Material laws for inelastic analyses on elbows of 12% or 0.5% Cr-steel up to creep rupture for 30 000 hours creep tests at 550°C

Abstract

Damages on elbows gave rise to a study of their creep and creep rupture behaviour comparatively by inelastic finite element analyses and 30000 hours creep tests. The procedure how the elastic-plastic and creep laws for the monotonic and constant loading have been deduced from test results gained by uniaxially loaded specimens, and the results, are outlined. The description of the creep behaviour due to uniaxial loading up to the rupture strain has been based on the concept of true strains and stresses and an extended and modified Theta-projection method. Among several functions selected for the approximation of experimental strain-time data, two expressions approach them up to rupture within an error of only a few per cent. The methods and results throw new light on the tertiary creep range, the stress dependence of the creep behaviour and the technique of creep tests. Representations showing the stress and time dependence of Norton's coefficients, evaluations of the Monkman—Grant relation and other relations based on the experimental and theoretical data, complete the insight into the creep behaviour of the ferritic steels concerned.