A novel creep test rig to study the effects of long-range residual stress, elastic follow-up and applied load

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. Assessments of the high-temperature integrity of practical structures are based on experiments carried out using standard laboratory-scale creep test specimens tested under either load- or displacement-controlled conditions. In practice, structures are subjected to combinations of residual and applied stresses which in turn lead to mixed boundary conditions. Conventional laboratory creep tests do not represent these circumstances. The purpose of this paper is to describe the design of a novel test rig to introduce long-range residual stresses at high temperature. The concept of rig is based on three-bar structure with an initial misfit introduced into the central bar to represent a long-range residual stress and 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 concept.