Load-time dependent relaxation of residual stresses

Tests with aluminum specimens of aircraft structural elements commonly show that lifetime is enhanced due to compressive residual stresses which occur as a result of plastic tensile deformation at a stress concentration. The present study describes cyclic tests which were conducted with 7075-T651 bare aluminum center-hole specimens in order to determine how these residual-stress benefits may be affected by a procedure of applying compressive loads, holding these loads for specified time periods, and then cycling to failure. Results indicate that all benefits due to residual stresses observed in aluminum specimen tests of a few days duration may not be observed in actual aircraft lower wing surface structures which sustain compressive loads of 24 hr and longer when parked.