The influence of partial peening of fatigue crack growth

Fatigue crack growth rate is an important factor for the life assessment of engineering components and structures. Various surface treatment techniques have previously been developed and employed in industrial applications to extend the fatigue life of engineering structures by implementing compressive residual stresses on material surfaces. Residual stresses can decelerate or accelerate fatigue crack growth in engineering structures, depending on their distribution profiles (i.e. tension or compression). In this thesis, the influence of partial surface peening on the fatigue crack growth of a high strength steel (HSS) material has been investigated by performing laboratory tests. The effects of partial peening on the fatigue crack growth behaviour of Optim700QL ferritic HSS have been experimentally investigated. Dog-bone shaped specimens were tested under tension-tension fatigue load and specimens with semi-circular notches were tested under 4-point bending conditions. An initial notch was machined in the middlewidth of all specimens to create a starter crack for fracture mechanics studies. Three distinct extents of partial shot peening and cavitation shotless peening, with respect to the crack tip and specimen symmetry line, were applied on the specimen geometry. The finite element modelling (FEM) was used to first calculate the shape function, and then the stress intensity factor (SIF), to develop a new model for 4-point bend specimen geometry with the dimensions considered in this project, the solutions of which are not available in the literature. Moreover, FEM was used to predict the stress concentration factor (SCF) for a dog-bone specimen to avoid reaching plasticity at the crack tip in load calculations for tension-tension tests. The fatigue crack growth results from the partially peened specimens have been compared with those obtained from similar specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the peening process play a significant role in the fatigue crack growth behaviour of the material, indicating that partial surface peening can be used as an effective method to decrease the fatigue crack growth rate under pure bending fatigue loading conditions, but is harmful under tension-tension loading conditions