A STATISTICAL INVESTIGATION OF FATIGUE CRACK INITIATION AND GROWTH PROCESS BASED UPON A LARGE SAMPLE SIZE EXPERIMENT

After finishing 102 replicate constant amplitude crack initiation and growth tests onLyl2-CZ aluminum alloy plate,a statistical investigation of the fatigue crack initiation and growthprocess is conducted in this paper.According to the post-mortem fractographic examination by scan-ning electron microscopy (SEM),some qualitative observations of the spacial correlation among fa-tigue striations are developed to reveal the statistical nature of material intrinsic inhomogeneity duringthe crack growth process.From the test data,an engineering division between crack initiation andgrowth is defined as the upper limit of small crack.The distributions of crack initiation life N.,growthlife N,and the statistical characteristics of crack growth rate da/dN are also investigated.It is hopedthat the work will provide a solid test basis for the study of probabilistic fatigue,probabilistic fracturemechanics,fatigue reliability and its engineering applications.     

基于多因素的焊接结构疲劳裂纹扩展速率分析

以Donahue等提出的疲劳裂纹扩展速率计算模型为基础,通过引入形状系数、张开比和残余应力等参数,建立了适用于焊接结构的疲劳裂纹扩展速率计算模型,分析了多种因素对焊接结构疲劳裂纹扩展速率的影响规律。结果表明,焊板厚度和焊缝余高的变化均会对焊接结构疲劳裂纹的扩展速率产生影响,在对焊接结构表面形状进行设计时应保有一定的焊缝余高;有效应力比的增大会降低焊接结构疲劳裂纹的扩展速率,且裂纹深度的变化不会改变有效应力比对焊接结构疲劳裂纹扩展速率的影响;残余应力的增大会提高焊接结构疲劳裂纹的扩展速率,且残余应力对疲劳裂纹扩展速率的促进作用随着裂纹深度的增加而增大,在对焊接结构的疲劳性能进行设计时须考虑残余应力对结构性能的影响。     

Experimental study and BEM analysis of fatigue crack growth in a cracked heterogeneous body

A model for analysing a soft-hard heterogeneous body with a crack in the hard region is presented in this paper. The result of fatigue experiments shows that mechanical heterogeneity affects the rate of propagation of fatigue crack. Meanwhile the results computed by BEM for cracked heterogeneous bodies under cycling loading indicate that the smaller the distance between the crack and the interface of hard and soft regions is, the smaller the amplitude of crack opening displacement, COD and ofJ-integral as well at the same step during the fatigue crack growth will be. The effect of heterogeneity on the rate of fatigue crack propagation is shown by the variation of J. The smaller the distance of the crack to the interface is, the smaller the rate of fatigue crack growth will be.     

The effect of intermittent heating on fatigue crack growth

The effects on fatigue crack growth of the application of heat treatment with and without an applied load are discussed. The investigation indicates that when heat treatment is performed simultaneously with an applied load, compressive residual stress will be introduced resulting in better resistance to fatigue crack initiation and growth.Fatigue crack growth tests were performed on single-edge-cracked plates. The fatigue life and crack growth rate for plates with no heat treatment and plates with heat treatment under zero stress are similar. However, for plates with heat treatment carried out under a static load equal to the maximum in the fatigue cycle, the fatigue life increased by 40% and the crack growth rate immediately after heat treatment decreased by 65%.     

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。     

狭长缺口根部萌生的小裂纹扩展分析

根据伴随着小裂纹延展过程中不同力学参数相互作用的分析研究基础之上,我们就疲劳小裂纹的扩展特性进行了分析,根据不扩展裂纹性质对小裂纹的长度范围进行了讨论,探讨了狭长缺口根部萌生的小裂纹扩展速率, 并研究了不扩展裂纹形成机理,推演出不扩展裂纹长度计算方法,发展出一种预测小裂纹扩展行为的方法,借助这种方法可以实现裂纹萌生寿命的预测,与疲劳设计相结合可以进一步提高构件安全性与可靠性．不扩展小裂纹的长度可以通过计算定量得到,它与材料的疲劳极限、裂纹门槛值及缺口尺寸等参数紧密相关．以理论数据为依据,可以设计出针对一定材料及尺寸的试样,使其在一定荷载条件下不扩展裂纹长度达到宏观可观测的范围,进而降低小裂纹测量方面的技术难度,进一步验证不扩张裂纹形成机理．     

Fatigue damage in polycrystals – Part 2: Intrinsic scatter of fatigue life

Part 2 deals with the evolution of plastic flow resistance with crack growth from its minimum value (fatigue limit) towards its saturated bulk value (cyclic yield stress). The far-field stress level, the geometry of the crack and the grain size distribution of the material are those parameters that control the area of crack tip plasticity and hence the rate towards saturation. The implication of the far-field stress is held responsible for the violation of the similitude concept and the failure of the stress intensity factor to describe conditions of short cracking. However, an engineering tool based on the stress intensity factor and being able to predict the fatigue life of short cracks can be constructed, considering that the distribution of crack growth rates is intrinsically defined by the material itself. The above allows the development of a set of equations able to construct the fatigue life scatter of the material.     

