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Ferrous Fibre Network Materials for Jet Noise Reduction in Aeroengines Part I: Acoustic Effects
Advanced Engineering Materials
Abstract
This paper describes an investigation into the acoustic effects of introducing tubular modules, made of highly porous ferrous materials, into the exhaust region of a small turbojet aeroengine. The modules are composed of sintered stainless steel fibre networks, with void contents in the approximate range of 80-95%. Some of the modules incorporated radial gradients of void content, produced by bonding together thin layers of different density. Measurements of acoustic intensity, as a function of frequency, have been made with various module geometries and locations, and data have also been acquired relating to engine thrust, engine speed and fuel consumption. It is shown that significant reductions in noise level (up to ~10 dB) can be achieved, at approximately constant thrust and fuel consumption, by the introduction of modules of this type. Such effects can arise both via modification to the gas flow field (and hence to noise generation characteristics) and by attenuation of acoustic waves within the modules. Reductions in noise creation will tend to be specific to the gas flow field within the engine concerned, but acoustic attenuation is expected to be a more general effect. In the second of this pair of papers, an investigation is presented into the thermo-mechanical stability of these fibre network materials in this environment.
... Random fiber networks (RFNs) are typical structures which can be found in natural and artificial materials, such as the cytoskeleton of eukaryotic cells (Gould et al., 2021), skins (Yang et al., 2015), tendons (Raspanti et al., 2005), paper (Ostoja-Starzewski and Stahl, 2000), metal fiber sintered sheets (Veyhl et al., 2013), carbon bonded carbon fibers composites , porous ceramics (Fan et al., 2022) and electrospun polymer membranes (Domaschke et al., 2020;Zundel et al., 2017). Because of their unique structures and properties, such as high porosity and flexibility, large specific surface area, great specific modulus and strength and excellent thermal insulation ability, RFNs are now widely used in separation and filtration, noise absorption, advanced wearable devices, catalytic reactions and aerospace engineering (Hu et al., 2016;Golosnoy et al., 2008;Du and Zhang, 2022;Rodriguez et al., 2021;Sanchez et al., 2022;Yang et al., 2022). Most fibers in these fiber network materials are not in an ideally straight state. ...
... A f and I f are the area and inertia moment of cross section. k s = 10/9 is the form factor of shear of circular section (Gere and Timoshenko, 1991). As shown in Fig. 6(c), the shearing energy is much smaller than the other two in orders so that it is neglected in later discussions. ...
... The base fibers can be flexibly selected to meet various needs, which are made of copper [1], titanium [2], carbon nanotube [3][4][5][6][7], and steel [8]. In particular, a number of stainless steel fibers [9][10][11][12][13][14][15][16] exhibit superior performance such as high temperature resistance, corrosion resistance, high surface to mass ratio and permeability. As the cell size can be accurately controlled with different fibers of diameter ranging from several to several hundred micrometers, they are widely applied in filtration and separation [17], gas infiltration [18], catalyst support [19], biomaterials [11,15], heat transfer [20], and sound absorption [10,21,22]. ...
... In particular, a number of stainless steel fibers [9][10][11][12][13][14][15][16] exhibit superior performance such as high temperature resistance, corrosion resistance, high surface to mass ratio and permeability. As the cell size can be accurately controlled with different fibers of diameter ranging from several to several hundred micrometers, they are widely applied in filtration and separation [17], gas infiltration [18], catalyst support [19], biomaterials [11,15], heat transfer [20], and sound absorption [10,21,22]. In addition, it has been shown that auxetic fiber networks can have various promising mechanical properties over traditional open-cell cellular materials in terms of high specific stiffness and strength, high shear and indentation resistance [13,[23][24][25], larger fracture toughness [26] and enhanced energy absorption properties [27], among others. ...
Sintered metal fiber sheets (MFSs) made by sequential-overlap method are transversely isotropic open-cell cellular materials with paper-like fiber network architectures, which exhibit auxeticity and are promising for various potential applications due to the reentrant micro-structure. The thickness effect on the out-of-plane auxeticity (negative Poisson's ratio) of MFSs samples of 2–20 mm thick subjected to in-plane tensile loading is investigated with digital image correlation technique. Furthermore, the deformation modes of fibers within MFSs during various loading stages are examined with X-ray tomography. It is found that in addition to the straightening of reentrant fibers, fiber layers with defects and joints failure induced slippage between adjacent layers leads to local shear and results in unique umbrella-like local deformation termed umbrella effect, which gradually dominates the auxeticity during tensile loading. Although remarkably increasing lateral deformation, the umbrella effect significantly diminishes the in-plane mechanical performance such as rigidity and strength. In particular, this effect is suppressed by sample thickness: the overall performance tends to stabilize with sample thickness greater than a certain value, provided that the MFS is uniform with all fibers randomly distributed. The finding facilitates wider application of auxetic MFSs with further understanding on the relationship between the thickness effect and performance.
... Open-pored metals are modeled and developed due to their functional properties (Golosnoy et al. (2008), Xie et al. (2004), Rösler and Mukherji (2005), Lefebvre (2008)). Pore size and porosity result from the production processes like sintering of metal fibers (Golosnoy et al. (2008)), coating of polymer foams (Paserin et al. (2004)), melt infiltration techniques (Brothers et al (2005)) or selective phase extraction (Rösler and Mukherji (2005), Rösler et al. (2005), Rösler and Näth (2010), Hinze et al. (2011)). ...
... Open-pored metals are modeled and developed due to their functional properties (Golosnoy et al. (2008), Xie et al. (2004), Rösler and Mukherji (2005), Lefebvre (2008)). Pore size and porosity result from the production processes like sintering of metal fibers (Golosnoy et al. (2008)), coating of polymer foams (Paserin et al. (2004)), melt infiltration techniques (Brothers et al (2005)) or selective phase extraction (Rösler and Mukherji (2005), Rösler et al. (2005), Rösler and Näth (2010), Hinze et al. (2011)). The selective phase extraction allows the production of membranes with pore sizes of several hundred nanometers and a porosity of around 50%. ...
