MESO-SCOPICAL EFFECTS DURING FATIGUE CRACK INITIATION
疲劳裂纹萌生的细观效应

Evident changes in X-ray diffraction patterns are observed in surface or sub-surface layers of 16Mn and 300M steel specimens which have been tested under cyclic stress with stress levels equal to fatigue limits of correspondent steels and did not broken until 107cycles. Such layers are located at the surface for surface-unhardened 16Mn and 300M specimens and their thickness are about 3 diameters of ferrite grain and 15 diameters of original austenite grain, respectively; but for shot peened 300M specimen, such layer is located beneath the hardened surface layer, in the region with tensile residual stress, and its thickness is about 12 diameters of original austenite grain. These facts show that, even under the critical conditions for fatigue crack initiation, there always exist meso-scopical pre-yielding regions which consist of rather many grains and sustain back and forth sliding during fatigue test.