13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- A New Method of Introducing Long Range Residual Stresses to Study Creep Crack Initiation A. M. Shirahatti*, Y. Wang, C. E. Truman, D. J. Smith 1 Solid Mechanics Group, Department of Mechanical Engineering University of Bristol, Bristol BS8 1TR, United Kingdom * Corresponding author: anil.shirahatti@bristol.ac.uk Abstract One of the many challenges in the behaviour of structures is to understand if the presence of residual stress plays an important role in contributing to failure of a structure operating at high temperature. A typical example is the reheat cracking, associated with the austenitic stainless steel welded components, where the presence of residual stress is seen as a major factor. A review of previous methods that introduce residual stresses into specimen indicated that a method that doesn’t introduce microstructural changes during the generation of residual stresses should be sought. The purpose of this paper is to describe a new method of introducing long range residual stresses at high temperature. The method uses a three bar structure with an initial misfit introduced into the central bar to represent a long range residual stress. The rig was designed so that the induced residual stresses could be characterised easily without using time consuming residual stress measurement techniques. Initial results demonstrated that the magnitude and the interaction of the residual stress with the applied loading is a function of the initial misfit displacements and the relative stiffness of the components of the system. Additionally, the subsequent behaviour of the system, with and without the application of additional loading, is governed by (a) the degree to which the misfit is accommodated by plastic and creep strain and (b) the elastic follow-up provided by the system. The paper describes the design of a test rig and laboratory tests conducted to validate the method. Keywords Residual stress, Creep, Elastic follow-up, 316H stainless steel 1. Introduction Residual stress plays an important role in the component life assessment of the structures. Such stresses may arise usually as a consequence of the manufacturing process and final fabrication. Welding is a typical manufacturing process where, unless the component is subjected to post-weld heat treatment, the residual stress can attain a value close to or equal to the yield stress. Fabrication can also lead to additional locked-in stresses developed from the fitting-up of the different parts of an assembly [1]. Residual stresses are usually treated as secondary stresses. However, in certain circumstances they must be classed as primary. For example, in a cracked structure where the fit-up residual stresses do not self-equilibrate across a ligament, the residual stresses may provide a significant contribution to the plastic collapse of the ligament. Whether they do or not depends on how the residual forces change as a crack grows and plastic deformation accumulates in the structure. This in turn depends on the level of elastic follow-up (EFU). A typical practical case where we expect to see the effect of EFU is shown in fig 1 for a pressurized piping system. As shown schematically, the system and its welds can be treated as a series of springs with the pipe having stiffness K1, K3 and the weld with stiffness K2. When the pipe is built-in and welded we would expect that long range residual stresses to be present and are represented as an initial far field displacement X. The pipe will also be subjected to internal pressure and consequently the system is also subjected to external load P. Elastic follow-up is expected when part of a structure (the area around the welds) reduces its stiffness (either through creation of plasticity and/or the growth of crack) relative (i.e. EFU) to the surrounding material. This would result in additional strain accumulation and relaxation of the initial stresses created during the welding and fit-up. Many methods have been proposed to generate well defined residual stress fields in laboratory test specimens. In the context of investigating the influence of residual stress on creep, the following
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