Nickel-based superalloys are predominantly used as structural materials in high-temperature applications due to their exceptional high-temperature strength resulting from precipitation hardening of the coherent γ’-phase. The self-assembly of the γ’-cubes to interconnected networks can be utilized by electrochemical leaching to produce nanoporous superalloy membranes with open porosity and, therefore, functional properties. So far, only γ’-membranes consisting of the intermetallic compound Ni3(Al, Ti, Ta) have been manufactured and analyzed. In this work, the production of the mirrored γ-membranes, consisting of the nickel solid solution, is described.
... Not only that, but the porous sound absorbing materials exhibit low density, high strength and flexibility, good thermal conductivity, excellent hightemperature resistance and wear resistance. Gradually, increasing number of scholars and engineers began to focus on the characterization and design of porous metal sound absorbing materials [7][8][9][10][11][12][13][14]. ...
A novel porous copper fiber sintered sheets (PCFSSs) with different porosities were fabricated using the cutting copper fibers with rough surface. The effect of fiber surface morphology, porosity, thickness and gradient pore structure of PCFSS on sound absorption performance was tested by using the transfer function method. Our results show that the copper fiber sintered sheets with rough surface exhibited better sound absorption performance compared with the copper fiber sintered sheets with smooth surface. The increase of sound resistance by increasing thickness or decreasing porosity could significantly improve the sound absorption coefficients in the low frequency region and widen its acoustic absorption bandwidth. Whereas, the sound absorption performance in the high frequency region was decreased with the increase of sound resistance. Moreover, the three-layer PCFSSs with gradient pore structure could cause a repeated propagation and dissipation due to the reflection of sound waves between the gradient interfaces, which can satisfy the requirements of noise control in different frequency region. Our work provides a method for improving sound absorption performance of porous metal material for different applications.
... This type of structure has a broader absorption bandwidth with relatively compact size. Therefore, bulk liners are applied widely in various fields: for example, wind tunnels [5], automotive mufflers [6], aircraft engines (auxiliary power unit [7], fan nacelle and casing treatment [8][9][10], exhaust for jet noise [11]), and heating ventilation and air conditioning systems in vehicles and buildings. ...
A mode-merging design method (MMDM) for bulk liners with porous materials is proposed to maximize transmission losses in flow ducts. The eigenequation and its partial derivative equation governing the coupled acoustic fields in the duct and backchamber are derived, from which the merging double eigenvalue is obtained at a target frequency for a single incident circumferential mode. The crucial liner parameters (namely. the chamber depth and the flow resistivity of the porous material) can be optimized simultaneously. A finite element propagation model is employed to evaluate the MMDM-based design for a finite-length liner, demonstrating the effectiveness of the method for selected typical design cases. It is found that mode merging can be realized for a nonlocally reacting liner, with or without a perforated plate, mainly for protecting the porous material inside its chamber. The MMDM, under the assumption of an infinite-length liner, can give an almost globally optimal design for a finite-length liner when the length exceeds five times the duct diameter. It is expected that the proposed MMDM can be useful in the preliminary designs for actual bulk liners due to its rapidness and accuracy.
... The point-reacting liner permits only plane wave propagating within its narrow back cavity along the cavity axis, and it usually possesses a strong frequency selectivity for noise absorption, especially for a single-layer liner [3,4]. The bulk reacting liner, named bulk liner hereafter for brevity, usually consists of a perforated facesheet with large porosity and extended back chambers filled with air or porous materials, and permits higher order acoustic modes propagating toward all directions within its chambers, such structures can bring about broader frequency band of sound absorption at a relatively low cost, therefore, the bulk liners are widely applied for sound control in various fields, e.g., wind tunnels [5], automotive mufflers [6], aircraft engines (the auxiliary power unit [7], the fan nacelle and casing treatment [8][9][10], the exhaust for jet noise [11,12]), and Heating Ventilation and Air Conditioning (HVAC) systems in vehicles and buildings. 1 PhD candidate, School of Energy and Power Engineering. 2 Associate professor, School of Energy and Power Engineering. ...
... Porous metals and ceramics can even stand adverse conditions such as high temperature and high sound intensity, a potential for industrial applications. Studies of the applicability of porous metals in engine liners have been reported in the literature [1][2][3] . This paper presents experiments on noise reduction in a flow duct with porous liners at low Mach numbers. ...
Porous materials can absorb sound effectively in a wide frequency range. Porous metals and ceramics can even stand adverse conditions such as high temperature and high sound intensity, a potential for industrial applications. Studies of the applicability of porous metals in engine liners have been reported in the literature. This paper presents experiments on noise reduction in a flow duct with porous liners at low Mach numbers. The main concern here is effects of noise reduction due to porous structure and facing sheet perforation at various flow speeds and sound pressure levels. Both transmission and insertion sound pressure level differences were measured in an anechoic chamber for sintered fibrous metal, padded fibrous metal and ceramic foams, with perforation ratios of 0.0567, 0.0641, 0.2267, at Mach numbers of 0.1, 0.17 and 0.25, respectively. Results seemed to indicate that perforation had on the noise reduction the most salient effect, which could be explained by the impedance effect of perforated facing sheets. While perforations keep around the same order, liners of porous materials in the tests roughly had similar effects.
... Its relevant transport properties are determined by the architecture of the solid and pore phases, which can be controlled and optimized by altering manufacture parameters. Therefore, it is important to study the microstructure and transport properties of porous media for improving their performances in industrial applications including filtration process, separation treatment, and fuel cells [1][2][3]. Especially, the flow laws through such media draw wide attention. In the exploration of creeping flow in microstructure at pore-scale level, the most common exploration method is numerical simulation. ...
... Porous metals exhibiting open porosity are of interest for many functional applications, such as filtration or noise attenuation [1,2]. These materials are mainly produced by sintering of powders or fibers [3], coating of polymer sponges [4] or melt infiltration techniques [5]. Resulting pore sizes of these "conventional" porous metals are typically in a range between 10 μm and 100 μm. ...
