Approximate creep rupture lifetimes for butt welded ferritic steel pressurised pipes

The creep rupture of butt welded ferritic steel pipes composed of a range of weld and heat affected zone materials has been previously analysed in detail (Leckie and Hayhurst, 1994). These analyses required substantial computational resources which cannot be justified during the preliminary phases of the design process. To reduce cost and improve speed an approximate method for the analysis of kinematically determinate structures, known as the modal method (Leckie and Hayhurst, 1974), has been developed to compute creep rupture lifetimes. This paper reports an extension of the method for the analysis of multi-material structures, such as weldments, and its implementation as a post processor to a stationary state finite element creep analysis. Histories of stress and damage have been determined using the modal method for weldments with a range of heat affected zone and weld material combinations. The lifetimes determined in this way are compared with those determined using complete continuum damage mechanics analyses, and are shown to be conservative; in addition the regions of intense damage have been shown to be accurately predicted. The modal method is also shown to be superior to more approximate reference stress methods.