The Influence of Residual Stresses on Constant and Variable Amplitude Fatigue Behaviour of Steel

Doktorsavhandling, 2011

Subjected to dynamic loading, truck components are exposed to variable amplitude fatigue. The fatigue life of components is to a large degree controlled by the behaviour of the surface zones. Different mechanical surface treatments such as shot peening have been developed to introduce compressive stresses close to the surface, thereby delaying failure of components exposed to fatigue loading. During shot peening the specimen is bombarded by small hard shots leading to plastic deformation of a thin surface zone. The inhomogeneous plastic deformation leads to changes in interplanar spacing of the crystal lattice as well as the density of dislocations in the surface zone. We can express this as development of compressive residual stresses and a change in the work hardening state. The compressive residual stress is thought to improve the fatigue properties by increasing the surface resistance to crack initiation as well as reducing the crack propagation rate. Tie rod arm as a truck component is the subject of present study. This component may be made of two different materials: medium carbon microalloyed steel or quenched and tempered steel. The former is continuously cooled leading to essentially pearlitic microstructure and the latter has a tempered martensite microstructure. Although the pearlitic arms made of microalloyed steel were the main focus of this study, the tempered martensite microstructure was also investigated to a certain extent. Strain as well as stress control push-pull fatigue tests were performed on cylindrical specimens made from the actual components. The tests were conducted in constant amplitude, with occasional overloading as well as variable amplitude conditions. The results showed that the life time of specimens at higher strain amplitudes (higher plastic strain amplitudes) is not effected by shot peening. However at smaller plastic strain amplitudes the peened specimens exhibit a longer life time than unpeened ones. The residual stress relaxation and changes of work hardening state were also monitored at the surface and in depth for different strain and stress amplitudes as well as cycle numbers. The influence of plastic strain amplitude on relaxation of residual stresses was studied in detail as a function of fatigue cycle number. In parallel whole components were also tested in constant and variable amplitude loading fatigue, with and without shot peening. The fatigue behaviour in bending as well as relaxation of the residual stresses were also studied for the tempered martensitic steel with different degrees of decarburisation and subsequent shot peening. Also in this case a positive influence of shot peening is mainly seen at small strain amplitudes.