Experimental and numerical study of creep and creep rupture behavior of PA6

Polyamide 6 (PA6) is a semi-crystalline polymer utilized in many structural components working under steady load. At room temperature, PA6 exhibits time dependent (viscoplastic) deformation. The aim of this work is to investigate the mechanical response and the crack growth of PA6 under creep conditions.The experimental database consists of tests carried out on smooth and notched round bars. Load versus displacement curves were recorded for monotonic tests tensile at various crosshead speeds. Then, creep tests at constant load were performed allowing the record of the creep strain history according to the applied load.Microscopic observations highlight the initial spherulitic structure.Smooth and notched specimens were utilized in order to identify and to validate material coefficients dedicated for analytical modeling. The non linear fracture mechanics for creeping solids was applied to results on “pre-cracked” specimens. For this kind of loading, use of the load parameter C^∗ is recommended.By plotting C^∗ values versus time to failure, a unique correlation was obtained. The knowledge of this master curve allows lifetime assessment of PA6 cracked bodies.     

Cyclic fracture behaviour of 304LN stainless steel under load and displacement control modes

Attempts have been made to understand cyclic fracture behaviour of AISI 304LN stainless steel used for nuclear piping materials under load vis‐à‐vis displacement controlled fracture tests; the former closely simulate the seismic loading conditions. The load controlled tests indicate that a material fails in a limited number of cycles even when the load amplitudes are sufficiently below the maximum load in a monotonic J–R test. The displacement controlled tests, on the other hand, show that the energy absorbing ability of a material gets severely reduced under cyclic loading conditions. The obtained results on standard laboratory specimens have been compared with similar available results on components in order to provide guidelines for maximum load bearing capability of engineering components under cyclic loading.     

The behaviour of Kevlar fibre-epoxy laminates under static and fatigue loadings. Part I—experimental

Kevlar 49 fibre and unidirectional Kevlar fibre reinforced plastic (KFRP) laminates both show an increase in stiffness under monotonic tensile loading. This stiffening effect is time-dependent and is reversible once the load is removed. In contrast, the modulus of a cross-ply KFRP laminate is affected primarily by matrix cracking of the transverse (90°) ply, and is sensitive to strain-rate and temperature. In cyclic (tensile) loading, however, the modulus of the cross-ply laminate depends on a combination of the fibre stiffening effect and transverse matrix cracking.     

Effects of Shear‐Transverse Coupling and Plasticity in the Formulation of an Elementary Ply Composites Damage Model,Part I: Model Formulation and Validation

Abstract: Continuum damage mechanics (CDM) models have been employed successfully in the literature to predict the response of laminated composite materials. Some sophisticated models can include the effects of non‐linear shear and transverse damage progression, plasticity and shear‐transverse damage coupling. However, these models require non‐standard test data for calibration that may not always be available to a modeller. In this two‐part study, we examine the effect of neglecting plasticity parameters, and also the effect of neglecting both plasticity and shear‐transverse coupling parameters in simplified CDM models for predicting monotonic tensile strength. In part I, we develop simplified versions of the CDM model and test their ability to accurately predict the failure response of angle‐ply laminates. In part II, we provide details of the experimental test series carried out to determine the input parameters for the models. It was found that neglecting plasticity requires some approximations in the damage development laws, but the resulting model can predict well the response of the angle‐ply laminates tested under monotonic loading to failure. Neglecting shear‐transverse coupling is acceptable for the some materials.     

