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A Barker-coded modulation thermal-wave imaging (BCTWI) with laser as an external stimulus source based on pulse compression technique is reported on carbon fiber reinforced polymer (CFRP) laminates delamination detection. CFRP samples with different size and depth flat bottom holes (FBHs) are prepared for providing simulated defects of CFRP interlayer delamination. Three kinds of thermal wave feature images (phase, delay time and peak) are formed and deeply studied on their capability of detectivity. Experimental results indicate that delay time and phase images are relatively suitable modality for low-power active thermography requirement due to insensitive to the various excitation power. BCTWI significantly improve SNR and defect depth resolution in comparison with linear frequency modulation thermography (LFMT).

In this paper, a combination active thermography and three-dimensional (3D) structural optical imaging is reported on the nondestructive inspection of curved clad composites with subsurface defects. Artificial points are marked on the background of test objective and helpful to obtain the mapping matrix from two-dimensional (2D) thermal wave characteristic imaging data to 3D spatial coordinate’s digital model. The 3D digital model is transformed into a new coordinate system and then reprojected onto the thermal-wave feature image exactly. The fusion of 2D thermal-wave characteristic data and the 3D digital model is achieved by bilinear interpolation. Specimens of carbon fiber reinforced polymer (CFRP) skin-honeycomb sandwich (CFRP-SHS) and High silicon oxygen phenolic resin (HSOPR) sheet with artificial defects are prepared for the experimental investigation. Experimental results indicate that the proposed method can reliably inspect the defects and accurately locate defect positions on the curved surface of CFRP-SHS and HSOPR laminate test sample, and it has a significantly potential for the use of aerospace and aircraft manufacturing and application.

In this work, a one-dimensional (1D) photoacoustic mathematic model of turbid medium in frequency domain is presented, and it is derived from the theories of optical absorption and radiation transfer, thermal-wave diffusion and mechanical fluctuant effect. The theoretical simulations are carried out to analyze the effects of absorption coefficient, the relative position of the absorber on the amplitude and phase of the photoacoustic signal. A frequency – domain photoacoustic imaging system is developed to carry out the position scanning inspection experiments. At the fixed laser modulation frequency, the absorption property of the medium is detected according to the positive correlation between the amplitude of the photoacoustic signal and the absorption coefficient. Based on the relationship between the relative phase of the photoacoustic signal and the relative position of the medium, the depth of the optical absorber inside the turbid medium is quantitatively detected. The experiment results indicate that the amplitude and phase of the photoacoustic signal have high sensitivity to the optical absorption coefficient and structural characteristic parameters of the turbid medium.

As an important part of solid-propellant missile engine, the quality assurance of cladding layer ‘high silicon oxygen phenolic resin (HSOPR) sheet’ is very important. In this paper, independent component analysis enhanced pulse thermography (ICA-PT) was introduced to detect the subsurface defect of HSOPR. The basic principle of ICA was described. The experiment of HSOPR sheet with simulated flat bottomed holes (FBHs) detected by ICA-PT was carried out. Moreover, the comparative experiments of different algorithms such as pulse phase thermography, continuous wavelet transform and principal component analysis were carried out. The SNR of the characteristic image was analyzed. The experimental results demonstrated that ICA algorithm can improve the detection SNR significantly. Therefore, the research on ICA-PT is of significant importance to quantitative evaluation the quality of the high silicon oxygen phenolic resin sheet.

Photothermal diffusion-wave imaging and thermal tomography are promising techniques for the analysis of debonding detection of the solid-propellant missile engine. In this paper, the dynamic thermal tomography based on continuous wavelet transform (CWT-DTT) was introduced to detect the defect and reconstruct the 3D (three-dimensional) visualization of the high silicon oxygen phenolic resin cladding layer. The basic principle of CWT-DTT and the tomography processing were described. Moreover, 3D thermal wave model stimulated by pulse signal was developed. The relationship between the SNR of the feature image and the scale factor of CWT was analyzed. The tomography experiment of cladding layers with simulated flat bottomed holes (FBHs) defects by CWT-DTT was carried out. The experimental results demonstrated that the measuring depth of all FBHs has a good agreement with the actual depth. Therefore, the research on CWT-DTT is of significant importance to quantitative evaluation the quality of the high silicon oxygen phenolic resin cladding layer.

