The macroscale properties of polymer-matrix composites depend immensely on the quality of the interaction between the reinforcement phase and the bulk polymer. This work presents a method to improve the interfacial adhesion between metal-oxides and a polymer matrix by performing surface-initiated polymerization (SIP) by way of a biomimetic initiator. The initiator was modeled after 3,4-dihydroxy-L-phenylalanine (dopa), an amino acid that is highly concentrated in mussel foot adhesive proteins. Mechanical pull out tests of NiTi and Ti-6Al-4V wires from poly (methyl methacrylate) (PMMA) were performed to directly test the interfacial adhesion. These tests demonstrated improvements in maximum interfacial shear stress of 116% for SIP-modified NiTi wires and 60% for SIP-modified Ti-6Al-4V wires over unmodified specimens. Polymer chain growth from the metal oxides was validated using x-ray photoemission spectroscopy (XPS), ellipsometry, scanning electron microscopy (SEM), and contact angle analysis.
This paper quantifies how the quality of dispersion and the quality of the interfacial interaction between TiO(2) nanoparticles and host polymer independently affect benchmark properties such as glass transition temperature (Tg), elastic modulus and loss modulus. By examining these composites with differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM), we were able to demonstrate changes in properties depending on the adhesive/wetting or repulsive/dewetting interactions the nanoparticles have with the bulk polymer. We further quantified the dispersion of TiO(2) nanoparticles in polymethylmethacrylate (PMMA) matrices by a digital-optical method and correlated those values to the degree of Tg depression compared to neat PMMA. Samples with the same weight percent of nanoparticles but better dispersion showed larger shifts in Tg.
The 2–2–0 single-domain 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 crystal/porous polyurethane composite is studied in which the orientation of the spontaneous polarisation vector Ps of crystal, its volume fraction and the volume fraction and shape of pores in polymer are varied to attain maxima (minima) of effective hydrostatic piezoelectric coefficients and , squared figure of merit and electromechanical coupling factor . This paper describes an effect of the Ps orientation and the shape of pores on the hydrostatic piezoelectric response of this composite. Advantages caused by large values of , , , and of the studied composite are discussed.
In the present investigation, the effect of three different stir casting routes on the structure and properties of fine fly ash particles (13 μm average particle size) reinforced Al–7Si–0.35Mg alloy composite is evaluated. Among liquid metal stir casting, compocasting (semi solid processing), modified compocasting and modified compocasting followed by squeeze casting routes evaluated, the latter has resulted in a well-dispersed and relatively agglomerate and porosity free fly ash particle dispersed composites. Interfacial reactions between the fly ash particle and the matrix leading to the formation of MgAl2O4 spinel and iron intermetallics are more in liquid metal stir cast composites than in compocast composites.
An approach to predicting delamination due to the free-edge effect in a curved beam specimen loaded in bending is presented. Stress-based criteria do not give satisfactory results because of the stress singularity at the free edge. The conventional fracture-mechanics approach for free-edge delamination cannot be used because the strain-energy release rate does not reach an asymptotic value. An improved approach is developed on the basis of an assumed initial defect. The strain-energy release rate is calculated by using finite elements and this is then used to predict the failure load. Good correlation with the experimental results is obtained.
A study of the growth of delamination due to bending in a [905/05/905] graphite/epoxy laminate is presented. A plane strain finite element analysis is used to determine the strain energy release rate during delamination in a three-point-bend specimen. A closed form expression for energy release rate is obtained from classical lamination theory and Timoshenko beam theory and is compared with the finite element analysis result. These results combined with the experimentally determined critical load, Pcrit, are used to calculate the critical strain energy release rate, GC. The critical energy release rate is also determined experimentally by the compliance method.
Microbuckling in composite laminates is thought to initiate by the elastic bending of fibres, loaded by resin matrix material in shear. The fibres rotate and break in two places, forming a kink band. The fibres then rotate further until the matrix between the fibres fails, and the kink band and hence the laminate loses its load carrying capability. The present work investigates existing criteria for fibre microbuckling failure in a 0° unidirectional carbon-fibre-reinforced plastic (CFRP) laminate loaded in compression. From simple arguments, it is concluded that fibres undergoing bending cannot fail in tension on their convex side but rather that they fail in compression on their concave side. Inferences are made on which failure criterion should be used to predict unidirectional laminate failure when the failure mode is by 0° fibre microbuckling (or fibre kinking).
Both glutaraldehyde (GA) and 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) can crosslink the cell wall polymers and dimensionally stabilize wood particles and the treated wood particles are thus expected to enhance the properties of the resulting wood particle/polypropylene composites. Compared to the composites filled with untreated particles, treatments of wood particles with both GA and DMDHEU showed a great reduction in water uptake and dimensional swelling of the resulting composites up to 39% and 46%, respectively. Both the flexural and tensile moduli increased due to wood particles treatments with GA and DMDHEU. Treatments of wood particles improved the tensile strength but moderately weakened the flexural strength and Charpy impact strength of the composites. Dynamic mechanical analysis and microscopy suggested an improved interfacial compatibility between wood particles and matrix due to GA and DMDHEU treatments. Chemical treatment resulted in smaller particle sizes and altered microscopic fracture appearance after composite production as compared to untreated particles. Morphological changes were attributed to embrittlement of wood particles, which may negatively influence the mechanical properties of the resulting composites.
