Lock-In Signal Post-Processing Techniques in Infra-Red Thermography for Materials Structural Evaluation

This paper describes the potential of off-line thermographic signal processing by means of Lock-In Correlation algorithms, in order to implement structural health monitoring and stress analysis techniques. Thermal datasets acquired by infrared thermocameras are locked-in and correlated numerically with opportune reference signals, and amplitude and phase values of various harmonics retrieved by means of a Fast Fourier Transform and time averaging based filtering. This information is then processed for NDT defect probing and for evaluating the Thermoelastic Effect induced temperature changes. Two case studies in particular are discussed, implementing the proposed signal lock-in processing: a Thermoelastic Stress Analysis of a Brazilian disc under cyclic compression, and NDT of a delaminated polymer matrix composite panel. The NDT case study evidences the ability of the lock-in algorithm to analyse the response of the component to multi-frequency modulated heat waves, while both case studies demonstrate the potentials of the off-line signal treatment to enable low cost thermal setups.