采用螺栓连接的工字形全装配式RC剪力墙试验研究

采用连接钢框、高强度螺栓将带有内嵌边框的纵横向预制钢筋混凝土（RC）剪力墙连接起来,形成工字形剪力墙。为研究该全装配式剪力墙的受力性能和抗震性能,进行了3榀墙体的单调加载试验和1榀墙体的低周反复荷载试验,分析了该全装配式工字形剪力墙的承载能力、刚度、延性性能、耗能能力、连接件的应变分布以及连接件间的相对滑移等,最后探讨了试件的极限抗剪抵抗机制。研究结果表明:该全装配式剪力墙具有较高的承载能力、较好的延性性能以及耗能能力,位移延性系数约为3~6;高强度螺栓的直径、连接钢框钢板的厚度对装配式剪力墙的抗侧刚度及峰值荷载有一定的影响;内嵌边框既传递了分布钢筋的应力,也起到了约束混凝土、增加RC剪力墙延性性能的作用。     

复合材料整体化多墙盒段渐进式失效分析和试验验证

为准确预测复合材料盒段的损伤起始和破坏过程,针对复合材料整体化多墙盒段研究了考虑材料失效的渐进损伤有限元分析技术。首先基于ABAQUS软件,利用标准试验获得的材料力学性能建立了盒段渐进损伤分析模型,分别采用三维Hashin失效准则和平方应力失效准则作为复合材料层板和连接界面的失效判据;然后,基于该模型完成了复合材料整体化多墙盒段后屈曲承载能力的求解,并利用破坏试验对分析模型进行验证。结果表明:结构承载能力和应力的有限元分析结果与试验值吻合良好,预测的失效模式也与试验结果一致,屈曲载荷和承载能力误差在5%以内。所得结论表明考虑层板失效和界面脱粘的分析模型能有效模拟整体化多墙盒段的破坏过程。      

Experimental investigation of three-dimensional woven composites

This paper summarizes an extensive experimental study of composites reinforced with three-dimensional woven preforms subjected to tensile, compressive and in-plane shear loading. Three innovative three-dimensional woven architectures were examined that utilize large 12 K and 24 K IM7 carbon tows, including two ply to ply angle interlock architectures and one orthogonal architecture. Additionally, a two-dimensional quasi-isotropic woven material was evaluated for comparison. Loads were applied in both the warp and the weft directions for tensile and compressive loading. Digital image correlation was used to investigate full field strains leading up to quasi-static failure. Experimental results including ultimate strengths and moduli are analyzed alongside representative failure modes. The orthogonal woven material was found to have both greater strength and modulus in tension and compression, though a ply to ply woven architecture was found to outperform the remaining three-dimensional architectures. Recommendations are made for improving the manufacturing processes of certain three-dimensional woven architectures.     

A progressive damage simulation algorithm for GFRP composites under cyclic loading. Part II: FE implementation and model validation

The FE implementation of FADAS, a material constitutive model capable of simulating the mechanical behaviour of GFRP composites under variable amplitude multiaxial cyclic loading, was presented. The discretization of the problem domain by means of FE is necessary for predicting the damage progression in real structures, as failure initiates at the vicinity of a stress concentrator, causing stress redistribution and the gradual spread of damage until the global failure of the structure. The implementation of the stiffness and strength degradation models in the principal material directions of the unidirectional ply was thoroughly discussed. Details were also presented on the FE models developed, the computational effort needed and the definition of final failure considered. Numerical predictions were corroborated satisfactorily by experimental data from constant amplitude uniaxial fatigue of multidirectional glass/epoxy laminates under various stress ratios. The validation of predictions included fatigue strength, stiffness degradation and residual static strength after cyclic loading.     

Stiffness and fracture analysis of laminated composites with off-axis ply matrix cracking

Matrix cracks parallel to the fibres in the off-axis plies is the first intralaminar damage mode observed in laminated composites subjected to static or fatigue in-plane tensile loading. They reduce laminate stiffness and strength and trigger development of other damage modes, such as delaminations. This paper is concerned with theoretical modelling of unbalanced symmetric laminates with off-axis ply cracks. Closed-form analytical expressions are derived for Mode I, Mode II and the total strain energy release rates associated with off-axis ply cracking in [0/θ]_s laminates. Stiffness reduction due to matrix cracking is also predicted analytically using the Equivalent Constraint Model (ECM) of the damaged laminate. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy and carbon/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed.     

