Transverse shear modulus and strength of honeycomb cores

The transverse shear mechanical behavior and failure mechanism of aluminum alloy honeycomb cores are investigated by the single block shear test in this paper. The transverse shear deformation process of honeycomb cores may be approximately categorized into four stages, namely elastic deformation, plastic deformation, fracture of cell walls and debonding of honeycomb cores/facesheets. The elastic deformation of unit cell under transverse shear displacement is also investigated by the finite element method, and the result shows that the bending deformation of the cell walls is similar to that of the cantilever beam. In order to precisely predict the equivalent transverse shear modulus and strength, not only shear deformation but also bending deformation of cell walls should be considered. Therefore, in the present paper, the equivalent transverse shear modulus and strength are predicted by application of the cantilever beam theory and thin plate shear buckling theory in conjunction with simplifying assumption as to the displacement in the cores. It is concluded that the contribution of bending deformation of cell walls to equivalent transverse shear modulus and strength is obvious with the decreasing height of cell walls.     

Damage tolerance assessment by bend and shear tests of two multilayer composites: Glass fibre reinforced metal laminate and aluminium roll-bonded laminate

The damage tolerance of an aluminium roll-bonded laminate (ALH19) and a glass fibre reinforced laminate (GLARE) (both based on Al 2024-T3) has been studied. The composite laminates have been tested under 3-point bend and shear tests on the interfaces to analyze their fracture behaviour. During the bend tests different fracture mechanisms were activated for both laminates, which depend on the constituent materials and their interfaces. The high intrinsic toughness of the pure Al 1050 layers present in the aluminium roll-bonded laminate (ALH19), together with extrinsic toughening mechanisms such as crack bridging and interface delamination were responsible for the enhanced toughness of this composite laminate. On the other hand, crack deflection by debonding between the glass fibres and the plastic resin in GLARE was the main extrinsic toughening mechanism present in this composite laminate.     

Synthesis and mechanical behavior of ternary Mg–Cu–Dy in situ bulk metallic glass matrix composite

In situ Mg phase reinforced Mg₇₀Cu₁₇Dy₁₃ bulk metallic glass (BMG) matrix composite with diameter of 3 mm was fabricated by conventional Cu-mold casting method. The results show that the Mg-based BMG matrix composite exhibits some work hardening except for initial elastic deformation, a high fracture compressive strength of 702 MPa, which is 1.125 times higher than single-phase Mg₆₀Cu₂₇Dy₁₃ BMG and some plastic strain of 0.81%. The improvement of the mechanical properties is attributed to the fact that the Mg phase distributed in the amorphous matrix of the alloy has some effective load bearing and plastic deformation ability to restrict the expanding of shear bands and cracks and produce its own plastic deformation, which was proved by the shear deforming and fracturing mode and the fracture surfaces characterized by the vein pattern, severe remelting, and the very rough and bumpy region of the alloy.     

Fractographic study of transverse cracks in a fibre composite

Transverse fracture of unidirectional fibre composites was studied in a model glass/epoxy composite in which 1 mm-diameter rods had been used in place of fibres. The fracture surface resulting from transverse cracking in this model system was studied by scanning electron microscopy (SEM). The interaction of the crack with the epoxy matrix resin and the glass rods was the following: Cracks in the resin appeared to have effected a debonding at the glassmatrix interface before reaching the glass. The debonding then propagated along the interface and induced secondary cracks ahead of the primary debonding crack. The confluence of the secondary and primary cracks resulted in sharp ridges being formed on the matrix resin surface, produced by plastic deformation of the rigid epoxy resin. These appeared as a field of parabolic marks. Considering the brittleness of the resin, the amount of plastic deformation indicated by the ridges was astonishing. As the debonding continued around the glass rod, a transverse corrugated texture developed on the resin surface, again produced by plastic deformation. Finally, the cracks reentered the matrix from small patches of polymer adhering especially strongly to the glass surface. The overall fracture energy of transverse cracking of unidirectional fibre composites is suggested to consist, therefore, of the following elements in addition to crack propagation in the matrix resin: (a) the glass-resin debonding before the incoming cracks reach the glass, (b) the initiation of secondary cracks or debonds at the interface, (c) the plastic deformation in generating the ridges on the rigid resin surface, appearing both as the paraboloids and the transverse corrugation, and (d) cracking of the matrix reinitiated at the opposite side of the glass. The use of an enlarged glass reinforcement in this study provided a more direct observation of the properties of transverse crack propagation in composite materials than would have been possible with the small, roughly 10m fibres.     