In this paper, the tensile properties and the low-cycle fatigue behavior of the 7005 aluminum alloy reinforced with 10 vol% of Al2O3 particles (W7A10A composite) and 6061 aluminum alloy reinforced with 20 vol% of Al2O3 particles (W6A20A composite) were studied. The microstructural analyses showed clustering of Al2O3 particles, irregularly shaped and with a non-uniform size. A significant increase of the elastic modulus and tensile strength in the MMCs, respect to the unreinforced alloys, was evidenced by the tensile tests, while the elongation to fracture decreased. The temperature effect on the tensile properties was not relevant up to 150 °C, while strength significantly decreased at 250 °C, mainly in the composite with the lower content of the ceramic reinforcement. The low-cycle fatigue tests showed no evidence of isotropic hardening or softening for the W7A10A, and a slight cyclic softening for the W6A20A. SEM analyses of the fracture surfaces showed that both the tensile and fatigue fracture was controlled by interfacial decohesion (expecially for the W7A10A composite), fracture of reinforcing particles (mainly for the W6A20A composite), void nucleation and growth. Also the presence of the MgAl2O4 spinel, probably, played a significant role in the mechanisms of failure in the W6A20A composite, by promoting void nucleation at the particles–matrix interfaces, interfacial decohesion, and also failure of the particles. These effects can be responsible of the slight softening observed in the W6A20A, under the low-cycle fatigue conditions.
The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) with ±45° fiber orientation was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 46 GPa and the ultimate tensile strength (UTS) was 55 MPa. Tensile creep behavior was examined for creep stresses in the 15–45 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in all test environments for creep stress levels ⩽35 MPa. At creep stresses >35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated.
The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress–strain behavior and tensile properties were evaluated at 1200 °C. Tension–tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 105 cycles at the frequency of 0.1 Hz and as 2 × 105 cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.
The resurgence of interest in metal matrix composites has been fuelled by the development of new fibres with high temperature characteristics. The new family of continuous fine ceramic fibres based on SiC or Al2O3 offers the possibility of producing high temperature composites with metal or ceramic matrices. The toughening of ceramics by these fibres is a particularly interesting prospect.Two types of continuous silicon carbide Nicalon monofilaments (NLP 101 and NLM 102) have been tested in air and argon up to 1300°C. Tensile and creep tests have shown that the tensile strength falls and the fibres creep above 1000°C. Different behaviour was found for the two types of fibres. The NLM 102 fibre was stronger and crept less at high temperature under small strains. However its creep lifetime was less than that of the NLP 101 fibres.These differences have been interpreted with the aid of a microstructural study. The fibres were found to contain silicon, carbon and oxygen (electron microphobe and Auger spectrometer) and SiC was also detected (X-ray diffraction and transmission electron microscopy). The modification of the amorphous and microcrystalline structures during creep have been investigated. A fine segregation of free carbon particles was detected (X-ray diffraction and ESR) and was seen to disappear during heat treatment in both types of environment studied.
To understand the mechanism of the ‘hybrid effect’ on the tensile properties of hybrid composites, single fiber type and hybrid microcomposites were fabricated by using Kevlar-149 as the low elongation fiber and S glass fibers as the high elongation fiber, in a DER 331/DER 732 epoxy mixture (70/30, w/w). Kevlar-149 fiber showed a significantly higher tensile strength in the microcomposite than as a single filament. For the hybrid, Kevlar-149 fibers usually broke one by one. A positive hybrid effect for the failure strain but a negative hybrid effect for the strength of the hybrid were observed. Tensile strength of the microcomposites predicted by Monte Carlo simulation agreed with the experimental results reasonably well. The tensile modulus of the hybrid followed the rule of mixture. The fiber/matrix interface properties were investigated by using single fibre pull-out from a microcomposite (SFPOM) test, which showed a significant difference between the interfacial shear strength (IFSS) of Kevlar fiber/epoxy in single-fiber type (SFT) and that in the hybrid at a constant fiber volume fraction, which shortened the ineffective length and contributed to the failure strain increase of Kevlar-149 fibers in the hybrid.
The stress-rupture failure of S-glass/Kevlar-149 seven-fiber microcomposites was modeled by employing a Monte Carlo method, taking into account matrix creep effects and fiber lifetime. While being obtained at low computational cost, the simulated data compared favourably with experimental results. The critical cluster sizes for the respective specimens were obtained directly from the simulation. Their frequency distribution was the subject of special attention. Parametric studies were carried out to examine the influence of matrix creep and the lifetime and stiffness of the single fiber on the failure process. It was found that failure of the hybrid microcomposites under consideration is driven by single-fiber failure rather than by matrix creep effects. However, it appears that the lifetime of a Kevlar-149 fiber is increased significantly when it is embedded in an epoxy matrix.
To increase the utilisation of composite material, new domain of application should be investigated such as intermediate temperature (up to 150 °C) for long term utilisation. Under these conditions, oxidation could not be neglected.This paper investigates material durability with the shape ratio influence of sample on oxidation behaviour during ageing at isothermal temperature. Weight loss evolution is discussed according to different shape ratio. Evolution of mechanical properties is also investigated to compare the thickness influence on the evolution of properties with ageing time.The weight loss due to oxidation could be assimilated to a degraded material flux going out of the sample from the exposed surface (and edges). A first estimation gives a coefficient of 2.3 for the degraded material quantity going out of the sample per cm2 of laminate edges compared to cm2 of laminate surface. In fact we have 2.3 times more degraded material going out of edges per cm2 than going out of surface per cm2.