Failure analysis in postbuckled composite T-sections

Blade-stiffened skin designs made of composite materials have the potential to produce highly efficient structures, when the large strength reserves in the postbuckling range are utilised. This paper investigates the failure under postbuckling deformations of T-section specimens cut from a blade-stiffened panel, by comparing experimental results to finite element models. In the experimental work, T-section specimens with a particular lay-up and geometry were tested to failure in antisymmetric and symmetric loading rigs. These loading rigs simulate deformations on skin-stiffener interfaces during panel postbuckling. For the numerical analysis, two-dimensional models of the interface cross-section were used with a strength-based criterion that monitored failure within each ply. The use of a zero-thickness layer of cohesive elements has also been investigated in order to simulate the delamination behaviour. The numerical predictions are compared to the experimental results in terms of the failure load, specimen stiffness and specimen behaviour. The analysis approach is shown to be capable of predicting the critical damage locations and initiation loads for both antisymmetric and symmetric loadings. The successful prediction of failure in skin-stiffener interfaces can be linked to a global-local approach for efficient analysis of large, fuselage-representative composite structures.     

Experimental Tests and Numerical Modelling of Hexagonal Concrete Specimens

This paper describes a novel experimental investigation into concrete cracking under complex and rotating loading paths. Hexagonally shaped specimens are loaded in compression until some damage has occurred and then unloaded and rotated before being loaded again until failure. The results from tests on specimens loaded in two directions are compared with control tests undertaken under monotonic loading. The development of the test setup and sample preparation procedure for these novel experiments is described. The results show that the diffuse cracking accumulated during the initial loading direction has the effect of lowering the peak load carrying capacity of the rotated specimen relative to the peak load obtained under monotonic loading. The results of the experiments performed are subsequently used as benchmark tests for numerical simulations, to test the ability of a constitutive model, as implemented in a finite element program, to simulate cracking under complex and rotating load conditions. Numerical simulations of the experiments are performed here using a recently developed plastic-damage-contact constitutive model (Jefferson (2003) Int J Solids Str 40:5973–5999; Jefferson (2003) Int J Solids Str 40:6001–6022; Jefferson (2004) Comput Concrete 1(3):261–284). This model couples the familiar damage and plasticity model frameworks, taking advantage of their relative strengths in modelling tensile and compressive stress states, respectively. In addition, this model employs a contact function to simulate crack closure and aggregate interlock behaviour. The numerical results manage to capture the essential features of the experimental behaviour, in particular, a reduction of the peak load attained as a result of damage initiated in the original loading direction. However, the constitutive model is unable to fully capture the unloading response. A relatively high value for the specific fracture energy G _f was employed in the simulations to emulate the relatively ductile response of the experiments. This relatively ductile response of the specimens is conjectured to be due to friction on formed crack planes, and currently local plasticity on these planes is not simulated within the model framework.     

一种结构静力重分析方法及其在层合板逐次失效分析中的应用

本文提出了一种结构静力重分析方法,通过引入结构刚体位移特征向量,可以消除刚度矩阵的奇异性,从而求出结构广义柔度矩阵,位移一般解可在边界条件尚未引入之前导出。对于给定载荷及边界条件的静力问题,可以通过求解一个阶数较小的线性系统来获得位移解。对于局部修改的结构,相对于新结构的广义柔度矩阵可以迅速加以修正。这种静力重分析方法可以用于载荷、边界条件及结构单元同时或分别改变时的静力重分析问题。 之后,这种重分析方法被应用于复合材料层合板逐次失效分析,用以进行受破坏结构的刚度重分析。用有限元法求得各铺层的应力之后,引入破坏准则来判断铺层失效及失效模式,在本构方程中引入三个参数来修改失效铺层的刚度。层合板的破坏扩展及极限强度通过逐渐增加载荷迭代求得。文中提供了多个算例,计算结果与实验数据及文献吻合良好。     