Elevated temperature tensile properties and failure of a copper-chromium in situ composite

A Cu-10 vol% Cr in situ composite was produced by melt processing and deformed by swaging to form rods with a total deformation true strain of 3.15. Scanning electron microscopy showed that the composite microstructure consisted of Cr fibres aligned with their long axes parallel to the rod axis. X-ray diffraction indicated that the Cr fibres had a strong <110> fibre texture. The mechanical properties of the composite were measured by tensile testing over the temperature range –70 to 600°C. Examination of fibre fracture and fibre-matrix debonding at and near the tensile test fracture surfaces indicated that a transition from localised to global damage occurred between 300 and 400°C.     

The flatwise compressive properties of Nomex honeycomb core with debonding imperfections in the double cell wall

Mechanical properties of Nomex honeycomb core are governed by not only its global dimensions, cell topology, material properties and proportion of the aramid paper and phenolic resin, but also possible manufacturing imperfections, such as the debonding between the two aramid paper sheets in the double cell wall. To account for the layered feature of the cell walls and the bonding conditions between aramid paper sheets, a three-dimensional unit cell model was proposed and developed in this study. The aramid paper sheets, the phenolic resin coating, the adhesive between the aramid paper sheets, and their bonding relationships were all explicitly modelled in accordance with their actual geometry and material parameters. The model was validated by comparing the predicted load-displacement curves and failure modes with the test results. The effects of representative bonding imperfections on both the collapse load and the related displacement of the honeycomb core under flatwise compression were evaluated. Through the analyses, it was found that the debonding imperfections have significant effects on the mechanical behaviour of the honeycomb core and that with the same debonding area the debonding at the outside edge of the adhesive printing line is the most critical. It was also found that debonding fracture may occur if adhesive is not strong enough or the debonding imperfection area is large.     

等腰梯形蜂窝芯玻璃钢夹芯板的面外压缩性能EI北大核心CSCD

利用材料试验机对玻璃钢(FRP)夹芯板面外压缩性能进行实验测试与模拟研究。结果表明:夹芯板面外压缩变形可分为弹性变形与断裂两个阶段。蜂窝芯中part 2胞壁厚度t1与part 2高度h比值t1/h较大时,夹芯板以屈服方式变形;t1/h较小时,夹芯板以屈曲方式变形。蜂窝芯中part 2为夹芯板主要承载构件,蜂窝芯中part 1与part 3对part 2起到固支作用,面板对蜂窝芯起到固支作用。蜂窝芯中part 2胞壁厚度为夹芯板面外压缩抗压强度影响的主要因素,蜂窝芯胞壁边长影响次之,而蜂窝芯中part 1,part 3与面板厚度的影响较小。夹芯板总高度一定时,随着蜂窝芯层数增加,夹芯板抗压强度逐渐增大。     

明胶鸟弹撞击复合材料蜂窝夹芯板试验

开展明胶鸟弹撞击复合材料蜂窝夹芯板试验,研究夹芯结构在软体高速冲击下的损伤形式,分析相关因素对结构动态响应结果的影响。通过CT扫描对复合材料蜂窝夹芯板内部进行检测可知,面板出现分层、基体开裂、纤维断裂、凹陷、向胞内屈曲等损伤形式,蜂窝芯出现芯材压溃、与面板脱粘的损伤形式;分析复合材料蜂窝夹芯板后面板的动态变形过程及撞击中心处位移-时间数据可知,复合材料蜂窝夹芯板在撞击过程中出现由全局弯曲变形主导和局部变形主导的两种变形模式;通过对比不同工况下的复合材料蜂窝夹芯板损伤程度可知,复合材料蜂窝夹芯板损伤程度随鸟弹撞击速度的增加而增大;蜂窝芯高度为10 mm的复合材料蜂窝夹芯板较蜂窝芯高度为5 mm的复合材料蜂窝夹芯板的损伤程度大;初始动能较大的球形鸟弹较圆柱形鸟弹对复合材料蜂窝夹芯板造成的冲击损伤程度更大。      

Interfacial cracking of a composite

The interfacial cracking, or debonding, of a composite has been studied both in tension and interlaminar shear, the fracture force being applied parallel to the interfaces in both cases. Application of the energy balance theory of brittle fracture has provided theoretical criteria for debonding failure. These equations have been verified experimentally using polymethylmethacrylate models. There were three conclusions: (1) interfacial cracks can propagate along the direction of the applied force in a theoretically predictable manner; (2) these interfacial cracks must be triggered by flaws, either edge cracks or internal defects; (3) it is wrong to characterise brittle interfacial adhesion by means of an interlaminar shear strength. Instead, the interfacial fracture energy should be used.     

