6.2.2 Full-Field Thermal Imaging and Calibration Thermal image capture is done with Telops FAST-IR 1000 high speed infrared camera, capable of 256 320 pixel resolution at speeds up to 1000 fps and a reduced pixel resolution at speeds up to 30,000 fps. The exposure time of the IR camera varied in the current tests from 100μsdown to5μs. With decreasing exposure time the IR camera can register higher surface temperatures, but at the same time the lowest measurable temperature increases so that the usable temperature range at each constant exposure time is ca. 250 K, as seen in Fig. 6.3. The resolution, speed and exposure time for all cameras used in the various strain rate tests are listed in Tables 6.1 and 6.2. To synchronize the Phantom visible camera and the IR camera, they are externally driven by a dual-channel Keysight 33,500 series waveform generator, which enables running the cameras at different but phase-synchronized frequencies. In practice, the visual camera typically is run at a frequency which is the same or a multiple of the IR camera frequency. The waveform generator sends TTL level timing pulses to both cameras in an externally triggered burst mode. In the low strain rate tests with the MTS frame, the external trigger to the waveform generator is sent through the digital output port of the controller unit 100 ms before the actuator starts to move. The data acquisition of the MTS controller starts at the same time as the trigger is sent, and therefore the images taken by both cameras as well as the load cell and LVDT signals of the load frame have a common recorded time base. In the intermediate strain rate apparatus the trigger signal is generated by a proximity sensor detecting the movement of the hydraulic actuator. The force signal obtained from the strain gage bridges is also synchronized with the cameras. The time of the force signal is shifted in order to account for the time it takes for the wave to propagate from the specimen end to the strain gage bridge location on the transmitter bar. With the Split Hopkinson Bar, Photron SA 1.1 camera is used instead of the Phantom camera because of its better resolution at the required higher frame rates. Since the Photron camera cannot be clocked externally, the synchronization with the IR camera is done by using one channel of the waveform generator to run the infrared camera and using the other channel to trigger the autonomous image acquisition of the Photron camera at a preset frequency (normally a multiple of the IR camera’s framing frequency). Fig. 6.3 Exposure time versus measurable temperature range for the Telops camera Table 6.1 Visual cameras with resolutions and frame rates used in the various tests Strain rate (s 1) Camera Pixel resolution Frame rate (fps) 0.01 Vision research phantom v7.3 800 by 600 5 0.1 Vision research phantom v7.3 800 by 600 70 1 Vision research phantom v7.3 800 by 600 500 200 Vision research phantom v7.3 512 by 128 20,000 3000 Photron SA 1.1 384 by 128 90,000 40 J.L. Smith et al.
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