A new method to adjust the pore size in superalloy membranes is shown, utilizing controlled cooling from solution heat treatment of the solid superalloy. Hereby, the nucleation rate and, thus, the size of the gamma'-precipitates can be varied to a large extent. This leads to a corresponding variation in the pore size once the membrane material is produced by directional coarsening of the gamma'-phase to an interconnected network and subsequent selective extraction of the gamma-phase. Furthermore, it was found that coherent and incoherent gamma'-precipitates can be used alike to fabricate superalloy membranes, and yet, result in vastly different pore morphologies. The findings widen the application range of this novel material class.
... Consequently, this study focuses on cellular metals, which can resist elevated temperatures as well as fatigue loading and, additionally, resemble geometrically conventional passive absorbers. Depending on the production processes, e.g., sintering of metal fibers [3], powder processing using a foaming agent [4], coating of polymer foams [5], or melt infiltration techniques [6], the characteristic dimensions of the porosity are usually in a range of 0.01 mm to 10 mm. Here, the relationship between sound absorption, pore size and porosity is analyzed and discussed. ...
Rising numbers of flights and aircrafts cause increasing aircraft noise, resulting in the development of various approaches to change this trend. One approach is the application of metallic liners in the hot gas path of aero-engines. At temperatures of up to 600 degrees C only metallic or ceramic structures can be used. Due to fatigue loading and the notch effect of the pores, mechanical properties of porous metals are superior to the ones of ceramic structures. Consequently, cellular metals like metallic foams, sintered metals, or sintered metal felts are most promising materials. However, acoustic absorption depends highly on pore morphology and porosity. Therefore, both parameters must be characterized precisely to analyze the correlation between morphology and noise reduction performance. The objective of this study is to analyze the relationship between pore morphology and acoustic absorption performance. The absorber materials are characterized using image processing based on two dimensional microscopy images. The sound absorption properties are measured using an impedance tube. Finally, the correlation of acoustic behavior, pore morphology, and porosity is outlined.
... Features of the architecture, such as the fibre orientation distribution and the fibre segment aspect ratio, can be controlled and characterised [15]. In addition to mechanical properties, thermal [16], acoustic [17] and magnetic [18,19] characteristics of such networks have also been investigated. ...
This paper concerns energy absorption during projectile penetration of thin, lightweight sandwich panels with metallic fibre cores. The panels were made entirely of austenitic stainless steel (grade 304). The faceplates were 0.4mm thick and the core (∼1–2mm thick) was a random assembly of metallic fibres, consolidated by solid state sintering. The impact tests were simulated using ABAQUS. Faceplate behaviour was modelled using the Johnson and Cook plasticity relation and a strain rate-dependent, critical plastic strain failure criterion. The core was modelled as an anisotropic, compressible continuum, with failure based on a quadratic, shear stress-based criterion. The experimental data show that, with increasing impact velocity, the absorbed energy decreased from the ballistic limit, reached a minimum value, and then underwent a monotonic increase. The FEM modelling demonstrates that this increase arises from the kinetic energy of ejected fragments, while the energy absorbed by plastic deformation and fracture tends to a plateau. Normalised absorbed energies have been compared to values for single faceplates. The sandwich panels are marginally superior to single plates on an areal density basis.
... In addition to the limited work on fracture of ceramic reinforced with steel fibres, there have been studies of fibre network materials, oriented towards acoustic damping [37], heat exchangers [38], cores of lightweight panels [39] and bioactive layers on prosthetic implants [40]. There has also been tomographic characterisation of fibre orientation distributions [41] and use of the fibre work of fracture (per unit length) to obtain network fracture energies [42,43]. ...
A model is presented for prediction of the fracture energy of ceramic–matrix composites containing dispersed metallic fibres. It is assumed that the work of fracture comes entirely from pull-out and/or plastic deformation of fibres bridging the crack plane. Comparisons are presented between these predictions and experimental measurements made on a commercially-available composite material of this type, containing stainless steel (304) fibres in a matrix predominantly comprising alumina and alumino-silicate phases. Good agreement is observed, and it’s noted that there is scope for the fracture energy levels to be high (∼20 kJ m−2). Higher toughness levels are both predicted and observed for coarser fibres, up to a practical limit for the fibre diameter of the order of 0.5 mm. Other deductions are also made concerning strategies for optimisation of the toughness of this type of material.
... Les réseaux de fibres métalliques réticulées présentent de nombreux avantages pour une utilisation en tant qu'échangeur thermique [11], isolant ou absorbant acoustique [12,13]. En effet, ces matériaux présentent une faible densité relative, une porosité importante et une simplicité d'élaboration par des moyens économiques. ...
Des matériaux à architecture poreuse et aléatoire ont été élaborés à partir de fibres de verre ou de carbone enchevêtrées en vue d'une application potentielle comme âme de panneaux sandwich ventilé. Les fibres de carbone ont été choisies pour leurs bonnes propriétés mécanique et les fibres de verre pour leur faible coût. Les contacts entre fibres sont bloqués par collage à la résine époxy. Un moyen d'élaboration original a été développé. Les fibres sont placées dans une enceinte et sont enchevêtrées par un flux d'air comprimée. La résine est vaporisée en fin de mélange. Les comportements en compression et traction ont été étudiés sur les deux types de fibres alors que seuls les matériaux élaborés à partir des fibres de carbone ont été étudiées en flexion 3 points.
We proposed a gradient phononic crystal-Helmholtz cavity structure for simultaneous noise and vibration reduction. By simplifying the structure into a typical Helmholtz cavity and a phononic crystal, we derived the resonance frequency of this structure. Next, we use COMSOL software to calculate the transmission loss and transmissibility. Results of our analysis confirmed that the structure can successfully simultaneous reduce noise and vibration within 2500 Hz, and the range of the reduction frequency is close to 500 Hz. We also show different structures to realize the adjustability of the band gap and improve the practical of noise and vibration reduction.
This is the simulation diagram of phononic crystal-Helmholtz cavities. From the perspective of multiple physical fields, vibration reduction belongs to solid mechanics fields, while noise reduction belongs to pressure acoustics fields. In our paper, we both studied these two fields, the results show that this structure can simultaneous reduce noise and vibration within 2500 Hz, and the range of the reduction frequency is close to 500 Hz. In addition, the relationship between reduction frequency and the change of gradient is further studied. Results show that the frequency of the bandgap decreases with the number of unit cell increased. The center frequency of the bandgap decreases from 2000 to 1400 Hz, and it becomes stable at about 1400 Hz eventually.
