随机超载对疲劳裂纹扩展迟滞效应的模拟

考虑超载的迟滞效应,对随机超载作用下的疲劳裂纹扩展进行了模拟计算．载荷谱为在基本恒幅循环载荷基础上加入一以泊松流发生的随机超载序列,超载的大小为均匀分布．采用裂纹闭合模型考虑超载的迟滞效应,认为裂纹张开应力在超载引起的塑性区内按线性规律衰减．循环续循环模拟计算出裂纹从初始长度一直到疲劳破坏的扩展曲线．据此,计算了各种超载发生强度和大小下的疲劳裂纹扩展寿命的平均值与标准差。     

一种经验型疲劳小裂纹扩展模型

金属材料疲劳寿命由裂纹萌生和裂纹扩展寿命两部分组成，其中对于萌生寿命中的小裂纹分析是精确描述裂纹萌生寿命的关键．而小裂纹在扩展过程中由于尺寸相对较小，导致传统线弹性断裂力学预测方法失效，需要对其进行改进，考虑裂纹尖端塑性区引起的残余压应力对小裂纹扩展速度的影响．本文针对此问题进行了初步分析，通过对塑性区引起的残余应力的量化，结合小裂纹门槛值特性，提出了一种经验型修正的小裂纹扩展模型，用于定量预测裂纹的萌生寿命．使用铝合金6082-T6缺口试样进行了疲劳实验，并与理论结果进行了对比，验证了所提模型的有效性．     

弹塑性疲劳微弯裂纹尖端塑性区问题

主要研究疲劳载荷作用下弹塑性裂纹弯曲延伸扩展问题.通过分析论证,比较精确地研究了疲劳载荷作用下弯曲延伸裂纹尖端塑性区域边界上交变应力的分布状况.综合考虑了疲劳作用应力,塑性区域交变应力,利用二阶摄动方法,研究计算了弯曲延伸裂纹尖端塑性区域的范围,并预测了疲劳载荷作用下弹塑性裂纹扩展路径.     

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。     

疲劳裂纹扩展时焊接残余应力分布的测定

本文采用切割法测定了沿焊缝横向残余应力的分布规律．由于切割试样可以近似地模拟疲劳裂纹扩展，通过测量释放的应力，计算出残余应力的分布，可发现残余应力对疲劳裂纹扩展速率的影响规律．     

裂纹闭合和循环塑性预应变及循环载荷压缩部分对裂纹扩展速率的影响

本文针对三种国产材料 Ｌｙ１１ｃｚ、 Ｌｙ１２ｃｚ 铝合金和 １８ Ｍｎ Ｈ Ｐ钢,通过实验初步考察了循环塑性预应变和循环载荷压缩部分对疲劳裂纹扩展的影响;采用电测法,测定了两种铝合金材料疲劳裂纹扩展的张开应力和有效应力强度因子幅值比 Ｕ。结果表明：（１）材料循环塑性预应变和循环载荷压缩部分,都使疲劳裂纹扩展速率提高;（２）常幅载荷下,在疲劳裂纹稳定扩展阶段,有效应力强度因子幅值比 Ｕ 与应力比 Ｒ 有关,与裂纹长度ａ 无关,并依赖于材料的力学性能。     

考虑材料循环塑性的疲劳裂纹扩展模拟

提出了一种考虑材料循环塑性性能的研究疲劳裂纹扩展与闭合行为的有限元模拟方法．对所选用的循环塑性本构关系进行了基本实验检验．探讨了在疲劳裂纹扩展有限元分析中网格尺寸的影响，给出了网格优化准则．研究了在循环硬化条件下考虑裂纹合效应时裂纹面张开廓形、裂纹尖端应力、应变场和正反向塑性区的演变规律．对于循环硬化和不同循环应力比Ｒ等因素对裂纹张开应力水平的影响也作了考察     

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．     

恒扭对汽车薄板Ⅰ型疲劳裂纹扩展的影响

本文针对SAPH440和DP780两种汽车薄钢板,对其在给定循环拉伸疲劳载荷、以及循环拉伸与不同恒定扭矩共同作用下的裂纹扩展行为进行了试验研究.试验结果表明恒定扭矩对汽车薄板Ⅰ型疲劳裂纹扩展有明显的迟滞作用.在一定范围内,恒定扭矩越大,迟滞作用越为明显;但超过使裂纹扩展最慢的临界扭矩值后,迟滞作用反而减弱.恒定扭矩使Ⅰ型疲劳裂纹扩展过程表现为裂纹扩展速率随扩展长度的增加而先保持低速后又加速,进而被划分为起始低速段和后续加速段;基于线弹性断裂力学理论,提出了一种等效应力强度因子变程及修正的Paris公式来描述该后续加速段的裂纹扩展速率.     

