Research on Fatigue Crack Arrest Assessment Criteria under Cyclic Thermal Stresses

今年，我们对这一主题进行了分析和实验。首先，我们研究了有限长厚壁(R_m/W=1)圆筒内环向裂纹在循环热冲击作用下的止裂深度。从均匀的温度分布中冷却圆柱体的内部。利用已有的系统评价方法，研究了换热条件对最大瞬时应力强度因子的影响。然后，结合巴黎定律，假定门槛应力强度范围ΔK<th>为临界值，并对循环热应力下的裂纹止裂长度进行了估算。结果表明，归一化裂纹扩展深度存在一个上限，与圆柱体材料无关。最后，通过等效热冲击载荷下的裂纹扩展试验，验证了疲劳裂纹止裂判据ΔK_(lt；max&gt；[小于或等于]ΔK_(Th)的有效性。结果表明，热冲击当量载荷对应的门槛应力强度范围近似等于本征ΔK<th>。验证了应用ΔK&lt；max&gt；[小于或等于]ΔK<th>进行循环热冲击止裂分析的有效性。

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.