In order to improve the environmental-friendly performance and sound absorption performance of petroleum polyurethane (PU) foam, a variety of polyols are used to prepare plant-based PU foams with different closed pore ratios. In this research, the microstructure of PU foam was replicated and a periodic unit cell (PUC) model with ideal conditions was created. The model was created as tetrakaidecahedrons with pores of different sizes in the different cytoderms. The non-acoustic parameters of the Johnson-Champoux-Allard (JCA) acoustic model can be obtained by finite element analysis of the PUC model under specific conditions. The theoretical sound absorption coefficient of different foams was obtained through the JCA acoustic model. The experimental results show that the increase of the closed pore ratio can improve the sound insulation performance of PU foam. Both the experimental results and the simulation results show that the increase in closed pore ratio can improve the sound absorption effect of PU foam on low-frequency noise. The simulation results further show that the sound absorption coefficient of PU foam first increases and then decreases with the increase in closed pore ratio. And the change of the closed pore ratio has a greater influence on the air-flow resistivity and tortuosity.
The quasi-static and dynamic mechanical behaviors of metal fiber sintered sheets (MFSSs) with six relative densities and three fiber diameters at the strain rates ranging from 0.001/s to 2000/s are investigated by using Universal Materials Testing System and split Hopkinson pressure bar (SHPB) system. The effects of relative density, strain rate on the yield strength and the energy absorption efficiency of MFSSs are evaluated. Finite element models (FEM) based on computed tomography (CT) images and idealized fiber networks are developed to simulate the compressive behavior at different strain rates. The measured responses of MFSSs are generally in agreement with that predicted by the 3D reconstructed model and the idealized model. Two deformation modes of MFSSs are explored based on stress wave theory and a critical speed is calculated to differentiate the two modes. Finally, an idealized lapping network is proposed to explore the compatibility condition of the geometrical characteristics and the effect of the strain rate and relative density on the mechanical behavior MFSSs.
A key characteristic of metals is their high thermal and electrical conductivity – due to the presence of free electrons. There are many applications in which these properties are exploited. For example, high thermal conductivity is useful in improving the resistance of materials to thermal shock and avoiding “hot spots” during localized heating. High electrical conductivity is desirable in power transmission lines, electrical and electronic components, and for electromagnetic shielding. A problem commonly arises in situations where such high conductivities need to be combined with good mechanical properties, since conventional strengthening by alloying commonly leads to sharp reductions in these conductivities. Metal matrix composites (MMCs) offer potential for control over property combinations. For example, ceramic reinforcement can strengthen without affecting matrix conductivity, so composite conductivity can remain high. There is thus interest in predicting how conductivity varies with reinforcement properties, volume fraction, aspect ratio, interfacial structure, etc. Combinations of high conductivity with low thermal expansivity confers resistance to distortion under transient heating or cooling conditions. Again, scope for tailoring property combinations is greater for an MMC than with conventional specification of alloy composition and heat treatment.
Novel stainless steel porous twisted short fiber materials (PTSFMs) with spatial composite intertexture structure are produced by compaction following with sintering of twisted short fibers. The stainless steel twisted short fibers are fabricated using a cutting stainless steel fiber ropes method with a self-developed rotary multi-cutter tool. The porous structure of the stainless steel PTSFMs exhibits a large difference in the through-thickness and in-plane direction, which results in differences in compressive behavior. During the compressive process, it is determined that the stainless steel PTSFMs exhibit typical elastic-plastic behaviors (three deformation stages) in the in-plane direction, but in the through-thickness direction, entered the compact densification zone after short-term nonlinear elastic deformation without a plastic platform stage. The compression deformation resistance in the through-thickness direction is obviously stronger than that in the in-plane direction at a given porosity. The compression strength and the anisotropy both decreased with increased porosity. The compression properties are anisotropic, which also result in different energy absorption efficiencies of the two directions. The relationship between the porosity and the energy absorption efficiency is not monotonically increasing or decreasing, but with the change of porosities, there is an optimal porosity, which has relatively high-energy absorption efficiency.
Engineers and scientists alike will find this book to be an excellent introduction to the topic of porous materials, in particular the three main groups of porous materials: porous metals, porous ceramics, and polymer foams. Beginning with a general introduction to porous materials, the next six chapters focus on the processing and applications of each of the three main materials groups. The book includes such new processes as gel-casting and freeze-drying for porous ceramics and self-propagating high temperature synthesis (SHS) for porous metals. The applications discussed are relevant to a wide number of fields and industries, including aerospace, energy, transportation, construction, electronics, biomedical and others. The book concludes with a chapter on characterization methods for some basic parameters of porous materials. Porous Materials: Processing and Applications is an excellent resource for academic and industrial researchers in porous materials, as well as for upper-level undergraduate and graduate students in materials science and engineering, physics, chemistry, mechanics, metallurgy, and related specialties. • A comprehensive overview of processing and applications of porous materials - provides younger researchers, engineers and students with the best introduction to this class of materials • Includes three full chapters on modern applications - one for each of the three main groups of porous materials • Introduces readers to several characterization methods for porous materials, including methods for characterizing pore size, thermal conductivity, electrical resistivity and specific surface area. © 2014 Tsinghua University Press Limited. Published by Elsevier Inc. All rights reserved.
A brief introduction of functional and structural applications is given in this chapter for porous metals. Based on this, some functional applications are mostly presented, which include filtration and separation, sound absorption, heat transferring, and porous electrodes. Moreover, applications of porous metals in transportation and iatrology are focused on. Of course, porous metals also serve many other important purposes, such as for use in light structures, energy management and vibration control, electromagnetic shielding, arresting flames, and catalytic engineering.
