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

Abstract

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 overloading 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 controlled single static plasticization for residual stress generation. This analysis identifies almost stable residual stresses for both materials under different cyclic strain-controlled load levels.

An experimental study on residual stress relaxation in low‐cycle fatigue of Inconel 718Plus

“… Residual stress relaxation appears only if the applied load condition is above a threshold value, expressed by a threshold strain amplitude ε a,th , which has been previously identified and derived from the static behavior of the material (Equation 7). Residual stress relaxation is concentrated in the first 10 cycles, most likely in the first cycle, as observed in several works in literature. Residual stress relaxation continues after the first 10 cycles, even if this phenomenon is minimal with respect to the initial one, and the continuous relaxation after the first 10 cycles cannot be neglected especially for higher strain amplitudes. Regarding the first statement, in a recent study, 52 static four‐point bending tests were performed to induce residual stresses in flat‐bending specimens. Subsequently, specimens were subject to several constant cyclic strain‐controlled bending loads, and the tests revealed almost cyclic stability of residual stresses for such loading, considering that the maximum strain amplitude applied was relatively low (0.20%).…”

“…Regarding the first statement, in a recent study, 52 static four-point bending tests were performed to induce residual stresses in flat-bending specimens. Subsequently, specimens were subject to several constant cyclic straincontrolled bending loads, and the tests revealed almost cyclic stability of residual stresses for such loading, considering that the maximum strain amplitude applied was relatively low (0.20%).…”