有效应力强度因子范围门槛值与载荷比关系的研究

不同载荷比下的有效应力强度因子范围门槛值△Keff.th往往被看作一个常数,然而疲劳实验数据结果通常散落在一个较窄的带宽范围内,而不是所期望的一条曲线。另外,基于改进的McEvily模型对△Keff.th进行的灵敏度分析表明△Keff.th对疲劳裂纹扩展率具有重要的影响,尤其是在占有大部分疲劳裂纹扩展寿命的近门槛值区域。因此,文章认为不同应力比下的△Keff.th为变量,并且通过三个方面对△Keff.th与应力比R的关系进行了深入的研究:(a)Schmidt和Paris提出的一个关于△Keff.th的简化模型和相应的试验数据;(b)基于传统的裂纹完全闭合概念的△Keff.th试验数据;(c)基于裂纹局部闭合模型的△Keff.th,p试验数据。分析结果表明,在应力比低于临界应力比时,随着应力比的增加,有效应力强度因子范围门槛值也相应增加;而应力比高于临界应力比时,有效应力强度因子范围门槛值随应力比的增加而减小。另外,通过对试验数据的曲线拟合分析表明,Lorentz分布能很好地描述相应的试验数据。

A Study on the Relation Between Threshold Effective Stress Intensity Factor Range and Load Ratio

The threshold effective stress intensity factor range,△Keff,th is normally viewed as a constant under different load ratios.However,the fatigue crack growth data always collapse into a relatively narrow band rather than a single curve as expected.On the other hand,sensitivity analyses for △Keff,th based on the extended McEvily model show that △Keff,th has significant effect on the fatigue crack growth rate especially near the threshold region where most of the fatigue life is consumed.Therefore,△Keff,th is regarded as a variable for different load ratios in this paper and the relation between △Keff,th and load ratio,R is further studied mainly based on the following three aspects:(a) the simple model of △Keff,th proposed by Schmidt and Paris and the corresponding experimental data; (b)the direct experimental data of △Keff,th with the conventional full crack closure concept; (c) the experimental results of modified △Keff,th with the partial crack closure model.Results show that △Keff,th will firstly increase with increasing load ratio below the critical load ratio,Rc and then decease above Rc.Besides,the function of △Keff,th against load ratio,R is further studied through the curve fitting method according to the experimental data.h is found that Lorentz distribution is in reasonably good agreement with the employed experimental data.