超声疲劳试验方法及其应用

超声疲劳是一种加速的疲劳试验方法,它的测试频率(20kHz)远远超过了常规疲劳测试频率(小于200Hz)．超声疲劳试验研究表明50^#车轴钢和40Cr钢直到10^10个应力循环后仍会发生疲劳断裂,并不存在常规疲劳试验曲线所示的“疲劳极限”,因此用10^7周次的疲劳试验数据进行疲劳强度设计并不安全．50^#车轴钢和40Cr钢超声疲劳性能优于常规疲劳性能．扫描电镜分析表明,超长寿命阶段50^#车轴钢裂纹萌生于次表面夹杂．介绍了超声疲劳试验系统、工作原理及超声疲劳试样的设计．     

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在空气和腐蚀介质中对低合金钢分别的进行疲劳试验,获得了S-N曲线,并对疲劳试样表面和断口形貌进行了观察。结果表明：与空气相比,腐蚀介质使低合金钢的疲劳强度显著降低;空气中疲劳试样只有一个萌生于试样表面基体的裂纹源,而腐蚀介质中的疲劳试样一般有多个裂纹源,多数裂纹源均萌生于点蚀坑;空气中疲劳裂纹扩展区以疲劳辉纹为主,而腐蚀疲劳则以沿晶开裂等脆性特征为主;此外还对空气中的疲劳极限进行了预测,预测值与试验值比较吻合     

Fatigue crack growth model for assessing reliability of box-girders for cable-stayed bridge combining SHMS with strain data

Enhancement of the computer algorithm developed for the Runyang cable-stayed bridge (RYCB) in China is made by incorporating the fatigue crack growth analysis in contrast to the SN curve approach. Strain data obtained from the structural health monitoring system (SHMS) and finite element calculations are used. This provides the application of a deterministic method in addition to the probabilistic approach with the added feature of crack growth. The choice of selecting the two-parameter fatigue crack growth criterion was based on the definition of reliability index β such that the new results can be compared with those using the SN curves. A gradual drop of the reliability index β with time with an upper limit was obtained for the crack growth model in contrast to the linear time relation for the SN curve model that had no upper limit. This difference is significant and reveals the importance for selecting the fatigue failure criterion. Deterministic and probabilistic crack growth models are used to assess the differences. The results are based on the box-girder component strain history data of the Runyang Cable-stayed Bridge (RYCB) in China, stress history recorded by structural health monitoring system (SHMS) is analyzed using the monitored stress amplitude, mean stress and stress ratio. Finite element calculations are used to supplement data at locations not accessible for measurements. Additional improvement with reference to damage accumulation and the physical meaning of the reliability index will be studies in relation to the fatigue damage of box-girder of long cable-stayed bridges.     

可靠性设计中的疲劳裂纹扩展随机模型

首先导出了疲劳裂纹扩展的确定性方程，然后将疲劳裂纹扩展确定性方程随机化，导出了疲劳裂纹随机扩展的概率公式，并推导了疲劳裂纹随机扩展的置信限公式，最后，给出了二种材料疲劳裂纹随机扩展数据的处理结果。     

Analysis of short crack growth from microscopic fatigue properties

Successful simulation of kinetics of small fatigue crack growth entails three aspects: Stage I, Stage II growth rate prediction and transition prediction. In this paper attention is focused on growth rate predictions. By using microstructurally-affected-zone and process zone concepts, microscopic fatigue behaviour of small fatigue crack propagation is logically linked with macroscopic fatigue behaviour, showing an intrinsic relation between small fatigue crack growth and macroscopic low-cycle fatigue properties during crack growth. Furthermore, variation of relatively big plastic zone size associated with a growing small fatigue crack is kinetically simulated. As a result a quantitative prediction model of growth rates for Stage I and Stage II growth has been developed whose explicit advantage is that the growth rate of small fatigue crack can now be predicted in terms of bulk fatigue properties in conjunction with local microstructural characteristics.     

Fatigue crack growth retardation assessment of 2024-T3 and 6061-T6 aluminium specimens

A fatigue crack growth retardation model is developed. It considers a strip plastic zone with material hardening effect which is taken as one of the basic mechanisms controlling fatigue crack growth. Crack growth is treated incrementally and corresponds to the failure of material elements ahead of an existing crack after a certain critical number of low cycle fatigue. Computed curves are correlated to test data obtained from the 2024-T3 and 6061-T6 aluminium specimens. Deviations from test data increase with increasing crack length.     

