Archive of Appled Mechanics manuscript No. (will be inserted by the editor) A phenomenological model for anisotropic creep in a multi-pass weld metal K. Naumenko, H. Altenbach Martin-Luther-University Halle-Wittenberg, Department of Engineering Sciences, D-06099 Halle, Germany The date of receipt and acceptance will be inserted by the editor Abstract Creep strength of welded joints can be estimated by continuum damage mechanics. In this case constitutive equations are required for different constituents of the welded joint: the weld metal, the heat affected zone and the parent material. The objective of this paper is the modelling of the anisotropic creep behavior in a weld metal produced by the multi-pass welding. In order to explain the origins of anisotropic creep a mechanical model for a binary structure composed from ”fine grained” and ”coarse grained” constituents with different creep properties is introduced. The results illustrate the basic features of the stress redistribution and damage growth in the constituents of the weld metal and agree qualitatively with experimental observations. The structural analysis of a welded joint requires a model of creep for the weld metal under multi-axial stress states. For this purpose the engineering creep theory based on the creep potential hypothesis, the flow rule and assumption of transverse isotropy is applied. The outcome is the coordinate-free equation for secondary creep formulated in terms of the Norton- Bailey-Odqvist creep potential and three invariants of the stress tensor. The material constants are identified according to the experimental data presented in the literature. Key words: Weld metal, Multi-pass welding, Anisotropic creep 1 Introduction For many structures designed for high temperature applications, e.g. piping systems and pressure vessels, an important problem is the assessment of creep strength of welded joints. The lifetime of the welded structure is primarily determined by the behavior in the local zones of welds, where time-dependent creep and damage processes dominate. Different types of creep failures occurred in the last years are discussed in [29], for exam- ple. The design of welded structures and their residual life estimations require engineering mechanics models which would be able to characterize creep strains, stress redistributions and damage evolution in the zones of welds. A weld is usually termed to be a metallurgical notch. The reason for this is the complex microstructure in the weld metal itself and in the neighboring heat affected zone. In the last years many research activities were directed to the study of welded joints. First, theoretical and experimental analysis are addressed to the welding process with the aim to predict the formation of the micro-structure of the welds and to analyze the residual stresses [7]. Second, the behavior of welded joints under the mechanical and thermal loadings were investigated [21]. Here one has to consider that the stress-strain response at the room temperature is quite different for the weld metal, the heat affected zone and the base metal (parent material), particularly if they are loaded beyond the Correspondence to: K. Naumenko (e-mail: konstantin.naumenko@iw.uni-halle.de)