Characterization of fatigue damage in unidirectional GFRP composites through acoustic emission signal analysis

Fatigue damage progression in composite materials is governed by different failure mechanisms, each of which contributes to the damage to a different extent. To assess the cumulative damage undergone by the material and to estimate the residual life, it is necessary to discriminate and characterize the failure mechanisms. This necessitates an on-line technique which can be used to monitor and measure the damage progression as it occurs. Acoustic emission (AE), an on-line monitoring tool, is ideally suited for this purpose. To understand fatigue damage in terms of different failure mechanisms using this technique, it becomes necessary to identify and establish their AE characteristics. This paper discusses the experimental investigations carried out on unidirectional glass fibre reinforced plastic (GFRP) composite specimens in which the acquired AE data was analysed utilizing pattern recognition (PR) techniques. The results obtained from the experiments show that three different failure mechanisms which primarily contribute to the damage at any given stage can be discriminated and characterized. Further, an attempt is made to estimate the cumulative damage to predict residual life.     

Non-destructive testing of barely visible impact damage in polymer matrix composites

Barely visible impact damage (BVID) is developed when polymer matrix composites are subjected to impact loading. The damage has an adverse effect on structural integrity, and potentially leads to catastrophic failure.Thus it is important to employ a variety of advanced non-destructive testing (NDT) techniques in parallel to unambiguously determine the integrity of composite systems. This study deals with damage evaluation using visual inspection, ultrasonic C-scan, electronic speckle pattern interferometry (ESPI), electronic shearography (ES) and optical deformation and strain measurement system (ODSMS). Internal damage was introduced using an instrumented falling weight impact test (IFWIT) machine by controlling the combination of striker mass and releasing height. It was found that different NDT techniques were successfully employed to identify and visualise the impact induced internal damage. Experimental results generated from these techniques show a good agreement in damage identification and determination. The features, capabilities and limitations associated with these techniques were briefly discussed.     

Ceramic-matrix composites fatigue and fracture

Fiber-reinforced ceramic-matrix composites (CMCs) have been shown to exhibit excellent high-temperature properties. There are some published data on the mechanical properties of Nicalon fiber-reinforced composites with various matrices, but much of the work was performed in bending, and there is little information on the failure modes in textile-reinforced CMCs, especially under cyclic-loading conditions. This article is an interim report on research that examines the tensile deformation, fracture, smoothbar fatigue, and fatigue crack-growth behavior of several CMCs. Unidirectional, two-dimensional eight-harness satin weave, and three-dimensional angle-interlock weave Nicalon fiber architectures infiltrated with polymers and then pyrolized were investigated and are compared with similar experiments on Nicalon-reinforced calcium-silicate glass-ceramic-matrix composites.     

基于连续损伤力学的高低周复合疲劳损伤

基于连续损伤力学(CDM)的经典损伤理论和不可逆热力学原理，分别对高周和低周疲劳载荷下的损伤演化模型进行了研究，进而推导出一个新的高低周复合疲劳损伤模型；将该模型编写为UMAT耦合到ABAQUS有限元分析软件中，实现了对缺口材料高低周复合疲劳损伤的模拟及裂纹萌生位置和萌生寿命的预测；同时研究了不同的高低周循环比对裂纹萌生寿命的影响.结果表明,裂纹容易在缺口根部应力集中处萌生；该模型考虑了高低周循环的交互作用，模拟计算结果更符合实际情况；同时通过研究发现高的循环比会使裂纹的萌生加速.     

铜晶体的疲劳损伤微观机制

对Cu单晶体、双晶体和多晶体疲劳损伤微观机制的总结结果表明：在中、低应变范围Cu单晶体的疲劳裂纹主要沿驻留滑移带萌生,而在高应变范围则沿粗大形变带萌生;Cu双晶体中疲劳裂纹总是优先沿大角度晶界萌生和扩展,而小角度晶界则不萌生疲劳裂纹;对于Cu多晶体,疲劳裂纹主要沿大角度晶界萌生,有时也沿驻留滑移带萌生,而孪晶界面两侧由于滑移系具有相容的变形特征而未观察到疲劳裂纹萌生．     

通过调整润湿性开发具有高抗损伤性能的仿珍珠贝壳层状2024Al/B4C复合材料

为了解决复合材料中B4C陶瓷相难以被金属铝润湿的问题,利用TiH2和B4C的原位反应引入TiB2,进而调节其润湿性和界面结合.通过将熔融合金压力浸渗到冷冻铸造法制备的多孔陶瓷支架中,制备具有层状结构的2024Al/B4C?TiB2复合材料.与2024Al/B4C复合材料相比,加入TiH2后复合材料的抗弯强度和裂纹扩展韧...     

