一、前言许多机械工程材料构件都在疲劳载荷条件下服役。由于铸造、锻造、焊接、热处理以及加工等原因,这些材料的表面或内部不可避免地存在着缺陷或裂纹。大量的疲劳裂纹扩展试验表明,当应力强度低于某一数值时,裂纹始终不扩展或扩展极其缓慢。人们把这个数值定义为界限应力强度系数幅度ΔK_(th9)即通常所说的门槛值。ΔK_(th)与工作应力幅Δ_σ、初始缺陷和裂纹形状因子f(y)之间建立了如下的关系: I. INTRODUCTION Many mechanical engineering materials are used under fatigue loading conditions. Due to casting, forging, welding, heat treatment and processing, the surface or the interior of these materials inevitably have defects or cracks. A large number of fatigue crack growth tests show that when the stress intensity is lower than a certain value, the crack does not expand or expand extremely slowly at all times. This value is defined as the magnitude of the limit stress intensity coefficient ΔK_ (th9) which is commonly referred to as the threshold value. The relationship between ΔK_ (th) and working stress amplitude Δ_σ, initial defect and crack shape factor f (y) is as follows: