| Thermal barrier coating is usually composed of a ceramic layer（YSZ）and bonding layer（Mcr Al Y）on the substrate materials such as aero-engine turbine blades.Due to its low thermal conductivity,high heat resistance and excellent corrosion resistance,thermal barrier coating can be used to protect the substrate materials from heat insulation and corrosion resistance.However,in the service process of aero-engine,due to the influence of high-temperature friction and sand erosion wear,thermal barrier coating will be thinner,which greatly reduces its thermal protection ability.Or due to the thermal expansion difference between the ceramic topcoat and the substrate material,thermal barrier coating will fall off,and even lead to catastrophic accidents in serious cases.To ensure aviation flight safety and eliminate safety hazards,photothermal nondestructive testing technology with outstanding advantages,such as non-contact,non-destructive,high-resolution,large detection area,and easy-to-implement,has been paid attention to in the field of aviation equipment in recent years.Therefore,this paper focuses on the theory and key detection techniques of photothermal nondestructive testing for thermal barrier coating of aero-engine.Firstly,an extrapolated model of Beer-Lambert law for accurately describing the photon-density-wave field distribution in semitransparent turbid media is proposed,and combined with the Green function method,the photothermal propagation mechanism and multi-physical FE simulation of thermal barrier coatings are investigated in depth.Then,an advanced nonlinear frequency modulation excitation waveform and a high-resolution,automatic photothermal detection technology for the subsurface defects in thermal barrier coating when the raw thermographic data is highly missing are proposed.Finally,the thermal barrier coating samples are designed and the laser-induced thermographic experiment for these samples is also carried out.Overall,this study not only improves the existing theory of photothermal wave propagation of thermal barrier coatings but breaks through the existing photothermal nondestructive detection technology of thermal barrier coatings.And it can also provide a powerful detection method or means for high-resolution,rapid and reliable nondestructive detection of the subsurface defects of thermal barrier coating of aero-engine.The specific research content and results of this paper are as follows:Firstly,an extrapolated model of Beer-Lambert law which can accurately describe the photon-density-wave field distribution in semitransparent turbid media is first proposed.The effects of the absorption coefficient,scattering coefficient,anisotropy factor and relative refractive index on the effectiveness of the proposed model are studied.The results show that although the absorption coefficient,scattering coefficient,anisotropy factor and relative refractive index affect the distribution of photon-density-wave field in the semitransparent turbid medium,its effectiveness is not affected.Also,compared with traditional methods,the proposed model has significantly higher accuracy,for example,for the typical highly absorption medium with an absorption coefficient of 10 cm-1and a scattering coefficient of100 cm-1,the accuracy can be improved by at least 3.7 times.This model overcomes the limitations of the traditional radiative-transfer-equation,such as the inability to accurately describe the distribution of the photon-density-wave field in the near field and its complexity of the calculation.Therefore,the model provides theoretical support for the photon-density-wave field in the semitransparent ceramic topcoat of thermal barrier coating studied.Then,based on the extrapolated model of Beer-Lambert law and combined with the Green function method,a photothermal coupling model for the three-layer thermal barrier coating consisting of a semitransparent ceramic topcoat and two-layer opaque metal solids is established.The dependence of the thermal wave behaviours on both the thickness and photothermal properties of the semitransparent coating is investigated using the model.Inspired by high-precision radar target detection,a nonlinear frequency modulation waveform is also proposed for the high-resolution detection of subsurface defects in thermal barrier coating.The results show that the waveform has significantly better pulse compression quality than conventional waveforms and is also easy to implement.In addition,to verify the effectiveness of the established model and the proposed waveform,the photothermal multi-physical field numerical simulation of thermal barrier coating is carried out,and the influence of the thickness and its thermal diffusivity of the semitransparent ceramic topcoat on the nonlinear frequency modulation radar is also analyzed.The results show that the depth resolvability of the nonlinear frequency modulation radar is positively correlated with the thermal diffusivity of the ceramic topcoat and negatively correlated with the thickness of the ceramic topcoat.Again,to verify experimentally the validity of the theoretical model and the proposed nonlinear frequency modulation excitation waveform,10 thermal barrier coating samples with different ceramic thicknesses and metal substrates are also designed.The laser-induced thermographic testing system is first used to conduct lock-in thermographic experiments on the above samples,and the effects of excitation frequency,topcoat thickness and metal substrate on the lock-in detection results of the subsurface defects of the thermal barrier coating are studied.Then,a comparative study of the proposed nonlinear frequency modulation waveform and other conventional waveforms is also carried out experimentally to verify its superiority.The influence of the topcoat thickness and metal substrate on the detection results of nonlinear frequency modulation radar of subsurface defects for thermal barrier coating is also revealed.Finally,the photothermal nondestructive detection technology based on morphological filtering and K-means clustering image segmentation algorithm are proposed respectively,which provides important technical support for the high-resolution and automatic detection of subsurface defects of the thermal barrier coating.Finally,to address the problem of missing thermographic data in thermographic NDT,and achieve rapid thermal imaging without losing the resolution of the thermal image,a photothermal NDT detection technology based on smooth low-rank tensor completion is proposed.The detection results of lock-in thermal imaging and nonlinear frequency modulation radar for the subsurface defects of thermal barrier coating with different topcoat thicknesses and metal substrates are systematically studied by the proposed technique.The results show that the technique can reduce the amount of raw thermal image sequence by at least 20 times on the premise of enhancing the detection resolution,so the amount of thermal image acquisition can be significantly reduced.In addition,it is also demonstrated that the proposed photothermal detection technology can be further combined with the K-means clustering algorithm,and achieve high-resolution and automatic detection of the subsurface defects in the thermal barrier coating when the raw thermographic data is highly missing. |
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