Analysis of fatigue damage evolution and associated anisotropic elastic property degradation in random short-fiber composite

A theoretical analysis of cyclic fatigue damage and associated anisotropic property degradation in a random short-fiber composite is presented. The fatigue damage takes various forms of microcracking, originated from microscopic stress concentrators in the highly heterogeneous material system. A probabilistic treatment of the microcracks is introduced to evaluate the statistical nature of the microscopic fatigue damage. Damage evolution and accumulation are analyzed through the development of probabilistic density functions of microcrack length and orientation during the cyclic loading history. Constitutive equations for the damaged fiber composite are then derived on the basis of a self-consistent mechanics scheme in conjunction with a three-dimensional elliptic crack theory and the microcrack density functions. Cyclic fatigue degradation and associated damage-induced anisotropy of composite material properties are determined and checked against experiments. The tensorial nature of material damage and composite stiffness changes during fatigue are evaluated explicitly. A power-law relationship between the rate of damage growth and the fatigue loading cycle is obtained. The rate of fatigue damage growth is found to decrease exponentially with load cycles—a phenomenon unique to the random short-fiber composite. This study provides a comprehensive analytical treatment of the homogeneous fatigue damage problem for random short-fiber composites. The fundamental mechanics and mechanisms of fatigue damage evolution and associated anisotropic property degradation of the composite are elucidated.     

考虑纤维随机分布的复合材料热残余应力分析及其对横向力学性能的影响

建立了考虑纤维随机分布并包含界面的复合材料微观力学数值模型,模拟玻璃纤维/环氧复合材料固化过程中的热残余应力。通过与纤维周期性分布模型的计算结果进行对比,发现纤维分布形式会对复合材料的热残余应力产生重要影响,纤维随机分布情况下的最大热残余应力明显大于纤维周期性分布的情况下。研究了含热残余应力的复合材料在横向拉伸与压缩载荷下的损伤和破坏过程,结果表明:热残余应力的存在显著影响了复合材料的损伤起始位置和扩展路径,削弱了复合材料的横向拉伸和压缩强度。在横向拉伸载荷下,考虑热残余应力后,复合材料的强度有所下降,断裂应变显著降低;在横向压缩载荷下,考虑热残余应力后,复合材料的强度略有下降,但失效应变基本保持不变。由于热残余应力的影响,复合材料的横向拉伸和压缩强度分别下降了10.5%和5.2%。      

Stochastic loading and material inhomogeneity in fatigue crack propagation

The Gaussian stationary stochastic load process is considered to cause fatigue crack growth in randomly inhomogeneous materials. The Paris-Erdogan model is applied and the time when the crack length reaches a critical value (lifetime) is determined as a random variable. It is shown that random variations of the lifetime are not significantly affected by stochastic fluctuations of loading and material inhomogeneity. These fluctuations, however, affect significantly the mean value of the lifetime. An example is carried out to show quantitatively the influence of the load and material randomness on the structural lifetime.     

考虑脱粘的复合材料加筋板屈曲后屈曲及承载能力数值分析

建立了复合材料层合加筋壁板的屈曲后屈曲有限元分析模型。该模型采用界面单元以有效模拟筋条和壁板之间的连接界面, 连接界面和复合材料层板分别采用Quads和Hashin失效准则作为失效判据, 引入材料刚度退化模型, 采用非线性有限元方法, 研究了复合材料加筋壁板在压缩载荷下的前后屈曲平衡路径及破坏过程。数值分析结果与实验结果吻合良好, 证明了该方法的合理有效性。详细探讨了筋条尺寸及界面单元强度等参数对加筋壁板屈曲后屈曲行为及承载能力的影响规律, 研究表明增加筋条截面惯性矩及筋条密度在一定程度上能有效提高加筋板的屈曲载荷与极限强度, 筋条密度增加到一定程度会引起结构破坏形式由失稳破坏?湮顾跗苹? 界面强度与铺层方式对极限强度有重要影响, 界面脱粘是引起加筋板最终破坏的重要因素。      

Compressive Behavior of 3D Woven Composite Stiffened Panels: Experimental and Numerical Study