Evolution of fatigue damage using the fatigue damage map method

Using only readily available material properties and the concept of the fatigue damage map the work attends to develop a methodology able to predict crack tip propagation rates characterising each of the fatigue stages, namely crack arrest, microstructurally and physically short crack (Stage I), long crack growth (Stage II) and Stage III growth. Crack tip propagation rates are predicted through the combination of an Elasto-Plastic crack tip opening displacement model and the crack blunting theory. The methodology provides a theoretical framework for a 3D version of the fatigue damage map for damage tolerance design. In addition, the work attends to explain a number of fundamental problems starting from crack tip propagation discrepancies, found in the short cracking stage, as well as to provide an insight towards the effect of grain size on the fatigue limit/threshold stress intensity factor.     

Dislocation pile-up model of fatigue thresholds for 2024- and 7075-alike aluminium alloys

Two parameters, K_max^* and δK_th^*, are presented to describe fatigue threshold behaviour and damage under any load ratio without invoking crack closure. Modelled are two fatigue thresholds that are coherently related to fatigue limit δσ_FL; they predict the fundamental threshold curves for aluminium alloys. By using a continuous configuration of dislocations in pile-up, fatigue limit behaviour is simulated as pile-up of dislocations against grain boundaries. A fatigue limit is determined in terms of a critical condition at which a fictitious microcrack associated with the pile-up corresponds to the onset of propagation. These two fatigue thresholds are attainable as the local stresses at the crack front approaching the fatigue limit. Microstructure is incorporated in the model to account for the effect on threshold behaviour. As a result, two fatigue threshold criteria are required. Quantitative assessment of the two criteria requires only knowledge of the conventional material properties in conjunction with microstructure. The micromechanical modelling exhibits a strong dependence of fatigue thresholds upon local microstructure.     

软夹硬力学不均匀焊接接头疲劳裂纹闭合行为的实验研究

力学性能不均匀是焊接接头的三大待征之一，本文采用柔度法研究了不同硬夹层宽度的软夹硬力学不均匀焊接接头疲劳裂纹的闭俣行为。研究结果表明，裂纹尖端附近软区的局部屈服在疲劳载荷卸载过程中促使裂纹闭合。随着硬夹层宽度的减小，这一影响越来越明显。     

抽油杆疲劳裂纹形状比变化规律

采用断裂力学方法研究了抽油杆杆体疲劳裂纹形状比在扩展过程中的变化规律，证明无论初始形状如何，抽油杆杆体疲劳裂纹形状比在扩展过程中趋于同一个稳定数值．通过裂纹扩展实验对直径为15．9mm的抽油杆疲劳裂纹形状比进行了实验测定，发现其裂纹形状比迅速靠近0．78．     

A fatigue life prediction methodology for cylindrical shell with an inclined crack under torsion

Considered is the fatigue life prediction of a cylindrical shell with an inclined crack under torsion. The effects of the initial angles of inclination and shell curvatures on fatigue crack growth rate are investigated. The mixed mode fracture parameters are extended to describe the fatigue crack growth behavior. A methodology for fatigue life prediction is suggested.     

TC4钛合金超高周次轴向振动疲劳试验

应用基于压电超声疲劳试验技术开发的20kHz轴向振动疲劳试验系统,完成了室温下TC4钛合金超高周疲劳试验,获得了TC4合金在107～109周次范围内的轴向振动疲劳寿命曲线(S-N曲线);运用C.Paris推导公式预测了TC4合金材料的寿命,得到各应力水平下破坏率为50％、95％、99％的安全寿命.结果表明:在疲劳循环大于107周次时,试件仍会发生疲劳断裂,疲劳强度随循环次数的增加而下降,并不存在明显的疲劳极限.TC4合金的S-N曲线在107～109周次的范围内呈连续下降型.在轴向振动超高周疲劳试验中,试件的裂纹扩展寿命只占其在50％破坏率下疲劳安全寿命的一小部分,其疲劳寿命主要由试件的裂纹萌生寿命决定.     

巴西圆盘试样中心斜裂纹疲劳扩展轨迹的边界元模拟

提出了一个平面弹性体多裂纹疲劳扩展模型. 它主要涉及到复合型加载情况下多裂纹尖端疲劳扩展的数学模型及杂交位移不连续法(一种边界元法). 在数值模拟中, 对每一裂纹扩展增量分析时,在其先前的边界上增添裂纹扩展增量, 且只对新增添的裂纹扩展增量划分单元, 同时, 按照这种边界元法的实施方法对一些单元特征进行调整, 就可以方便地模拟裂纹扩展. 用这种数值方法模拟了巴西圆盘试样中心斜裂纹疲劳扩展轨迹,数值结果说明了预报模型的有效性, 揭示了裂纹体几何对疲劳扩展的影响.