Nomex fabric composites filled with the particulates of polyfluo150 wax (PFW) and nanoparticles of SiO2, respectively, were prepared by dip-coating of Nomex fabric in a phenolic resin containing particulates to be incorporated and the successive curing. The friction and wear behaviors of the pure and filled-Nomex fabric composites sliding against AISI-1045 steel in a pin-on-disk configuration were evaluated on a Xuanwu-III high temperature friction and wear tester. The structure of the composites, and the morphologies of the worn surfaces and of the counterpart steel pins were analyzed by means of scanning electron microscopy. The adhesion and tensile strength of the unfilled, PFW or nano-SiO2 filled Nomex fabric composites were evaluated with a DY35 universal material tester. The results showed that the addition of PFW and nano-SiO2 significantly improved the wear resistance and decreased the friction coefficient, moreover the PFW as a filler is better than nano-SiO2. The improved tribological performance of filled-Nomex fabric composites when compared with the unfilled one, can be attributed to the self-properties of filler, such as the self-lubricative of PFW, the bonding strength between the Nomex fabric and the adhesive resin adopted with the different particles and the special characteristic of transfer film.
The effects of isothermal exposure on the failure mechanisms of composites with a titanium aluminide matrix have been investigated through the evaluation of their tensile properties at room temperature after high-temperature heat treatments. The composites have been processed by hot pressing unidirectional arrays of SiC filaments between Ti3Al based foils.Owing to the importance of the role played by the filament/matrix interfacial zone on the mechanical behavior of this type of semi-brittle composite, the chemical interaction between matrix and reinforcement during the isothermal exposure has been identified and correlated with the degradation of mechanical performance. The mechanisms of damage have been explained thanks to the assessment of residual thermomechanical stresses and to the mechanical characterization of each component of the composites before and after undergoing various temperatures and durations of heat treatment. The failure mechanisms of the 1D-SCS6/super-α2 composites appear to be mainly controlled (1) by the fiber/matrix reaction zones and matrix embrittlement with regard to yield strength and damage development, and (2) by filament degradation with regard to the fracture strength.
This paper gives details of the input data and a description of the laminates provided to all participants in an exercise to predict the strength of composite laminates. The input data include the elastic constants and the stress/strain curves for four unidirectional laminae and their constituents. Six types of laminates, chosen for the analysis, are described together with the lay-up, layer thicknesses, stacking sequences and the loading conditions. Consideration is given as to why these six laminates were selected and of the challenges imposed by the selected problems. The detailed instructions issued to the contributors are also presented.
Orthotic devices have been commonly made from plastics like polypropylene. However, insufficient mechanical properties, labour intensive preparation process, long client’s visiting times urge us to seek an alternative. An orthotic device made from a light-curable composite material is expected to overcome the aforementioned shortcomings by allowing the shaping and hardening of the orthosis to be performed during one fitting directly on the patient. This paper examines the mechanical properties of the composite material made with a 2.5-dimensional (2.5-D) woven fabric and a custom light-curable resin designed specifically for orthotic applications. A corresponding 2-dimensional (2-D) composite was also made for comparison purpose. Static (flexural strength and modulus) and dynamic (impact and fatigue) mechanical properties of the 2.5-D and 2-D composites were investigated. It was found that both 2.5-D and 2-D composites exhibited much higher flexural strength and modulus than polypropylene. In addition, they were non-brittle in nature. Though inferior to the 2-D composite in static flexural properties, the 2.5-D composite was superior to the corresponding 2-D composite in terms of impact and fatigue properties. Other comparisons between the 2.5-D and 2-D composites were also made and scanning electron microscopy (SEM) observation was conducted in this work.
The paper presents a macroscopic model of the non-linear stress/strain behaviour and the ultimate failure of a needled, woven carbon/carbon (C/C) composite, under tensile and shear loads parallel to the plies. The model is based upon continuum damage mechanics concepts for the description of damage evolution, and the approach to plasticity for the description of inelastic strains induced by matrix damage, according to the general approach developed by Ladevèze for composites. Modelling was guided by the experimental data on the matrix damage modes obtained from extensive microscopy examination of the test specimens under load. The model involves three scalar damage parameters including two tensile parameters, and one shear parameter resulting from a combination of the tensile ones. Identification of the damage parameters requires one off-axis (at 45° in the plane of the plies), and two on-axis (parallel to tow directions) uniaxial tensile tests. The model was validated with tensile stress/strain curves measured from various off-axis tests. The ultimate failure was predicted by the use of damage accumulation criteria.
A high-temperature fibre-testing apparatus has been designed. It is dedicated to the determination of various properties at very high temperatures, including electrical conductivity, Young's modulus, thermal expansion coefficient, strength. Test temperatures as high as 3000 °C can be applied to carbon fibres. Two types of carbon fibres (a PAN-based and a Rayon-based fibre) have been investigated at temperatures up to 2000 °C. The measured properties are discussed with respect to microstructural features.