正交异性钢桥面板肋-面板焊缝疲劳分析

基于焊缝的局部三维断裂力学模型和超重多轴货车的载荷谱,进行正交异性钢桥面板的肋-面板焊缝表面裂纹的疲劳寿命分析。采用Schwartz-Neuman交替法计算肋-面板焊缝处半椭圆表面裂纹的应力强度因子,基于裂尖反向塑性区模型考虑循环载荷中压应力对疲劳裂纹扩展的作用。正交异性钢桥面板的肋-面板焊缝的应力计算结果表明:超载货车作用下肋-面板焊缝处的横向应力峰值和应力幅都有明显增加;相比于标准疲劳荷载车,超载货车作用下肋-面板焊缝处半椭圆表面裂纹的裂纹扩展率增大了6.1倍;对应于正交异性钢桥面板的肋-面板焊缝处的拉-压循环应力,平面应变状态下的裂尖反向塑性区使裂纹扩展率增加了3.7倍;基于所得裂纹扩展速率,本研究给出仅在严重超载的五轴和六轴货车作用下正交异性钢桥面板的肋-面板焊缝疲劳寿命不足20年,远远低于桥梁的设计寿命。因此,考虑超载多轴货车的载荷谱和循环载荷中的压应力对肋-面板焊缝疲劳裂纹扩展的影响十分重要。     

The effects of overloads on sustained-load crack growth in a nickel-base superalloy: part I—analysis

Analytical procedures are presented for predicting the retardation effects of cyclic overloads on the sustained load crack growth rate in Inconel 718 at 649°C. The Wheeler model is used in a crack growth computer program, CRACKS, by representing sustained load by equivalent fatigue cycles per unit time and an equivalent stress ratio, R. A new model, the exponential overload (EXPOL) model, is developed based on the concept of a crack growing at a retarded rate through an overload plastic zone. The analytical procedures use a minimum of empirical constants and are capable of accurately representing the time-dependent sustained load crack growth behavior with single or Multiple cyclic overloads.     

Reverse shear on inclined planes at the tip of a sharp crack

Plane strain plastic yielding at a crack tip has been represented by edge dislocations with Burgers vectors parallel to symmetrical planes inclined at 70° and 45° to the plane of the crack. The plastic displacement and the stresses near the crack tip were calculated by a numerical method and the effect of a reduction in applied stress was determined. Removal of the whole or a part of the initial load produces reverse shear in regions of the slip band nearest the crack tip. The amount of reverse shear depends only on the reduction in the load and not on its initial value. The reverse shear is associated with the presence of negative dislocations and the stresses near the crack tip may become compressive even though the applied (remote) stress is still tensile. The degree and extent of compression depends on the reduction in applied stress and on its original value. It is argued that the residual compressive stresses produced under fluctuating loads may produce crack closure and crack arrest. The effect of residual plasticity in a slip band left behind a growing crack has been estimated. It is shown that after an overload the excess residual plasticity opposing crack opening rises to a maximum value when the crack tip has advanced some distance from the point where the overload was applied.     

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.     

A reexamination of plasticity-induced crack closure in fatigue crack propagation

The crack closure concept is often used to consider the R-ratio and overload effects on fatigue crack growth. The presumption is that when the crack is closed, the external load produces negligible fatigue damage in the cracked component. The current investigation provides a reassessment of the frequently used concept with an emphasis on the plasticity-induced crack closure. A center cracked specimen made of 1070 steel was investigated. The specimen was subjected to plane-stress mode I loading. An elastic–plastic stress analysis was conducted for the cracked specimens using the finite element method. By applying the commonly used one-node-per-cycle debonding scheme for the crack closure simulations, it was shown that the predicted crack opening load did not stabilize when the extended crack was less than four times of the plastic zone size. The predicted opening load was strongly influenced by the plasticity model used. When the elastic–perfectly plastic (EPP) stress–strain relationship was used together with the kinematic hardening plasticity theory, the predicted crack opening load was found to be critically dependent on the element size of the finite element mesh model. For R = 0, the predicted crack opening load was greatly reduced when the finite element size became very fine. The kinematic hardening rule with the bilinear (BL) stress–strain relationship predicted crack closure with less dependence on the element size. When a recently developed cyclic plasticity model was used, the element size effect on the predicted crack opening level was insignificant. While crack closure may occur, it was demonstrated that cyclic plasticity persisted in the material near the crack tip. The cyclic plasticity was reduced but not negligible when the crack was closed. The traditional approaches may have overestimated the effect of crack closure in fatigue crack growth predictions.     

Intergranular strain evolution near fatigue crack tips in polycrystalline metals

The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.     

Numerical analysis of fatigue striations

A computational model was developed to numerically analyse fatigue striations. The inclined strip yield model with continuous distributions of infinitesimal dislocations was utilized to express the crack tip plasticity in this model. The fatigue crack tip blunting process was approximated by sequential activation of two slip lines under loading, and crack closure during unloading was taken into account. The plastic zone at a growing fatigue crack tip at the maximum load was independent of the crack growth up to ten cycles while the reversed plastic zone decreased in a size to one twentieth of that at the maximum load as the crack grew. The ratio of these plastic zone sizes and also the crack tip opening displacement were quite different from the simple prediction by J.R. Rice for a stationary crack. The computed striation spacings were compared with the observed ones and moderate agreement between them obtained.     

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.