Issue 50

A. Kostina et alii, Frattura ed Integrità Strutturale, 50 (2019) 667-683; DOI: 10.3221/IGF-ESIS.50.57 668 I NTRODUCTION eramic coatings are widely applied in various fields of industry as thermal barriers which protect structures from high-temperature influence. They provide corrosion defense, improve the reliability of metallic details and have high resistance to fuels, lubricants and other chemical substances. The technological process of the coating consists of the adding of the one or several layers of ceramic particles on a solid surface. Apparently, adhesion of the coating with the main structure is not uniform. Initiation of structural defects such as delamination, voids, inclusions, cracks and other heterogeneities can substantially decrease the efficiency of the protection. Accurate identification of coating defects will improve the operational reliability of the structure and reduce the risk of disastrous consequences induced by its failure. Pulsed thermography is an active non-destructive technique which uses an optical excitation source (usually, xenon lamp or laser) to stimulate heating of the object under investigation. The registration of the temperature during cooling is carried out by an infrared camera which can be positioned on the same side with the excitation source (reflection mode) or on the opposite side (transmission mode). The duration of the heating is significantly less than the observation time. Thermal properties of the subsurface defect (air) are different from the properties of the main material. Therefore, the diffusion rate of the structure in case of the presence of the defect is reduced which reflects in higher values of the surface temperature above the discontinuity. The main parameter which is applied to the analysis of the data obtained during pulsed thermography is a temperature contrast. This value is estimated as the difference between the mean surface temperature of the area above the discontinuity and the mean temperature of the sound area (temperature in the reference region without defects). Temperature contrast depends on the size of the specific defect and its depth and usually susceptible to the surface noise induced by non-uniform heating, variation of emissivity and reflections from the environment [1-2]. Therefore, accurate prediction of the defect location requires the development of relatively simple and effective techniques of signal processing. Pulsed thermography can be applied to the identification of subsurface defects, their depths and sizes in metals, ceramics and composite materials [3-6] while mathematical models can provide a deeper understanding of this process and obtain optimal parameters for the experimental investigations. In [7] numerical simulations were performed to obtain optimal values of excitation parameters with respect to the evaluation of the paint coating thickness. A numerical model of the pulsed thermography for heterogeneous materials has been developed in [8] on the base of the finite volume method. The significant influence of such parameters as irradiation power density, non-uniform heating and geometric characteristics of the defects on the value of the temperature contrast was established with the use of the proposed model. M. Grosso et al. [9] applied a pulsed thermography model to the investigation on the limits of this method with regard to the adhesive composite joint. The sensitivity of this method to the thickness of the material, heating time and excitation energy was studied and the limiting values of the defect depths were obtained. R. C. Waugh et al. [10] developed a finite-element model of pulsed thermography procedure and applied it to several materials (aluminum, carbon fibre-reinforced plastic and adhesively bonded joints). The obtained results have shown that the main limiting factor of the pulsed thermography is the uncertainty in thermal material properties. In [11] an algorithm for an accurate simulation of the pulsed thermography is proposed. This algorithm updates several parameters (material properties, pulse duration, heat power and ambient temperature) using smoothed and refined experimental data. The proposed algorithm demonstrated a large optimization of the calculation time. However, not all of the material properties were defined properly and the accuracy of the prediction was affected by signal noise. More recent approaches for identification of subsurface defects are based on adaptive algorithms and artificial neuron networks. In [12] an artificial neuron network is applied to the estimation of the defect’s depth. Results of the numerical simulation obtained according to the mathematical model of the pulsed thermography procedure were used for generation of ideal (without noise) sets of training data. Verification of the proposed algorithm was based on its application to the experimentally obtained data. It has been shown that the accuracy of the prediction in the worst case is equal to 90%. In this work, a three-dimensional numerical model of the pulsed thermography method is proposed for the evaluation of the temperature contrast in steel plate with a ceramic coating containing artificial defects of various depths and sizes which represent delamination of the coating from the main structure. The developed model takes into account complex heat exchange which includes heat transfer by convection, conduction and radiation. The verification of the model was based on the experimental results presented in [13]. The first part of the paper is devoted to the investigation of influence of various forms and durations of the heating pulse on the dynamics of the temperature evolution. In the second part of the paper, the processing technique for thermal contrast data has been developed. The approach is based on the Kalman filter adapted to the problem of subsurface defect detection by pulsed thermography. The proposed methodology has C