The structural behavior and damage propagation of 3D woven composite stiffened panels with different woven patterns under axial-compression are here investigated. The panel is 2.5D interlock woven composites (2.5DIWC), while the straight-stiffeners are 3D woven orthogonal composites (3DWOC). They are coupled together with the Z-fibers from the stiffener passing straight thought the thickness of the panel. A “T-shape” model, in which the fiber bundle structure and resin matrix are drawn out to simulate the real situation of the connection area, is established to predict elastic constants and strength of the connection region. Based on Hashin failure criterion, a progressive damage model is carried out to simulate the compressive behavior of the stiffened panel. The 3D woven composite stiffened panels are manufactured using RTM process and then tested. A good agreement between experimental results and numerical predicted values for the compressive failure load is obtained. From initial damage to final collapse, the panel and stiffeners will not separate each other in the connection region. The main failure mode of 3D woven composite stiffened panels is compressive failure of fiber near the loading end corner.     

Damage mechanisms and life prediction in high temperature fatigue of a unidirectional SiC–Ti composite

Isothermal fatigue tests are performed in the longitudinal direction at 450°C on a unidirectional SiC/Ti composite. Three major damage mechanisms are identified: the matrix cyclic softening which overloads fibers leading to their progressive rupture; the interfacial degradation of these broken fibers and their oxidation by the environment. Damage kinetics are estimated using microscopic observations and acoustic emission. Finally, a micromechanical model is used in order to understand the respective influence of these damage mechanisms. It is shown that, at a sufficient load level, fatigue fracture of the composite strongly depends on the interfacial degradation kinetics and that fiber oxidation by the environment generates more progressive damage and considerably reduces the composite fatigue life compared to loading under vacuum.     

2D-SiC/SiC复合材料损伤耦合力学行为

基于轴向和45°偏轴加载实验,分别获得2D-SiC/SiC复合材料在单一轴向应力和复合应力状态下纤维束轴向方向上的拉伸、压缩和面内剪切应力-应变行为,计算分析材料在复合应力状态下的损伤耦合力学行为。结果表明,在45°偏轴拉伸和压缩复合应力状态下材料损伤耦合力学行为的起始应力分别约为40MPa和-100MPa。复合应力状态下材料纤维束轴向方向上的拉伸损伤和面内剪切损伤进程间具有相互促进作用,面内剪切损伤对压缩损伤进程具有促进作用,但是压缩应力分量对面内剪切损伤进程具有明显的抑制作用;上述损伤耦合作用随着应力水平的增加而越发显著。由试件断口电镜扫描结果可知,复合应力状态下材料纤维束轴向方向上3个应力分量对材料内部0°/90°和45°3种取向基体裂纹开裂损伤进程的影响作用,是2D-SiC/SiC复合材料产生损伤耦合力学行为的主要细观损伤机制。     

Accumulation of fatigue damage and lifetime prediction under irregular loading conditions with overloads and random cycle asymmetry

The prediction of service life with an experimentally established relation for correcting the magnitude of cumulative fatigue damage on overloads is discussed. The results of block loading tests of cylindrical aluminum alloy specimens with different stress concentrators were used to quantitatively estimate the effect of cycle asymmetry of basic loading and overload steps on the lifetime. It was demonstrated how the lifetime decreased in passing from block to random variations of loading conditions. It is shown that the estimation of service life can be based on experimental models for the correction of cumulative damage under random loading conditions simulating the operating ones. Translated from Problemy Prochnosti, No. 2 pp. 49–55, March–April, 1997.     

Damage mechanisms of SiC-coated C/C composites subjected to cyclic loading in combustion gas environment

A SiC-coated C/C composite was subjected to cyclic loading in a combustion wind tunnel with flame temperature of 1300 °C. Both the ratchetting strain and hysteresis changes showed that the majority of damage was produced in the first 50 cycles, and then the rate of damage accumulation gradually approached a steady value as the cycles proceeded. Lamellar structure of the C/C composite was observed after oxidation, which was caused by the difference in reactivity between the carbon fiber and carbon matrix. Acoustic emission activities throughout the tensile tests of the cycled specimens showed that little damage was produced before the applied tensile load exceeded the maximum history load of the cyclic loading tests.     

