Decreasing and increasing the value of the compressive residual stresses induced by high frequency mechanical impact treatment during cyclic loading

The beneficial effect of compressive residual stresses induced by mechanical surface treatment for fatigue life improvement is strongly related to their stability under cyclic loading. High Frequency Mechanical Impact (HFMI) treatment is a relatively new mechanical surface treatment process used to improve the fatigue by inducing compressive residual stresses. In previous investigations different residual stresses states were created at the same level of work hardening by using the stress peening method for the HFMI-treatment of notched specimen made of steel grades S355J2+N and S960QL. High fatigue strength was reached in cyclic bending tests for notched specimens after HFMI treatment. The residual stresses were determined after the test in the unloaded state. The current study presents investigations on the changes of the residual stress state after cyclic loading (from unloaded condition to half lifetime). Therefore, the residual stresses were determined by a combination of X-ray and neutron-diffraction techniques. Cyclic 4-point bending tests were done under pure tensile load (stress ratio R=0.1). After these tests an increase of the values of compressive residual stresses were observed at the notch root depending on the initial residual stress condition and the cyclic yield strength of the material. The experimental determination of the residual stresses was complemented by Finite Element Analysis (FEA) of the cyclic loading. The FEA showed an increase of the amount of compressive residual stress as well. The high fatigue life of the notched steel specimen could be explained by this effect.