Abstract Aim of the paper is to understand the effect of microstructural features of SA508 Grade 3 Class I low alloy steel (LAS) on short crack propagation rate under cyclic loading. The complex upper bainitic microstructure of this LAS consists of low angle bainitic ferrite lath boundaries and high angle prior austenite grain boundaries (PAGBs). Compared to bainitic ferrite lath boundaries, the PAGBs provided major hindrance to short fatigue crack propagation in the subject LAS. The high angle PAGBs strongly resist the dislocation motion ahead of the crack tip as the crack tip approaches the PAGBs compared to that of low angle bainitic ferrite lath boundaries. This restriction of dislocation motion ahead of the crack tip based on hindrance provided by PAGBs resulted in retardation in short fatigue crack propagation rate along the crack path. The short fatigue crack propagated at stress intensity factor (SIF) range ‘ Δ K ’ values lower than threshold SIF range ‘ Δ K t h ’ for the long cracks. The growth rate of short fatigue cracks cannot be predicted by Paris law which is applicable for long crack growth. This is due to the fact that crack growth rate undergoes acceleration and retardation in short crack regime because of microstructural effects.

中文翻译：

---

显微组织特征对SA508 3级I类低合金钢疲劳短裂纹扩展行为的影响

摘要 本文旨在了解SA508 3 级I 类低合金钢(LAS) 的显微组织特征对循环载荷下短裂纹扩展速率的影响。该 LAS 的复杂上贝氏体显微组织由小角度贝氏体铁素体板条边界和大角度原始奥氏体晶界 (PAGB) 组成。与贝氏体铁素体板条边界相比，PAGBs 对 LAS 中的短疲劳裂纹扩展提供了主要障碍。与小角度贝氏体铁素体板条边界相比，当裂纹尖端接近 PAGB 时，大角度 PAGB 强烈抵抗裂纹尖端前方的位错运动。这种基于 PAGB 提供的阻碍的裂纹尖端前位错运动的限制导致沿裂纹路径的短疲劳裂纹扩展速率的延迟。对于长裂纹，短疲劳裂纹在应力强度因子 (SIF) 范围 'ΔK' 值低于阈值 SIF 范围 'ΔK th ' 处传播。适用于长裂纹扩展的巴黎定律无法预测疲劳短裂纹的扩展速率。这是由于在短裂纹范围内，由于微观结构的影响，裂纹扩展速率会加速和减速。