This paper is concerned with both crack initiation and crack propagation behaviour over a field of residual stresses that varies strongly from crack initiation site to final fracture zone.
The stress field is strongly influenced by the process parameters. The main processing operations on the components were: turning and grinding of the bulk material; grid blasting on the bulk material; electroplated chromium coating after the grid blasting; and finally a grinding finishing process of the coating. Traditional turning and grinding process parameters introduce tensile residual stresses. The grid blasting operation is known to introduce compressive residual stresses while the electroplated chromium coating and the final grinding process, both introduce tensile residual stresses, causing eventually some surface microcracks. However these eventual microcracks on the coating, due to the grinding process, cause a relaxation of the coatings residual stresses. When the component is ready for use, a variable residual stress field exists along the thickness direction of the component. This stress field will strongly influence both crack initiation as well as crack propagation. The high surface residual tensile stresses produce, at the beginning of the fatigue test, a high density of microcracks. Thus, crack initiation is very fast. However, this effect causes a reduction on the overall tensile stress field. Then, crack propagation proceeds and is also affected by the stress field in the inner part of the component (due to internal compressive stresses introduced by the grid blasting and tensile stresses introduced by the turning and grinding operations in the bulk material). The main fatigue crack is able to start, propagate, and eventually stop (if it finds a compressive stress barrier), or may propagate with variable propagation rates.

The main issue of this paper is to evaluate the influence of the residual stress profile on crack initiation and on crack propagation behaviour.