Thermoelastic Stress Analysis by Means of a Standard Thermocamera and a 2D-Fft Based Lock-In Technique

Abstract

The thermoelastic effect in a generic orthotropic body under adiabatic and linear elastic stress conditions induces small and reversible temperature changes that can be correlated with the stress field by means of linear relationships. Infrared techniques are mainly used to measure temperature changes on the component surface [1], while adiabatic conditions are generally achieved by applying cyclic loads above a threshold frequency. The resolution resulting from implementing a Thermoelastic Stress Analysis (TSA) technique is a combination of the infrared acquiring system resolution and of the signal post-processing procedure. State of the art infrared detectors have NETD (Noise Equivalent Temperature Difference) values ranging between 0.1 and 0.01 K. In order to further enhance the resolution and to filter out noisy components affecting the small temperature changes induced by the thermoelastic effect, the most employed signal processing technique is a lock-in analysis. This results in a narrow-band filter where the components of the measured signal at frequencies different from a reference one are identified and rejected. If the reference frequency is the loading frequency, the harmonic filtered is the one carrying the amplitude and phase information of the thermoelastic signal.