An investigation has been carried out to study the low-cycle fatigue lives and cyclic stress response characteristics of a particulate SiC-reinforced 2024Al and its unreinforced matrix alloy at 22 and 190°C. The specimens were cyclically deformed with fully-reversed loading under plastic-strain amplitudes. The test results showed that the cyclic stress response characteristics of the composite and the 2024Al alloy were similar to each other in spite of changing the test temperature. The composite and its unreinforced counterpart generally exhibited cyclic hardening at 22°C and cyclic softening at 190°C. Increase in low-cycle fatigue resistance for both the composite and the aluminium alloy was observed as the test temperature rose from 22 to 190°C. For a given temperature the low-cycle fatigue endurance of the composite was lower than that of the unreinforced matrix alloy in the high and middle strain regions, however, at low strains the difference in fatigue endurance between the composite and the aluminum alloy decreased. The mechanism of strain in the composite, i.e. the strain in the composite was nearly completely sustained in the soft matrix, and the strain concentration adjacent to the reinforcement were two important factors that led to the shorter strain-fatigue life for the composite.
The main objective of the present work was to establish a friction stir welding (FSW) process parameters envelope for an AA 6061 alloy reinforced with 20% of Al2O3 particles, and determine properties of the obtained joints. After a brief description of the FSW technique, and the difficulties in joining MMCs, experimental procedure is illustrated. Microstructure has been observed with optical microscope, and images have been analysed with an image analysis software. Microhardness and tensile tests have been also carried out. The tool’s stirring effect has a substantial influence on the reinforcement particles distribution and shape. Tensile testing revealed joint efficiencies over 80% for the Rp0, 2 and of slightly more than 70% for the Rm, with failure outside the stir zone. The parameter envelope determined in the present study resulted in defect free, high strength welds. Results also indicate that the alloy investigated could be welded with process parameter comparable to those used for its unreinforced counterpart.
The use of aluminium-based particulate reinforced MMCs for automotive components and aircraft structures have been shown to be highly advantageous over their unreinforced alloys, due to their high specific strength and stiffness and superior wear resistance in a wide temperature range. The aim of this paper was to evaluate the effect of the hot forging process on the microstructure and tensile properties (at room and high temperature) of a MMC based on the aluminium alloys AA2618 reinforced with 20 vol.% of alumina particles (Al2O3p). Microstructural analyses of the as-cast and heat-treated composite showed large grain size of the aluminium alloy matrix and a quite non-homogeneous distribution of the reinforcing particles. The forging process led to an evident grain refinement, while it did not lead to significant variations in the size and distribution of the reinforcement particles. Regarding the effect of the forging process on the mechanical properties, it induced a slight increase in hardness, tensile strength, elastic modulus and an evident increase in tensile elongation. SEM analyses of the fracture surfaces of the tensile specimens showed substantially similar morphologies for the as-cast and forged composites, both at room and high temperature. The mechanism of damage was mainly decohesion at the matrix–particle interface.
Static mechanical properties of 2124 Al/SiCp MMC have been measured as a function of solution temperature and time. An optimum solution treatment has been established which produces significant improvements in static mechanical properties and fatigue crack growth resistance over conventional solution treatments. Increasing the solution treatment parameters up to the optimum values improves the mechanical properties because of intermetallic dissolution, improved solute and GPB zone strengthening and increased matrix dislocation density. Increasing the solution treatment parameters beyond the optimum values results in a rapid reduction in mechanical properties due to the formation of gas porosity and surface blisters. The optimum solution treatment improves tensile properties in the transverse orientation to a greater extent than in the longitudinal orientation and this results in reduced anisotropy.
Oxidation resistance tests were carried out on hot-pressed ZrB2–20 vol%SiC using an oxyacetylene torch. The temperature of the oxidized specimens exceeded 2200 °C. The mass and linear oxidation rates of the ZrB2–20 vol%SiC composites for 10 min were −0.23 mg/s and 0.66 μm/s, respectively. The surface layer appeared dense and adherent with the exception of a few burst bubbles and craterlets. No macro-cracks or spallation were detected after oxidation, suggesting that these composites possess a super oxidation resistance. The microstructure of the surface and cross-section of the oxidized specimens were studied by scanning electron microscopy along with energy dispersive spectrometry and X-ray diffraction. The oxidation mechanism was also discussed.
Because of their ultra small, nanometer scale size and low density, the surface area to mass ratio (specific area) of carbon nanotubes (CNTs) is extremely large. Therefore, in a nanotube-based polymeric composite structure, it is anticipated that high damping can be achieved by taking advantage of the interfacial friction between the nanotubes and the polymer resin. In addition, the CNT’s large aspect ratio and high elastic modulus features allow for the design of such composites with large differences in strain between the constituents, which could further enhance the interfacial energy dissipation ability. Despite their wonderful engineering potential, the damping properties of CNT-based composites have not been examined in any detail. The purpose of this paper is to investigate the structural damping characteristics of polymeric composites containing single-walled carbon nanotubes (SWNTs), with a focus on analyzing the interfacial interaction between the CNT and the resin materials. The system is modeled using a four-phase composite, composed of a resin, voids, and bonded and debonded nanotubes. A micromechanical model is proposed to describe interfacial debonding evolution. To characterize the overall behavior, Weibull’s statistical function is employed to describe the varying probability of nanotube debonding under uniaxial loading. To address damping effects, the concept of interfacial “stick-slip” frictional motion between the nanotubes and the resin is proposed. The developed method is extended to analyze composites with randomly oriented nanotubes. The analytical results show that the critical shear stress, nanotube weight ratio and structure deformation are the factors affecting the damping characteristic. Experiments are also performed to verify the trends predicted by the analysis. Through comparing with neat resin specimens, the study shows that one can enhance damping by adding CNT fillers into polymeric resins. It is also observed that SWNT-based composites could achieve higher damping than composites with several other types (different size, surface area, density and stiffness) of fillers. These results confirm the possible advantage of using CNTs for damping enhancement.