We use the copper fibers of diameters of 0.22 mm, 0.3 mm and 0.62 mm to prepare a single-layered copper fibrous porous plate and incorporate it into double and three layered sound absorption structure. We also study the effects of the depths of rear cavity and central cavity and of the depth of both types of cavity in action at the same time on the sound absorption performance of the sound absorption structure. The study results and their analysis show preliminarily that: (1) the increase of the depth of rear cavity helps improve the medium and low frequency sound absorption performance, yet there is an apparent valley of sound absorption between the first and second absorption peaks, thus reducing the sound absorption performance in the frequency range between 1 000 Hz and 3 000 Hz; (2) the increase of the depth of central cavity moves the absorption curve to the low frequency and reduces the resonance absorption coefficient, with the absorption valley between the first and second absorption peaks being deeper and its width being smaller, thus having less effect on the sound absorption performance of the sound absorption structure between 1 000 Hz and 3 000 Hz; in both cases, the sound absorption coefficient is more than 0.5; (3) the increase of the depth of both types of cavity can effectively suppress the depth of the absorption valley and enhance the sound absorption performance in the medium and low frequencies. The sound absorption coefficient curve of the three-layered porous copper fibrous structure with double cavities completely encompass that of the double-layered porous copper fibrous structure, while the sound absorption performance of the former is superior to that of the latter.
A novel porous metal fiber sintered sheet (PMFSS) with a three-dimensional reticulated structure was fabricated by multi-tooth cutting and high-temperature solid-phase sintering process with copper fibers. A uniaxial tensile test was conducted to investigate the effect of fiber length and natural aging factor on the tensile properties of the PMFSS. Results indicated that, under given stress, the increase of fiber length helped reinforce the tensile strength. The elongation of the PMFSS with medium length fiber of 15 mm exhibited the optimal performance, reaching about 13.5%. After natural aging treatment for a month, the tensile strength of PMFSS significantly decreased, but the change of elongation was negligible except for the one with the shortest fiber length of 5 mm, whose elongation was effectively improved. The morphological fracture features of PMFSSs were also characterized.
IntroductionSound AbsorptionCharacterization of Pore CharacteristicsDiscussionsConclusion
Acknowledgements
In the present paper, highly porous fibre networks made of 316L fibres, with different fibre volume fractions, are characterized in terms of network architecture, elastic constants and fracture energies. Elastic constants are measured using quasi-static mechanical and modal vibration testing, yielding local and globally averaged properties, respectively. Differences between quasi-static and dynamic elastic constants are attributed to through-thickness shear effects. Regardless of the method employed, networks show signs of material inhomogeneity at high fibre densities, in agreement with X-ray nanotomography results. Strong auxetic (or negative Poisson’s ratio) behaviour is observed in the through-thickness direction, which is attributed to fibre kinking induced during processing. Measured fracture energies are compared with model predictions incorporating information about in-plane fibre orientation distribution, fibre volume fraction and single fibre work of fracture. Experimental values are broadly consistent with model predictions, based on the assumption that this energy is primarily associated with plastic deformation of individual fibres within a process zone of the same order as the inter-joint spacing.
Aircraft noise is of great importance for aircraft passengers and residents near airports. Excellent candidates as metal fiber felts or metal foams to absorb engine noise are experimentally investigated in acoustic absorption measurements. Additionally, the empirical Delany-Bazley model is applied to receive simulation results. The comparison of measured and simulated results shows good agreement.
Nickel-based superalloys are predominantly used as structural materials in high-temperature applications due to their exceptional high-temperature strength based on precipitation hardening by the coherent γ′-phase. In modern superalloys, the γ′-phase fraction can amount to 75%, at which γ′-precipitates align as cubes parallel to the 〈001〉 directions of the crystal lattice. At high temperatures and under a mechanical load, e.g. during service in gas turbines, the γ′-cubes coalesce to γ′-rafts, generating an interpenetrating microstructure of γ and γ′. By extracting one phase of this interpenetrated network, nanoporous superalloy membranes containing channel-like interconnected pores are produced. So far, this can only be achieved by applying simultaneously thermal and mechanical loads during tensile creep deformation. Here, a new production process is presented. Due to internal stresses, load-free aging of single-crystalline superalloys, e.g. CMSX-4, also generates an interpenetrating microstructure of γ and γ′. This can be utilized to manufacture nanoporous superalloy membranes in the absence of an external mechanical load. The advantage of this process is its simplicity and the potential to fabricate larger membranes than possible by the costly tensile creep deformation process currently used.
Since many natural and biological materials are cellular, often with relatively high porosity levels (although sometimes these pores are partly filled with a fluid or a soft and compliant solid in the natural state), various types of porous materials are of interest for biological applications. A key feature for such applications is the space afforded for invasion, first by cells and ultimately by osseous tissue and vasculature. The surface should be chemically and topologically suitable for cells to penetrate and interlock. There is evidence that fine scale topographic features can affect both the adhesion and ingrowth of cells. One way of creating topographic features, such as terraces, is to employ suitable heat treatments so as to expose preferentially the low surface energy crystallographic planes via surface diffusion. The topography and crystallography of surface terraces, generated on solid-state-sintered ferritic stainless-steel fibre networks, have been characterised by electron back-scattered diffraction, atomic force microscopy and scanning electron microscopy. Initial work on the effect of these fine scale topographic features on cell proliferation shows encouraging results.
Chevron nozzles currently offer one of the most feasible methods of reducing jet exhaust noise in medium to high-bypass turbofan engines. Tests were conducted in the University of Cincinnati Nozzle Acoustic Test Facility, simulating a separate flow exhaust system to provide insight into some of the basic mechanisms and trends of this emerging technology. For this study, a baseline inner nozzle and three chevron nozzles were tested over a wide range of operating conditions, including dual and single How. Chevrons with varying numbers of lobes and levels of penetration were selected for this study to provide insight into the impact of these geometric parameters on the noise level. Spectral and directivity results from heated, coaxial flow tests showed that the chevron nozzles are most effective at lower frequencies and at aft directivity angles. Reductions in overall sound pressure level (SPL) ranging from 3 to 6 dB were documented. Calculations of perceived noise level directivity also showed 4-6 dB reduction at aft angles. The data also illustrated clear and consistent trends with respect to the chevron geometric parameters. Specifically, the chevron penetration was determined to be a primary factor in controlling the tradeoff between low-frequency reduction and high-frequency SPL increases. Although slight differences were observed with varying chevron lobe numbers at a fixed penetration, it appears that the effect is less significant than the penetration. Finally, the data indicated clear dependence of the chevron benefit on the velocity difference between the inner and outer streams.