玻璃纤维增强树脂基复合材料夹芯板-钢板连接接头的弯曲性能

以PVC泡沫或Balsa轻木为芯材的玻璃纤维增强树脂基复合材料（GRP）夹芯板目前广泛应用于船舶与海洋工程结构中。论文设计不同参数的GRP夹芯板-钢板混合接头模型,进行四点弯曲加载下的静力及疲劳试验研究,同时运用ABAQUS软件结合MSC.fatigue软件对接头的静态及疲劳弯曲失效进行数值模拟,分析了接头的弯曲强度、刚度和失效模式,并研究了接头填充区材料及长度、钢板嵌入填充区长度等参数对接头弯曲性能的影响。结果表明：弯曲载荷作用下接头破坏发生在连接结合部,失效模式则因填充区的不同设计而不同;对提高接头的弯曲性能较为明显的设计参数包括将钢板延伸到接头填充区或者选择Balsa轻木替代PVC泡沫芯材;对于受到疲劳弯曲载荷的接头模型,在较大疲劳载荷水平下,所有试件在未达到10⁶次循环时均发生了疲劳破坏;而在相对较小的疲劳载荷水平下,经过10⁶次循环后所有试件全部完好,并且接头的剩余强度与疲劳试验前的静强度相近,表明小载荷水平下接头的疲劳次数对其承载能力无影响。     

Evaluation of the mechanical and damage behaviour of tufted non crimped fabric composites using full field measurements

This paper investigates the effect of tufting on the mechanical properties of non crimped fabric (NCF) composites. In-plane behaviour is examined under tension and compression in the axial [0/90] and shear [±45] directions. Cyclic experiments in the bias and axial directions combined with a digital image correlation method (DICM) allow the investigation of damage distribution through the reduction of apparent stiffness and operations on the strain field. The out of plane mechanical response is studied via delamination tests in mode I and mode II. After studying each loading case individually, small structures of both composites are subjected to multi-loadings. Experimental results show that tufting reduces both the in plane stiffness and the strength in the axial direction (by approximately 10%), while it greatly enhances delamination resistance in the normal and shear directions. On the other hand tufts influence on in plane properties is moderate in the bias direction. But large differences were monitored between compression and tension response in the bias direction for standard and tufted composites. Discs punched tests inducing multi-loading (in and out of plane loading) show greater energy absorption but lower failure load for tufted specimens than for untufted. It was found that cyclic loading experiments monitored with DICM yields damage maps that offer a useful insight into damage development. Large damage differences are recorded between different load cases and lay ups. The results also show that the tufts influence the damage progression in the NCF.     

不锈钢T形件螺栓连接承载性能试验研究

通过开展14组不锈钢T形件螺栓连接试件的单调拉伸试验,得到了各试件的极限承载力、破坏模式和撬力发展规律,分析翼缘厚度、翼缘材料、螺栓直径和螺栓预拉力等因素对不锈钢T形件承载性能的影响。结果表明:螺栓预拉力对试件的极限承载力没有明显影响,但会提高试件的初始刚度; T形件的破坏模式取决于翼缘和螺栓的相对关系,与螺栓预拉力无明显关系;撬力值随着翼缘厚度和螺栓直径的减小而增大,但极限承载力对应的撬力值与螺栓预拉力无关。将试验结果与现行欧洲、美国和中国规范的计算结果进行比较,表明现行国内外规范的相关计算公式均较保守,其中美国规范由于翼缘采用极限抗拉强度,计算结果与试验值最为接近。     

A progressive damage simulation algorithm for GFRP composites under cyclic loading. Part I: Material constitutive model

A life prediction algorithm and its implementation for a thick-shell finite element formulation for GFRP composites under constant or variable amplitude loading is introduced in this work. It is a distributed damage model in the sense that constitutive material response is defined in terms of meso-mechanics for the unidirectional ply. The algorithm modules for non-linear material behaviour, pseudo-static loading-unloading-reloading response, Constant Life Diagrams and strength and stiffness degradation due to cyclic loading were implemented on a robust and comprehensive experimental database for a unidirectional glass/epoxy ply. The model, based on property definition in the principal coordinate system of the constitutive ply, can be used, besides life prediction, to assess strength and stiffness of any multidirectional laminate after arbitrary, constant or variable amplitude multi-axial cyclic loading. Numerical predictions were corroborated satisfactorily by test data from constant amplitude fatigue of glass/epoxy laminates of various stacking sequences.     

