Ductile Fracture Analysis Under Large Amplitude Cycles

Demonstration of large components integrity under seismic loading is based up to now on monotonic tearing resistance curves. However, it is well known that cycles decrease the fracture resistance of the material, mainly according to the loading ratio. Most studies use monotonic methods to analyze reversible cyclic loading and the associated increase of crack propagation: Delta J-R curves are largely used. For monotonic loadings, Turner [1] proposed a decomposition of the rate of dissipated fracture energy. This decomposition led on the determination of an energetic criterion for ductile fracture [2]. This intrinsic criterion allows the fracture prediction on large components. This paper aims to propose an analysis of cyclic ductile fracture which should allow the determination of an energetic criterion under large amplitude cycles. For that purpose, compact tension specimens are taken from a carbon steel pipe (Tu42C) used in the secondary circuit of French PWR. A series of cyclic tearing tests are carried out under quasi-static loadings. The effects of loading ratio and incremental plastic displacement are quickly studied. Here, we present an energetic analysis which take into account the crack closure and crack opening. Indeed, displacement fields around the crack tips are measured with digital image correlation and linked with electric potential measurement. That allows an accurate determination of crack closure and crack opening and let a precise calculation of fracture energy possible. The energetic fracture criterion will be confirmed with crack propagation prediction on different geometry like CT specimen and a through-wall-cracked pipe under cyclic reversed loadings.