Development and use of an anisotropic damage model for the high-temperature life assessment of a P91 weldment

Abstract

This paper describes the development of a finite element (FE) continuum damage mechanics methodology to deal with anisotropic creep and anisotropic damage for a P91 weld metal. The method employs a second-order damage tensor, together with a novel rupture stress approach to define the evolution of this tensor. The method takes advantage of the transverse isotropic nature of the weld metal, to achieve a reduction in the number of material constants required from test data (and hence tests) to define the damage evolution. Hill's anisotropy potential theory is employed to model the secondary creep. An FE implementation of the model is applied to the creep life assessment of an internally pressurized axisymmetric P91 pipe weldment. The study also considered the effect of various material mismatch ratios in the various zones of the weldment. Here material mismatch is considered to be a combination of the difference in uniaxial minimum strain rates and in the uniaxial rupture times.

Interpretation of small ring creep test data

“…Small specimen creep testing is useful in a number of practical engineering situations (see Parker et al, 7 Sturm et al 9 and Garzillo et al 13 ). It can be used to obtain the current creep strength of a service-aged material using, for example, scoop samples removed from a component, 5 or to obtain the creep properties of a HAZ region of a weld (see Peravali et al, 4 Garzillo et al 13 and Dorner et al 14 ), or to obtain the relative creep properties of materials produced as part of an alloy development programme and so on. For example, the MSR obtained from a service-aged material can be used with the Monkman-Grant relationship to estimate component life.…”

“…Components in chemical plant, power generation plant, aeroengines and so on, operate at temperatures and stresses that are high enough to cause creep to occur. 1 In the quest for improved efficiencies, lower emissions and fuel diversification, new materials are being introduced, and these are being subjected to increasingly severe loading conditions (see Saber et al, 2 Tanner et al 3 and Peravali et al 4 ). In order to ensure the safe operation of components, improved understanding of the creep behaviour of the materials being used to manufacture them and improved inspection techniques are required.…”

An anisotropic tertiary creep damage constitutive model for anisotropic materials

“…A number of researchers have developed constitutive models for transversely-isotropic materials; however, most formulations do not accurately model intermediate material orientations (when the longitudinal grains are not parallel or perpendicular to the load direction) [3,28,29]. For anisotropic microstructure materials, the effect of material orientation must be included in the both the creep strain rate and damage evolution equations [30].…”