Abstract:
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.
