Experimental Mechanics @ Michigan State University
		EM Basics: Thermoelasticity
Raw thermoelastic data from a mixed mode crack in aluminium				Raw uncalibrated thermoelastic data from a hole in aluminium plate.
Basic principles: When a body is subjected to a tensile stress the temperature of the body reduces slightly, and conversely when subjected to a compressive stress a slight increase in temperature occurs. When the body is in a cyclic loading regime, for instance during repeated loading or vibration, the small cyclic change in surface temperature of the body can be measured. This temperature change is directly proportional to the change in the sum of principal stresses on the component surface. Thermoelastic detection systems output the magnitude of this cyclic temperature change and also the phase of the cycle compared to a reference signal, commonly taken from a strain gauge applied to the component surface or from a load cell of a servo-hydraulic loading frame. This output can, with calibration, be directly related to the change in the sum of the principal surface stresses. The only surface preparation required is to ensure the uniform emissivity of the component surface in the infrared wavelengths to which the detector being used is sensitive, which can usually be achieved by the application of a thin layer of matt black paint. Because the detectors are measuring photon emission from a surface, the technique can also be used obliquely, with satisfactory results obtainable at up to 60 degrees from the axis normal to the surface.
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