Effect of Microstructures of steels on Fracture Toughness and Its Scatter

对于结构的可靠性或安全性评估，了解断裂特性的概率分布是非常重要的。本文基于钢解理断裂强度的判据是由最大拉应力给出的观点，对几种钢进行了实验，研究了解理断裂应力和断裂韧性的概率性质。此外，还研究了几种具有不同显微组织、晶粒尺寸和化学成分的钢的解理断裂应力/流动特性与断裂韧性之间的定量相关性。光滑和缺口试样的解理断裂应力服从威布尔分布，但缺口试样的断裂应力值高于光滑试样，且两种试样的威布尔参数不同。缺口或应力梯度对解理断裂的影响可以用基于最薄弱环节理论的统计有限元分析定量解释。这种统计方法也被用于预测解理断裂韧性的概率分布。假设当断裂应力超过一定的特征距离时，裂纹尖端会发生解理断裂。在此准则的基础上，对几种具有不同显微组织的钢的断裂韧性和材料的流动/断裂性能得到了良好的相关性。

For the assesment of reliability or safety of structures, it is very important to know the probabilistic distribution of fracture properties. In the present work, based on the view that the criterion for the cleavage fracture strength of steels is given by the maximum tensile stress, experiments were performed on several steels to study the probabilistic nature of the cleavage fracture stress and fracture tough-ness. Furthermore, quantitative correlation batween the cleavage fracture stress/flow properits andthe fracture touhgness has been investigated for several steels which have different microstructures, grain size and chemical composition.The cleavage fracture stresses in the smooth and notched specimens obey the weibull distribution, but the fracture stress in notched specimens takes higher values than the one in smooth specimens and the weibull parameters are different in both specimens. Such effect of notch or stress gradient on the cleavage fracture can be quantitatively explained by statistical FEM analysis based on Weakest Link Theory. This statistical approach has also been applied to predict the probabilistic distribution of the cleavage fracture toughness.It was assumed that the cleavage fracture at the crack tip would be initiated when the fracture stress is exceeded over a certain characteristic distance. Based on this Criterion good correlations have been obtained between the fracture toughness and flow/fracture properties of the materials for several steels which have various microstructures.