中文摘要：

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与应力比尺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。

英文摘要：

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.It is found that Lorentz distribution is in reasonably good agreement with the employed experimental data.