Transition from necking to cavitation driven tertiary creep with length scale in constrained ductile metal joints.

This study investigates the effect of length scale on the tertiary creep behavior of elastically constrained ductile metal joints. Creep tests were performed in tension on Sn–Cu joints of thicknesses ranging from 1.4 mm to 170 μm. Sn being more creep compliant as compared to Cu undergoes creep deformation, whereas Cu remains in the elastic state. The overall creep rate and the amount of strain accumulated during the tertiary creep decreased with reduction in joint thickness. This was accompanied by a reduction in the extent of lateral contraction, especially near the fracture surfaces of the joints. Fractographs revealed a transition in the mode of creep failure from pure necking in thick joints to cavitation along with constrained necking in thin joints. A joint size dependent tertiary creep model incorporating necking and cavitation was developed to explain the observed transition in tertiary creep behavior and final failure. The model captures the effects of both joint size dependent triaxial stress state and a decrease in the size of the necking ligament with reduction in joint size. Overall, the decrease in strain accumulated during tertiary creep is attributed to smaller size of the neck and higher triaxiality in thin joints, which enhances the strain due to cavity growth and reduces that due to necking as the joint thickness reduces. [ABSTRACT FROM AUTHOR]