has been observed for inclined fibers and related to fiber inclination angle. The experimental results showing the influence of fiber inclination angle are discussed and compared with theoretical analyses to provide insight into crack bridging by inclined fibers/whiskers. Implications for toughening by whisker bridging are also discussed.
In this study, as a part of research to characterize the tensile properties of steel fiber reinforced ultra-high strength cementitious composites, pullout tests of steel fiber were performed to evaluate the effect of fiber inclination angle on the load direction and an analytical pullout model was derived considering this effect. The fiber inclination angles considered in the pullout tests were 0°, 15°, 30°, 45°, and 60°. From the pullout tests, it was observed that the largest peak load was obtained at an angle of 30° or 45°, and the peak slip increased as the fibers were oriented at a more inclined angle. Based on the experimental results, an analytical pullout behavior model considering fiber inclination was proposed. In order to take into account the effect of fiber inclination in the pullout model, apparent shear strengths (τ(app)) and slip coefficient (β) were introduced to express the variation of pullout peak load and the augmentation of peak slip as the inclined angle increases. These variables are expressed as functions of the inclined angle (ϕ).
Much information now exists on factors affecting toughness in composites. Theoretical expressions for fracture energy also abound, in response to the many factors that have been identified as contributing to toughness in fibre reinforced materials.
This material is reviewed from the point of view of the effect of aspect ratio on toughness. Expressions relating fracture energy to aspect ratio are derived and compared with experimental data. It is shown that in many cases aspect ratio should be as large as possible. There are a few cases, however, where the aspect ratio should be as close to the critical value as possible.
The strengths of solids are examined, and the various factors that determine them discussed. The ideal strength comes directly from the atomic forces but the practical strength depends on the toughness of the material and the effects of stress concentrations. The types of microstructures most suitable for realizing such strengths are considered. It is concluded that metals densely reinforced with strong whiskers or fibres of light, refractory, non-metallic solids have great promise.
Measurements of the work of fracture of composites of polyester resin reinforced with chopped steel wires of various lengths are compared with the theory developed by Cooper. For composites containing aligned wires the results agree well with the model except where there is excessive resin cracking. The toughness of composites containing wires which are randomly distributed in the resin can be significantly greater than that of aligned composites with wires of similar length. This is probably due to the plastic shearing of wires not lying parallel or normal to the specimen axis.
The effect of the orientation of metal wires on the opening of a crack in a brittle-matrix composite has been studied. The force arising from the plastic bending of a wire which is weakly bonded to the matrix and which crosses the matrix crack at an angle to the crack face normal has been measured in model resin-wire composites and good agreement is found with a simple theory based on the calculation of the force needed to produce a plastic hinge in a cantilever beam. The force passes through a maximum at a small crack opening, of the order of one wire diameter, and decreases with further crack opening. The The largest force is obtained for a value of of approximately 45. For wires whose length approaches the critical length, the force and the total work of fracture arising from the bending of the wire are small compared to the values arising from the interfacial shear stress resisting pull-out; the contributions due to bending and interfacial shear stress are of comparable magnitudes for wires which are approximately one-fifth of the critical length.