In this paper, a one-dimensional (1D) thermal-wave model coupled diffuse-photon-density-wave for three-layer dental tissues using modulated laser stimulation was employed to illustrate the relationship between dental caries characteristic (i.e. caries layer thickness, optical absorption coefficient and optical scattering coefficient) and photothermal radiometry (PTR) signal. Experimental investigation of artificial caries was carried out using PTR scanning imaging. The PTR amplitude and phase delay were increased with dental demineralized treatment. The local caries characteristic parameters were obtained by the best-fitting method based on the 1D thermal-wave model. The PTR scanning imaging measurements illustrated that the optical absorption coefficient and scattering coefficient of caries region were much higher than those of the healthy enamel area. The demineralization thickness of caries region was measured by PTR scanning imaging and its average value shows in good agreement with the digital microscope. Experimental results show that PTR scanning imaging has the merits of high contrast for local inhomogeneity of dental caries; furthermore, this method is an allowance to provide a flexibility for non-contact quantitative evaluation of dental caries.

Heterodyne lock-in thermography (HeLIT) is a highly sensitive method to detect early demineralized in dental tissues, which is based on nonlinear photothermal phenomena of dental tissues. In this paper, the nonlinear photothermal phenomena of dental tissues was introduced, and then the system of HeLIT was developed. The relationship between laser modulated parameters (modulated frequency and laser intensity) and heterodyne lock-in thermal wave signal was investigated. The comparison between HeLIT and homodyne lock-in thermography (HoLIT) for detecting the different types of dental caries (smooth surface caries, proximal surface caries and occlusal surface caries) were carried out. Experimental results illustrate that the HeLIT has the merits of high sensitivity and high specificity in detecting different types of early caries.

The coating damage of SiC-coated carbon–carbon (C/C) composite is directly influenced on the anti-oxidation attack and ablation performances of carbon–carbon (C/C) composite substrate. It is unprecedentedly reported that infrared thermal-wave tomography (IR-TWT or TWT) approach with laser as external stimulation source is successfully employed for the evaluation of the SiC damage coated on the C/C composite substrate. The stimulation laser intensity is varied according to a chirp-pulsed chain, and employed to induce the chirp-pulsed radar thermal-wave signal. The thermal-wave signals of SiC-coated C/C composite dependence on coating thickness degradation and substrate material are deeply analyzed by using of one dimensional (1D) thermal-wave transfer analytical model. A temporal amplitude of thermal-wave signal as function of delay time is obtained by using of dual-orthogonal demodulation (DOD) algorithms, and it can be employed to represent thermal-wave transmission and to perform tomography. Experimental results confirm that TWT can be used to accurately and reliably assess the SiC coating damage characterization from the C/C composite substrate.

Laser excitation chirp pulsed photothermal radar imaging (CP-PRI), as a novel nondestructive testing and evaluation (NDT&E) approach, is employed to recognize the surface crack at glass fiber reinforced polymer (GFRP) composite materials. The mechanism of crack detection has been demonstrated in detail by a photothermal method and the principle of CP-PRI. A GFRP specimen with artificial surface crack is prepared for NDT&E by CP-PRI. First-order differential processing was employed to enhance edge sharpening of the amplitude image. The effects of the laser power, excitation pulsed width, reference pulsed width, delay time and chirp frequency on the signal-noise-ratio (SNR) of cracks are completely investigated. The results illustrated that the SNR of the crack is increased with the increase of laser power, the opening width of crack and excitation pulse width. The reference pulse width has few implications for the SNRs of the smaller opening width cracks (~100 μm). However, the reference initial delay time and chirp frequency had a significant influence on the SNRs of cracks. The comparison experiments between thermal wave radar imaging (TWRI) and CP-PRI has been carried out, its results indicated that the CP-PRI has the higher SNRs of cracks than that of TWRI. Consequently, CP-PRI is available for the detection of GFRP surface cracks as a result of its high detectability.

