Numerical and experimental investigations of effects of residual stresses on crack behavior in Aluminum 6082-T6

In the structural integrity assessment, residual stresses play an important role. The residual stresses affect both the crack driving forces and the crack-tip constraint. To investigate the interaction of residual stresses with mechanical loading during the onset of crack growth in Aluminum 6082-T6, modified single edge-notched bending specimens were used. Aluminum 6082 has the highest strength of the 6000 series alloys with excellent corrosion resistance. A residual stress field was created in the specimens by pre-loading. To accurately quantify the residual stress field created during this test procedure, the strains were measured during loading and unloading and compared with finite element results. After the introduction of the residual stress field, the specimens were tested under three-point bending to determine the load versus displacement behavior and fracture toughness. Also, a post-processor for finite element calculation was developed to enable determination of the J-integral values for the specimens having residual stresses. The constraint parameters Q and R were calculated at the crack-tip to describe the stress field in this region. The parameter Q is used to characterize the loading and geometry constraint, and the parameter R is used for characterizing the crack-tip constraint due to residual stresses. It is observed that tensile residual stresses around the crack-tip increase the crack-tip constraint and decrease the fracture toughness of the bodies. By increasing the external load, the constraint parameter R goes toward zero and the effects of residual stresses on the crack growth resistance become negligible.