13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- methods have been developed: pre-compression [2-6], quenching [7], side punching [8], Borland specimens [9], ring-weld specimens [10] and electron beam (EB) welding [1, 11, 12]. These methods produce either long-range or short-range residual stress fields. Quenching, Borland specimens and ring welding methods result in specimens with a residual stress field throughout the entire volume. On the other hand, side punching, in-plane compression and EB welding methods result in a residual stress field being set up in a very localised part of the specimen. The second group of methods rely on local, rather than global, incompatible displacements to set up the residual stress field. Another important observation is that the in-plane compression and EB welded specimens require the introduction of a sharp notch by for example electro-discharge machining, prior to creep testing. Introducing such a notch redistributes the residual stresses and so, care must be taken to ensure that the required levels of residual stress remain in the specimen after this redistribution. Consider the in-plane compression method as an example. Here, a volume of material local to the semi-circular stress raiser deforms plastically and this plastic zone resists the relaxation of the surrounding material. If a long notch is introduced such that it extends through the plastic zone and into the surrounding elastic zone then the elastic zone is free to relax and the residual stress field is lost [11]. (a) (b) Figure 1. (a) Built-in welded pipe bend (P is load, X is displacement) (b) Stress-strain behaviour of a local volume In all the above methods the magnitude and distribution of residual stress are found by time consuming methods i.e. neutron diffraction. Also, in all the cases various and nominally identical specimens are manufactured to determine the stress distribution. For example in case of cylinder quenching by Hossain [7], three specimens were manufactured and the neutron diffraction method was used to measure the stress distribution after quenching, short term ageing (1.25 hrs) and long term ageing (1800 hrs). In all of the above methods, residual stress is introduced at room temperature and when the specimen is subjected to high temperature the magnitude of residual stress is reduced drastically (30% reduction) [2-8] due to the lower yield strength at high temperature. In all the cases, the residual stress at high temperature was determined using finite element analysis.
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