汽车车架疲劳断裂损坏的焊接修复

详细分析了车架疲劳断裂损坏的原因,介绍了车架的焊接修复事项、要点和焊接工艺,并例举了车架裂纹的修复实例.     

CuW80铜钨合金疲劳损伤过程的研究

借助扫描电镜研究了CuW80铜钨合金电触头材料在循环载荷作用下微观组织的变化过程,用系统分析的方法定义CuW80铜钨合金疲劳损伤参数,并对其三点弯曲疲劳损伤过程进行了定量跟踪分析.试验结果表明,CuW80铜钨合金疲劳的损伤过程可以分为三个阶段,具有双线性损伤过程的特征,分别对应于疲劳裂纹萌生阶段(EF)和扩展过程(FH)两个线性阶段,且分别占疲劳总寿命的60%和33%;在循环载荷作用下CuW80铜钨合金的疲劳裂纹主要萌生于铜钨合金铜相的晶体内,而非发生于铜、钨界面,并且疲劳裂纹在铜相晶体内不断扩展直至断裂.     

氧化铝陶瓷疲劳损伤过程的数字描述

用非对称正弦波动态载荷变化的方式跟踪了不同粘土结合剂添加量对利用铝型材厂废渣合成的氧化铝陶瓷的疲劳损伤过程的影响,用系统分析方法定义的疲劳损伤参数作为主要评价指标,结合试样的晶相结构和显微结构分析讨论了影响氧化铝陶瓷疲劳特性的因素,确定了最佳的结合剂添加量.结果表明,影响氧化铝陶瓷疲劳寿命的主要因素为试样中玻璃相的含量,当粘土添加量为18%时,试样的裂纹萌生寿命达到最高,为22500周,此时,体系内刚玉相含量为75%,玻璃相含量为20%.     

Thermal fatigue resistance of discontinuously reinforced cast aluminum-matrix composites

The thermal fatigue resistance of AlSi alloys and discontinuously reinforced Al-matrix composites containing graphite, silicon carbide, and fly ash particulates, and short alumina (Saffil) fibers was characterized by measuring the total length of microcracks on gravity-cast and squeeze-cast test specimens as a function of number of thermal cycles (1000–5000 cycles, 270 K amplitude). In each thermal cycle, the test specimens were heated and stabilized in air at 375 °C, water quenched, and air stabilized. In all specimens, the total crack length on a specified region increased with increasing number of thermal cycles. Whereas among monolithic alloys, squeeze-cast Al-12SiCuNiMg alloy exhibited better resistance to thermal cracking than Al-25Si and Al-20SiNi alloys, among the composites, squeeze-cast Al-alumina and Al-fly ash composites exhibited the best thermal fatigue resistance. The theoretical estimates of the thermal fatigue resistance of these composites are consistent with the experimental observations.     

一个金属材料高周疲劳损伤力学模型(英文)

基于不可逆热力学提出一个新的高周疲劳损伤力学模型,该模型考虑载荷频率对疲劳寿命的影响。模型中的参数H和c对于无频率效应的材料是常数,而对于有频率效应的材料则是与频率有关的函数。同时,讨论了不同应力比时模型的表达形式。利用AlZnMgCu1.5和AMg6N两种材料在不同频率下的疲劳实验数据验证提出的模型。结果表明,该模型能够准确预测材料在不同加载频率和应力比条件下的疲劳寿命。     

镁合金早期疲劳损伤的非线性超声在线检测与原位微观观察(英文)

开发了一套在材料疲劳实验机上直接在线测量超声非线性系数的实验系统。利用该系统进行3组不同加载应力下的镁合金试件疲劳在线非线性超声检测,同时,利用光学显微镜对镁合金材料随着疲劳周数增大的微观结构变化进行原位观察。结果表明,在疲劳寿命的55%之前,随着疲劳周数的增加,位错滑移产生的驻留滑移带增多,微裂纹萌生扩展,超声非线性系数随之增大;在疲劳寿命的55%之后,宏观裂纹的出现使超声衰减系数增大,超声非线性系数有减小趋势。微观观察结果可以很好地验证超声实验结果。另外,高周和低周疲劳以及拉-拉和拉-压等疲劳模式的变化对实验结果没有明显的影响。     

Monitoring and modeling stress corrosion and corrosion fatigue damage in nuclear reactors

Despite the serious impact of stress-corrosion cracking and corrosion fatigue, the relevant design and operational guidelines for light-water nuclear reactors are often just rules of thumb which identify susceptibility or codes which only quantify the effects of stress. However, quantitative understanding of the crack advance process is unfolding for several alloy types, and modeling now permits accurate prediction of crack growth over a wide range of material, environmental and stress conditions.     

