不同铺层转角T700/E44 复合材料拉伸强度有限元分析

碳纤维复合材料的失效行为与复合材料内部的应力状态有关,不同铺层转角的单向碳纤维复合材料层合板的性能具有明显差异。文中利用HyperWorks 商用有限元软件建立了T700/E44 复合材料层合板拉伸模型,基于Chang-Chang 复合材料失效模型对不同铺层转角复合材料层合板的 X 向及 Y 向拉伸性能进行了数值模拟分析。研究结果表明,复合材料层合板以45° 铺层转角对称结构层合时,复合材料有着最佳的综合拉伸性能。这对高性能雷达中复合材料部件的铺层结构设计具有重要的指导意义。     

Inelastic behavior and strength of fiber-metal hybrid composite: Glare

Off-axis inelastic and fracture behavior of a new type of fiber-metal composite called GLARE 2, a hybrid laminate of unidirectional glass/epoxy laminae (GFRP) and aluminum-alloy sheets, have been examined under static tensile-loading conditions. The classical laminated plate theory (CLT) was applied for describing the off-axis inelastic behavior of GLARE 2 laminate. Irrespective of the off-axis angle, a sharptransition caused by the yielding of aluminum alloy layers appears in the stress-strain relations. The nonlinearity of the behavior of GLARE 2 after this transition increases more significantly as the off-axis angle becomes larger. The tensile-fracture strength of the GLARE 2 laminate is almost two times as large as that of the monolithic aluminum alloy in the fiber direction, and it is about five times the value of the GFRP in the transverse direction. The anisotropy for the tensile fracture strength of the GLARE 2 laminate can be predicted using the Tsai-Hill theory. For the off-axis angles > 5°, transverse cracks develop in the GFRP layers before the GLARE 2 laminate fractures. A CLT-based model, which takes into account the transverse failure in the GFRP layers to cause an instantaneous degradation of transverse and shear elastic moduli, accurately describes the characteristic deformation behavior of the GLARE 2 laminate.     

The effect of Al2O3 nano additions on failure of GFRP plate with two parallel pin loaded holes

Journal of Mechanical Science and Technology - The aim of this study is to scrutinize the effect of nano-additions on failure modes and loads of glass-epoxy laminate subjected two parallel...     

复合材料层合板疲劳寿命的可靠性分析

基于唯象的剩余强度衰减模型与蔡－希尔静强度判据,建立单向层合板在任意复杂面内应力作用下的疲劳失效准则。利用蒙特卡洛模拟方法计算层合板的疲劳寿命,并对其进行可靠性评估。根据Ｔ３００／ＱＹ８９１１的三种典型层合板［０］１６、［９０］２４和［±４５］３Ｓ的拉—拉疲劳试验结果,利用蒙特卡洛模拟方法计算层合板的疲劳寿命,结果表明寿命服从Ｗｅｉｂｕｌ分布。     

基于最终失效强度的层合板结构的鲁棒优化分析

采用有限元方法分析复合材料层合板结构的最终失效强度，并以此对结构进行优化和鲁棒优化分析。层合板结构同时承受面内和面外载荷，每个单层板考虑基体失效和纤维断裂两种失效模式。当某一单层失效后采用比率退化法计算结构新的刚度，然后重新进行结构分析，直到求得结构的最终失效强度。以纤维方向角和层合板厚度作为设计变量，最终失效强度为目标，对结构进行优化。在此基础上，考虑了层合板结构的鲁棒优化分析。     

Influence of fiber orientation on abrasive wear of unidirectionally reinforced carbon fiber–polyetherimide composites

