Thermal correction of thermoelastic data by a direct calibration of the thermal signal

Abstract

In thermoelastic stress analysis (TSA) the temperature change induced in a solid material as a result of the thermoelastic effect is measured with an infrared detector. Common infrared detectors used in TSA have a nonlinear signal to temperature relation. If a measurement is calibrated with a calibration constant determined previously at a specimen with a different absolute temperature, as well as if the measured surface has a non-uniform absolute temperature, the sensors' nonlinear response will cause significant errors. The thermoelastic signal therefore has to be corrected by the amount of signal change caused by the sensors' nonlinear response. This can be done by applying a temperature correction term to the thermoelastic signal S. As the absolute temperature is necessary to calculate the temperature correction term the thermal signal SDC has to be calibrated against temperature. It was reported that the temperature correction term depends on the system settings applied when a measurement is performed, more precisely on the electronic iris. In this work, the thermoelastic signal is corrected by a direct calibration of the sensors' thermal signal SDC. Therefore the sensors' absolute temperature to signal relation is determined with respect to the sensors' exposure time (ET). Based on the found thermal signal to temperature relation, a formulation is given which shows that the uncalibrated thermal signal and the thermoelastic signal are linearly related and that the slope of this linear relation is independent of the camera settings applied when the measurement is performed. This linearity is used for a thermal correction of thermoelastic data calibrated against stress. To calibrate the thermoelastic signal against the load-induced temperature change the found thermal signal to temperature relation is implemented in the DeltaVision calibration file. The results of the thermoelastic signal calibrated against temperature are compared with a calibration against the physical properties of the measured material.