13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- to the tensile residual force in middle bar. During the cyclic loading process, the tensile residual force in middle bar relaxed from 5.65 KN to 3.8 KN while the compressive residual force in the side bars relaxed from 2.8 KN and 1.9 KN. Figure 7 shows the load acting on C(T) specimen against total load line displacement measured during the cyclic loading phase of the test. The following points should be noted. First at point ‘A’ the C(T) specimen was subjected to a tensile load of 5.65 KN while the total load applied to the assembly was zero. This tensile load corresponds to the initial level of preload in the assembly at the start of the cyclic loading phase of the test. Second, five unloading lines did not return to the CMOD point ‘A’ from which the test started. This was due to the accumulation of plastic deformation in the specimen. It can be seen that about 0.06 mm of plastic CMOD had accumulated at the final unloading step. Also, the gradient of each of the unloading lines remained constant. This shows there has been no crack growth during cyclic loading and the same was recorded by PD system. Third, the line AB corresponds to the locus of unloaded points and reveals that the initial preload relaxed, as plastic deformation accumulated in the specimen thereby reducing the misfit. At point B, with an applied load of zero, the 3.8 KN load on the C(T) specimen corresponds to the level of preload remaining in the assembly i.e. 33% reduction in the initial preload level. An important feature of the behaviour of this assembly is that the relaxation line AB has a slope dependent on the relative stiffness of the assembly and in turn corresponded to the EFU associated with the structure. To achieve different values of EFU, different combinations of diameters of the middle and the side bars can be used. For example, in the present rig if the dimensions are changed so that the middle and side bars diameter is 10 mm and overall height is 864 mm, one can achieve overall EFU of 6.61 (1/Z=0.151) as shown in fig 3. 5. Concluding remarks Four methods of inducing residual stress in laboratory creep specimens were reviewed. These include quenching, side punching, in-plane compression and welding methods. It was found that the ability of these methods to provide insight into the effect of residual stress on creep in engineering structures is limited. These methods cause micro-structural changes to the material as well as inducing residual stress. Also, the short-range residual stresses produced by some of the methods do not accurately represent the long-range residual stresses found in many engineering structures. In each of these methods residual stress measured at room temperature before the start of test will change when the specimen is subjected to high temperature. None of the methods determine the effect of residual stress on creep and complete structure when the residual stress relaxes. We therefore conclude that new methods which can induce long-range residual stress without causing micro-structural change in the material and can represent combine boundary condition are required. A new method based on a three bar structure illustrated that residual stresses can be induced into a specimen at high temperature in a controlled manner and can be characterised easily without the use of time consuming measurement techniques. The proposed method does not cause any micro-structural change in the specimen and provides details of residual stresses distribution in complete structure at any time. Calibration tests revealed that the structure replicates mixed boundary conditions. Different combinations of diameters of the middle and side bars can be used to achieve different elastic follow-up factors and can therefore study the influence of EFU on initial residual stress and structure as a whole. The new method and the test rig designed can be used for both cracked and uncracked specimens to carry out short and long term creep tests.
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