A composite with continuous carbon fibers (CF) (80% by vol.) and high performance thermoplastic polyetherimide (PEI) matrix was developed and evaluated for various mechanical properties as a function of fiber orientation angle (0°, 30°, 45°, 60° and 90°). It was observed that Young's modulus, Poisson's ratio, toughness and % strain decreased with the increase of fiber orientation angle with respect to loading direction. In-plane shear modulus was highest for fibers with 45°. Overall, unidirectional (UD) CF reinforcement enhanced all strength properties of PEI significantly. Composites with fibers in 0° (parallel to loading direction) proved best in almost all the properties. Tribological evaluation in abrasive wear mode under different loads and fiber orientations indicated that coefficient of friction (μ) and specific wear rate (K₀) decreased with load, in general. Comparatively low specific wear rate (K₀), (in the order of 0.7 1×10^?9 m³/Nm) was observed for 0° fiber orientation, while fibers in 90° showed almost three times higher wear rate. Overall fiber reinforcement in 0° orientations proved beneficial from both strength and tribological performance point of view. SEM proved useful to correlate wear rate with surface topography.     

TEM studies of anti-wear films/wear particles generated under boundary conditions lubrication

Friction and wear performance of engine oil were studied in presence of Zinc-dialkyldithiophosphate (ZDDP) and ZDDP–iron fluoride (FeF₃) combination using a ball-on-ring wear testing device under boundary conditions. Friction and wear performance of engine oil improves in presence of ZDDP–FeF₃ combination. In order to understand the wear mechanisms the microstructure and the chemical composition of wear debris generated during wear process were investigated using TEM together with EDX analyzes. Novel observations on the wear debris generated at different testing loads are presented. Independent of normal loads, amorphous debris containing P, O, Fe and Zn elements and crystalline debris of Fe₂O₃ are formed. No trace of S is present in amorphous debris under low load (2.32 GPa) conditions while S is a dominating element under high loaded (3.68 GPa) conditions. On the other hand, at lower loads a few iron oxide is formed while at higher loads larger sizes of iron oxides are formed resulting in larger friction and wear.     

基于HyperWorks 的不同铺层方式复合材料剪切强度有限元分析

剪切强度和剪切韧性是反映复合材料构件在复合受力状态下承载能力及耗能能力的重要指标,不同铺层方式的单向玻璃纤维与短切玻璃纤维混杂增强复合材料层合板的层间剪切性能有明显差异。文中基于HyperWorks 商用有限元软件建立了精确的复合材料层合板模型,通过数值模拟分析不同铺层方式复合材料层合板的层间剪切性能。研究结果表明,铺层材料对复合材料层合板的层间剪切性能影响较大,而铺层顺序对复合材料层合板的层间剪切性能影响较小。     

Uniformly loaded, clamped, cross-ply laminated, elliptic plates—An initial failure study

An exact polynomial solution for a uniformly loaded, antisymmetric cross-ply laminated, clamped elliptic plate is combined with the Tsai-Hill failure criterion for an initial flexural failure analysis of GFRP and CFRP plates. Non-dimensionalised initial failure loads and associated plate centre deflections—both of which are significantly dependent on the direction of the applied loading—are presented graphically for the complete range of aspect ratios for plates consisting of varying numbers of laminae. It is also demonstrated that changes in the in-plane boundary conditions produce negligible changes in the initial failure response of the plate.     

A dynamic asymmetrical crack model of bridging fiber pull-out in unidirectional composite materials

As a rule, when a crack happens in composite materials, the fibrous system will generate bridging fibers resulted in the asymmetrical extending of the crack. In this paper, a dynamic asymmetrical crack model of bridging fiber pull-out in unidirectional composite materials is built for analyzing the distributions stress and displacement with the internal asymmetrical crack under the loading conditions of an applied non-stress and the traction forces on crack faces yielded by the bridging fiber pull-out model. Thus the fiber failure is determined by the maximum tensile stress, the fiber ruptures, and hence the crack propagation should also occur in self-similar modality. The formulation involves the development of a Riemann-Hilbert problem. The analytical solution of an asymmetrical extension crack in unidirectional composite materials under the conditions of moving increasing loads Pt²/x² and Px²/t is concluded, respectively. Based on relative material properties, the variable law of dynamic stress intensity factors was depicted perfectly. After the conclusion of analytical solutions with the superposition theorem, the solutions of arbitrary complex problems could be acquired.     