单丝复合体系渐进损伤过程模拟及组分性能对损伤过程的影响

为研究单丝复合体系在单向加载过程中的应力传递及损伤演化规律,基于剪滞模型建立了渐进损伤过程的三维数值分析模型。单丝复合体系的渐进损伤过程曲线和临界状态下的纤维段数、应变载荷及纤维轴向应力分布均与文献试验结果非常吻合,表明本文所提出的单丝复合体系渐进损伤模型能够有效模拟单丝断裂过程中的损伤起始、损伤演化和断裂临界状态。研究了模型中组分材料的模量和强度对损伤过程的影响。结果表明：保持组分材料强度不变,增加纤维的模量能够加快损伤过程,基体模量和界面模量的增加对单丝复合体系渐进损伤过程影响不大;在组分材料模量及界面强度不变的情况下,随着纤维强度的增加,单丝复合体系渐进损伤过程的起始应变载荷和临界应变载荷均增加,临界状态下的纤维断点数减少。     

Fatigue crack growth with retardation under stationary stochastic loading

The crack propagation under Gaussian stationary stochastic loading in presence of crack growth retardation effects is considered. The Wheeler retardation model is applied. The crack growth process is modeled by Markovian diffusion process. Some properties of envelope and clustering effect of the load process are taken into account in calculation of parameters of the Kolmogorov diffusion equation. The mean time to failure when the crack reaches its critical length is determined. The analysis of the mean lifetime equation and results of an example show that the stochastic fluctuations of the stationary load process alone do not affect significantly random variations of the lifetime. A quasi-deterministic relation between the features of the fatigue fracture and some material and load parameters is proposed. It gives a potential to consider effectively the fatigue crack propagation with retardation effects as an additional failure mode in reliability analysis.     

模拟混凝土滞回行为的各向异性损伤模型

该文引入相互独立的拉损伤、压损伤演化规律,分别描述受拉开裂和受压破碎导致的混凝土损伤行为,然后采用非线性卸载．线性重加载模拟滞回行为,建立了改进的各向异性损伤模型。该文还建议了模型关键参数的率定方法。应用该模型模拟混凝土受拉和拉一压往复加载试验,计算获得的应力-应变曲线与试验曲线具有良好的可比性,能够反映损伤导致的刚度...     

Fatigue life prediction under variable loading based on a new damage model

To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T 6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This paper contains the results of mono-axial random load fatigue tests with different mean and amplitude values performed on 6082 T 6 aluminium alloy specimens. Cycles were counted with rainflow algorithm and damage was cumulated with a new model proposed in this paper and with the Palmgren–Miner model. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.     

Damage detection in holed composite laminates using an embedded FBG sensor

This paper discusses damage detection in a holed CFRP laminate under static and cyclic loading using an embedded fiber Bragg grating (FBG) sensor. In order to detect the damage extension in the laminate, the change in the spectrum shape was measured using an embedded FBG sensor and was compared with that obtained by numerical simulation. The shape of the reflection spectrum did not change during the cyclic load test; however, it did change with increased strain in the static load test, due to damage around the hole. To clarify this difference, the polished surface of the cross section of the specimen was analyzed. Debonding was observed between the optical fiber and matrix during the cyclic load test. These results lead us to conclude that fatigue damage around a hole in a composite laminate may not be detected with an FBG sensor due to the debondings.     

Micromechanics of diffusion-induced damage evolution in reinforced polymers

In this work we numerically investigate the nucleation and evolution of micromechanical damage in reinforced glassy polymers under transient hygro-mechanical loading. In particular, we demonstrate the role that fiber distribution plays in the evolution of overall damage due to fiber-matrix interfacial debonding under moisture ingress. The heterogeneity of fiber distribution (clustering) is characterized by the coefficient of variation C_v of the center-to-center distances between interacting fibers, determined by identifying a cut-off radius around a typical fiber. The initial moisture diffusion-induced damage provides synergistic conditions for the rapid evolution of debonding under subsequent mechanical loading. The results indicate that microstructural heterogeneity strongly affects the moisture diffusion characteristics that in turn hurt the overall load carrying capacity of a composite due to aggravated damage. The strong dependence of the moisture-induced damage evolution on the fiber arrangement suggests that one should not resort to using simplistic unit cell models that assume regular fiber arrangements in such cases.     

The effects of explosive blast load variability on safety hazard and damage risks for monolithic window glazing

Although the modelling of built infrastructure subject to blast loading has been well developed, considerable uncertainty remains with respect to explosive loading parameters and structural response. This paper focuses on facade glazing – as this poses significant safety hazards when affected by explosive blast loads. A structural reliability analysis is used to calculate probabilities of glazing damage and safety hazards conditional on given threat scenarios. The analysis considers the variability of explosive blast loading; in particular, from variations in explosive weight, explosion effects in terms of pressure, stand-off distance, inherent blast load variability and model error. Uncertainties in structural response (including the variability in glazing stress limits, situational geometry, fragment drag coefficients and modelling error) are then considered in the analysis. This allows the prediction of likelihood and extent of damage and casualties. It was found that damage and safety hazard risks are very sensitive to the accuracy of the blast loading prediction model and the inherent variability of blast loading.     