Developing hybrid polymer composites with embedded shape-memory alloy wires

Adaptive composites integrate actuating and sensing materials into a structural material (e.g., the integration of thin shapememory alloy [SMA] wires can function as actuating elements into fiber-reinforced polymer composites). SMA composites can open new perspectives on the development of engineering structures and devices with adaptive shape, stiffness, damping, and other properties. Materials design involving the constitutive modeling of SMA composite thermomechanical responses is a key topic in the development of the adaptive composites. As such, example simulation results obtained using a recently developed composite model are presented. For more information, contact P. Šittner, Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague 8, 182 21, Czech Republic; 4202 66052657; fax 4202 821227; e-mail sittner@fzu.cz.     

Mechanisms governing deformation and damage during elevated-temperature fatigue of an aluminum-magnesium-silicon alloy

The cyclic deformation and fracture characteristics of aluminum alloy 6061 are presented and discussed. The specimens were cyclically deformed using fully reversed tension-compression loading under total strain-amplitude control, over a range of temperatures. The alloy showed evidence of softening to failure at all test temperatures. The degree of softening during fully reversed deformation increased with test temperature. The presence of shearable matrix precipitates results in a microstructure that offers a local decrease in resistance to dislocation movement, causing a progressive loss of strengthening contributions to the matrix. At elevated temperatures, localized oxidation and embrittlement at the grain boundaries are exacerbated by the applied cyclic stress and play an important role in accelerating crack initiation and subsequent crack propagation. The fracture behavior of the alloy is discussed in light of competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic plastic strain amplitude and concomitant response stress, and the test temperature.     

Progress in studying the fatigue behavior of Zr-based bulk-metallic glasses and their composites

Zr-based amorphous alloys with a high glass-forming ability and thermal stability were discovered in 1990. In the following years, the alloy design increased the critical casting thickness to several centimeters, and a homogeneous dispersion of nanoscale particles was found to improve the ductility. Therefore, Zr-based bulk-metallic glasses (BMGs) are being studied widely because of their potential as structural materials. The fatigue behavior is an important characteristic of structural materials. The current review documents the fatigue studies of Zr-based BMGs and their composites. The fatigue characteristics of these alloys in different loading conditions and environments are summarized and compared in this paper. The factors affecting the fatigue behavior of the Zr-based BMGs and their composites are discussed. The mechanisms of fatigue-crack initiation and propagation in BMGs are addressed in this review. In order to broaden the scope of applications of BMGs, a fundamental understanding of the fatigue behavior is important for the design of new alloy systems and the development of the processing techniques.     

Low-cycle impact fatigue of SiCw/7475Al composite

Important uses in the future for metal-matrix composites are in aerospace, weaponry, and high-speed power plants in which the inertial force produced by great acceleration is a load of high strain rate. Therefore close attention is given to the mechanical behavior of a composite at high strain rates. This paper reports a study of the behavior and mechanisms of a SiC _w /7475 composite in low-cycle impact fatigue (LCIF). The LCIF and impact tension tests were conducted by using the push-pull impact fatigue apparatus developed by the authors, in which the loading assembly was actually a combination of a Hopkinson’s pressure bar and an extension bar. In the apparatus the trapezoidal stress wave loads were produced. The strain rates in specimens may reach 400 s ^-1 .The results show that for a SiC _w /7475 composite, the strain-rate effects on yield stress, ductility, cyclic hardening and softening, δ _e /2-N _f relation, and transition life were slight. In low-cycle impact fatigue the cracks often initiated within or near the SiC particles, which mingled in the composite. The SiC _w /7475 composite was found to be less ductile than its alloy matrix; in low-cycle fatigue brittleness appears. Therefore great attention must be given to the behavior of the composite when it is used as a structural material.