Effect of biaxial cyclic tension on the fatigue life and damage mechanisms of Cr–Mo steel

• Cyclic biaxial tension tests were run at various biaxialities on 2.5%Cr–1%Mo steel.
• Moderate biaxialities (B = 0.25 and 0.5) had a beneficial effect on fatigue lives.
• By contrast equibiaxial tension slightly reduced the fatigue lives.
• Various endurance criteria failed to capture both mean stress and biaxiality effects.
• An alternative fatigue criterion extending Gerber’s parabola was proposed.

Combined cyclic tension and internal pressure tests with various proportions of each loading were run on a 2.5%Cr–1%Mo steel to investigate the effect of positive stress biaxiality on fatigue lives and damage mechanisms. While moderate stress biaxiality had a beneficial effect on fatigue lives, attributed mainly to a retardation of crack initiation, equibiaxial tension had a slightly detrimental effect, attributed to a “pseudo size effect”. Intergranular facets associated with temper and hydrogen embrittlement were observed on the fracture surfaces. The evolutions of their surface fraction with ΔK and load biaxiality suggested a possible reduction in crack growth rate at moderate biaxialities. Several popular multiaxial fatigue criteria failed to describe all fatigue data. Thus, a new fatigue criterion based on Gerber’s parabola has been proposed. It captures the evolution of the endurance limit under the combined effects of a positive mean stress and positive biaxiality.