The mechanical properties and the fracture mechanism of composites consisting of ZA-27 alloy reinforced with Zircon particles were investigated with the primary objective of understanding the influence of the particulate reinforcement on the behaviour of the ZA-27 alloy. The zircon particle content in the composites ranged from 1 to 5% in steps of 2% by weight. The composites were fabricated by the ‘compocasting’ technique in which the molten mass was squeeze-cast under pressure. The study revealed improvements in ultimate tensile strength, yield strength, hardness, and Young's modulus of the composites, but at the cost of ductility and impact strength, which decreased with increase in zircon content. The fracture behaviour of the composites was altered significantly by the presence of the zircon particles. Crack propagation through the matrix and the reinforcing particles resulted in the final fracture. Scanning electron microscopy and fractography analyses have been carried out to furnish suitable explanations for the observed phenomena.
The non-linear mechanical behavior of a CVI-processed 2D woven C/SiC composite has been investigated by means of an ultrasonic method. This method provided the complete variation of the stiffness tensor of the material required to fully identify anisotropic damage, which was otherwise inaccessible by classical strain measurements. The various damage mechanisms induced by mechanical loading and their influence on the tensile behavior were determined and analyzed by comparing the variations of the components of the stiffness tensor obtained from the ultrasonic measurements with the prediction of microcracks by a system of slit cracks derived from a micromechanical model. Two damage modes were thus emphasized: transverse microcracking characterized by a deterministic accumulation and a random development of longitudinal microcracking, i.e. fiber/matrix and bundle/matrix debonding. Comparisons with the results obtained in Part I from both classical strain measurements and microstructural observations are also made and discussed, whenever possible.
Analytical models for textile materials are typically unit cell abstractions, which are incapable of assessing the effects of common fiber architecture defects. Experimental characterization of common defects is thought to be a way to assess the relative importance of a feature for inclusion in more complex analytical models. This paper addresses moiré interferometry experiments on idealized textile samples of decreasing abstraction (increasing reality of local features). Since interferometry measures displacements (strains) at a free surface, one must be careful to separate the effects due to textile architecture from those due to free-edge effects. Moiré interferometry techniques are shown to be ineffective for testing edges (surfaces which include interlaminar boundaries) of most real-world (non-idealized) textiles due to the inability to characterize the mechanical response due to local variations in microstructure. Thus, there is a need for fabricating samples of a very idealized nature.
The Hasselman–Johnson (H–J) model for predicting the effective transverse thermal conductivity (Keff) of a 2D-SiCf/SiC composite with a fiber-matrix thermal barrier was assessed experimentally and by comparison to numerical FEM predictions. Agreement within 5% was predicted for composites with simple unidirectional or cross-ply architectures with fiber volume fractions of 0.5 or less and with fiber-to-matrix conductivity ratios less than 10. For a woven 2D-SiCf/SiC composite, inhomogeneous fiber packing and numerous direct fiber–fiber contacts would introduce deviations from model predictions. However, the analytic model should be very appropriate to examine the degradation in Keff in 2D-woven composites due to neutron irradiation or due to other mechanical or environmental treatments. To test this possibility, expected effects of irradiation on Keff were predicted by the H–J model for a hypothetical 2D-SiCf/SiC composite made with a high conductivity fiber and a CVI-SiC matrix. Before irradiation, predicted Keff for this composite would range from 34 down to 26 W/(m K) at 200 and 1000 °C, respectively. After irradiation to saturation doses at 200 or 1000 °C, the respective Keff-values are predicted to decrease to 6 or 10 W/(m–K).
The use of fiber, interphase, CVI SiC minicomposites as structural elements for 2D-woven SiC fiber-reinforced chemically vapor infiltrated (CVI) SiC matrix composites is demonstrated to be a viable approach to model the elastic modulus of these composite systems when tensile loaded in an orthogonal direction. The 0° (loading direction) and 90° (perpendicular to loading direction) oriented minicomposites as well as the open porosity and excess SiC associated with CVI SiC composites were all modeled as parallel elements using simple Rule of Mixtures techniques. Excellent agreement for a variety of 2D woven Hi-Nicalon™ fiber-reinforced and Sylramic-iBN reinforced CVI SiC matrix composites that differed in numbers of plies, constituent content, thickness, density, and number of woven tows in either direction (i.e, balanced weaves versus unbalanced weaves) was achieved. It was found that elastic modulus was not only dependent on constituent content, but also the degree to which 90° minicomposites carried load. This depended on the degree of interaction between 90° and 0° minicomposites which was quantified to some extent by composite density. The relationships developed here for elastic modulus only necessitated the knowledge of the fractional contents of fiber, interphase and CVI SiC as well as the tow size and shape. It was concluded that such relationships are fairly robust for orthogonally loaded 2D woven CVI SiC composite system and can be implemented by ceramic matrix composite component modelers and designers for modeling the local stiffness in simple or complex parts fabricated with variable constituent contents.
