13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 3 shows the three different working coil configurations on cylindrical solid specimen. The results showed that the working coil configuration A (6 turns with gap in the middle) gives larger thermal gradients (30~40°C) at 500°C. A configuration of longitudinal opposite-direction working coil (Fig. 3-5 B) was then tested, the result showed oppose thermal gradient (500°C in the center, 520~530°C at 12.7 mm away from the center). After several trial and error experimentations, the third configuration C was designed. Two separate working coils were attached on the power unit, and each with three turns. From the experimental result of this configuration, the dynamic thermal gradient almost disappears (within 5~10°C) along the axial direction at 500°C. Therefore, through this series of experimentations of working coil configuration, two separate working coil gives the minimal thermal gradient in axial direction. 2.2. Control System To conduct in-phase strain-controlled TMF test, a program was written in the control system to accurately manage all the components. The dynamic temperature of the specimen is measured by the spot-welded thermocouple in the center of the gage length, and the temperature controller sends the measurement to the signal controller synchronistically. The signal controller send the commands to the heater and air jet during the heating and cooling phases, and it also controls the axial strain to feedback the hydraulic piston to apply the load. All the data is stored by the data acquisition on timed-basis. The strain is controlled by the dynamic temperature signal as shown in Fig. 4. The thermal cycling is carried out at beginning of each test, and thermal strains at Tmax and Tmin are recorded. The total strains at Tmax and Tmin are calculated: max max T T Tot Thermal mech ε ε ε = + (1) min min ( ) T T Tot Thermal mech ε ε ε = + − (2) A strain-temperature range ratio δ is defined as: ( ) ( ) max min max min T T Tot Tot T T T ε ε ε δ − ∆ = = ∆ − (3) both ε and T in voltages. The ratio of input to output voltages of the gain amplifier and attenuator is then adjusted to the ratio of δ. The temperature cycling between Tmax and Tmin will also generate a voltage which is equal to total strain at the corresponding temperature. After the test, thermal strain is fitted as a function of temperature in a polynomial equation. The mechanical strain can be obtained by subtracting the thermal strain from the total strain.
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