切应力状态下疲劳短裂纹的试验研究

通过扭转疲劳试验和塑料复型膜技术研究了中碳钢试样的疲劳短裂纹特性。疲劳短裂纹发生在长度方向与最大切应力相近的片状铁素体中，并在初期迅速扩展。但当其达到晶界时，裂纹扩展速度降低，甚至停止增长，成为不扩展裂纹。只有当应力强度因子满足ＬＥＦＭ条件时，裂纹才会克服晶界障碍继续扩展，最后形成主导裂纹导致试样破坏。疲劳断裂包含有全部三种断裂模式。

A EXPERIMENTAL STUDY ON FATIGUE SHORT CRACK UNDER SHEAR STRESS CONDITION

The initiation stage and short crack behaviour in torsional fatigue of medium carbon steel was investigated by using a replication technique.The fatigue crack initiation was found in the maximum shear stress direction in ferrite phase and its growth rate at this stage is fast.It is possible for the short crack to retard its growth or even arrest at the grain boundary.When the range of the stress intensity factor is large enough to satisfy LEFM condition,the crack could overcome the obstacles and propagate again,furthermore,at length,a dominant crack would be formed and finally leads the specimen broken.Fatigue failure mode has revealved all three fracture modes.