13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 3.2. Calibration tests A series of calibration tests were conducted and divided into two categories; preliminary tests and an load-unload test. Tests were carried out at high temperature and using a 316H stainless steel C(T) specimen. The C(T) specimen was manufactured as per ASTM 1457 but using a screw fitting arrangement rather than pins to load the specimen. This was adopted to ensure accurate measurement of stiffness rather than conventional pin loading. To restrict the crack growth, a 1 mm diameter hole was introduced at the end of standard 0.1 mm wide EDM notch. Preliminary tests were conducted to determine the stiffness of bars, specimen and determine the overall elastic follow-up value for the rig experimentally. Two applied load tests, within the elastic region, were conducted such that in each case the temperature of the C(T) specimen was maintained at 5500C. In the first test only the middle bar with the C(T) specimen was connected to both end pieces. In the second test both the side bars are connected to the top and bottom end bars but the middle bar with C(T) specimen was connected only to the top end piece such that it was free to move (i.e. used to record the C(T) specimen temperature). In each case, the load cell, the temperature at different locations, strain gauge, total displacement and CMOD were measured. The results from these tests are discussed later. The second category of test was a load-unload test undertaken to understand the relaxation of residual force with applied load. First, a residual stress was introduced into the structure. The middle bar with the C(T) specimen was connected to both end pieces. Side bars were connected to only the top end piece and were free to move through the clearance holes in the bottom end piece. All instruments were then connected to the test rig and the furnace was heated to achieve 550 0C for the C(T) specimen. This arrangement permitted free thermal expansion of the bars and the specimen. When a stable temperature was achieved, nuts S11 and S21 (shown in fig 5) were screwed down, so that the top and bottom end pieces were forced apart. This resulted in the middle bar loaded in tension and the side bars carried the balancing compressive forces. The force in each bar was determined via the strain gauges. Finally when the desired residual force was introduced into the structure, nuts S12 and S22 on side bars were fixed. Having introduced the desired residual stress into the C(T) specimen, the entire assembly was repeatedly loaded and unloaded to progressively higher load levels. The residual force in all three bars, load line displacement of C(T) specimen, potential drop readings and overall extension of the rig were recorded for both load and unload path. Table 1. Properties of materials for three bar structure Material Young’s Modulus (GPa) Yield Strength (MPa) Tensile Strength (MPa) 316H stainless steel at 5500C 151 172 444 Nimonic 80A at 5500C 187 875 1210 EN24T Steel at room temperature 210 680 925 Table 2. Comparison of theoretical and experimental values Ks N/mm Kin N/mm Kout N/mm Keff N/mm Β αeff Zs Zeff Z Theoretical 69696 155047 85139 48083 2.23 3.54 1.45 1.28 1.86 Experimental 75781 155647 81761 50967 2.05 3.21 1.48 1.31 1.95
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