Multiaxial fatigue behaviour of a short‐fibre reinforced polyamide – experiments and calculations

This paper deals with the fatigue behaviour of a short fibre reinforced thermoplastic under multi‐axial cyclic stress. Based on experimental results on notched and plain specimens, limits of existing methods for the fatigue life estimation in the design process of components exposed to complex multi‐axial loads were investigated. During the manufacturing process of short fibre reinforced thermoplastic components, a moderately anisotropic behaviour in stiffness and strength arises. Because of the material's anisotropy, classical failure hypotheses for the assessment of multiaxial load cases do not apply. In this study, a fatigue failure hypothesis was implemented that assesses the stress components in accordance with the correlating fatigue strengths in the material coordinate system, considering potential interaction between the stress components. Striving for a verified multi‐usable fatigue life assessment method, multiaxial load cases were examined experimentally. The experimental results on unnotched and notched specimens and the fatigue life estimation on the basis of the Tsai‐Wu‐failure hypothesis will be presented.     

Statistical model of the transverse ply cracking in cross-ply laminates by strength and fracture toughness based failure criteria

Cross-ply laminate subjected to tensile loading provides a relatively well understood and widely used model system for studying progressive cracking of the transverse ply. This test allows to identify material strength and/or toughness characteristics as well as to establish relation between damage level and the composite stiffness reduction. The transverse ply cracking is an inherently stochastic process due to the random variability of local material properties of the plies. The variability affects both crack initiation (governed by the local strength) and propagation (governed by the local fracture toughness). The primary aim of the present study is elucidation of the relative importance of these phenomena in the fragmentation process at different transverse and longitudinal ply thickness ratios. The effect of the random crack distribution on the mechanical properties reduction of the laminate is also considered. Transverse ply cracking in glass fiber/epoxy cross-ply laminates of the lay-ups [0₂/90₂]_s, [0/90₂]_s, and [0/90₄]_s is studied. Several specimens of each lay-up were subjected to uniaxial quasistatic tension to obtain crack density as a function of applied strain. Crack spacing distributions at the edge of the specimen also were determined at a predefined applied strain. Statistical model of the cracking process is derived, calibrated using crack density vs. strain data, and verified against the measured crack spacing distributions.     

Schwingfestigkeit der keramikverstärkten Kolbengußlegierung GP-ALSi 12 CuMgNi bei erhöhten Temperaturen

Fatigue Strength of Fibre-Reinforced Squeeze Cast Piston Alloy GP-AlSi 12 CuMgNi Under Elevated Temperatures On specimens made of squeeze-cast piston alloy GP-AlSi 12 CuMgNi without reinforcement and Al₂O₃-fibre-reinforced, thermal fatigue was investigated. Several mechanical properties such as tensile and compressive yield strength, incremental-step-method based monotonic and cyclic stress-strain curves in the temperature range of 20 to 400°C were determined. Moreover, fatigue tests in the range of 5 · 10⁴ and 2.5 · 10⁷ were carried out for various stress ratios in the same temperature range on unnotched specimens and S-N-curves were designed. Haigh-diagramms show the mean stress sensitivity of the materials. Obviously the fibre reinforced material is superiour at all regarded temperatures. The considerable gain of material strength obtained by fibrereinforcement entails even better conditions for the use of newly developed pistons under increased thermal load.     

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.