复合材料修补结构的疲劳寿命预测方法

针对复合材料修补结构应力分布复杂的特点,在剩余强度衰减模型的基础上引入局部应力应变法的分析思想,建立起复合材料修补结构的疲劳寿命预测模型.在剩余强度衰减模型和希尔提出的各向异性材料塑性条件基础上建立单向层合板在任意应力状态下的疲劳破坏准则.     

Stability behaviour of arbitrarily laminated composite plates with free and elastically restrained unloaded edges

In this paper, the initial buckling loads and the corresponding buckling modes of symmetric rectangular laminated plates are investigated. The considered laminates are supposed to have a uniform thickness, are subjected to a linearly distributed inplane compressive normal load and are simply supported at the two loaded edges with one free unloaded plate edge and with one simply supported unloaded edge where elastic rotational restraints are considered. Unlike in many other investigations, the composite laminates presently under consideration may have arbitrary yet symmetric lamination schemes with bending–torsion coupling. The initial buckling loads of such plates are calculated using the RITZ-method for which some especially adjusted displacement shape functions are employed. Since a series expansion of the buckling shape is performed in the load direction only while in the perpendicular direction one single displacement function can be shown to be sufficient, the present approach is numerically very efficient when compared to approaches in which a series representation is chosen with respect to both inplane directions. Comparison with reference results and with finite element computations leads to an excellent agreement. Some new findings on the general stability behaviour of this class of laminated plates are presented as a closure.     

Effect of prestress level on the strength of CFRP composite laminate

The effect of fiber prestressing has been investigated on the tensile strength, modulus strength, flexural properties and residual stresses of Carbon fiber reinforced polymer (CFRP) composites. Unidirectional carbon fiber in an epoxy resin has been examined under different prestressing levels (0, 5, 10, 15, 20, 30, 45, 60, 80 MPa) at ambient temperature and 50 % fiber volume fraction. A new method was implemented to produce the prestressed laminates for several standard tests. The results showed that prestressing on 3-ply unidirectional carbon fibers improved the fiber tensile strength to 99 % and the tensile modulus to 31 %. Similarly on 8-ply unidirectional carbon fibers has improved the fiber flexural strength to 63 % and flexural modulus to 81 %. A new technique was used to measure the residual stresses and tensile modulus of the composite laminate by recording the final extension and the remaining load directly after the curing process and releasing the applied load.

     

Elastoplastic buckling of rectangular plates in biaxial compression/tension

This paper examines the elastoplastic buckling of a rectangular plate, with various boundary conditions, under uniform compression combined with uniform tension (or compression) in the perpendicular direction. The analysis is based on the standard linear buckling equations and material behaviour is modelled by the small strain J₂ flow and deformation theories of plasticity. A detailed parametric study has been made for Al 7075 T6 over a range of plate geometries (a/b=0.25–4,a/h≈20–100) and with three sets of boundary conditions (four simply supported boundaries and the symmetric combinations of clamped/simply supported sides). For sufficiently thin plates we recover with both theories the classical elastic results. However, for thicker plates there is a remarkable difference in the buckling loads predicted by these two theories. Apart from the expected observation that deformation theory gives lower critical stresses than those obtained from the flow theory, we report on the existence of an optimal loading path for the deformation theory model. Buckling loads attained along the optimal path—specified by particular compression/tension ratios—are the highest possible over the entire space of loading histories. By contrast, no similar optimum has been found with the flow theory. This discrepancy in the buckling behaviour, obtained from the two competing plastic theories, provides a possibly new illustration of the plastic buckling paradox.     

The load-carrying capacity of initially imperfect eccentrically loaded plates

Theoretical analyses of compressed plates with various loading and boundary conditions previously published by the authors are extended to include the effects of initial imperfections. Comparison of the theoretical results is made with experiment. It is shown that for plates with supports on both unloaded edges the simple criterion that collapse occurs when the maximum membrane stress reaches yield, gives accurate estimates of failure for eccentrically loaded plates. The collapse modes of plates which have one edge free are found to be rather complex in nature and vary with plate thickness and loading eccentricity.A simplified design analysis for the prediction of collapse loads of eccentrically loaded simply supported plates is formulated and shown to predict experimental results very accurately.     

