Pulsed Phase Thermography Research Articles

Flash pulse phase thermography for a paint thickness determination

The contribution describes a fast contactless measurement of a paint thickness non-uniformity using flash pulse thermography. Specimens sprayed by a paint were thermally excited by a flash lamp and temperature responses were recorded by an infrared camera. The recorded sequences were post-processed with Fast Fourier Transform to obtain phase angles. Differences in the resulting images showed phase differences which corresponded to a paint thickness non-uniformity. Furthermore, the phases were correlated with the thickness by means of calibration curve so that the paint thickness could be determined with flash pulse phase thermography measurement. The method showed a promising potential in the contactless evaluation of the paint thickness. Average error of the thickness determination was less than 10 % for samples with paint thickness from 41 to 74 µm on AISI 304 substrate. Advantages, disadvantages and limitations of described method were discussed.

Debonding defects detection of FMLs based on long pulsed infrared thermography technique

Fiber metal laminates (FMLs) are the popular materials in the fields of aerospace, automobile and rail transportation. In the process of preparation and service, the defects of matrix, fiber fracture and fiber matrix debonding are easily produced, which poses a threat to the safety of FMLs’ quality, performance and the operational safety of the aircraft and other equipment using the material. Aiming at developing a method for defects detection in FMLs by long pulsed infrared thermography, test was applied to FMLs with debonding defects. Halogen lamps were used as active heat source into the sample, and the infrared image sequence was collected with an infrared camera A325sc. Thermal wave signal reconstruction, pulsed phase thermography and principal component analysis methods were used to process the original image sequences. Experimental results show that the signal-to-noise of processed images by these three methods are all increased, while that of principal component analysis method increases significantly under the same given test parameters, and it can be used to detect the defects in FMLs.

Image enhancement method for laser infrared thermography defect detection in aviation composites

A new infrared preprocessing modality called sequence differential processing is proposed. Different from the cold image subtraction which only decreases the background noise, the proposed modality takes the parallel test into account to preprocess the raw thermal data sequence prior to applying the advanced postprocessing techniques such as principal component thermography and pulsed phase thermography. The results show that the preprocessing method not only effectively reduces the influence of uneven laser energy distribution on detection efficiency but also enhances internal defect information. Moreover, the combination of the proposed preprocessing method with the pulsed phase thermography and principal component analysis algorithms improves the ability of laser infrared thermography to detect defects inside aviation carbon-fiber-reinforced plastics.

Improvement of the Non-Destructive Testing of Heritage Mural Paintings Using Stimulated Infrared Thermography and Frequency Image Processing.

Within the framework of conservation and assistance for the restoration of cultural property, a method of analysis assistance has been developed to help in the restoration of cultural heritage. Several collaborations have already demonstrated the possibility of defects detection (delamination, salts) in murals paintings using stimulated infrared thermography. One of the difficulties encountered with infrared thermography applied to the analysis of works of art is the remanence of the pictorial layer. This difficulty can sometimes induce detection artifacts and false positives. A method of thermograms post-processing called PPT (pulse phase thermography) is described. The possibilities offered by the PPT in terms of reducing the optical effects associated with the pictorial layer are highlighted first with a simulation, and then through experiments. This approach can significantly improve the study of painted works of art such as wall paintings.

Reconstruction of Simulated Cylindrical Defects in Acrylic Glass Plate Using Pulsed Phase Thermography

The results of testing of acrylic glass, in which cylindrical defects were simulated at different depths by applying Pulsed Phase Thermography, are presented in the paper. To ensure better visibility of the simulated defects, suitable thermal images were selected and then processed by using two different procedures. In the first procedure, reduced thermal image sequences were generated by uniform extraction from the basic sequence, to enable analysis at different sampling frequencies. The second procedure was based on the application of a window function, which ensured that only uniformly selected thermal images took part in the evaluation of the basic sequence. The remaining thermal images were not used, but they did participate in the determination of the length of the analyzed sequence; in other words, their existence was registered through the number of samples used in Fast Fourier Transformation. The second procedure yielded much better results with regard to the estimation of the shape of a defect and the depth at which it was located. To provide better insight into the development of the thermal process in the defect area, an additional analysis of pixel intensity variation in the time domain was undertaken.

Pulsed Phase Thermography Approach for the Characterization of Delaminations in CFRP and Comparison to Phased Array Ultrasonic Testing

Pulsed phase thermography (PPT) is a well-established algorithm used for processing thermographic data in frequency domain with the aim to extract information about the defect size and depth. However, few works demonstrated the capability of PPT technique in defects evaluation in real components. The aim of this work is the assessment of capability of PPT technique in determining delaminations in CFRP components used in aeronautics. The component chosen for implementing the technique has a non-uniform geometry and the defects inside it are not simulated, but they are real and generated during the production process. The specimen has been investigated through the application of both the ultrasonic technique and the thermographic one. Thermographic phase images elaborated with a suitable computational processing have been compared with Ultrasonic C-scan images and, the agreement between the location and depth of defects has been verified. Besides, the ultrasonic technique has been used to validate the PPT results.

Evaluation of anomaly detection capabilities using a non-orthogonal camera angle in pulse-phase thermography

Pulse-phase thermography (PPT) is widely used to nondestructively inspect internal defects in fiber reinforced polymers. However, the challenges using PPT for complex shapes is poorly documented in literature. Only small changes in the object distance have been considered.Complex parts can have significant variations in object distance and thus, in detected radiation. In this contribution, the effect of a non-orthogonal camera angle with respect to a flat sample, leading to varying object distances and an inhomogeneous sound background area in phasegrams, is investigated. Samples with artificial round and square defects of different sizes are positioned under varying angles with respect to the camera, representing geometric properties of complex parts. The construction of the thermographic system and the experimental setup to systematically vary the angle between camera and specimen is presented. We investigated the change of the signal-to-noise ratio (SNR) of artificial delaminations in PPT measurements under varying object distances. The SNR in a distance of 136 mm out of the focal plane is sufficiently high for image feature extraction. Phasegrams are exported to a colored representation, leading to a higher contrast in distinct color channels. An algorithm which extracts and merges defect information from three different color channels is developed. Challenging lighting conditions lead to a noisy background having artifacts. The developed filter performs better in defect detection and size quantification than a global or local threshold in grayscale phasegrams under those conditions.

Infrared Thermography of Steel Structure by FFT

Steel bottles, most commonly used by scuba divers, are prone to rust and need to be maintained properly and preventively examined during usage. Due to the geometrical characteristics of the tank (rounded wall) and the inaccessible inner surface of the wall where corrosion damages and initial cracks are expected to be found, the conventional nondestructive techniques do not yield usable results. Herein, emphasis is placed on non-destructive testing (NDT) by means of infrared thermography. The Pulse Phase Thermography (PPT) has been used for composite polymer matrix materials, but with the development of IR cameras, the method has found its application even for NDT of thin metal plates. In this paper, the Fast Fourier Transform (FFT) of infrared images is applied on severely corroded scuba divers tank. The amplitude and phase images contain information on damages or/and other structures inside the inspected part as these lead to a shift in the amplitude and phase of heat waves. In damaged areas heat wave passes shorter path due to differences in the thickness of the metal wall resulting in different heating/cooling curves. As a result, heat waves have different phase delay and defects can be seen as different color zones on phase images in characteristic pit corrosion oval shape.

Improving the probing depth of thermographic inspections of polymer composite materials

The maximum depth for which a defect of a given size can be detected in thermographic inspections is known as the ‘probing depth’. For materials such as polymer composites, with low through-thickness thermal diffusivity, inspections are limited to thin materials or near surface inspection. With the aim of improving probing depth, the paper describes how established signal processing techniques are adapted for thermographic inspections. The procedures are implemented for both pulse thermography and pulse phase thermography inspections of laminated composite materials and sandwich structures. It is demonstrated that the adaptation significantly improves the probing depth, identifying defects that could not be identified using existing procedures. The applicability of the new approaches is discussed, paying particular attention to systematic and random errors resulting from equipment setup. Simple and efficient compensation methods to reduce the effect these errors are presented.

Nondestructive evaluation of low-velocity impact-induced damage in basalt-carbon hybrid composite laminates using eddy current-pulsed thermography

Recently, basalt-carbon hybrid composite structures have attracted increasing attention due to their better damage tolerance, if compared with carbon fiber-reinforced polymer composites (CFRP). Low-velocity is considered as one of the most severe threats to composite materials as it is usually invisible and it occurs frequently in service. With this regard, nondestructive testing (NDT) techniques, especially emerging modalities, are expected to be an effective damage detection method. Eddy current-pulsed thermography (ECPT), as an emerging NDT technique, was used to evaluate the damage induced by low-velocity impact loading in a CFRP laminate, as well as in two different-structured basalt-carbon hybrid composite laminates. In addition, ultrasonic C-scan and x-ray computed tomography were performed to validate the thermographic results. Pulsed phase thermography, principal component thermography, and partial least squares thermography were used to process the thermal data and to retrieve the damage imagery. Then, a further analysis was performed on the imagery and temperature profile. As a result, it is concluded that ECPT is an effective technique for hybrid composite evaluation. The impact energy tends to create an interlaminar damage in a sandwich-like structure, while it tends to create an intralaminar damage in an intercalated stacking structure.

A Comparative Study of Enhanced Infrared Image Processing for Foreign Object Detection in Lightweight Composite Honeycomb Structures

The interest toward lightweight composites in the aeronautical industry grows year by year. The challenge is the identification and characterization of defects by using an integration of different techniques. The use of the infrared thermography (IRT) method for the inspection of lightweight composites is poorly documented in the open literature, due to the low heat diffusion through the honeycomb cores. In this study, IRT was used to retrieve the unknown positions of internal inserted objects in three different lightweight composite structures, by using three new thermal image processing integration methods. The corresponding post-processed results were compared against the traditional principal component thermography and pulsed phase thermography image processing techniques. The comparison showed that the integrated signal smoothing processing enhanced the image quality compared to the established techniques. Then, a thermal-physical analysis corresponding to the experimental results was conducted, in order to explain the experimental results. Finally, the advantages and disadvantages of the different presented methods when applied on the different lightweight composite structures were summarized.

A Quantitative Comparison among Different Algorithms for Defects Detection on Aluminum with the Pulsed Thermography Technique

Pulsed thermography is commonly used as a non-destructive technique for evaluating defects within materials and components. In the last few years, many algorithms have been developed with the aim to detect defects and different methods have been used for detecting their size and depth. However, only few works in the literature reported a comparison among the different algorithms in terms of the number of detected defects, the time spent in testing and analysis, and the quantitative evaluation of size and depth. In this work, starting from a pulsed thermographic test carried out on an aluminum specimen with twenty flat bottom holes of known nominal size and depth, different algorithms have been used with the aim to obtain a comparison among them in terms of signal to background contrast (SBC) and number of detected defects by analyzing different time intervals. Moreover, the correlation between SBC and the aspect ratio of the defects has been investigated. The algorithms used have been: Pulsed Phase Thermography (PPT), Slope, Correlation Coefficient (R2), Thermal Signal Reconstruction (TSR) and Principal Component Thermography (PCT). The results showed the advantages, disadvantages, and sensitivity of the various thermographic algorithms.

Remote line scan thermography for the rapid inspection of composite impact damage

An investigation of the feasibility of inspecting impact damage in polymer composite aircraft components using remote line scan thermography (LST) is presented. A numerical study is undertaken to determine the minimum heat-source beam width required to detect barely visible impact damage (BVID) above a threshold size. It is shown that the beam width requirement can be met with a non-laser-based heating source. A light-weight incandescent focusing-optic luminaire meeting this requirement is designed, developed and experimentally validated. A new image processing strategy for line-scan thermography is introduced. This method, called Dynamic Pulse Phase Thermography (DPPT), does not require static reconstruction of LST data streams and therefore is less restrictive than established processing methods for real-time applications. Experiments on carbon epoxy laminates, one containing flat-bottom-hole defects and another BVID, are used to demonstrate the performance of DPPT compared to Static Pulse Phase Thermography. It is shown that an LST inspection system comprising a light-weight luminaire and a relatively compact low-cost microbolometer, in conjunction with DPPT processing, is able to detect BVID in an aerospace grade carbon-epoxy laminate, at a scan rate of 50 mm/s and a stand-off distance of 500 mm. The significance of this finding for remote LST inspection via an unmanned aerial vehicle is discussed.

A design of multi-mode excitation source for optical thermography nondestructive sensing

Optical thermography is an important non-destructive testing (NDT) method which has been widely used in the fields of modern aerospace, renewable energy, nuclear industry, etc. The excitation source is a crucial device for the optical thermography system whose performance has a decisive effect on the detection results. Previous thermography NDT studies mainly focused on the physical mechanism, applications and signal processing algorithms. However, the design of the excitation source is rarely discussed. Due to the wide frequency range as well as the high power excitation requirements, it is a challenging task to develop a multi-mode excitation source for thermography NDT. This paper presents a novel design of the excitation source with a structure topology that combines the circuit with low frequency sinusoidal generation and a chopper circuit. It intimately satisfies the requirements of multiple-mode excitation for optical thermography. These include pulsed thermography (PT), lock-in thermography (LT), step heating thermography (ST), pulsed phase thermography (PPT), frequency modulated thermal wave imaging (FMTWI) and barker coded thermal wave imaging (BCTWI). The proposed topology, operating principle and the design procedure of the circuit have been investigated in details. A 2 kW prototype with a frequency range of 0.01 Hz–100 kHz has also been implemented. Validation of the proposed method has been undertaken to detect inner defects of both on a composite sample and a lead-steel sample with bonded structure.

Comparison and evaluation of different processing algorithms for the nondestructive testing of fiber-reinforced plastics with pulse thermography

Abstract When making components out of fiber-reinforced plastics, the composite material itself is formed during the manufacturing of the component, so that suitable methods for process monitoring and quality assurance of each manufactured component regarding its structural integrity are necessary. An alternative approach to the commonly used ultrasonic testing is pulse thermography, which is suitable for the inspection of large areas. But this method is restricted regarding inspection depth, on one side, and thermal contrast and depth resolution with increasing depth, on the other side. In this paper we compare several data processing methods which can be used to overcome the limitations of this method and to optimize the contrast for detection of deeply buried defects. (1) differential absolute contrast, (2) thermographic signal reconstruction, (3) pulse phase thermography, (4) principle component thermography and (5) high order statistics are compared regarding their capability of detecting defects, e. g. delaminations, in different depths. As a measure for the achievable contrast of the different methods designed to detect various defects we compared the respective signal-to-noise-ratio. Additionally, we discuss further approaches for improving the detection capability by applying different evaluation methods in sequence or combining them with image processing methods based on geometric filters. For this purpose a CFRP specimen with 18 artificial defects was examined.

Independent component analysis enhanced pulse thermography for high silicon oxygen phenolic resin (HSOPR) sheet with subsurface defects

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.

Evaluation of interlaminar delaminations in titanium-graphite fibre metal laminates by infrared NDT techniques

Fibre Metal Laminates (FMLs) describe a number of aerospace-grade layered structural materials where thin metal alloy foils are interleaved with Fibre Reinforced Polymer (FRP) plies. Such material hybridisation at the layer meso-scale poses a challenge for non-destructive testing (NDT). On the other hand, the need for NDT inspection of FMLs arises due to the potential development of interlayers disbonds, both during fabrication or in-service. This work describes the successful implementation of two Infrared-NDT approaches to evaluate a defected FML panel made of Titanium/Graphite (Ti-Gr). One technique is based on the modulation of the deployed heat from a low-cost halogen lamp, to retrieve defects as phase-contrast signatures from harmonics in the frequency domain. The second technique uses a flying laser heat-source, raster scanning the surface, searching for defect signatures on the heat distribution trail of the moving laser. The work explores also the opportunity to collimate the laser heat source as a round spot or a linear distribution. The proposed non-conventional IR-NDT approaches are also compared with a well-established Pulsed Phase Thermography (PPT) algorithm, demonstrating a similar ability of defects evaluation, but offering some favourable features, which are highlighted in the work.

Reducing inspection time of pulse phase thermography by using phase data at higher frequency range

The defect detection capability of pulse phase thermographic (PPT) non-destructive inspection depends on the frequency of Fourier transformed phase images. Conventionally, phase images at lower frequencies are required to detect deeper defects or defects in objects with lower thermal diffusivity. However, in order to obtain phase images at a lower frequency, a longer inspection time is required. In this paper, we suggest using the phase contrast between defective and non-defective areas that appears in a higher frequency range in order to reduce the inspection time of the PPT method. The numerical calculations and experimental results revealed that the phase data have a small peak (PH) in the high frequency range and that defects can be detected in phase images at the PH frequency because a discernible phase contrast is observed there. We derived a relationship among the frequency of the phase peak, defect depth, and thermal diffusivity of the test object through numerical calculations and dimensional analysis. In addition, the phase contrast at the peak was revealed to vary depending on the data point of the thermal data before Fourier transform. By using the phase images at the phase peak in a high frequency range, the inspection time can be reduced by approximately one order of magnitude.

Detectability of Subsurface Defects with Different Width-to-Depth Ratios in Concrete Structures Using Pulsed Thermography

The active thermography technique is one of the most effective nondestructive tests for evaluating subsurface delaminations in concrete structures. The limitation of this method, which has been studied for some time, is that the width of the smallest detectable defect should be at least two times larger than its depth. However, controversy on this matter remains for concrete material with largely uncertain homogeneity, although the development of the infrared (IR) detector technology improved the above-mentioned limitation. In this study, the pulsed thermography (PT) technique is therefore conducted in the laboratory to investigate the detectability of delaminations with the width-to-depth ratio (w2d) ranging from 1.0 to 7.9 by using a long IR wavelength detector with a focal plane array of 640 $$\times $$ 480 pixels. The study focuses on the w2d ratio lower than 2.0. A concrete specimen was made with 12 embedded simulated delaminations having different sizes and depths. The results showed that a combination of PT and pulsed phase thermography can be used to detect delaminations with a w2d ratio equal or greater than 1.25. In addition, the absolute contrast above the delamination increases with the higher w2d ratio, indicating that even for a relatively deep delamination, it is still detectable if a delamination is provided by appropriate heat energy and its size is sufficiently large. Finally, the study also indicates that as the amount of heating energy provided is increased, the greater accuracy in predicting the depth can be obtained.

Excitation-invariant pre-processing of thermographic data

There is a large number of industries that make extensive use of composite materials in their respective sectors. This rise in composites’ use has necessitated the development of new non-destructive inspection techniques that focus on manufacturing quality assurance, as well as in-service damage testing. Active infrared thermography is now a popular nondestructive testing method for detecting defects in composite structures. Non-uniform emissivity, uneven heating of the test surface, and variation in thermal properties of the test material are some of the crucial factors in experimental thermography. These unwanted thermal effects are typically coped with the application of a number of well-established thermographic techniques including pulse phase thermography and thermographic signal reconstruction. This article addresses this problem of the induced uneven heating at the pre-processing phase prior to the application of the thermographic processing techniques. To accomplish this, a number of excitation invariant pre-processing techniques were developed and tested in this article addressing the unwanted effect of non-uniform excitation in the collected thermographic data. Various fitting approaches were validated in light of modeling the non-uniform heating effect, and new normalization approaches were proposed following a time-dependent framework. The proposed pre-processing techniques were validated on a testing composite sample with pre-determined defects. The results demonstrated the effectiveness of the proposed processing algorithms in terms of removing the unwanted heat distribution effect along with the signal-to-noise ratio of the produced infrared images.