In this paper, the crack propagation characteristics in AISI 4340 steel with different strength and toughness have been investigated in quasi-situ via 3D X-ray tomography during fracture toughness testing. Three kinds of crack propagation characteristics were observed by the 3D image analysis, which were determined by the competition between cleavage fracture and voids growth/coalescence. It is found that the main crack would connect the micro-voids near the crack tip by cleavage fracture if the local stress achieves the cleavage fracture strength, resulting in rapid crack propagation. However, the growth and coalescence of micro-voids ahead of the crack tip would be linked with the main crack by strain-controlled, leading to the ductile fracture. The above two mechanisms would exist simultaneously in materials, which might be explained by the competition balance between strain and stress. Studying on transition of crack propagation mechanisms in materials would further understand the essence of fracture toughness. It will provide new insight into the fracture behavior of engineering materials and some new materials such as nanostructured materials and metallic glasses. Additionally, based on the energy principle, we propose a quantitative relationship between the microscope fracture surface morphology and fracture toughness of materials.

在本文中，通过3D X射线断层摄影术在断裂韧性测试过程中，准时研究了具有不同强度和韧性的AISI 4340钢的裂纹扩展特性。通过3D图像分析观察到三种裂纹扩展特征，其由解理断裂与空隙生长/聚结之间的竞争来确定。研究发现，如果局部应力达到劈裂断裂强度，主裂纹将通过劈裂断裂连接裂纹尖端附近的微孔，从而导致裂纹的快速扩展。然而，裂纹尖端前方的微孔的生长和聚结将通过应变控制与主裂纹相关联，从而导致韧性断裂。材料中会同时存在以上两种机制，这可以用应变和压力之间的竞争平衡来解释。研究材料中裂纹扩展机理的转变将进一步理解断裂韧性的本质。它将为工程材料和一些新材料（例如纳米结构材料和金属玻璃）的断裂行为提供新的见解。此外，基于能量原理，我们提出了显微镜断裂表面形态与材料断裂韧性之间的定量关系。它将为工程材料和一些新材料（例如纳米结构材料和金属玻璃）的断裂行为提供新的见解。此外，基于能量原理，我们提出了显微镜断裂表面形态与材料断裂韧性之间的定量关系。它将为工程材料和一些新材料（例如纳米结构材料和金属玻璃）的断裂行为提供新的见解。此外，基于能量原理，我们提出了显微镜断裂表面形态与材料断裂韧性之间的定量关系。