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Detailed Observation of Change in the Strain near Fatigue Crack Tip

疲劳裂纹尖端附近应变变化的详细观察

基本信息

批准号：
08455469
负责人：
YOSHINARI Hitoshi
金额：
$ 4.74万
依托单位：
University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1996
资助国家：
日本
起止时间：
1996 至 1997
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-08455469/
关键词：
Fatigue Crack Propagation Mechanism Fatigue Crack Opening Closure Phenimenon Compliance Hysteresis RPG (Re-tensile Plastic zone's Generated) point Change Rate of Compliance Fatigue Crack Propagation Parameter コンプライアンス変化率 RPG(Re-tensile P

项目摘要

In this study, for the purpose of proper understanding of the fatigue crack propagation mechanism, detaild observations of change in strain near the fatigue crack tip were carrind out. And successful measurement of the hysteresis curve of load (P) vs. strain (epsilon') with high accuracy was made.First, from introducing the curve of compliance (depsilon'/dp) and the change rate compliance (d^2epsilon'/dp^2) during one cycle, the following observations have been obtained concernig the fatigue crack opening/closure phenomenon.1) Unloading part of the hysteresis curve is found to be "S" shape, and the point of inflection of the unloading part is considered to be the crak closure point. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of compliance, the crack closure point can be measured automatically as the maximum point of the unloading part of this curve.2) Loading part of the curve of compliance is found to be "S" shape, and the point of inflection … More of the loading part is considered to be RPG (Re-tensile Plastic zone' Generated) point. So if the accuracy of measurement is high enough to be able to obtain a amooth curve of the change rate of compliance, the RPG point can be measured automatically as the minimum point of the loading part of this curve.Secondly, from dividing the loading and unloading processes into several characteristic areas on the basis of the tendency which the compliance and the change rate of compliance show, the following observations have been obtained concerning the fatigue crack propagation parameters.3) In unloading process, it seems to be impossible to isolate the area corresponding to the creation of the re-compressive plastic zone and there is also doubt about using the stress intensity factor because of the influence of the crack closing. So it seems to be rational to adopt the area in loading process, corresponding to the creation of the re-tensile plastic zone, for the fatigue crack propagation parameter.4) According to the experimental results, it is found to be clear that the re-tensile plastic zone's generated point disagrees with the crack opening point.5) From the facts described above 3) & 4), it may be concluded that DELTAK_<RP>, , not DELTAK_<OP> nor DELTAK_<C1>, is considered the most suitable parameter which controls the fatigue crack propagation. Less

为了更好地理解疲劳裂纹扩展机制，本研究对疲劳裂纹尖端附近的应变变化进行了详细的观察。并成功测量了载荷(P)与应变（epsilon’）的滞回曲线，测量精度较高。首先，通过引入一个周期内的柔度曲线（depsilon'/dp）和变化率柔度曲线（d^2epsilon'/dp^2），得到了关于疲劳裂纹开/闭现象的以下观察结果。1)发现迟滞曲线卸荷部分呈“S”形，将卸荷部分的拐点视为裂纹闭合点。因此，如果测量精度足够高，能够获得光滑的柔度曲线，则可以自动测量裂纹闭合点作为该曲线卸载部分的最大值点。2)发现柔度曲线的加载部分为“S”形，且弯曲点为…，更多的加载部分为RPG（再拉伸塑性区生成）点。因此，如果测量精度足够高，能够获得柔度变化率的光滑曲线，则可以自动测量RPG点作为该曲线加载部分的最小点。其次，根据柔度和柔度变化率的变化趋势，将加载和卸载过程划分为几个特征区域，得到了疲劳裂纹扩展参数的如下观察结果。3)在卸荷过程中，似乎不可能隔离出与再压缩塑性区形成相对应的区域，并且由于裂纹闭合的影响，使用应力强度因子也存在疑问。因此，采用加载过程中对应于再拉伸塑性区形成的面积作为疲劳裂纹扩展参数似乎是合理的。4)实验结果表明，再拉伸塑性区产生点与裂纹张开点不一致。5)由以上3)和4)可知，控制疲劳裂纹扩展最合适的参数是DELTAK_<RP>，而不是DELTAK_<OP>和DELTAK_<C1>。少