Stability and Relaxation Behaviour of Shot Peening Induced Residual Stresses in AISI 4140 During Bending Fatigue

Author:  Holzapfel, Schulze, Vohringer, Macherauch
Source:  Conf Proc: ICSP-6, (p.413-423)
Doc ID:  1996087
Year of Publication:  1996
Abstract:  
Shot peening induced residual stresses can be relaxed by supplying sufficiently high amounts of thermal and/or mechanical energy, which converts the residual elastic strains to microplastic strains. In order to better understand this relaxation behavior, the steel AISI 4140 (German grade 42CrMo 4) in a normalized condition and a quenched and tempered condition was investigated in bending fatigue experiments at 25 deg C, 250 deg C and 400 deg C. Characterization of the residual stress changes in shot peened flat specimens during fatigue ws realized by x-ray stress measurements. In all cases the residual stress relaxation occurs in different regimes. First, thermal relaxaton reduces the residual stresses during specimen heating. Next, relaxation during the first cyclic can be discussed on the basis of the effects due to quasistatic loading. Due to cyclic creep effects the interval between the irst cycle (N=1) and the number of cycles to crack initiation N, is characterized by linearly decreasing residual stresses with the logarithm of N. Finally for N > N, the reduction of residual stresses with logarithm of N is stronger than linearly. In both heat treatment conditions the residual stress relaxation at the three temperatures is similiar and is not accelerated by simultaneous higher temperature and cyclic loading effects. The similar relaxation behavior for the different steel conditions investigated is influenced by the separate effects of thermal and cyclic residual stress relaxation. The last one is essentially influenced by dynamic strain ageing processes.