In this paper, the detection capability of laser-generated bidirectional thermal wave radar imaging (BTWRI) inspection for carbon/glass fiber reinforced polymer (C/GFRP) defects was quantitatively evaluated by signal-to-noise ratio (SNR) analysis of characteristic images. A low-power experimental system was devised, and the C/GFRP specimens with multi-scale diameter-to-depth ratio were tested. An improved imaging algorithm (analytical chirp correlation, ACC) was presented by applying analytical bidirectional chirp signal to extract the correlation phase and amplitude of thermal wave radar signal corresponding to the expected delay time. The optimal ACC detection image is obtained by the SNR comparison of characteristic images frame by frame. The conventional imaging inspection results of using cross-correlation (CC), cross-correlation phase (CCP), chirp lock-in (CLI), and Hilbert transform mean (HTM) algorithms were performed for SNR comparison. Experimental results indicate that the ACC algorithm displays almost the same detection capability as the traditional CCP algorithm by a simpler calculation process, and the optimal ACC detection image has a significant enhancement on SNR performance for C/GFRP subsurface defects compared with conventional imaging algorithms.

The linear frequency modulated ultrasound excitation thermal wave imaging (LFM-UTWI) was investigated on detection of subsurface defects of metal sheet. A numerical finite element analysis is carried out to calculate thermal wave signal dependence of time by linear frequency modulated ultrasonic wave excitation. Cross-correlation operation in time domain and frequency domain are used to extract the main peak value and the corresponding delay time, respectively. Fourier transform (FT) is applied to calculate the amplitude and phase angle of harmonic component of thermal wave. Experimental results show that various deep subsurface defects are readily detected using LFM-UTWI with once excitation, and LFM-UTWI has an advantage of better defect detectability compared to ultrasound lock-in thermography (ULIT).

In this paper, the thermoelastic stress analysis (TSA) on a multilayered composite structure (MCS) was investigated by means of lock-in thermographic image technique (LITI). The application of thermoelastic stress analysis on MCS becomes particularly complicated due to consisting of different material components, which determines the different thermoelastic coupling response depended on material thermal-physical property. The thermoelastic coupling constants (TCC) of GFRP, medium-carbon steel and foam were obtained through thermomechanical calibration experiments, respectively. An artificial neural network was proposed to determine the component of MCS. Comparisons between finite element analysis (FEA) and LITI measurement are reported. It is found that the stress distribution of MCS can be evaluated with good accuracies using LITI measurement.

The ultrasound lock-in thermographic technique was developed to detect interfacial defects and cracks in a metal plate. Discrete correlation method (DCM), Fourier transformation method (FTM), Short Time Fourier transformation (STF) and Discrete Wavelet Transformation (DWT) algorithms were used to extract the characteristic information of the thermal wave signal generated by ultrasonic wave modulated. It is found that STF and DWT algorithms are available for analyzing the thermal wave signal generated by ultrasonic wave modulated due to higher signal–noise ratio. Experiments were performed to investigate the effect on the amplitude contrast and phase angle contrast by the ultrasound transducer position, initial sonotrode action force and modulation frequency, respectively. Experimental results show that transducer position closed to the defect, higher initial sonotrode action force loaded and optimal modulation frequency selected are help to detect the defects of metal plate using ultrasound lock-in thermographic technique.

Lock-in thermography, an active IR thermography technique for NDT, is based on propagation and reflection of thermal waves which are launched from the surface into the inspected component by absorption of modulated radiation. In this paper, thermal wave image sequences were sampled by a Cedip JADE MWIR 550 FPA infrared camera. Thermal wave signal processing algorithms are investigated to obtain information on subsurface defects. The Fourier transform, four-point correlation, and digital lock-in correlation algorithms are applied to extract the amplitude and phase of thermal wave’s harmonic component. A novel method called the time constant method (TCM) is proposed to analyze subsurface defects by using lock-in thermography. The experimental results confirm the thermal wave signal processing algorithms’ efficiency on subsurface defect detection.