Failure modes and fractographic study of glass-epoxy composite under dynamic compression

Unidirectional glass-epoxy composite has been tested under dynamic compressive loading conditions to study the different modes of failure and characterize them fractographically. Specimens of six fibre orientations = 0, 10, 30, 45, 60 and 90° with respect to the loading axis were loaded on Kolsky bars at an average strain rate of 265 sec^–1. The failure occurs on three different types of plane such that the fibre direction is preserved in all cases. Type A planes are tensile split planes and 0° specimens fail only in this mode. 10, 30 and 45° specimens shear on Type B planes by the combined action of normal and shear stresses. 60° and 90° specimens also fail by shear by the combined action of normal and shear stresses but on different types of planes called Type C planes. In these specimens the normal of the failure plane is found to make an angle lying between 55° and 70° with respect to the loading axis. The fractographs indicate intense matrix deformation and breaking up of fibre-matrix bonds for shear failure and comparatively clean fracture surfaces for tensile failure.     

The effects of phase morphology and adhesion between phases on fracture of ethylene-vinylalcohol copolymer/nylon 6/12 copolymer blends

The effects of phase morphology and the adhesion between phases of ethylene-vinylalcohol copolymer(EVOH)/nylon 6/12 copolymer blends on the fracture properties were estimated. Films of the blends which were obtained by extrusion processing showed different phase morphologies depending on the composition of the nylon 6/12 copolymer. The morphology of the partially miscible blend (EVOH and nylon 6f-nylon121-f where f=0.8) was needle-like in appearance. On the other hand the immiscible blend (EVOH and nylon 6f-nylon121-f where f=0.5) had equiaxed particles of nylon 6/12. The plastic deformation of films of the blends was observed using transmission electron microscopy. Deformation zones were observed for both blends but extensive debonding of particle interfaces was observed in the immiscible blend system. These observations are reinforced by our measurements of the interfacial fracture energy, Gc, between EVOH and nylon 6f-nylon121-f made using a double cantilever beam test. Gc decreases monotonically as 1–f increases. The fracture toughness of the partially miscible blend film measured at low temperature (–80°C) was higher than that of EVOH alone and there was fractographic evidence of a larger crack tip plastic deformation zone. In contrast, the fracture toughness of the immiscible blend was lower than that of EVOH and there was fractographic evidence of extensive debonding of the second phase nylon particles. This result suggests that it is important to have good adhesion between phases to achieve the optimum fracture toughness of these polymer blends. © 1998 Chapman & Hall     

Influence of hydrostatic pressure on the mechanical behavior and properties of unidirectional, laminated, graphite-fiber/epoxy-matrix thick composites

This paper reviews and gives new insight into earlier work by the author and his co-workers on the experimental investigation of the influence of superimposed hydrostatic pressure on the mechanical behavior and properties of the epoxy used for the matrix and unidirectionally laminated, graphite-fiber/ epoxy-matrix thick composites. The direction of the fibers was, respectively, 0°, 45° and 90° for the compressive test samples and 0°, 45° -45° and 90° for the shear samples.

Hydrostatic pressure induces very significant, often dramatic changes in the compressive and shear stress/ strain behavior of composites, and consequently in the elastic, yielding, deformation and fracture properties. The range of pressures covered for the compressive experiments was 1 bar to 4 kbar, and for the shear tests 1 bar to 6 kbar. The shear modulus (G) of the epoxy increased bilinearly with pressure, with the break, or the discontinuity point, occurring at 2 kbar. The compressive elastic modulus (E) and the shear modulus (G) of the composites increase in the same manner as for the epoxy. The break, which is located at 2 kbar, represents a pressure at which physical changes in the molecular motion of the matrix epoxy occur. That is, segmental motion of molecules between the cross-links is frozen in by 2 kbar pressure. This pressure is known as the secondary glass transition pressure of the epoxy at room temperature. Alternatively, the sub-zero secondary glass transition temperature of the epoxy is shifted to ambient temperature by 2 kbar pressure. The increase in the moduli may also be given a mechanical interpretation. The elastic or shear modulus of an isotropic, elastic material due to small compressive or shear deformations, respectively, superimposed on a finite volume deformation, which is caused by hydrostatic pressure, increases with pressure. Such an increase in E or G has been predicted using finite deformation theory of elasticity.

