13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Bespoke control software was also developed to perform and record these tests. 300 350 400 450 500 550 600 650 700 750 0 100 200 300 400 500 Time (seconds) Temperature (°C) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Normalised Load Temp (°C) Normalised Load Figure 3. The desired load (normalised) and temperature cycle for the measurement of crack growth rate 3.1 Temperature Measurement Dynamic temperature measurement during TMF, when using radiant lamp furnaces, requires a reliable experimental method and standard mineral insulated N type thermocouples have been used but these require careful control of shielding from direct radiant light on the thermocouple tip. Spot-welded N type thermocouples (i.e. thermocouple wires spot-welded onto the specimen surface ~1 mm apart, as recommended in [5] and location shown in Figure 4) have been used to calibrate the thermal behaviour of the test. These are thought to provide the most accurate test piece temperature but cannot be used during the test because of the risk of inducing cracks. Figure 4. Schematic of the placement of the thermocouples for the TMF thermal trials 3.2 Thermal Cycle Calibration Prior to TMF testing, there were two calibration trials required: • to demonstrate that the temperature variation (gradient) was less than 10oC throughout the desired thermal cycle, using the spot welded thermocouples located as shown in Figure 4. Thermocouple Clamp Thermocouple Clamp 2mm 2mm Defect Sheathed Thermocouples Spot Welded Thermocouples 6.5mm 6.5mm Not To Scale TC1 TC2 TC3 TC3 TC4 TC5 TC6 TC7 TC7 BCtrl TMon 1mm TC1 & TC2 here Cross-section through test piece at defect
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