Analysis of residual stress relaxation of aluminum alloys EN AW 6061/-82 T6 under cyclic loading

Stress relaxation describes the reduction of stress under static or cyclic loading
at a constant strain level. Several processes induce intentionally residual
stresses, for example, autofrettage of thick-walled pressurized tubes to improve
their fatigue life. This well-known process induces residual compressive
stresses at the critical inner surface by using a single static but controlled over-
loading internal pressure. Relaxation of residual stresses due to cyclic loading
in service would endanger the effectiveness of autofrettage and could finally
lead to unexpected fatigue failure. In this study, strain-controlled experiments
up to 500,000 load cycles and amending nonlinear finite element simulations
were done for the aluminum alloys EN AW 6061 T6 and EN AW 6082 T6 to
study potential cyclic stress relaxation in four-point bending tests after con-
trolled single static plasticization for residual stress generation. This analysis
identifies almost stable residual stresses for both materials under different
cyclic strain-controlled load levels.