Crack tip heating in short-fibre composites under fatigue loading conditions

Crack tip heating in cyclically loaded short-fibre polymer matrix composites occurs by a combination of hysteretic heating and frictional heating. While the former mechanism is caused by plastic and viscoelastic deformations within the polymeric matrix, the latter is due to interfacial friction between matrix and fibres, and crack surface interference associated with crack closure. The relative contribution of these two principal mechanisms depends upon a number of variables including the viscoelastic and plastic characteristics and frictional properties of the matrix polymer, the degree of interfacial adhesion, the fibre content and fibre orientation distribution and the loading conditions. The results confirm that even in a tension/tension loading mode, frictional heating may play a dominant role, at least in some systems.     

Damage evolution of novel 3D textile-reinforced composites under fatigue loading conditions

For the simulation of the material degradation process during multiaxial fatigue loading of 3D textile-reinforced composites a new physically based damage model is developed based on the fracture mode concept (FMC) of CUNTZE and the continuum damage mechanics. For the damage analysis and the model parameter identification cyclic tests under superposed tension/compression–torque loading are performed.     

Creep-compensated fatigue testing of polyethylene under reversed loading conditions

A machine capable of testing polymers under creep-compensated strain-controlled cyclic deformation has been constructed, and used initially to study fatigue behaviour in low-density polyethylene. Reversed loading conditions have been used with total strain amplitude up to 6.5%. Crack propagation is inhibited by the compressive component of the deformation. Within the range of testing employed, the slowest crack-growth rates are obtained for a particular tensile amplitude when the compressive component is such that minimal accumulated dynamic creep is measured.     

Damage evolution and real-time non-destructive evaluation of 2D carbon-fiber/SiC-matrix composites under fatigue loading

Electrical resistance acquisition, acoustic emission (AE) monitoring and infrared thermography were employed to evaluate damage evolution of 2D carbon-fiber/SiC-matrix composite under fatigue loading. Damage evolution was discussed on the basis of the calculation results of the modulus and mechanical hysteresis variation. At lower stress levels, the majority of damage was produced in the first few cycles and then the rate of damage accumulation gradually approached a steady value as the cycles proceeded. When the applied stress exceeded the endurance fatigue limit, extensive damage took place and led to failure of the composite. Changes of composite electrical resistance, AE activity and surface temperature had fairly well agreement with the modulus and hysteresis responses. It can be concluded that it is possible to employ these real-time non-destructive evaluation methods as in-situ damage evolution indicators for this kind of composites under fatigue loading.     

Reliability assessment of polysilicon MEMS structures under mechanical fatigue loading

This paper examines the current status and methodologies of study of material and system reliability in Microelectromechanical Systems (MEMS). This includes: a review of the current literature in the area of MEMS regarding failure analysis experimental investigations; testing methods and philosophies for material characterization and possible mechanistic analytical solutions for estimating material properties. The paper proposes a reliability framework that encompasses all the available information. This statistical platform will enable the MEMS design engineer to distill all the available information in the literature into a stand-alone semi-empirical material reliability model, and a holistic system-level model for a complete system.     

复合材料层合板冲击后压-压疲劳寿命预测方法

针对冲击后复合材料层合板, 发展了含冲击初始损伤层合板的压-压疲劳寿命预测方法。该方法基于无损单向板的力学性能和疲劳特性, 对不同铺层参数、 不同几何尺寸以及不同冲击条件下层合板的疲劳寿命进行预测。为消除人为假设冲击损伤造成的误差, 对层合板在冲击载荷及冲击后疲劳载荷作用下的破坏进行全程分析, 即把冲击后层合板的实际损伤状态直接作为疲劳分析的初始状态。同时基于逐渐损伤思想, 推导了含冲击初始损伤层合板的应力分析过程, 建立了相应的三维逐渐累积损伤模型, 开发了参数化的复合材料层合结构冲击及冲击后疲劳破坏模拟程序, 为复合材料层合结构的抗冲击设计及其疲劳损伤扩展行为研究提供了较好的技术平台。      

Computer simulation of fatigue crack propagation under random loading conditions

The aim of this study is to simulate fatigue crack propagation under random loading conditions using a simple algorithm based on the Wheeler model [Wheeler O. Spectrum loading and crack growth. J Basic Eng D 1972;94:181–86]. To create the computer simulation, a model based on the mechanical properties of the material has been used. These properties include the yield stress (σ_y) and Paris’s constants C and m. The loading conditions (baseline loading ratio R, baseline stress intensity factor range ΔK and overload stress intensity factor K_ol, R_ol) are also required. The present model is validated with fatigue crack growth test data conducted on 12NC6 steel samples with four different heat treatments in order to have different types of mechanical behavior. The computer simulation and experimental results for crack propagation for different overload distributions (a single overload, a repeated overload, different overload magnitudes, random overload) are in good agreement.     

