Creep Crack Growth Testing Under Secondary and Combined Loading

Abstract

Two novel fracture mechanics specimens were developed to enable creep crack growth (CCG) studies, under secondary and combined loading, to be carried out in laboratory controlled environments. Test samples were produced by the insertion of wedges into the mouths of C(T) specimens such that the geometrical misfit of the wedge induced on effective residual stresses. It is shown that the extent of crack tip plasticity generated during wedge insertion was limited which made these specimens ideal for CCG testing under secondary loading. Samples were also made by strategically placing electron beam welds in C(T) specimens, such that the weld induced residual stresses provided the crack driving force in subsequent CCG tests. These specimens were used to perform crack growth studies under secondary and combined loading conditions in Type 316H stainless steel at 550°C for up to 1,300 h, where crack extensions of up to ≈5 mm occurred. The experimental results are presented and compared to relevant CCG tests published in literature. It is shown that CCG testing under secondary and combined loading conditions requires the creep ductility of the material to be sufficiently low to ensure crack initiation occurs. A low creep ductility was achieved in this study by uniformly pre-compressing the material to 8% plastic strain.