Fatigue crack growth behavior of Al7050-T7451 attachment lugs under flight spectrum variation

This paper discusses an analytical and experimental investigations of the fatigue crack growth behavior in attachment lugs subjected to a randomized flight-by-flight spectrum. In the analysis, the stress intensity factors for through-the-thickness cracks initiating from lug holes were compared by weight function method, boundary element method (BEM), the interpolation of Brussat’s solution. The stress intensity factors of a corner crack at a transition region were obtained using two parameter weight function method and correction factors. Fatigue life under a load spectrum was predicted using stress intensity factors and Willenborg retardation model considering the effects of a tensile overload. Experiments were performed under a load spectrum and compared with the fatigue life prediction using the stress intensity factors by different methods. Changes of fatigue life and aspect ratio according to the clipping level of the spectrum were discussed through experiment and prediction. Effect of the spectrum clipping level on the fatigue life was experimentally evaluated by using beach marks of fractured surface.     

表面裂纹疲劳扩展的数值模拟(Ⅱ)

根据Paris疲劳裂纹扩展规律,对拉伸和纯弯曲疲劳载荷下表面裂纹扩展进行了数值模拟。数值模型中,用三次样条函数曲线拟合裂纹尖端,在裂纹扩展增量计算中考虑了裂纹闭合影响。裂纹形状演化的模拟结果与Newman和Raju经验公式预测结果进行了比较,表明了所采用的数值模拟方法的实用性。研究发现,裂纹闭合对疲劳裂纹扩展过程中的裂纹形状演化以及裂纹尖端的应力强度因子(SIF)分布都有明显影响。同裂纹形状演化一样,疲劳裂纹扩展过程中裂纹尖端的SIF分布表现出明显的特征。最后,建议了一个简单函数来统一描述表面裂纹尖端的SIF分布。     

Prediction models of fatigue crack closure and growth under random loading

The effects of plastic zones both in front of and behind crack tip on crack closure have been analysed. The total residual deformations of crack surfaces involve two parts, that is, the amount of plastic blunting of crack tip and the residual deformation in the wake of the tip. This paper presents a fatigue crack closure model in which the influences of compressive load on closure stress are discussed. The model is applied to random loading conditions by the assumption of limited memory properties. The fatigue lives are predicted using the proposed crack growth model for CCT plane stress specimen cut from 2219-T851 aluminum alloy under flight spectrum loadings, and the prediction values agree with the test results.The project was supported by the natural science foundation of China.     

焊趾表面裂纹的形态发展曲线与疲劳寿命预测

以作者建立的焊地椭圆表面裂纹应力强度因子数据库以及复杂应力场中焊践半随圆表面裂纹前缘应力强度因子分布计算的基本模式法为基础上,给出了复杂应力场中焊践表面表纹在疲劳扩展过程中形态变化规律及寿命的工程分析方法。     

A model for predicting the retardation effect following a single overload

Many engineering components are subjected to variable amplitude loading history. It is well known that retardation in fatigue crack growth occurs due to application of single overloads in a constant amplitude loading block. Many models have been proposed to capture this counter intuitive phenomenon which has resulted in improved understanding of retardation effect following tensile overloads and consequently resulting in better life prediction models. The proposed study is focused on to evaluation of retardation in fatigue life due to application of a single overload. A model for prediction of crack growth and crack growth rate following single overloads is presented. Several modifications to Wheeler’s growth idea are proposed, which incorporate a consideration for effective stress intensity factor, based on Elber’s concept of crack closure, relationship between overload ratio and the Wheeler’s exponent, and fatigue growth rate calculations. The results presented here show that plastic zone interaction following overload and the consideration of crack closure explain retardation effect following a single overload. Correlation between analysis and experimental data obtained from several sources in literature show that the scheme, is robust and provides an insight into the nonlinear aspect of crack growth results. The model has been tested for 2024-T3 aluminum alloy and 6061-T6 aluminum alloy and thorough calibrations performed, established the fidelity of the program.     