A novel porous metal fiber sintered felt (PMFSF) with a three-dimensional reticulated structure has been produced by the solid-state sintering of copper fibers. The copper fibers, with several microstructures distributed onto the surface, were fabricated using the cutting method. The Scanning Electron Microscope (SEM) results revealed that there were two kinds of sintering joints present in the PMFSFs: fiber-to-fiber surface contact and crossing fiber meshing. In the sintering process, the surface microstructures of the fibers helped to improve the forming process of the PMFSFs, as a result of high surface energy. Furthermore, the effect of different sintering parameters on the forming process of the PMFSFs was studied in detail, including the sintering temperature and holding time. The sintering temperatures had a significant influence on the surface microstructures of single fiber and specific surface area of the PMFSFs, but the holding time did not. The optimal PMFSF with a three-dimensional reticulated structure and larger specific surface area was produced by sintering copper fibers at 800°C for 30 minutes in the reduction atmosphere.
The mechanical behaviour of nanoporous superalloy membranes is analysed for the first time and discussed in conjunction with their microstructure, consisting of ligaments of the γ′ phase and channel-like pores a few hundred nanometers wide. Tensile tests and vibrating reed experiments are performed to determine the mechanical properties. It is shown that they are strongly dependent on the processing procedure, the orientation of the γ′ ligaments and interdendritic heterogeneities. A microstructural model of the membrane materials is then used to analyse the findings in more detail and rationalize the observed elastic behaviour. Following these results, unique metallic membrane materials can be produced, combining excellent strength on a macroscopic scale with extremely fine porosity on a microscopic scale.
This paper is devoted to the determination of the acoustic characteristics of a porous medium saturated by air. The analysis of sound propagation in such a medium is performed using an homogenization technique. This theory is suitable since acoustic wavelengths are much greater than the usual pore size. The macroscopic descriptions involve the effects of air viscosity, inertial forces and heat transfer.The first part of the paper deals with single porosity materials. Two cases are investigated : (i) a medium with large pores in which thermal exchanges are negligible ; (ii) a medium with smaller pores for which thermal exchanges must be accounted for.The second part is concerned with dual porosity media, i.e. when the grains themselves are also porous. Neglecting heat transfer first yields a simplified macroscopic description. This simply dual porosity model is then improved by considering thermal effects.These results show that new porous materials could be evolved by introducing a microporosity structure that would give enhanced absorption properties over a wide range of frequencies.
Lotus-type porous copper with large number of unidirectional cylindrical pores was fabricated by unidirectional solidification of melt dissolving hydrogen in a pressurized hydrogen atmosphere. The sound absorption coefficient of the porous copper plate, which has many open pores, was measured by standing-wave method in the frequency range up to 4 kHz. The absorption coefficient increases with increasing frequency. The absorption coefficient increases with increasing porosity and specimen thickness, while it decreases with increasing pore diameter. In addition, it was understood that the absorption coefficient of lotus-type porous materials could be evaluated by using the attenuation constant.
The possibilities for manufacturing metal foams or other porous metallic structures are reviewed. The various manufacturing processes are classified according to the state of matter in which the metal is processed-solid, liquid, gaseous or ionised. Liquid metal can be foamed directly by injecting gas or gas-releasing blowing agents, or by producing supersaturated metal-gas solutions. Indirect methods include investment casting, the use of space-holding filler materials or melting of powder compacts which contain a blowing agent. If inert gas is entrapped in powder compacts, a subsequent heat treatment can produce cellular metals even in the solid state. The same holds for various sintering methods, metal powder slurry foaming, or extrusion and sintering of polymer/powder mixtures. Finally, electro-deposition or metal vapour deposition also allow for the production of highly porous metallic structures. The various ways for characterising the properties of cellular metals are reviewed in second section of this paper. Non-destructive as well as destructive methods are described. Finally, the various application fields for cellular metals are discussed. They are divided into structural and functional applications and are treated according to their relevance for the different industrial sectors. (C) 2001 Elsevier Science Ltd. All rights reserved.
A study of the propagation of waves in porous media with an interconnected network of pores and micropores of very different characteristic sizes, saturated by a compressible Newtonian fluid, is proposed. With this aim, the homogenization technique for periodic separated scales media, is applied to realistic double porosity materials with motionless skeleton. From preliminary explicit estimations of wavelengths in the two fluid networks, it is shown that the macroscopic descriptions depend on the contrast of static permeability between pores and micropores and on frequency. The local equations are solved in the cases of low and high contrasts of permeability, and two main macroscopic behaviors are obtained. In the low contrast situation, the macroscopic flow is given by a kind of generalized Darcy's law involving both pores and micropores, and their respective characteristic frequencies. Regarding compressibility effects, both pore networks act in parallel. The high permeability contrast reveals that the macroscopic flow law is governed by the pores. The microporous part of the material is submitted to pressure diffusion effects, bringing dissipation, and modifying the dynamic bulk modulus of the material. The two situations of coupling are illustrated for simple geometry of double porosity materials, including perforated--and slits--microporous materials.
A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc. i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid-solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials.
A method of calculating the overall acoustic impedance of multiple layered absorbing systems is presented. Good agreement between measured (impedance tube) and computed impedances has been obtained. The acoustic impedance programs have been used to consider aspects of designing flat-walled composite anechoic linings.
Chevron nozzles currently offer one of the most feasible methods of reducing jet exhaust noise in medium to high-bypass turbofan engines. Tests were conducted in the University of Cincinnati Nozzle Acoustic Test Facility, simulating a separate flow exhaust system to provide insight into some of the basic mechanisms and trends of this emerging technology. For this study, a baseline inner nozzle and three chevron nozzles were tested over a wide range of operating conditions, including dual and single How. Chevrons with varying numbers of lobes and levels of penetration were selected for this study to provide insight into the impact of these geometric parameters on the noise level. Spectral and directivity results from heated, coaxial flow tests showed that the chevron nozzles are most effective at lower frequencies and at aft directivity angles. Reductions in overall sound pressure level (SPL) ranging from 3 to 6 dB were documented. Calculations of perceived noise level directivity also showed 4-6 dB reduction at aft angles. The data also illustrated clear and consistent trends with respect to the chevron geometric parameters. Specifically, the chevron penetration was determined to be a primary factor in controlling the tradeoff between low-frequency reduction and high-frequency SPL increases. Although slight differences were observed with varying chevron lobe numbers at a fixed penetration, it appears that the effect is less significant than the penetration. Finally, the data indicated clear dependence of the chevron benefit on the velocity difference between the inner and outer streams.
