Evaluation and modeling of fracture toughness based on the mezzo-scopic elastic-plastic fracture mechanics

基于统计局部断裂准则方法，对钢的解理断裂韧度进行了细观分析。在裂纹尖端区域的统计威布尔应力准则表明，钢的解理断裂韧性可以描述为屈服应力的函数，和材料的其他机械性能。根据理论预测，通过对缺口圆棒试样的解理断裂韧度与解理断裂应力之间的相关性的研究，首次对解理断裂韧度的公式进行了检验。通过该公式分析了韧性的分散性、应变速率和试样厚度对韧性的影响、冷加工和辐照对韧性的影响以及Ni的加入对韧性的改善，并基于统计局部断裂准则方法讨论了韧性评定中的试样尺寸要求。将解理断裂的Weibull应力准则应用于用韧性比例模型（TSM）描述的裂纹尖端约束损失场。提出的新模型表明，TSM是一个特殊的情况下，材料的威布尔形状参数，m和应变硬化指数，n的特定组合。分析预测表明，试样尺寸对韧性的要求与材料的n、m值有很大关系。进行了二维和三维有限元分析，以证实上述分析预测。

Cleavage fracture toughness of steels was mezzo-scopically analyzed on the basis of the statistical local fracture criterion approach. The statistical Weibull stress criterion at the crack tip region suggests that the cleavage fracture toughness of steels can be described as a function of the yield stress, and other mechanical properties of the materials. Formulation of the cleavage fracture toughness was first examined through an investigation on correlation between the cleavage fracture toughness and the cleavage fracture stress obtained in notched round bar specimens in accordance with the theoretical prediction. Then, scatter of the toughness, strain rate and specimen thickness effect on the toughness, deterioration of the toughness due to cold working and irradiation, and improvement of the toughness caused by the Ni addition, were analyzed through the formulation above.The specimen size requirements in toughness qualification are also discussed on the basis of statistical local fracture criterion approach. The Weibull stress criterion for cleavage fracture is applied to the constraint loss field at the crack tip described with the toughness scaling model (TSM). The new model proposed demon strates that the TSM is a particular case for a material with a particular combination of the Weibull shape parameter, m and the strain hardening exponent, n. Analytical prediction indicates that the specimen size re quirements in toughness must be significantly depending on the value of n and m of the material. The 2-and 3- dimensional FE analysis were performed to confirm above analytical prediction.