腐蚀环境下复合材料修补件力学特性研究

采用复合材料补片胶接修补含裂纹LY12CZ铝合金板,开展了试验室大气环境和加速预腐蚀环境下复合材料修补件静强度拉伸和疲劳裂纹扩展对比试验研究.结果表明,复合材料补片均能显著提高损伤结构的拉伸强度和疲劳寿命,且短周期的预腐蚀环境对修补件两种力学特性的影响可以忽略不计.同时,基于Paris公式和Rose分析模型,建立了常规环境和预腐蚀环境下疲劳裂纹扩展寿命预测模型,通过与试验结果的对比证明了该模型的工程有效性.     

随机载荷下疲劳裂纹的闭合和扩展估算模型

本文分析了裂尖前,后方塑性区对闭合性能的影响,提出从裂尖塑性钝化量和尾迹区残余塑性变形两个方面来确定裂纹面的残余变形,并讨论了压缩载荷对闭合应力的影响,由此建立了一个疲劳裂纹闭合模型,然后通过模型“有限记忆”性质的假定将它应用到随机加载情况。用此模型对铝合金2219——T851受飞机谱载荷的CCT平面应力试件进行了疲劳寿命估算,估算值与实验结果接近。     

拉压循环载荷下正交异性钢桥面板肋-面板焊缝裂纹扩展分析

采用四步法计算了考虑循环载荷中压应力影响的正交异性钢桥面板的肋-面板焊缝表面裂纹扩展。第一步是基于正交异性钢桥面板的疲劳分析模型,计算肋-面板焊缝处的应力,第二步是通过肋-面板焊缝的三维局部模型,用Schwartz-Neumann交替法计算焊缝表面裂纹的应力强度因子分布,第三步是用二维断裂力学模型和增量塑性损伤模型,计算循环载荷中的压应力对裂纹扩展的影响,第四步是用第二步中的三维裂纹分析结果和第三步中的二维断裂力学模型得到的裂纹扩展公式,计算钢桥面板的肋-面板焊缝表面裂纹扩展。计算结果表明,对应于正交异性钢桥面板肋-面板焊缝处的循环应力,本文所用模型的裂纹尖端反向塑性区导致裂纹扩展率增加50%以上。研究结果为正交异性钢桥面板肋-面板焊缝裂纹的疲劳寿命分析提供了研究基础。     

交变载荷下含两孔洞的铝合金长条的五种疲劳破坏型式

通过试验发现交变载荷下含两孔洞的铝合金长条,由于两孔洞的方位不同,至少有五种疲劳裂纹引起的破坏型式.在五种不同的破坏情况下,疲劳裂纹初始寿命和疲劳裂纹扩展寿命均不同,必须分阶段评估,这也直接影响到整体疲劳寿命的预计;此时常规的、经典的疲劳裂纹扩展理论难以直接应用.     

含缺口试件疲劳全寿命预测一种建议的方法

传统的研究含缺口构件的疲劳的方法是将疲劳启裂和疲劳裂纹扩展两个过程完全独立起来,用不同的方法来模拟,相互间并没有定量的关系。本文是基于最新发展的多轴疲劳损伤理论,建立了一种适用于各种载荷条件下的疲劳启裂和裂纹扩展的普适方法。根据从弹塑性分析中得到的应力应变,确定疲劳损伤模型,建立能够预测疲劳启裂、裂纹扩展速率和扩展方向的新方法。整个模拟可以分为两步:弹-塑性应力分析得到材料的应力应变分布;再运用一个通用的疲劳准则预测疲劳裂纹启裂和裂纹扩展。通过对1070号钢含缺口试件的疲劳全寿命预测,得到了与实验非常吻合的模拟结果。     

