13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 2.2. Behavior of the structure with applied load Figure 4 shows loading conditions and relaxation during plasticity for a representative three bar structure. AA’ gives the initial misfit, OB and OB’ give the residual force in middle and outer bars respectively. When the structure is further subjected to external load, the force in the middle bar increases from B to C. At point C yielding of specimen starts and the specimen takes no further load and follows line CD while outer bar remains in the elastic range and follows path A’D’. Until yielding at point C, the initially induced residual force remains constant in the middle and outer bars and follows paths B-1 and B’1’ respectively. As the applied load increases and the amount of plastic deformation in the specimen becomes equal to the initial misfit, the residual force reduces to zero. The rate of residual force relaxation for a mixed boundary condition follows path 1-2. The gradient of relaxation curves (1-3,1-2 or 1-4) depend upon the elastic follow-up factor. Structures having high EFU will follow path 1-4 while low EFU structures will follow path 1-3. Hence, the influence of initial residual stress present in a structure is dependent on the associated level of EFU. Figure 4. Loading and subsequent relaxation of a three bar 3. Experimental studies 3.1. Design of test rig An experimental three bar test rig was designed based on the conceptual model discussed in the earlier section. Since the design was centered around a conventional cylindrical furnace with a maximum internal diameter of 130 mm for a creep test frame, the design was constrained to the available maximum diameter of the furnace, the overall length of the structure (1000 mm), the ease of assembly, the ability to introduce known residual stress at high temperature and then to apply a predetermined load to the assembly. The overall arrangement of the test rig is shown in fig 5 and the material properties of the components for the test rig are tabulated in table 1. The middle bar was a combination of 316H stainless steel C(T) specimen and a Nimonic bar of diameter 25.4 mm while the side bars were Nimonic bars of diameter 19.05 mm. These dimensions are selected to provide an EFU value of about 2 (as shown in fig 3). All bars were screwed to a top and bottom end piece
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