Stress analysis of multi-layer electronic and mechanical systems (MEMS) under fatigue and impact loading conditions

Detailed micro electronic and mechanical systems (MEMS) for a mobile microprocessor complex shape were modeled using Finite Element (FE) processing. Fatigue and impact conditions were performed on the Ball Grid Array (BGA) Integrated Circuit (IC) using AbaqusCAE finite element analysis software. The main objective of this research is to make sure that BGA products can endure the roughness of the daily usage, where a portable electronic product is habitually coupled with potential damage of functional failure when the device falls.     

Deformation behaviour of elastomeric matrix composites under static loading conditions

Damage mechanisms of elastomeric matrix composites (EMCs), propose a complex interplay between material properties and service conditions. The occurrence of defects such as the cavitations in EMCs specimens in using conditions is an important problem. This situation requires the well understanding of the damage mechanisms of EMCs used in automotive and aeronautical fields.Elastomeric matrix composites subjected to static and fluctuating loads basically fail due to the initiation and growth of defects (cracks, cavities, etc.). In fact, high hydrostatic pressures influence mechanical behaviours of EMCs. This paper reviews the damage mechanism of EMCs under static loading. In order to evaluate mechanical properties and fracture behaviour of EMCs, in situ observations were made by using X-rays computed tomography (CT). Two types of specimens are investigated in this work; Natural rubber, NR vulcanised and reinforced by carbon black, and synthetic rubber (styrene-butadiene-rubber), SBR. A detailed study was carried out by scanning electron microscopy (SEM) for a better understanding the damage mechanisms and confirming the CT results.     

Lifetime estimation of mechanical assemblies under constant amplitude fretting fatigue loading

In the present paper, a new analytical methodology to estimate both crack path and lifetime of metallic structural components under fretting fatigue elastic partial slip loading condition is proposed. Such a methodology consists in the joint application of (a) the criterion by Carpinteri et al for metallic structures under multiaxial constant amplitude fatigue loading in high‐cycle fatigue regime, (b) the critical direction method by Araújo et al, and (c) the critical distance method by Taylor, in the form of the line method. The accuracy of the above methodology is evaluated through experimental tests available in the literature, performed employing two cylindrical fretting pads pressed with a constant normal load against a dog‐bone type test specimen subjected to a cyclic axial load. All these components are made of Al 7075‐T651 aluminium alloy.     

History effect in fatigue crack growth under mixed-mode loading conditions

Plastic deformation within the crack tip region introduces internal stresses that modify subsequent behaviour of the crack and are at the origin of history effects in fatigue crack growth. Consequently, fatigue crack growth models should include plasticity-induced history effects. A model was developed and validated for mode I fatigue crack growth under variable amplitude loading conditions. The purpose of this study was to extend this model to mixed-mode loading conditions. Finite element analyses are commonly employed to model crack tip plasticity and were shown to give very satisfactory results. However, if millions of cycles need to be modelled to predict the fatigue behaviour of an industrial component, the finite element method becomes computationally too expensive. By employing a multiscale approach, the local results of FE computations can be brought to the global scale. This approach consists of partitioning the velocity field at the crack tip into plastic and elastic parts. Each part is partitioned into mode I and mode II components, and finally each component is the product of a reference spatial field and an intensity factor. The intensity factor of the mode I and mode II plastic parts of the velocity fields, denoted by dρ_I/dt and dρ_II/dt, allow measuring mixed-mode plasticity in the crack tip region at the global scale. Evolutions of dρ_I/dt and dρ_II/dt, generated using the FE method for various loading histories, enable the identification of an empirical cyclic elastic–plastic constitutive model for the crack tip region at the global scale. Once identified, this empirical model can be employed, with no need of additional FE computations, resulting in faster computations. With the additional hypothesis that the fatigue crack growth rate and direction can be determined from mixed-mode crack tip plasticity (dρ_I/dt and dρ_II/dt), it becomes possible to predict fatigue crack growth under I/II mixed-mode and variable amplitude loading conditions. To compare the predictions of this model with experiments, an asymmetric four point bend test system was setup. It allows applying any mixed-mode loading case from a pure mode I condition to a pure mode II. Initial experimental results showed an increase of the mode I fatigue crack growth rate after the application of a set of mode II overload cycles.     