The deformation fields and kinematics of woven composite material systems, due to impact loads, were analyzed and characterized for various structural and load parameters. Target plates comprising of woven composites with 3D and 2D preforms were considered. Kinetic energies in the range of 18–39,000 J, due to projectile velocities in the range of 2–1000 m/s, were investigated. The impact problem model accounts for geometrical details of the flat target plates and the hemispherical projectile. Contact solutions at dissimilar surfaces were modeled with gap elements, and the solution of the nonlinear dynamic problem was obtained by the finite element method. In the present study, we investigated wave propagation effects, and how their spatial and temporal distribution is related to the evolution of multi-dimensional elastic fields and potential damage modes. Unit cells representative of the 2D and 3D woven composites were used to obtain estimates of the overall elastic moduli. It was found that the compression wave induced by impact reflected several times between the free surfaces of the target plate before fiber failure initiated, and that this was one of the major mechanisms leading to penetration. At low velocity impact, the deformations were similar to quasi-static bending deformation modes, and failure is predicted to be due to fiber breakage at the backside of the target plate. At higher impact velocities, wave propagation effects are more significant and lead to penetration at the impact face. For all material systems, localized shear damage in 3D woven systems and extensive shear delamination in 2D woven systems preceded complete penetration.
An investigation has been undertaken to determine the damage mechanisms and the associated mechanical response of a 2D reinforced composite of carbon fibers in an SiC CVI-processed matrix subjected to uniaxial tensile and compressive loadings at room temperature. Under tension loading, an extended non-linear stress/strain response was evidenced and related to a multi-stage development of damage involving transverse matrix microcracking, bundle/matrix and inter-bundle debonding as well as thermal residual stress release. This tensile behavior proved to be damageable-elastic with respect to a fictitious thermalstress-free origin of the stress/strain axis lying in the compression domain. In compression, after an initial stage involving closure of the thermal microcracks present from processing, the composite displayed a linear-elastic behavior until failure. The extent of damage over the material was characterized quantitatively at the microscale by the decrease of the average transverse microcrack spacing and at the macroscale by the decrease of both the longitudinal Young's modulus and the in-plane Poisson's ratio.
The matrix cracking of a variety of SiC/SiC composites has been characterized for a wide range of constituent variation. These composites were fabricated by the two-dimensional lay-up of 0/90 five-harness satin fabric consisting of Sylramic fiber tows that were then chemical vapor infiltrated (CVI) with BN, CVI with SiC, slurry infiltrated with SiC particles followed by molten infiltration of Si. The composites varied in number of plies, the number of tows per length, thickness, and the effective-size of the tows. This resulted in composites with a fiber volume fraction in the load-bearing direction that ranged from 0.12 to 0.20. Matrix cracking was monitored with modal acoustic emission in order to estimate the stress-dependent distribution of matrix cracks. It was found that the general matrix crack properties of this system could be fairly well characterized by assuming that no matrix cracks originated in the load-bearing fiber, interphase, chemical vapor infiltrated SiC tow-minicomposites, i.e., all matrix cracks originate in the 90° tow regions or the large unreinforced SiC–Si matrix regions. Also, it was determined that the higher fiber-count tow composites had a much narrower stress range for matrix cracking compared to the standard tow size composites.
Fatigue behavior was examined for three kinds of carbon fiber reinforced carbon matrix composites (C/Cs). The fatigue-tested materials included unidirectionally reinforced (UD), symmetric cross-ply laminated (CP) and symmetric quasi-isotropically laminated (QI) C/Cs. The fatigue fracture behavior of the C/Cs was classified into three regions based on applied strain, irrespective of the C/C types. In high applied strain region, fatigue fracture occurred in the same manner as in the static tensile tests. In the second region, where the applied stress is up to the fatigue limit, the strength of the C/Cs decreased slightly due to the degradation of the fiber strength caused by frictional wear along the fiber/matrix interface. The third region was at lower applied strain than the fatigue limit, in which no fatigue fracture occurred and the residual strength was recovered and sometimes enhanced due to damage at the fiber/matrix interface.
Due to their improved mechanical properties, 3D multi-layer spacer fabrics could be used for lightweight applications such as textile-based sandwich preforms. Modern flat knitting machines using high performance yarns are able to knit complex 3D multi-layer spacer fabrics consisting of individual surface and connecting layers. This paper reports on the development of 3D flat knitted spacer fabric for 3D thermoplastic composites using hybrid yarns made of glass (GF) and polypropylene (PP) filaments. Moreover, mechanical properties of reinforcement yarns, 2D knit fabrics and 2D composites manufactured using various integration methods of reinforcement yarns were also studied. The integration of reinforcement yarns as biaxial inlays (warp and weft yarns) is found to be the best solution for knitting, whereas the tuck stitches show optimal results.
In order to understand the oxidation resistance of a 2D C/SiC composite protected with self-healing filler in the inter-bundle pores, SiB4 particles as the filler were infiltrated into the inter-bundle pores of 2D C/SiC composite by slurry infiltration process. The SiB4 particles were combined with SiC during the chemical vapor deposition (CVD) SiC coating process. Isothermal oxidation tests of the as-received modified composite were carried out in air at temperatures ranging from 500 to 1000 °C. SEM and EDS results showed that all the open inter-bundle pores of the C/SiC composite can be filled with SiB4 particles. The SiB4 filler can hinder the inwards diffusion of oxygen in air, and protect the carbon fibres and carbon interphase from oxidation. As a result, the modified composite lost weight slowly, and showed no obvious decrease in flexural strength at the temperatures of 500–900 °C for an oxidation time of 10 h.