The NASA Glenn Research Center recently completed an experimental study to reduce the jet noise from modern turbofan engines. The study concentrated on exhaust nozzle designs for high-bypass-ratio engines. These designs modified the core and fan nozzles individually and simultaneously. In comparison with chevrons, tabs appeared to be an inefficient method for reducing jet noise. Data trends show that interaction between fan flow and the core cowl could strongly impact noise and cruise performance irrespective of the mixing device installed. The study demonstrates that modifications of the core nozzle are generally more advantageous than of the fan nozzle. Even greater advantage in noise reduction and associated cruise thrust penalties is demonstrated through simultaneous modification of both nozzles. The best nozzle design had a 0.06% cruise thrust loss and, corrected for takeoff thrust loss, a 2.7-EPNdB reduction for the effective perceived noise level metric. This design simultaneously employed chevrons on the core and fan nozzles. Last, five nozzle configurations with cruise thrust loss of less than 0.5% and noise reductions of over 2.5 EPNdB are identified as candidates for full-scale engine and flight demonstrations.
Some active impedance control experiments in an anechoic chamber are reported. Further active methods are developed for the design of locally controlled absorption liners in air, whose basic principles were described by Olson and May and subsequently investigated practically by Guicking and his colleagues at the beginning of the 80's. Two methods are described and tested. In the first, processing of the acoustic pressure and velocity information from close to the membrane of a control loudspeaker is used to produce a desired impedance. In the second active and passive means are combined: the impedance of the rear face of a porous layer is actively controlled so as to make the front face normal impedance take on a prescribed value. The impedance matching performance of both systems subject to an incident acoustic field including a single secondary loudspeaker is studied for both normal and oblique incidence. The combination of active and passive methods is a pragmatic approach, the aim of which is to simplify the control system for impedance control over extensive areas of wall. Indeed, the association of active control with a porous material allows the active system to be reduced in complexity to a simple active pressure release. Even though somewhat sub-optimal for sound absorption, the hybrid passive/active systems support feedback methods and lead to highly absorptive coatings.
The behavior of multilayer systems with porous and viscoelastic materials is of utmost importance to noise and vibration control inside vehicles, airplanes and other environments. In this paper, some configurations are analyzed and experimental results of surface impedance and absorption are compared with those obtained by numerical simulations. The influence of some characteristics, such as stiffness, tortuosity and flow resistivity in the performance of porous materials is evaluated.
Metallic foams have a combination of properties that make them attractive for a number of engineering applications, including lightweight structural sandwich panels, energy absorption devices, and heat sinks. For many potential applications an understanding of the mechanical behavior of these foams is essential. Recently, there has been substantial progress in identifying the mechanisms of deformation and failure in metallic foams. Here, we summarize the current understanding of the elastic moduli, uniaxial strength, yield criterion, creep, and fatigue of metallic foams.
The absorption performance of single layer perforated panel system has been usually estimated by equivalent electro-acoustic circuit analysis based upon the analogy between electric circuit and acoustic system. In the case of multiple layer perforated panel system, however, the transfer matrix method is more convenient than the equivalent circuit analysis.Hence, in this paper the transfer matrix method widely used for the one-dimensional acoustic analysis of engine exhaust muffler is presented. The absorption coefficient is estimated from the overall transfer matrix obtained by multiplying unit transfer matrices for perforated panels or airspaces. The proposed transfer matrix method is confirmed by comparing the estimated absorption coefficient with the measured value. In addition, the effect of dimension and arrangement of the perforated panels on the acoustical performance is discussed.
This article presents the results of a study of the microstructure and strength of open-cell 6101 aluminum alloy fans with three different levels of pore size (measured in pores per inch (PPI)). The macrostructures and microstructures of open-cell foam struts are characterized using a combination of optical and scanning electron microscopy (SEM). The compositions of the individual phases are also determined via energy-dispersive spectroscopy (EDS). The variations in strut microhardness are then measured using Vickers microindentation techniques. Following the measurement of the mechanical properties of foams, a modified model is used to estimate the relative density from measured strut dimensions. The mechanisms of compressive deformation are then elucidated before presenting a discussion on unit-cell modeling, strut plastic deformation, and the relationships between strut microstructure and microhardness.
The paper describes a new determination of the equal-loudness relations for pure tones in free-field conditions which has been carried out at the National Physical Laboratory as a result of requests from organizations interested in various aspects of the acoustics of hearing. The equal-loudness contours are of considerable importance in this field, being fundamental to a proper understanding of aural judgments of the loudness of sounds of all kinds. They are also concerned in numerous practical applications in the study of noise.
The first set of contours for free-field conditions was given by Fletcher and Munson in 1933, and a second determination was carried out by Churcher and King in 1937, but these two investigations showed considerable discrepancies over parts of the auditory diagram. The present work has been carried out on a more extensive scale, using a large team of otologically normal persons, and new techniques have been introduced enabling reliable measurements to be made over a wider range of intensity than has hitherto been possible.
The new results cover a range of frequency of from 25 to 15 000 c/s and of sound pressure level up to about 130 dB relative to 0.000 2 dyn/cm2. The data show a greater degree of regularity than the former results, and allow the equivalent loudness of a pure tone of any frequency to be expressed by formulae quadratic in the sound pressure level, the coefficients varying smoothly with frequency. The results include a new determination of the normal threshold of hearing in free field, which is highly consistent with the equal-loudness contours. At frequencies above 1000 c/s account needs to be taken of variations due to the age of the observers, which become of particular importance at the upper end of the frequency range.
The new results have indicated the causes of some of the discordant features in the earlier determinations and it is hoped that the work will facilitate agreement on a standard set of equal-loudness contours. On account of its relevance to noise measurement, some extension to equal-loudness relations for bands of noise is being undertaken.
