Abstract:Based on the results of notch 3PB fatigue tests and the observation of metallographic surfaces and fracture surfaces, it is found that for fatigue bending tests, the fatigue crack initiated and extended directly from the notch root, then extended step by step by the fatigue bending loads. When a crack extended to the length, which acts as a Griffiths crack and matches the loading stress, the crack propagated catastrophically through entire specimen. In the fatigue bending tests at the certain stress amplitude values, the crack length increased with increasing of fatigue unloading cycles, and the damage produced by fatigue process increased, the fatigue regions became wider, the fracture mechanism is that the fatigue regions propagate along a single orientation, the cleavage regions propagate dispersedly to different orientations. However, for shorter fatigue cycles the fracture mechanism is that initiation origins are produced at the notch root, then propagate dispersedly to final fracture. No any differences appear between the fatigue and the cleavage regions on the propagation path.