Measurement Principles of TSA

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 in compression and decrease in temperature when in tension. As such, the change in temperature is directly proportional to change in stress.

Diagram of the Thermoelastic Effect
Figure 1 – Diagram of the Thermoelastic Effect

The key requirement for coupling the volumetric deformation and change in temperature is through the application of an oscillatory/dynamic load on the object being imaged. The load signal or suitable proxy (such as a strain, displacement or acceleration signal) must be fed into the ThermoESA sensor. Such signals may be sourced from either; a load cell, LVDT, DIC system, laser displacement sensor, strain gauge, and/or accelerometer.

Graph of load (blue) and temperature (black) vs. time, indicating the relationship between the two measured parameters.
Figure 2 – Graph of load (blue) and temperature (black) vs. time, indicating the relationship between the two measured parameters.

With these conditions in place, the TSA system performs a synchronous cross-correlation sampling process (aka. Lock-In) on the phase data from the load signal and infrared video stream. This extracts the stress related signal (temperature response) at each pixel location, whilst diminishing all extraneous components including random noise. Using a process known as “ensemble averaging” blocks of data are acquired, and the averaged temperatue (signal) response is maximizsed, whereby the temperature amplitude deviation (change in stress) can be determined with near certainty.

TSA measurement principle
Figure 3 – TSA measurement principle

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