This is one of the fundamental applications of shot peening. The energy transfer due to plastic deformation creates dislocations in the grain structure along with this the plastic deformation introduces compressive stress. The combination of these two effects provides a highly effective method of introducing negative stress states in the surface and subsurface of the material.
As cracks usually form at the surface or sub surface, if this region can have a negative stress induced by Shot Peening then the probability of crack initiation is greatly reduced.
Where the crack is initiated by a force overload in that the load is greater than the yield strength of the material the shot peening will not improve the situation. The mechanisms of crack growth and initiation have to be within the Low Cycle Fatigue (LCF) range for shot peening to have an effect. Where the component is subject to High Cycle Fatigue (HCF) the shot peening can often have a more significant effect on component life.
Whilst shot peening can significantly increase fatigue life the mechanisms for this are complex, the compressive stress introduced into the component by shot peening not only reduces the initiation rate though, but also has the effect of slowing crack growth.
It must be remembered that the interaction between the process of shot peening and the material being shot peened as well as the component geometry are all significant factors in determining the best parameters to use for processing parts. If shot peening is carried out with the wrong parameters or poorly executed the fatigue properties can be significantly reduced!
See the graph of data from research paper by Toshiya Tsuji, Yuji Kobayashi SINTOKOGIO, LTD. , Japan, email@example.com on ‘Influence of shot peening and carbon content for low cycle fatigue strength of carburized steel’
Its good evidence of the improvement on LCF life of carburised steels.