Development of a standardized procedure for the characterization of interlaminar delamination propagation in advanced composites under fatigue mode I loading conditions

A round robin exercise on opening mode I fatigue delamination propagation has been performed with the aim of developing a standardized test procedure. The material chosen for the test was one type of carbon–fiber reinforced polymer–matrix laminate (IM7 fiber, 977-2 epoxy). The Double Cantilever Beam specimen from the quasi-static mode I delamination resistance test (ISO 15024) has been used for the fatigue test. Test set-up, measurements and data acquisition have been defined with an emphasis on applicability in an industrial test environment. Selected test parameters have been varied in order to investigate their effect on the results. Three different approaches for delamination length determination have been compared. Visual determination of delamination length, a compliance-based approach and an effective delamination length calculation based on a separate measurement of the modulus of elasticity yield reasonable agreement. This agreement suggests that further development of the test procedure to incorporate automated data acquisition and analysis may be worthwhile.     

Residual strength evaluation of concrete structural components under fatigue loading

This paper presents methodologies for residual strength evaluation of concrete structural components using linear elastic and nonlinear fracture mechanics principles. The effect of cohesive forces due to aggregate bridging has been represented mathematically by employing tension softening models. Various tension softening models such as linear, bilinear, trilinear, exponential and power curve have been described with appropriate expressions. These models have been validated by predicting the remaining life of concrete structural components and comparing with the corresponding experimental values available in the literature. It is observed that the predicted remaining life by using power model and modified bi-linear model is in good agreement with the corresponding experimental values. Residual strength has also been predicted using these tension softening models and observed that the predicted residual strength is in good agreement with the corresponding analytical values in the literature. In general, it is observed that the variation of predicted residual moment with the chosen tension softening model follows the similar trend as in the case of remaining life. Linear model predicts large residual moments followed by trilinear, bilinear and power models.     

Damage mechanisms under tensile and fatigue loading of continuous fibre-reinforced metal-matrix composites

Metal-matrix composites are gaining increasing attention for structural applications. However, the database relating to their mechanical properties and microstructural characterization remains limited. In the present study, aluminium-matrix composites reinforced with SiC fibres, α-Al₂O₃ fibres and carbon fibres have been investigated. Continuous fibre reinforcements, unidirectional in the 0° and 90° directions, were used. Tensile and compression tests were performed using specially designed test equipment for metal-matrix composites. The best results were achieved when cylindrical hour-glass shaped specimens were used. Fatigue testing of the composites showed that a pronounced improvement in the fatigue behaviour can be achieved upon the addition of fibre reinforcement.     

Damage tolerant evaluation of cracked stiffened panels under fatigue loading

This paper presents the methodologies for damage tolerant evaluation of stiffened panels under fatigue loading. The two major objectives of damage tolerant evaluation, namely, the remaining life prediction and residual strength evaluation of stiffened panels have been discussed. Concentric and eccentric stiffeners have been considered. Stress intensity factor for a stiffened panel has been computed by using parametric equations of numerically integrated modified virtual crack closure integral technique. Various methodologies for residual strength evaluation, namely, plastic collapse condition, fracture toughness criterion and remaining life approach have been described. Effect of various stiffener sizes and stiffener type (concentric and eccentric stiffeners) on remaining life and residual strength has been studied under constant amplitude load. From the studies, it has been observed that the predicted life is significantly higher with concentric and eccentric stiffener cases compared to the respective unstiffened cases. The percentage increase in life is relatively more in the case of concentric stiffener compared to that of eccentric stiffener case for the same stiffener size and moment of inertia. From the studies, it has also been observed that the predicted residual strength using remaining life approach is lower compared to other methods, namely, plastic collapse condition and fracture toughness criterion and hence remaining life approach will govern the design. It is noted that residual strength increases with the increase of stiffener size.     

动态载荷下C/C复合材料的压缩破坏行为

利用电子万能试验机以及Split Hopkinson Compressive Bar(SHPB)测试了2DC/C复合材料在准静态、动态载荷下的压缩性能,结合光学显微镜分析了其在不同应变率下的破坏形貌、讨论了应变率对压缩破坏形貌的影响。结果表明:与准静态(10-4/s)相比,动态载荷下(5×102/s)复合材料的压缩强度提高了55%,压缩刚度提高了66%,具有较强的应变率效应;在准静态载荷下,C/C复合材料沿40°角剪切破坏,断口上炭纤维破坏具有溃散及剪切破坏特征,而在动态载荷下,C/C复合材料破坏成大小不一的碎片,其炭纤维破坏具有劈裂特征。C/C复合材料破坏模式的不同可归结为基体及界面强度的应变率效应。     

On the mechanical behaviour under cyclic loading of ceramic matrix composites

In this paper, a constitutive law is presented to model the mechanical behaviour of ceramic matrix composites. It allows matrix-cracking, interfacial debonding, sliding and wear to be accounted for in the framework of continuum mechanics. Based upon micromechanical studies, a 1D and 2D model was derived. An application was performed on a [0,90] SiC/SiC composite.