A statistical model for the time dependent failure of unidirectional composite materials under local elastic load-sharing among fibers

In this paper we develop a statistical model for the time dependent failure of a unidirectional composite material under tensile loads. We consider probability distributions for lifetime in both stress-rupture and cyclic-fatigue and for strength under a linearly increasing load. We begin with a probability model for the failure of fibers which has its roots in the statistical kinetics of molecular failure, and we relate the parameters of our model to certain thermal activation quantities. At the micromechanical level we assume localized, elastic, load-sharing among failed and non-failed fiber elements in a cross section of the composite. As in earlier static versions we model the composite as a chain-of-short-bundles. Using asymptotic techniques we obtain accurate approximations to the probability distributions for composite lifetime with parameters that depend on certain fiber kinetic and variability parameters, fiber load-sharing constants, and on a “critical crack size” parameter that emerges as crucial.     

复合材料帽型加筋壁板的失效机制分析与改进设计

为了准确预测复合材料帽型加筋壁板的后屈曲承载能力,针对压缩载荷下筋条端头斜削的复合材料帽型加筋壁板结构的失效机制及失效载荷进行了研究。首先利用物理试验,研究了端头斜削的复合材料帽型加筋壁板失效过程,然后构建了考虑蒙皮/缘条胶接界面以及复合材料层板失效的非线性损伤分析模型,详细地研究了损伤起始、扩展和失效的全过程。在此基础上,提出了包覆层对蒙皮/缘条界面进行增强的设计方案,并基于数值仿真和试验研究了包覆层对复合材料帽型壁板的破坏模式和承载能力的影响。数值分析和试验结果表明,包覆层设计能够明显提高结构的屈曲载荷和后屈曲承载能力,分析结果与试验值吻合良好,且预测的破坏模式也与试验结果一致。     

Constitutive modeling of creep-fatigue interaction for normal strength concrete under compression

Conventional approaches to model fatigue failure are based on a characterization of the lifetime as a function of the loading amplitude. The Wöhler diagram in combination with a linear damage accumulation assumption predicts the lifetime for different loading regimes. Using this phenomenological approach, the evolution of damage and inelastic strains and a redistribution of stresses cannot be modeled. The gradual degration of the material is assumed to not alter the stress state. Using the Palmgren–Miner rule for damage accumulation, order effects resulting from the non-linear response are generally neglected.In this work, a constitutive model for concrete using continuum damage mechanics is developed. The model includes rate-dependent effects and realistically reproduces gradual performance degradation of normal strength concrete under compressive static, creep and cyclic loading in a unified framework. The damage evolution is driven by inelastic deformations and captures strain rate effects observed experimentally. Implementation details are discussed. Finally, the model is validated by comparing simulation and experimental data for creep, fatigue and triaxial compression.     

Zyklische Beanspruchung drahtverstärkter Verbundstrangpressprofile mit Aluminiummatrix

Cyclic loading of wire‐reinforced aluminium matrix composite extrusions Aluminium matrix composite extrusions reinforced with wires featuring high strength and stiffness represent an innovative materials concept for lightweight structures. The use of reinforcing elements should improve the mechanical properties and hence enhance the performance of the lightweight structures. Composite extrusions made from the aluminium alloy EN AW‐6060 reinforced with reinforcing elements made from the spring steel 1.4310 and the cobalt‐base alloy Haynes 25 were examined under cyclic loadings which are of vital importance for the desired applications. Initially, load controlled multiple step tests at a load ration of R = ‐1 allowed for the determination of the cyclic stress‐strain‐curve. Afterwards, lifetime predictions were determined from these results by using mechanical models proposed by Morrow and Basquin, which were reviewed Woehler tests without mean stress. Furthermore, light and electron microscopy served for the clarification of damage and failure mechanisms. The investigations have been carried out with varying materials, configurations and surface treatments of the reinforcing elements. The investigations strived for the identification of the parameters’ influence on the lifetime behaviour to optimize the materials systems regarding the fatigue behaviour.