13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- In order to study the effect of residual stress, it is desirable that the method chosen to induce residual stress in the specimen causes no other changes which might influence creep. It is also desirable that residual stress fields set up in the laboratory creep specimens are representative of the long range residual stress fields found in engineering structures. Of the various methods reviewed only side punching, in-plane compression and EB welding have been used to study the effect of residual stress and applied load on creep under load control conditions. These techniques can also be used to carry out displacement controlled tests. In practical circumstances relaxation of residual stress in one section is compensated by changes in residual stress distribution in other sections to keep the complete structure in equilibrium, i.e. components are often subjected to combined displacement and load controlled situations as shown in fig 1b. It is not possible to study this effect using the reviewed samples and techniques. A new method is presented that introduces residual stress in a controlled manner such that the stress can be calculated easily at any time and without the use of time consuming residual stress measurement techniques. 2. Three bar structure The new method is based on a classical three bar model and is developed to introduce long range residual stresses through strain incompatibility. This model (or system) has several key features relevant to the high temperature problems of creep crack initiation and growth. The magnitude and the interaction of the residual stress with the applied loading are a function of the initial misfit displacements and the relative stiffness of the components of the system. The subsequent behaviour of the system, with and without the application of additional loading, is governed (a) by the degree to which the misfit is accommodated by plastic and creep strain and (b) the elastic follow-up provided by the system. 2.1. Model Figure 2 shows the three bar structure model consisting of two outer bars ‘B’ and a central bar combination of bar ‘A’ and a compact tension, C(T) specimen. The bars A and B are able to deform elastically and have stiffness Kin and Kout respectively. An initial misfit, ‘X’ exists between the bars so that joining the bars together introduces fit-up residual stresses into the system, with tension in bar A and balancing compression in bar B. The residual force in the middle bar does not self equilibrate across a section but the tensile residual force in middle bar is in equilibrium with the net compressive force in the outer bars. The structure can be subjected to the applied load ‘P’ so that when plasticity, creep or crack growth occurs in the C(T) specimen and the overall EFU factor, Z is given by ܼ ൌܼ ܼ ௦ (1) where, ܼ ൌ൬ଵା∝ ∝ ൰ and ܼ ௦ ൌቀଵା ఉ ఉቁ ߚ ൌ ೞ , ଵ ൌ ଵ ଵ , ߙ ൌ ଶೠ (2) and Ks is the stiffness of the specimen.
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