The paper discusses the absorption of non-homogeneous thin macro-porous materials. In particular, the acoustic absorption performance of three-dimensional (3-D) porous media made up from thin porous patches, with different acoustic properties, is investigated using a numerical model. The presented model considers the configuration wherein the material is bonded onto the hard-walled termination of a semi-infinite rectangular waveguide. It couples a finite element description for the porous material to a modal description in the waveguide and uses a power balance approach to accurately quantify the absorption performance of the material. Experimental results are presented to validate the model in the special case of a macro-perforated porous material. Using this model a parameter study is presented. It is shown that absorption may be increased at low frequency by using non-homogeneous patch-works.
Measurements have been made of the acoustic properties of a typical mineral wool fibrous material at temperatures up to 500C. In addition to acoustic impedance and propagation constant the flow resistance of the material was measured over the same temperature range. With use of a modified form of the Delany-Bazley empirical formula good agreement was found between predicted and measured values of the acoustic impedance and propagation constant at various temperatures up to 500°C.
The architecture of bonded fibre networks produced by sintering of short stainless steel fibres has been characterised using computed X-ray microtomography. Two important characteristics of such networks are the distributions of fibre segment length and fibre orientation. These have strong influences on the mechanical, thermal and electrical properties. To extract quantitative architectural data from the reconstructed fibre networks, a 3-D skeletonisation algorithm was used to convert the reconstructed fibre surfaces into their corresponding medial axes.
The concept of meso-perforations in appropriately chosen porous media can help enhance their sound absorption performance. The meso-perforated materials are also referred to as “double porosity materials” since they are made up of two interconnected networks of pores of different characteristic size. Several theoretical, numerical and experimental works have been accomplished on the subject by the authors. The purpose of this paper is to give a synthetic review of these works and establish practical design rules to develop optimized noise control solutions based on this concept. The paper presents two complementary models to deal with this kind of materials: an analytical model based on homogenization techniques and a numerical model relying on a finite element discretization of the domains. The limits of these models are discussed. The choice of the design parameters is then been investigated in order to provide practical design rules. This choice relies on a criterion which is evaluated from the knowledge of the resistivity, porosity and tortuosity of the micropous medium, and the calculation of a geometrical parameter defined from the chosen mesoscopic structure. Experimental and numerical results regarding the influence of the mesopore profile along the thickness performed in a appropriately chosen substrate microporous medium are presented. The agreement between the models and the experiments is satisfactory. Results show that significant enhancements of the absorption properties can be obtained over a selected frequency band by adjusting the mesopore profile. It is also shown that interesting absorbing properties can be obtained when coating a double porosity medium with an impervious screen.
Standard fracture mechanics tests were carried out on two different types of aluminium foam, ALPORAS® foams and ALULIGHT® foams, with a variety of densities. Standard fracture toughness tests on compact tension (CT) specimens with widths from 50 mm to 300 mm and in situ tests in the scanning electron microscope were performed. Fracture toughness values in terms of the critical stress intensity factor, KIC, the critical J-integral, JIC, and the critical crack-tip opening displacement, COD5,i, were determined. To identify the fracture process, local deformation measurements were performed on the foam surfaces with a digital image processing system.From the deformation measurements it is evident that the deformation is strongly localised on different length scales. A relatively large fracture process zone, 6–8 cells in height, is developed, where only few of them are heavily deformed. On the cell wall level the deformation is again strongly localised to the thinnest parts of the cell wall, where cracks initiate and propagate. The crack propagates through the foam, building many secondary cracks and crack bridges. The comparison of K vs. Δa (crack extension), J vs. Δa and COD vs. Δa with the current fracture processes at the crack tip and the load–displacement response reveals that COD gives the most reliable measured values to characterise the fracture toughness.
Bonded networks of metal fibres are highly porous, permeable materials, which often exhibit relatively high strength. Material of this type has been produced, using melt-extracted ferritic stainless steel fibres, and characterised in terms of fibre volume fraction, fibre segment (joint-to-joint) length and fibre orientation distribution. Young’s moduli and yield stresses have been measured. The behaviour when subjected to a magnetic field has also been investigated. This causes macroscopic straining, as the individual fibres become magnetised and tend to align with the applied field. The modeling approach of Markaki and Clyne, recently developed for prediction of the mechanical and magneto-mechanical properties of such materials, is briefly summarised and comparisons are made with experimental data. The effects of filling the inter-fibre void with compliant (polymeric) matrices have also been explored. In general the modeling approach gives reliable predictions, particularly when the network architecture has been characterised using X-ray tomography.
A new type of acoustic liner developed for broadband noise reduction in flow ducts is considered in this paper. It combines passive absorbent properties of a porous layer and active control at its rear face. The complete design procedure of this hybrid passive/active liner is developed here. The passive part is first considered with the determination of a suitable porous material and the cut-off frequency separating the active low frequency regime from the passive high frequency one. The control system is then presented: a digital adaptive feedback control is performed independently cell by cell, allowing an easy subsequent increase of the liner surface. The entire optimization process has been successfully applied to a laboratory flow duct: both predictions and measurements show the interest of the hybrid liner to reduce the noise radiation.
The diversity of porous materials is noted. However, this study is particularly relevant to the use of sound absorbent materials in architectural acoustics. The theory of sound propagation within an idealised porous material consisting of a rigid matrix through which run parallel cylindrical pores normal to the surface is reviewed. Extensions to pores of arbitrary orientation and cross-section are achieved by introducing physically-measura ble microstructural constants rather than phenomenological bulk parameters that might be frequency dependent. By comparison of several theories that account for sound propagation within an elastically-framed porous material a basis is laid for an improved formulation, that takes into account both viscosity and heat conduction.The application of various propagation theories to model the reflection and transmission of sound at porous boundaries is considered. Particular attention is paid to the common assumption of local reaction and to the adequacy of modelling the porous interface as that of a quasi-homogeneous fluid. Finally the most widely used methods of measuring acoustical characteristics of porous materials at normal and oblique incidence and of obtaining their values by empirical means are surveyed.
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