航空导管弯曲试验的疲劳裂纹萌生寿命

飞行器液压导管受接头和卡箍等约束，在使用的振动环境中，会因弯曲应力而导致破裂，影响到飞行安全．本文对飞行器液压系统通用的不锈钢导管的裂纹萌生寿命进行了试验研究．首先在对8 mm、12 mm 无缺陷导管和含U 型缺口8 mm 导管的疲劳试验和有限元分析的基础上，得到了导管的最大拉应变-裂纹萌生寿命数据．然后采用基于强度极限和弹性模量估算法的Manson-Coffin 公式来预测导管裂纹萌生寿命．最后引入加载类型修正系数、表面质量修正系数、试样尺寸修正系数、应力集中敏感系数和有效应力集中系数，使修正后的公式对三种类型的导管均有较好的裂纹萌生寿命预测精度．     

Fatigue crack initaition and propagation from hole defects

The aim of this study is to investigate the interaction of two hole defects affecting fatigue crack initiation life and propagation behavior. The location of two hole defects was characterized by an angle of alignment and the distance between the centers of two hole defects. The fatigue cracking behavior is studies under bending for AI 5086. When defects are located close to each other, the fatigue crack initiation lives are varied with their relative locations. In the experiments, the area of the local plastic stratin strongly played a role in the fatigue crack initiation lives. Therefore, the authors introduce a parameter which contains the plastic deformation are at stress concentrations and proposed a fatigue crack initiation life prediction curve. In addition, the directions and propagation rates of fatigue cracks initiated at two hole defects were studied experimentally.     

基于子模型法的带有表面裂纹钢丝应力强度因子研究

钢丝裂纹应力强度因子是进行钢丝疲劳断裂寿命评估、疲劳裂纹扩展分析和钢丝断裂强度评估等工作的重要参数。本文首先介绍了裂纹扩展分析软件FRANC3D,然后基于子模型法模拟研究了拉伸荷载作用下带有表面裂纹钢丝的应力强度因子,裂纹种类包括直线形裂纹和半圆形裂纹,最后拟合得到拉伸荷载作用下带表面裂纹钢丝的应力强度因子形状修正系数表达式,分析了利用该公式进行承载力评估时产生误差的原因。研究结果表明,利用子模型模拟分析拉伸荷载作用下带有表面裂纹的钢丝应力强度因子时计算精度高,计算速度快,对计算机硬件要求低;利用该方法得到的钢丝裂纹应力强度因子,在进行索承式桥梁吊索安全性能评估时,评估结果更精确。     

基于裂纹闭合效应的高载迟滞预测模型修正

裂纹闭合效应通常是导致I 型裂纹扩展在高载作用下发生迟滞效应的主要因素之一．本文采用汽车薄板QSTE340TM 材料，针对不同应力比，高载比条件下疲劳裂纹扩展行为进行了实验研究．论文通过断面分析，针对各参数对裂纹闭合效应的具体影响进行了分析讨论，认为裂纹作用区域随裂纹扩展而动态变化，从而提出了一种对有效应力强度因子幅的修正方法．通过在原有模型中引入幂函数形式的动态变量α，表征裂纹闭合效应的作用比例随裂纹长度的动态变化，取得了较好的效果．     

EFFECT OF LOADING TYPE ON FATIGUE CRACK GROWTH BEHAVIOR OF SOLDER JOINT IN ELECTRONIC PACKAGING

Fatigue cracking tests of a solder joint were carried out using in-situ scanning electron microscopy （SEM） technology under tensile and bending cyclic loadings. The method for predicting the fatigue life is provided based on the fatigue crack growth rate of the solder joint. The results show that the effect of the loading type on the fatigue crack growth behavior of a solder joint cannot be ignored. In addition, the finite element analysis results help quantitatively estimate the response relationship between solder joint structures. The fatigue crack initiation life of a solder joint is in good agreement with the fatigue life （N50%） of a totally electronic board with 36 solder joints.     

Growth of part-through and through-thickness fatigue cracks in sheet glassy plastics

Linear-elastic fracture mechanics is used to interpret observations of through-thickness fatigue crack growth in sheet specimens of polymethylmethacrylate, polycarbonate and an unfilled epoxy, and to correlate measurements of the growth of part-through and through-thickness fatigue cracks in sheet polymethylmethacrylate. It is shown that at least one of these materials may be useful for model studies of the growth of part-through thickness fatigue cracks in metallic components.     