The normally brittle epoxy develops yielding when the superimposed hydrostatic pressure exceeds 2 kbar. The shear yield stress (1% off-set) of the epoxy increases linearly with pressure above 2 kbar. This kind of yielding behavior can be predicted by a pressure-dependent yield criterion. The compressive yield strength of the 45° and 90° composites increases bilinearly with pressure, and the shear yield strength of the 0°, 45° and 90° composites also increases bilinearly with pressure. This bilinear behavior is also due to the secondary glass transition pressure of the matrix epoxy, being located at 2 kbar. The fracture strength of the composites also increases with pressure linearly and the greatest increase occurs in the 45° composite in compression and in the −45° composite in shear. The fracture modes of the composites undergo changes with increasing hydrostatic pressure. For instance, the 0° composite undergoes a brittle-ductile transition under shear stress, while no such transition appears to set in under compressive stress. The fracture mode of the 45° composite changes from matrix failure at lower pressures to fiber failure at high pressures under shear stress.     

Synthesis and property of short tungsten fibre/Zr-based metallic glass composite

The short tungsten fibre reinforced Zr_41.2Ti_13.8Ni_10.0Cu_12.5Be_22.5 bulk metallic glass composites with macroscopic isotropic mechanical properties were prepared by infiltration and rapid solidification. The diameters of the tungsten fibres are 300, 500 and 700?µm with a length of 1000?µm and the fibre volume fraction in three kinds of composites is between 60 and 65%. The room temperature compressive deformation behaviours of these composites were investigated systematically. The results show that the strength and plasticity of the composites increase with the decrease in the tungsten fibre diameter. The maximum compressive strength and plastic strain of the composite with 300?µm fibre, respectively, reach 3079?MPa and 37%. The fracture modes of all the composites are shear fracture. The superior compressive property of the composites with short tungsten fibre is due to the competition among different fracture modes and the inhibition effect of the interface on the shear band extension.     

Discrete element modelling of unidirectional fibre-reinforced polymers under transverse tension

The mechanical behaviour of unidirectional fibre-reinforced polymer composites subjected to transverse tension was studied using a two dimensional discrete element method. The Representative Volume Element (RVE) of the composite was idealised as a polymer matrix reinforced with randomly distributed parallel fibres. The matrix and fibres were constructed using disc particles bonded together using parallel bonds, while the fibre/matrix interfaces were represented by a displacement-softening model. The prevailing damage mechanisms observed from the model were interfacial debonding and matrix plastic deformation. Numerical simulations have shown that the magnitude of stress is significantly higher at the interfaces, especially in the areas with high fibre densities. Interface fracture energy, stiffness and strength all played important roles in the overall mechanical performance of the composite. It was also observed that tension cracks normally began with interfacial debonding. The merge of the interfacial and matrix micro-cracks resulted in the final catastrophic fracture.     

Virtual fracture testing of composites: A computational micromechanics approach

The use of multiscale finite element simulations to perform virtual fracture tests of composite materials is demonstrated. The fracture behavior of a fiber-reinforced composite beam in presence of a notch perpendicular to the fibers was modeled using an embedded cell approach in three dimensions. The representation of the material in front of the notch tip - where damage was going to be concentrated - included the actual fiber-matrix topology, while the remainder of the beam was modeled as a linear thermo-elastic, transversally-isotropic homogeneous solid. The damage and fracture micromechanisms which controlled the onset of fracture (namely, plastic deformation of the matrix, brittle fiber fracture and fiber/matrix frictional sliding) were included in the behavior of the different phases and interfaces, and the corresponding micromechanical parameters governing their behavior were independently measured. Virtual fracture tests were carried out at 25 °C, 200 °C and 400 °C to determine the maximum load carried by the notched beams, and the apparent fracture toughness of the composite beams was computed from the maximum load and the initial notch length in the simulations. The predictions of the evolution of the fracture toughness with temperature were in good agreement with the experimental data without the need of adjustable or “tuning” parameters, which shows the potential of this approach to carry out “virtual” tests to predict the fracture behavior of novel microstructures and materials.     

