Electro-thermal Modeling and Matlab-simulink Simulation of Lock-in Thermography for Non-destructive Characterisation

This paper proposes and validates a novel simulation for thermal non-destructive characterization. An electro-thermal modelling of the sample is adapted for MATLAB-SIMULINK simulation. Though the technique is capable of considering any heat stimuli on an arbitrary sample, the special case of lock-in thermography (LT) with periodic heating is taken up for illustration. A mild-steel sample having defects at different depths is taken as a test sample. Phase of the reflected thermal wave having great importance in LT, is obtained by measuring time delay of the reflected thermal wave. Predictions from the proposed simulation technique are shown to match well with experimental results. Introduction: Presently two different types of active thermography [1] are mainly in use: Pulse and Lock-in thermography. In pulse thermography, the examined material is warmed up with a short energy pulse (light, eddy current, or ultrasonic pulse) and the thermal response recorded with time. The resultant infrared image sequence indicates material defects at different depths, but it is also affected by local in-homogeneities of the material surface as well as non-uniform heating. Lock-in thermography uses sinusoidal intensity modulated thermal excitation in order to derive information from the observed phase and magnitude of reflected thermal wave. The phase angle has the advantage that it is independent of local variations of illumination or of surface emissivity. The present work describes a MATLAB-SIMULINK [2] simulation for lock-in thermography. The defect depth is estimated by measuring the phase of the reflected thermal wave. Simulated results show good match with experimental results. The basic concept of Lock-in thermography is the periodic heating of the sample surface by a sinusoidal intensity modulated heat source as shown in Fig.1. This causes a heat distribution to propagate into the material. Reflection of the thermal wave is caused, if it finds a different thermal impedance medium. For the purpose of analysis, the sample is considered as semi-infinite, on to which a uniform heat source periodically deposits heat at a modulating angular frequency ω. Then, neglecting convection losses, the temperature T, as a function of depth z and time t is given by [3], T (z, t) = T 0 e-(z/µ) cos(2πz/λ – ωt) (1) where T 0 is amplitude of the oscillating temperature, z the depth below the surface, λ is the wavelength of thermal wave, ω the angular frequency and µ is the thermal diffusion length [3], defined as follows µ = λ/2π = [2α/ω] 1/2 …