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Study on theoretical assessment method of K_<th> and assessment method of fatigue life for a short crack

K_<th>理论评估方法及短裂纹疲劳寿命评估方法研究

基本信息

批准号：
02452183
负责人：
TOYOSADA Masahiro
金额：
$ 2.05万
依托单位：
KYUSHU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1990
资助国家：
日本
起止时间：
1990 至 1992
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-02452183/
关键词：
Fatigue Crack Propagation Crack Opening and Closing RPG Load DELTAK_<RP>Short Crack Fatigue Life Assessment by Unified Theorem 寿命評価表面き裂 △K_<RP>再引張降伏発生荷重下限界応力拡大係数範囲疲労き裂伝播速度 Reーtensile Plastic zone's Generating Load Crack Open

项目摘要

From the consideration about the small change of compliance for a cracked body under cyclic loadings, it can be postulated that fatigue crack dose not grow if no damage occurs in the vicinity of a crack tip.Then we propose an effective stress intensity factor range (DELTAK_<RP>) corresponding to the period in which re-tensile plastic zone's generates at a crack tip for a fatigue crack propagation parameter. Moreover an automatic controlled system was developed to measure Re-tensile Plastic zone's Generated load (RPG load).Then the various types of fatigue crack propagation test were conducted by using above developed system. For a long crack, it becomes clear that DELTAK_<RP> gives the quantitative effect of stress ratio on fatigue crack propagation rate (da/dN) and the logarithmic curve of DELTAK_<RP> - da/dN appears to be linear in the wide range from very slow crack growth rate region to stable crack growth region, which means that DELTAK_<th> is not a material constant. It is also confirmed that the logarithmic curve of DELTAK_<RP> - da/dN for a physically small crack is in good agreement with that for a long crack, considering that fatigue cracks initiate and propagate as suface cracks.For a microstructual short crack, it is postulated that dislocations do not excess barrieres such as a grain boundary and a pearlite zone. Then equevalent DELTAK is defined from a plastic zone size and equivalent DELTAK_<RP> is obtained from measured RPG load. Therefore crack propagation curves are calculated for a crack emanated from a notch root without distingurishing crack initiation and propagation life and are well in agreement with exprimental ones. This result seems to mean the establishment of fatigue life assessment method by unified theorem.Moreover the simulated program by a fatigue propagation model in an arbitrary stress distribution field based upon the Dugdale model is also developed.

从裂纹体在循环载荷作用下柔度变化很小的观点出发，假定裂纹尖端附近无损伤时，疲劳裂纹不会扩展，提出了一个有效应力强度因子范围（DELTAK_<RP>），它对应于裂纹尖端再拉伸塑性区的产生周期。此外，还开发了一套用于测量再拉伸塑性区产生载荷（RPG载荷）的自动控制系统，并利用该系统进行了各种类型的疲劳裂纹扩展试验。对于长裂纹，DELTAK_<RP>给出了应力比对疲劳裂纹扩展速率（da/dN）的定量影响，并且DELTAK_<RP>- da/dN的对数曲线在从非常慢的裂纹扩展速率区到稳定裂纹扩展区的宽范围内呈线性，这意味着DELTAK_<th>不是材料常数。同时也证实<RP>了小裂纹的DELTAK_ - da/dN对数曲线与长裂纹的DELTAK_ - da/dN对数曲线是一致的，认为疲劳裂纹是以表面裂纹的形式萌生和扩展的;对于微结构短裂纹，位错不超过晶界和珠光体区等势垒。然后根据塑性区尺寸定义等效DELTAK，并<RP>根据测量的RPG载荷获得等效DELTAK_。因此，在不区分裂纹萌生和扩展寿命的情况下，计算了缺口根部裂纹的扩展曲线，并与实验结果相吻合。这一结果似乎意味着疲劳寿命评估方法的统一定理的建立，并在Dugdale模型的基础上，开发了任意应力分布场中疲劳扩展模型的模拟程序。