Photoelastic study of the durability of interfacial bonding of carbon fibre-epoxy resin composites

The physical techniques of polarizing microscopy, including the quantitative measurements of small optical retardations, have been used to investigate elastic fields adjacent to short carbon fibres in epoxy resin composites. The elastic fields associated with shear stress distribution along the fibre-matrix interface have been employed to monitor the initiation of interface debonding during hot (100 °C) water uptake. By examining the development of stress birefringence during resin swelling in the resin adjacent to individual fibres, the differences in the durability of interfacial bonding and the fibre failure modes for differently coated fibres have been obtained. The results show that the state of self-stress in model composites, comprising a single carbon fibre in a film of epoxy resin, can, by immersion in hot distilled water, be enhanced to such an extent that the axial tension in the fibre can be sufficient to initiate fibre fracture. The results also show that, for fibres that have been given certain proprietary surface treatments, the fibre fractures by different failure modes.     

An Investigation of the Effects of Layer Architecture on Tensile Damage Mechanisms in a Silicon Carbide (SiC) Fiber-Reinforced Titanium Matrix Composite

This paper presents the results of recent studies of the effects of layer architecture (±45° and 0/±45°) on deformation and cracking phenomena in Ti-15-3/SCS-6 (SiC) composites. Deformation and cracking phenomena were elucidated by microscopic observations during incremental loading to failure. The initial damage occurred early via debonding between the fibers and the predominantly TiC reaction layer. This was followed by a complex sequence of damage that included: matrix cracking, sub-grain formation, stress-induced alpha phase precipitation, microvoid nucleation and coalescence, fiber fracture and catastrophic failure. Fracture in the [±45°]_2s composite occurred by shear mechanisms. However, an axial failure mode was observed in the [0±45°_2s composite. The composite strengths are compared with empirical estimates obtained from the Tsai–Hill criterion.     

Influence of strain-rate on the uniaxial compressive behavior of 2-D braided textile composites

The uniaxial compressive stress–strain response and the associated observed micro-structural deformation mechanisms of three braided textile composite sheets were experimentally investigated at quasi-static and high-strain-rates. The compressive loads were applied in a direction perpendicular to the plane of the textile composites. For quasi-static loading, the three composites exhibited a non-linear elastic response followed by a load-drop at the on-set of matrix failure. With further increase in load, the axial and bias braids were stretched in the matrix damaged zones resulting in inelastic deformation at a constant stress level. Under high-strain-rate loading conditions, the yield strength increased upto 60%, and the overall stress did not unload as a function of strain. Composites with a greater number of lay-ups exhibited nonuniform deformation along the thickness direction in the form of shear fracture through several lay-ups.     

An experimental study on the effect of tow variations on compressive characteristics of plain weave carbon/epoxy composites under compressions

The effect of tow deformation on the static and fatigue characteristics of fabric composites under compression was investigated by experimental approach. Sheared specimens made of plain weave carbon/epoxy prepregs were prepared using a picture frame rig and the shear angles were 0°, 16°, 26°, 34°, 46°. To verify the effect of the tow variations of the fabric composites on compressive characteristics, the unidirectional composite specimens composed of the same fibre and matrix system with the same stacking sequences as the fabric composites were prepared for comparison. The static compressive test results showed that the static compressive strength of sheared fabric specimens was lower than that of the unidirectional specimens with the same stacking angle. On the other hand, the fatigue test results showed that fatigue life of sheared fabric specimens was higher than that of the unidirectional specimens for mild shear deformation cases. It was proved that these results were fully affected by the tow deformation caused by the shear deformation of the fabric specimens. The compression–compression fatigue behaviours of sheared fabric specimens were verified by appropriate unit-cell models.     

An assessment of mechanical behavior and fractography study of glass/epoxy composites at different temperatures and loading speeds

Effect of loading rate on fracture and mechanical behavior of autoclave cured glass fiber/epoxy prepreg composite has been studied at various loading (striking) rates (0.01-10³ mm/min). The maximum load carrying capacity and strain at yield continuously increases with increasing loading speed. The interlaminar shear strength (ILSS) value is high at low loading speed and becomes low at high loading speed with the transition of loading rate at approximately 300 mm/min. The formation of steps, welt interfacial failure and cleavage formation on matrix resin i.e. localized plastic deformation processes were dominating mechanisms for specimens tested at low loading rates, while brittle fracture of fiber, fiber pull-out and impregnation were dominating mechanisms for specimens tested at loading rates of 800 mm/min or higher.