Quantitative Investigation of Brittle Out-of-plane Fracture in X70 Pipeline Steel

Abstract

Brittle out-of-plane cracking (by delamination or brittle tilted fracture) affects the impact toughness of pipeline steels. It has been investigated on notched tensile specimens using a local approach to fracture. Smooth and notched bars taken along four directions (including the short transverse direction, ND) have been tested in tension at temperatures between 20°C and -196°C. Delamination partly results from the lower value of the critical cleavage stress along ND, linked to the microtexture anisotropy, but also from the presence of ductile microcracks acting as stress concentrators triggering fracture along the rolling plane. Brittle tilted fracture was associated to a relatively lower value of the critical cleavage stress in that plane, but prior delamination was necessary to trigger it. The relevance of a macroscopic critical stress criterion for delamination is finally discussed.

A study of the mechanism of delamination fracture in bainitic magnetic yoke steel

“…ACCEPTED MANUSCRIPT 13 probability of cleavage fractures. The path of crack propagation (transgranular cleavage) was determined by microscopic yield stress around the fracture.…”

“…Tankoua et al [13] pointed out that the delamination in X70 pipeline steel resulted from the lower value of the critical cleavage stress and the presence of ductile micro-cracks, which were stress concentration triggering fractures along the rolling plane. Furthermore, considerable researches on delamination phenomena in composites are also currently under way.…”