Creep cavitation failure.

Following theoretical and experimental studies of multi-axial creep fracture with particular reference to the behaviour of copper at 250°C, a polycrystalline model simulating grain boundary cavitation is examined. In the model, a modified form of Kachanov's damage concept is combined with the Hull and Rimmer vacancy diffusion mechanism to yield a novel rate equation that is critically sensitive to the spacing between cavities. Adoption of various relationships between spacing and applied stress is shown to permit prediction of various stress criteria for creep failure time. The model also incorporates equations expressing the equilibrium of forces at grain boundaries, and compatibility of the displacements arising from mass transfer at cavities. With the aid of numerical integration, these equations show the model behaviour to be relatively insensitive to grain geometry, material properties, and initial and final states of damage, although marked changes in grain shape or ultimate strength can significantly change fracture times. Experimentally, fracture times for copper tested at 250°C in uniaxial tension, biaxial tension and torsion (tube and disc specimens) are found to be a power function of maximum principal applied stress. Model simulation of these fracture times is demonstrated on the adoption of a simple expression, relating void spacing to maximum principal applied stress, suggested by metallographic examination of tapered tensile specimens subsequent to creep tests. Apart from wide variations observed in the damage at any given stage of creep, the main microstructural features of cavitation under steady load are also well simulated. For prediction, the spacing-stress relationship is found inadequate on two counts: it cannot be independently determined by metallo-graphic techniques with sufficient accuracy, and it is inconsistent with the behaviour of tensile specimens briefly preloaded at twice the creep stress.