An Anisotropic Creep Damage Model for Anisotropic Weld Metal

Abstract

Past studies from creep tests on uniaxial specimens and Bridgman notch specimens, for a P91 weld metal, showed that anisotropic behavior (more specifically transverse isotropy) occurs in the weld metal, both in terms of creep (steady-state) strain rate behavior and rupture times (viz., damage evolution). This paper describes the development of a finite element (FE) continuum damage mechanics methodology to deal with anisotropic creep and anisotropic damage for weld metal. The method employs a second order damage tensor following the work of Murakami and Ohno (1980, “A Continuum Theory of Creep and Creep Damage,” Creep in Structures, A. R. S. Ponter and D. R. Hayhurst, eds., Springer-Verlag, Berlin, pp. 422–444) along with a novel rupture stress approach to define the evolution of this tensor, taking 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. The theoretical model is implemented in a material behavior subroutine within the general-purpose nonlinear FE code ABAQUS (ABAQUS User’s Manual, Version 6.6, 6006, Hibbitt, Karlsson and Sorenson, Inc., Providence, RI). The validation of the implementation against established isotropic continuum damage mechanics solutions for the isotropic case is described. A procedure for calibrating the multiaxial damage constants from notched bar test data is described for multiaxial implementations. Also described is a study on the effect of uniaxial specimen orientation on anisotropic damage evolution.

An anisotropic tertiary creep damage constitutive model for anisotropic materials

“…For anisotropic microstructure materials, the effect of material orientation must be included in the both the creep strain rate and damage evolution equations [30].…”

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

“…The above definition of damage rate considers the change in direction of maximum principal stress during multiaxial loading and permits damage to be determined for arbitrary orientations. A more detailed description has been provided in reference [18]. When r 5 0 and v (1) ?n 5 1, equation ( 10) reduces down to an isotropic damage formulation with direction-independent rupture properties B 1 , x 1 , and a 1 .…”

“…equation ( 10) and the associated scalar rupture terms in equation ( 11), and on the other hand to possible assumptions about the relative propensities of void or cavitation damage growth under tension-dominated and compression-dominated loading. The present material model is developed primarily for application to pressurized welded connections, which are typically tensile hoop and tensile axial stress dominated; see reference [8], for example, for a detailed parametric analysis of the stress distributions in a circumferentially welded pipe using an anisotropic steady state creep material model based on equations (17) and (18). The particular choice of stress dependence of equations ( 11) is well established for isotropic damage-based life predictions for notched specimens and for pressurized piping applications (see, for example, reference [4]).…”