Ultra-high temperature video extensometer: System development and experimental validation

The video extensometer has been widely advocated for tensile/compressive strain measurement in high-temperature material testing due to its advantages of non-contact measurement, wider measuring range, and larger applicable temperature over traditional clip-on mechanical extensometers. However, existing video extensometers, despite the adoption of active imaging devices, cannot adapt to the rapidly changing thermal radiation from the heated sample and surrounding heating elements during the high-temperature tests. This is because due to the significantly intensified thermal radiation, the decorrelated images degrade the measuring accuracy or even destroy the analysis. To address this problem, we developed an ultra-high temperature video extensometer that can automatically adjust the camera exposure time for reliable thermal strain measurement. Based on the pre-established image quality evaluation criteria, the camera will choose an appropriate exposure time according to the detected thermal radiation within the region of interest, thus ensuring high-quality images for real-time strain measurement. Static tests of tungsten-molybdenum alloy samples at different temperatures were performed to evaluate the noise level of the established ultra-high temperature video extensometer. The effectiveness and accuracy of the developed ultra-high temperature video extensometer were validated through the tensile tests of tungsten-copper, tungsten-potassium, and tungsten-molybdenum alloy samples at high temperatures up to 2000 °C.