X-ray Microstructure And Resid.stress Analysis Of Shot Peened Surface Layers During Fatigue Loading

Author:  Bergstrom, J. and Ericsson, T.
Source:  Conf Proc: ICSP-3, (p.221-230)
Doc ID:  1987108
Year of Publication:  1987
Fatigue is basically a surface related phenomenon as the fatigue cracks usually initiate at the surface and grow from there into the material. Surface hardening is therefore often used to improve fatigue properties and an important example of this is shot peening. Shot peening work hardens the surface layer and induces compressive residual stresses. Roughly speaking, the residual stress acts as an applied mean stress and a compressive residual stress will therefore retard fatigue crack initiation and growth. The work hardening results in an increased dislocation density which hinders dislocation movements due to the fatigue load and supresses localized plastic deformation which is a starting feature for crack initiation. Residual stresses have a greater influence on hard metals than on soft. For steels it is well known that residual stress relaxation will occur due to cyclic loads for low strength grades and the resistance against relaxation will increase with increasing strength. There is also a medium strength interval where neither the amount of relaxation nor its mechanisms are well defined. Cold work gives a larger increase in fatigue strength at low strength level, i.e. a softer material work hardens more than a harder one. The effect of work hardening may also be reduced due to so-called cyclic softening under cyclic loads. The two effects, residual stress relaxation and cyclic softening are interdependent. By using X-ray diffraction technique residual stress can be measured in the surface layer and stress relaxation can be followed. By doing so called profile analysis of X-ray intensity peaks the degree of cold work and cyclic softening can be studied. Thus X-ray diffraction offers a mean to study the interdependent processes of residual stress relaxation and cyclic softening. Here such a study is presented for a shot peened quenched and tempered steel subjected to constant amplitude loading with R=-1 and R=0 and R=-1 with sinngle overload peaks. The object of the study has been to i) map the residual stress behaviour, ii) evaluate its determining parameters, iii) evaluate the inelastic strain history by determining dislocation structure changes and iv) to correlate these with residual stress behaviour.

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