CTS试件中复合型疲劳裂纹扩展

针对复合型循环载荷作用下的金属构件中的裂纹扩展问题进行了实验分析和理论建模. 首先 采用紧凑拉剪试件(CTS)和 Richard研制的复合型载荷加载装置，对承受复合型循环载荷的裂纹进行了实验研究. 实验选择了两种金属材料试件，分别承受3种形式的复合型循环载荷的作用，在裂纹尖端具 有相同的初始应力场强度的条件下考察复合型循环载荷对裂纹扩展规律的影响. 实验结果表明，疲劳裂纹的扩展速率与加载角度有关. 对于同样金属材料的试件，当裂尖处 初始应力场强度相等时，载荷越接近于II型，裂纹增长速率越快. 采用等效应力强度 因子(I型和II型应力强度因子的组合)、裂纹扩展速率及复合强度等参数，以实验数据为 基础，建立了一个疲劳裂纹扩展模型，用来预测裂纹在不同模式疲劳载荷作用下的扩展速率. 为验证其有效性，该模型被应用于钢制试件的数值模拟计算中. 实验结果与模拟计算曲线保 持一致，表明该模型可以用来估算带裂纹金属构件的寿命.     

Constant amplitude and variable load history fatigue test results and predictions for cruciform and lap welds

Cruciform and lap welds were fatigue tested under constant amplitude axial load and SAE Bracket spectrum load conditions. For the cruciform joints, fatigue cracks generally initiate at the root but may initiate at the toe if higher bending stresses are induced by joint distortion. For lap welds, the stress ratio ($\text{R}$) and weld shape are the major factors influencing the fatigue crack initiation site.The fatigue test results were compared with predictions made using an analytical model developed by the authors, and good agreement between experiment and theory was observed. The model for the predictions assumes that the fatigue crack initiation period, which is the number of cycles for the initiation of a fatigue crack and its early growth and coalescence into a dominant fatigue crack, is the main portion of the total fatigue life at long lives.     

DETERMINATION OF CRACK SURFACE DISPLACEMENTS FOR A RADIAL CRACK EMANATING FROM A SEMI-CIRCULAR NOTCH USING WEIGHT FUNCTION METHOD

A radial crack emanating from a semi-circular notch is of significant engineering importance. Accurate determination of key fracture mechanics parameters is essential for damage tolerance design and fatigue crack growth life predictions. The purpose of this paper is to provide an efficient and accurate closed-form weight function approach to the calculation of crack surface displacements for a radial crack emanating from a semi-circular notch in a semi-infinite plate.Results are presented for two load conditions: remote applied stress and uniform stress segment applied to crack surfaces. Based on a correction of stress intensity factor ratio, highly accurate analytical equations of crack surface displacements under the two load conditions are developed by fitting the data obtained with the weight function method. It is demonstrated that the WuCarlsson closed-form weight functions are very efficient, accurate and easy-to-use for calculating crack surface displacements for arbitrary load conditions. The method will facilitate fatigue crack closure and other fracture mechanics analyses where accurate crack surface displacements are required.     

A STUDY OF CRACK CLOSURE IN ELECTRIC-FIELD-INDUCED FATIGUE IN FERROELECTRIC CERAMICS

Considering the influence of the domain switching near the tip of a crack and apply-ing the idea of multiscale singularity fields in piezoelectric fracture,we have obtained an empiricalcriterion for the crack closure.Based on the domain switching in the electric yield region,referringto Yang's results on the small scale yield model for the electrical fatigue crack,a model of thecrack closure during electric-field-induced fatigue is developed to analyze the crack growth.Interms of the model we have obtained the formula of the rate of the crack growth under cyclicelectric loading.Finally we compare the theoretical predictions with the results given by Cao andEvans experimentally.It should be pointed out that the model proposed is empirical and needsto be verified by more experimental results.