A theory of autofrettage with applications to creep and fatigue

Abstract A new theory of autofrettage is proposed for a closed-end cylinder which takes into account work-hardening in the presence of small plastic strains based upon equivalent stress-strain data. Comparisons with experiments on carbon and alloy steels shows that more realistic predictions are possible than with the simplified non-hardening theory. Residual stress distributions are determined for given elastic-plastic radii in the wall of the pressurised cylinder assuming purely elastic unloading. The greatest beneficial effect that autofrettage has on fatigue strength is identified with an optimum pressure for which the equivalent residual stress in the bore does not exceed the reversed yield stress. When the latter is determined from the kinematic hardening rule it is shown that considerable improvement can be expected in the fatigue life of a cylinder subjected to a fluctuating internal pressure. The possible beneficial effect of a residual stress state to creep resistance is also considered. It is postulated that when an equivalent hydrostatic stress state exists at the bore under steady-creep conditions the reductions in equivalent creep rate and the rate of damage accumulation will result in an increased creep life.