Creep-fatigue life assessment of cruciform weldments using the linear matching method

Abstract

This paper presents a creep-fatigue life assessment of a cruciform weldment made of the steel AISI type 316N(L) and subjected to reversed bending and cyclic dwells at 550 °C using the Linear Matching Method (LMM) and considering different weld zones. The design limits are estimated by the shakedown analysis using the LMM and elastic-perfectly-plastic material model. The creep-fatigue analysis is implemented using the following material models: 1) Ramberg–Osgood model for plastic strains under saturated cyclic conditions; 2) power-law model in “time hardening” form for creep strains during primary creep stage. The number of cycles to failure N⋆ under creep-fatigue interaction is defined by: a) relation for cycles to fatigue failure N∗ dependent on numerical total strain range Δεtot for the fatigue damage ωf; b) long-term strength relation for the time to creep rupture t∗ dependent on numerical average stress σ¯ during dwell Δt for the creep damage ωcr; c) non-linear creep-fatigue interaction diagram for the total damage. Numerically estimated N⋆ for different Δt and Δεtot shows good quantitative agreement with experiments. A parametric study of different dwell times Δt is used to formulate the functions for N⋆ and residual life L⋆ dependent on Δt and normalised bending moment M˜, and the corresponding contour plot intended for design applications is created.