中文摘要：

通过对淬火和退火两种不同热处理状态45钢材料进行Ⅰ-Ⅱ复合型断裂试验，宏观上观察其断裂方向和断裂路径的发展变化规律，细观上电镜分析同一材料不同复合比和不同材料同一复合比时断裂机理的变化特征，发现加载方式从Ⅰ型到Ⅱ型变化过程中，淬火45钢的断裂机理为准解理脆断，其断裂危险点与断裂方向主要受复杂组织结构影响；退火45钢断裂形式从准解理脆断主导逐渐改变为孔洞正拉断主导，危险点位于裂端应力三维度最大值处，断裂方向与其上最大拉应力作用面一致。并得到结论，材料发生的准解理断裂和孔洞正拉断均为脆性断裂，材料的细观组织结构和应力三维度Rσ大小是影响断裂机理变化的主要因素。当材质均匀性较差、晶粒粗大时，细观缺陷上应力集中程度高，脆断的初始断裂方向主要受局部组织结构影响，难以用宏观应力场说明；但在连续介质假设适用的一般情况下，脆断危险点在应力三维度最大值处，断裂面方向与最大拉应力作用面一致，三轴拉应力是促使材料发生脆断的主要因素。

英文摘要：

I- II mixed mode fracture experiments of 45 steel in two different heat treatment condition （quenching and annealing） were made, through examining variety laws of fracture direction and fracture path under the macroscopic angle, analyzing change features of different fracture mechanisms in the microscopic aspect, it was found that with loading regime changing from I to II , for quenching 45 steel, the fracture mechanism belong to quasi-cleavage fracture, the dangerous point and fracture direction were influenced mainly by its complex microstructure; for annealing 45 steel, fracture mode changes from quasi-cleavage fracture to traction fracture with voids, the dangerous point was located at the maximum of stress triaxiality in crack tip, the fracture direction was the same to the direction of major principal stress. The result shows that, both quasi-cleavage fracture and traction fracture with voids belong to brittle fracture, microstructure and stress triaxiality are main factors which affected the fracture mechanism of materials. When crystals of material are massive or value of stress triaxiality large, then there is little plastic deformation in fracture procedures, and measure of stress concentration is high in microscopic defective, so the dangerous points and fracture directions are mainly affected by complex microstructures, and it is difficult to indicate using field of stress. However, in the condition of continuous medium assumption applicable, the dangerous point of brittle fracture is located at the maximum stress triaxiality, fracture direction is consistent with the maximum principal stress, triaxial tensile stress is the main factor of brittle fracture.