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

从裂纹体在循环载荷作用下柔度的微小变化出发，假设在裂纹尖端附近未发生损伤时，疲劳裂纹不会扩展，进而提出了疲劳裂纹扩展参数的有效应力强度因子范围(DELTAK；lt；Rp&gt；)与裂纹尖端产生再拉伸塑性区的周期相对应。此外，还研制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；lt；Rp&gt；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&lt；Rp&gt；-da/dN的对数曲线在从很慢的裂纹扩展速率区到稳定的裂纹扩展区的较宽范围内呈线性，这意味着DELTAK<th>不是材料常数。考虑到疲劳裂纹是以表面裂纹的形式萌生和扩展的，物理小裂纹的DELTAK&lt；Rp&gt；-da/dN与长裂纹的对数曲线符合得很好；对于微观组织的短裂纹，假设位错不会产生晶界和珠光体区等位错。然后，根据塑性区大小定义等值DELTAK，并由测量的RPG载荷得到当量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，与实验结果吻合较好。这一结果似乎意味着建立了统一理论的疲劳寿命评估方法。此外，还开发了基于Dugdale模型的任意应力分布场中疲劳扩展模型的模拟程序。

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.