Thermoelastic stress analysis by means of an infrared scanner and a two-dimensional fast Fourier transform-based lock-in technique

An infrared thermographic experimental set-up has been proposed and evaluated towards the capability to measure thermoelastic-effect-induced temperature changes. A standard infrared thermocamera with a nominal noise-equivalent temperature difference (NETD) resolution of 0.12 K has been employed to measure the temperature from unidirectional glass-reinforced plastic tensile coupons under cyclic sinusoidal loads. The raster scanning mode of the camera single detector produces a time delay in acquiring the signal from two succeeding pixels on the same row, and from consecutive scanned rows. By exploiting the acquired dwell times, it was possible to produce a periodic pattern on the thermal maps, caused by and correlated with the thermoelastic-effect-induced temperature changes. The acquired raw data have then been post-processed with a lock-in algorithm implemented in MATLAB® and based on a two-dimensional fast Fourier transform analysis. The filtered thermoelastic component from the lock-in analysis showed good linearity with the load applied, up to values of the temperature change one order of magnitude lower than the NETD resolution limits of the thermocamera. In the light of this the present experimental set-up and processing methodology can be proposed as a potential low-cost tool for thermoelastic stress analysis investigations.