关键词: 球芯；疲劳断裂；裂纹；疏松；应力集中

Abstract: A 40Cr steel ball core fails twice in a row during the fatigue test of an automotive thrust rod. To determine the cause of the failure, this article conducts a detailed analysis using methods such as macroscopic observation, microscopic observation, metallographic structure analysis, chemical composition analysis, and hardness testing. The results show that the fracture failure modes are all fatigue fracture, and the chemical composition of the ball core and Rockwell hardness meet the design requirements. The micro-cracks existing inside the surface decarburization layer and the loose surface structure and other surface defects cause the stress concentration coefficient at the structural mutation location to increase, forming stress concentration points. Under the continuous cyclic loading force, the cracks start to initiate from the defect and continuously expand inward until fracture. This research has certain reference value for the failure modes of the same type of 40Cr steel ball cores.

Key words: ball core; fatigue fracture; crack; loose; stress concentration