Hydroforming of aluminum extrusion tubes for automotive applications. Part I: buckling, wrinkling and bursting analyses of aluminum tubes

Three possible failure modes have been identified in tube hydroforming: buckling, wrinkling and bursting. A general theoretical framework is proposed for analyzing these failure modes as an elastoplastic bifurcation problem. This framework enables advanced yield criteria and various strain-hardening laws to be readily incorporated into the analysis. The effect of plastic deformation on the geometric instability in tube hydroforming, such as global buckling, axisymmetric wrinkling and asymmetric wrinkling, is precisely treated by using the exact plane stress moduli tensor. A mathematical formulation for predicting the localized condition for bursting failure is established herein. Furthermore, the critical conditions governing the onset of buckling, axisymmetric wrinkling and asymmetric wrinkling are derived in closed-form expressions for the critical axial compressive stresses. Closed-form solutions for the critical stress are developed based on Neale–Hutchinson's constitutive equation and an assumed deformation theory of plasticity. It is demonstrated that the onset of asymmetric wrinkling always requires a higher critical axial compressive stress than the axisymmetric one under the context of tube hydroforming with applied internal pressure and hence the asymmetric wrinkling mode can be excluded in the analysis of tube hydroforming. Parametric studies show that buckling and axisymmetric wrinkling are strongly dependent on geometric parameters such as t₀/r₀ and r₀/ℓ₀, and that axisymmetric wrinkling is the predominant mode for short tubes while global buckling occurs for long slender tubes.     

Micromechanical initial failure analysis of symmetrically laminated cross-ply plates

A first-ply failure analysis is given for a symmetric, cross-ply laminate under flexural loads. The effects of fibre content and type, voids and hygrothermal conditions are investigated on the failure load and its location by means of micromechanical relations. Numerical results are given for graphite-, boron- and E-glass-epoxy laminates.     

Post-buckling and delamination propagation behavior of composite laminates with embedded delamination

Delamination occurred due to poor manufacturing process or in-service actions significantly affects the mechanical and failure behavior of laminated composite structures. In this study, the buckling and post-buckling delamination behavior of laminated composite with an embedded initial delamination under in-plane compression was studied experimentally and numerically. First, compression tests for laminated composite specimens with embeded initial delamination were performed and the buckling and delamination responses were obtained. Then the experimental test was numerically simulated using finite element methods with the progressive failure accounted for by using cohesive zone modeling. The load-displacement curve, strain behavior and delamination shapes of experimental specimens obtained from load cells, strain gages installed at different locations, and C scan images, respectively, were compared with the FEM results, and good agreements were attained. The effect of the buckling modes, laminate stacking sequence and shape of initial delamination on the buckling load and propagation behavior was studied by considering different ply stacking and shapes of initial delaminations. It was found that the buckling mode determined the growth direction of the delamination propagation, and the stacking sequence influenced the extent of the propagation area, while the orientation of the delamination affected the buckling loads.     

Characterization of the residual mechanical properties of woven fabric reinforced composites after low-velocity impact

An investigation is undertaken to examine the residual mechanical properties of crowfoot-weave carbon/epoxy laminates subjected to a transverse central low-velocity impact load. It is found that the residual strength and stiffness of impacted laminates decrease with increasing impact damage area. Experimental data indicates that flexural stress constitutes the basis of a failure criterion that also describes damage severity, if localized damage at the region of impact is not serious. By formulating a simple model involving the motion of a rigid impactor, together with fundamental stress analysis of a transversely loaded plate, the effects of impactor mass, impact velocity, impactor tip radius, laminate thickness and lay-up on low-velocity impact damage are identified. Based on the experimental observations, it is found that the residual mechanical properties can be approximated by a linear relationship with a single damage severity parameter Q, where Q is a function of incident impact energy, impactor tip radius and laminate thickness. The theoretical results are verified by experimental data.