TSA relies on a coupling between volumetric deformation and a reversible change in temperature, called the thermoelastic effect. This means, during elastic deformation, a solid will increase in temperature when undergoing compression (shown as pink) and decrease in temperature when undergoing tension (shown as yellow). As such, the change in temperature is directly proportional to change in stress.
The key practical requirement for determining the bulk (volumetric) stress via a temperature measurement is the application of an oscillatory/dynamic load to the object being imaged. The load signal is then read-into the ThermoESA sensor where it is synchronized with the IR-camera signal recording stream (at each pixel). Providing the object is relatively stationary and has a high surface emissivity, a TSA measurement can be performed. Suitable proxies for the load signal, include;
- force/load from a load cell
- displacement from an LVDT or laser displacement sensor
- displacement or strain from a DIC system
- strain from a strain gauge, and/or
- acceleration from an accelerometer
With the load signal & IR-camera signal recording stream, the ThermoESA / MiTE SW performs an in-situ, “lock-in” ensemble-averaging cross-correlation. The IR-camera temperature response signal is cumulatively averaged at discrete time intervals (phased-locked) over N-blocks. Using this method the noisy IR-camera signal is effectively cleaned over time via averaging the noise effects.
