Thermoelastic Stress Analysis

Thermoelastic Stress Analysis is a form of differential infrared thermography which, by monitoring the change in temperature of a component under transient load, allows the change in stress on the surface of a component to be determined. This is because small changes in stress result in small changes in temperature, and these fluctuations in temperature can be detected using sensitive infrared camera systems.

TSAfundamentals

The Thermoelastic Principle

Everything has a temperature, and at any point above absolute zero the atoms which make up a substance exhibit Brownian motion. In elastic solids, atoms are associated with their neighbours by electron bonds, which are easily thought of as tiny springs holding everything together. The combination of Brownian motion, and the electron bond ‘springs’, make up a constantly vibrating network of atoms.

At a particular temperature (e.g. ambient laboratory temperature) the atoms are vibrating and continually absorbing and re-emitting infrared photons due to their temperature, and everything is in equilibrium. But if the springs between the atoms are stretched very slightly (applied tensile strain) the Brownian motion of the atoms decreases, and the infrared emission decreases accordingly: the atoms ‘calm down’ slightly. Similarly, if the springs are compressed, the Brownian motion increases and the infrared emission increases: the atoms become slightly more ‘frantic’.

In practice, if an elastic solid is subjected to a reasonably rapid change in strain, it exhibits an observable change in surface temperature. Reasonably rapid typically translates into a cyclic test frequency greater than a couple of cycles a second. This temperature change, for typical engineering materials below the yield stress, will be of the order of a few tenths of a degree centigrade (several hundred mK).

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The example data above show: (left) the thermoelastic response of a uniaxially loaded plate with a central hole - the hole creates a stress concentration, which is revealed by the thermoelastic method, and with calibration can describe the full distribution of principal stress sum over the plate surface; (right) the thermoelastic response of a mode I crack growing across a planar aluminium specimen, which with analysis of the stress distribution around the crack tip can yield the crack tip stress intensity factors KI and KII.

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