The creep rupture of butt welded ferritic steel pipes composed of a range o
f weld and heat affected zone materials has been previously analysed in det
ail (Leckie and Hayhurst, 1994). These analyses required substantial comput
ational resources which cannot be justified during the preliminary phases o
f the design process. To reduce cost and improve speed an approximate metho
d for the analysis of kinematically determinate structures, known as the mo
dal method (Leckie and Hayhurst, 1974), has been developed to compute creep
rupture lifetimes. This paper reports an extension of the method for the a
nalysis of multi-material structures, such as weldments, and its implementa
tion as a post processor to a stationary state finite element creep analysi
s. Histories of stress and damage have been determined using the modal meth
od for weldments with a range of heat affected zone and weld material combi
nations. The lifetimes determined in this way are compared with those deter
mined 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 t
o more approximate reference stress methods. (C) 2000 Editions scientifique
s et medicales Elsevier SAS.