High temperature nonproportional low cycle fatigue using fifteen loading paths

• Nonproportional straining significantly reduced fatigue lives of Type 304 at 923 K.
• Nonproportional low cycle fatigue lives depended on strain history.
• Reduction in fatigue life by nonproportional loading about 80% compared with proportional loading.
• Nonproportional strains and energy parameter successfully correlated proportional and nonproportional low cycle fatigue lives.
• ASME equivalent strain gave a significantly unconservative life estimates.

This paper discusses effects of nonproportional loading on low cycle fatigue lives for Type 304 stainless steel hollow cylinder specimens at 923 K. Strain controlled axial–torsion low cycle fatigue tests were performed using fifteen proportional and nonproportional strain paths at a strain rate of 0.1%/s. Nonproportional straining significantly reduced fatigue lives of the steel at 923 K. Reduction in fatigue life due to nonproportional straining reached about 80% compared with the proportional straining in specific strain paths. The nonproportional strains, ΔɛNP and ΔεNP∗, and energy parameter, ΔσIΔɛI, successfully correlated proportional and nonproportional fatigue lives but ASME equivalent strain gave significantly unconservative estimates.