Research on Fatigue Crack Arrest Assessment Criteria under Cyclic Thermal Stresses

今年，我们对这一课题采取了分析和实验相结合的方法。首先，研究了有限长厚壁（R_m/W =1）圆柱内表面周向裂纹在循环热冲击作用下的止裂深度。气缸内部由均匀的温度分布冷却。采用系统的评价方法研究了传热条件对该问题最大瞬态SIF的影响。然后，结合Paris定律，假设阈值应力强度范围ΔK_<th>，并对循环热应力作用下的裂纹止裂长度进行了评价。结果表明，与圆柱体材料无关，归一化裂纹扩展深度存在上限。并将其主要参数归纳为结构参数。最后进行了等效热冲击载荷下的裂纹扩展试验，验证了ΔK_<max>【小于或等于】ΔK_<th>疲劳止裂准则的有效性。得到热冲击等效载荷所对应的阈值应力强度范围近似等于本征值ΔK_<th>。验证了应用ΔK_<max>【小于或等于】ΔK_< The >进行循环热冲击下裂纹止裂分析的有效性。

This year, we made both analytical and experimental approach to the subject. First, we made a study of the crack arrest depth under cyclic thermal shock for an inner-surface circumferential crack in a finite-length thick-walled (R_m/W =1) cylinder with rotation-restrained edges. The inside of the cylinder is cooled from a uniform temperature distribution. The effects of heat transfer conditions on the maximum transient SIF for the problem were investigated with formerly developed systematical evaluation methods. Then, a tentative value of the threshold stress intensity range ΔK_<th> was assumed together with the Paris law and the evaluation of the crack arrest length under cyclic thermal stress was carried out. The results suggest the existence of an upper limit for the normalized crack growth depth, independent of the cylinder material. And the dominating parameters for this were concluded as the structural ones.Finally, crack propagation tests under equivalent thermal shock loads were conducted to confirm the validity of the criterion ΔK_<max> 【less than or equal】 ΔK_<th> for fatigue crack arrest. The result obtained was that the threshold stress intensity range corresponding to a thermal shock equivalent load was approximately equal to the intrinsic ΔK_<th>. The validity of applying ΔK_<max> 【less than or equal】 ΔK_<th> for crack arrest analysis under cyclic thermal shock was confirmed.