The strength and reliability of ceramic-matrix composites may be expected to be dependent upon various factors including the size of the stressed volume, the loading history and the loading conditions. The influence of these factors on the strength of 2D woven SiC/SiC composites is examined in this paper. Tensile, three-point flexure and four-point flexure tests were performed on batches of test specimens. Tensile tests were also performed on broken parts of tensile specimens (successive failure tests). The statistical distributions of strength data exhibit important features such as a narrow scatter, no significant dependence on the stressed volume, no dependence on the previous loadings and a certain dependence on the loading conditions. These features are related to the damage mechanisms which limit the contribution of flaws to ultimate failure, and to variability in the local stress-state.
Fibre-orientation measurement by two-dimensional (2D) image analysis of polished cross-sections is a rapid and highly efficient method for determining the fibre orientation distribution over large sample areas. In a recent paper, a new technique was presented for measuring fibre orientation to a high level of accuracy by the use of a confocal microscope. In this paper, the confocal technique is used to evaluate independently the performance of the 2D image analysis technique and the errors are presented in full. The results reveal a significant systematic error in orientation measurements and the effect of image resolution is considered. A method is proposed for correcting this systematic error and its validity is verified experimentally. A technique for determining the sampling error of a fibre-orientation measurement is presented, enabling the calculation of confidence limits about the derived orientation tensor components. It is shown how confidence limits aid the comparison of two fibre-orientation measurements of similar samples. Furthermore, the sampling error should enable a more meaningful comparison of numerical simulation of injection moulded composites and their experimentally manufactured counterparts.
High-temperature creep behavior of symmetric angle-ply laminates made of unidirectional T800H/3631 carbon/epoxy composite is examined at relatively high stress levels. Constant-stress creep tests in tension are performed at 100 °C for 5 h on plain coupon specimens of three types of angle-ply laminates [±30]3S, [±45]3S and [±60]3S under load control conditions. For each angle-ply laminate, creep tests are carried out at three different stress levels. Creep strain recovery following the 5-h creep is also observed for 5 h at the same temperature, after completely removing the creep stress. Creep responses are clearly observed in all kinds of angle-ply laminates. The creep strain rate in the angle-ply laminates tends to rapidly disappear as the creep strain increases. The transient creep is thus dominant in the angle-ply laminates, regardless of the fiber orientations. The prior creep strain does not completely recover with time after removing the creep stress, which indicates certain inelastic mechanisms have operated with creep. Similar features are also observed for the off-axis creep behavior of the unidirectional laminates of the same composite system. A whole history consisting of the prior instantaneous elastoviscoplastic behavior at a constant strain rate and the subsequent creep response at a constant stress level is simulated using the classical laminated plate theory and a phenomenological viscoplasticity model for individual plies. Material constants involved by the ply viscoplasticity model are identified on the basis of the off-axis creep behavior for unidirectional laminates. It is demonstrated that excellent agreements between the predicted and observed results are obtained by additionally taking into account the fiber rotation induced by deformation.
The tensile fatigue properties of specific types of 3D woven, stitched and z-pinned composites with through-thickness reinforcement are compared in this paper. Tensile tests under monotonic and cyclic loading were performed on the 3D composite materials to determine the influence of the z-reinforcement type – woven z-binder, stitch or z-pin – on the tensile modulus, strength and fatigue life. The in-plane Young’s modulus of the composites was not affected by the type or volume content of the z-reinforcement. The tensile strength of the 3D woven and stitched composites was also not affected by the z-reinforcement, however the strength of the z-pinned composite dropped steadily with increasing volume content of z-reinforcement. The fatigue life of the 3D composites was reduced by the z-reinforcement, regardless of whether they were woven z-binders, stitches or z-pins. The fatigue lives of the 3D composites decreased with increasing volume content of z-reinforcement. The tensile fatigue properties are degraded by the z-reinforcement causing damage to the microstructure of the 3D composites. The fatigue damage mechanisms caused by the different types of z-reinforcement are described. The results indicate that through-the-thickness reinforcement is detrimental to the tensile fatigue life, although the study was restricted to specific types of materials and further research into a wider variety of 3D woven, stitched and z-pinned composites is required for a general assessment of their fatigue performance.
The paper studies tension–tension fatigue behavior of a single-ply non-crimp 3D orthogonal weave E-glass composite and of a laminated composite reinforced with four plies of a standard plain weave fabric. Both composites have same total thickness and very close fiber volume fraction. The paper presents the description of the materials, the results of quasi-static tensile and of tension–tension fatigue tests, including the damage development during fatigue tensile loading. The non-crimp 3D woven fabric composite, loaded in both principal in-plane directions (warp and fill), shows the best quasi-static tensile properties and, when loaded in the fill direction, exhibits much longer fatigue life than its laminated plain weave counterpart. During both quasi-static and fatigue loading, the latest damage initiation is observed for the 3D woven composite in both in-plane directions. The PW laminate develops delamination between the plies for each maximum stress in the cycle considered. Contrary to that, the 3D composite is not affected by delamination neither under quasi-static nor under fatigue loading conditions.
A method for the prediction of fracture under dynamic loading for small structures made of a 3D carbon/carbon composite is proposed. Owing to the length scale of the loading, the notion of homogenised material is meaningless and our choice then is to model, identify and compute the material at an intermediate or mesoscale (fibre-strands/matrix-blocks). The idea here is that, because of the small size of the meso scale, the previous description of the damage mechanisms should be valid even for high-rate loading. This work is focused on the application of this approach to the case of plate/plate impact by making use of a simplified analysis.
Monotonic, multi-step and cyclic short beam shear tests were conducted on 2D and 3D woven composites. The test results were used to determine the effect of z-yarns on the inter-laminar shear strength as well as the multi-loading behavior. The presence of z-yarns was found to affect not only the inter-laminar shear strength of the composite but also the behavior of the composite beyond the elastic limit. Microscopic examination of the damaged specimens revealed large delamination cracks in 2D woven composites while delamination cracks were hindered by z-yarns in 3D composites. This crack arrest phenomena resulted in a reduction in inter-laminar crack lengths and a higher distribution of the micro-cracks throughout the 3D composite. The multi-step and cyclic loading tests are found to be useful in the monitoring of specimen behavior during short beam shear testing. The induced damage was quantified in terms of the loss of strength and stiffness during each loading cycle. It was found that while the 2D composites have higher damage resistance, the 3D composites have a higher damage tolerance.
Numerical micromechanical investigations of the mechanical behavior and damage evolution of glass fiber reinforced composites are presented. A program code for the automatic generation of 3D micromechanical unit cell models of composites with damageable elements is developed, and used in the numerical experiments. The effect of the statistical variability of fiber strengths, viscosity of the polymer matrix as well as the interaction between the damage processes in matrix, fibers and interface are investigated numerically. It is demonstrated that fibers with constant strength ensure higher strength of a composite at the pre-critical load, while the fibers with randomly distributed strengths lead to the higher strength of the composite at post-critical loads. In the case of randomly distributed fiber strengths, the damage growth in fibers seems to be almost independent from the crack length in matrix, while the influence of matrix cracks on the beginning of fiber cracking is clearly seen for the case of the constant fiber strength. Competition between the matrix cracking and interface debonding was observed in the simulations: in the areas with intensive interface cracking, both fiber fracture and the matrix cracking are delayed. Reversely, in the area, where a long matrix crack is formed, the fiber cracking does not lead to the interface damage.
The carbon/silicon carbide brake materials were prepared by chemical vapor infiltration (CVI) combined with liquid melt infiltration (LMI). The carbon fiber preform was fabricated with the three dimension needling method. The microstructure, mechanical, thermophysical, and frictional properties of C/SiC composites were investigated. The results indicated that the composites were composed of 65 wt%C, 27 wt%SiC, and 8 wt%Si. The density and porosity were 2.1 g cm−3 and 4.4%, respectively. The C/SiC brake materials exhibited excellent toughness. The average dynamic friction coefficient and static friction coefficient of the materials were about 0.34 and 0.41, respectively. The friction coefficient was stable. The fade ratio of the friction coefficient under moist conditions was about 2.9%. The linear wear rate was less than 1.9 μm side−1 cycle−1. These results show that C/SiC composites have excellent properties for use as brake materials for aircraft.
This work is aimed at elucidating the processing–microstructure–property relationships of three-dimensional thermoplastic composites. The material processing involves fabric formation and hot-press molding. A powder-impregnated Nylon/carbon yarn was used to form three-dimensional fabrics. Two types of fabrics have been made in the present work, including 3-axis orthogonal woven and two-step braided. The woven fabric is characterized by more evenly distributed fibers along three orthogonal directions, whereas the braided fabric contains most fibers along the axial direction. Compression molding was employed for composite consolidation. Matched molds were designed for making the composites with predetermined thicknesses. The molding thickness and molding temperature were varied to examine their respective effects on the resulting properties. Yarn geometries in the molded composites were studied through microscope observations. The molding significantly distorted the through-thickness yarns of the woven fabric, and the mechanisms of yarn distortion were identified and related to the fabric structure. Material characterization was conducted by means of flexure tests. The loading curves show significant non-linearity with the development of damage. The molding thickness is a critical parameter governing the flexural modulus, flexural strength, and damage modes. A special mounting method was used to permit the examination of the fractured interiors of specimens. How surface loops affect damage modes and how damage grows within these non-uniform materials are discussed in detail.
An automated optical section method for quantifying the orientation state of short fibers in injection molded parts is presented. The method is based on imaging tracer fibers in index of refraction-matched transparent composites. The experimental methods and performance characteristics of the method are described. The method is non-destructive, operator-independent, economical and rapid. Scanning to depths of about 1.3 mm without cutting is possible at 10 μm spatial resolution at processing rates of < 10 min mm−3. Straightforward modifications should allow processing rates >1 mm3 min−1. Calibration tests suggest that fiber orientation is quantified to within the accuracy expected based on the degree of solid angle discretization chosen for analysis (5 ° here). Reproducible fiber orientation distributions are realized for sample domain sizes of about 2 mm × 2 mm × 0.15 mm. The orientation distribution function is quantified as well as the orientation tensor components. Convenient graphical visualizations of the 3D orientation distribution function and tensor representations are provided.
A new numerical model is proposed for simulating the mechanical behavior of unidirectional composites which is based on a three-dimensional (3D) shear-lag model. The 3D shear-lag model considers the micro-damage phenomena of interfacial debonding and interfacial yielding. In order to confirm the validity of the model, the calculated stress concentration is compared with the HVD model (Hedgepeth JM, Dyke P. Local stress concentrations in imperfect filamentary composite materials. J Comp Mater 1967;1:294–309) in the appropriate limit. Monte Carlo simulations with the present shear-lag model were then conducted to obtain the ultimate tensile strength (UTS) as a function of fiber strength and interfacial properties. The damage progression and formation of clusters versus the type of interfacial damage, and the size-scaling of the tensile strengths, are carefully examined. Coupled with a size-scaling analysis, model predictions for tensile strength show good agreement with experiment.