Laser Thermography Research Articles

A fast deployable model for crack identification with laser thermography testing

This paper presents a novel feature extraction method, enabling efficient training and deployment of neural networks for rapid identification of crack defects in Laser Array Spot Thermography (LAST). We trained the crack defect identification model based on pixel-level features, encoding each pixel as a feature vector using Frangi filter, and classifying them using a neural network. Experimental results demonstrate that Frangi features are an effective method for distinguishing cracks, speckles, and background noise interference in the experiment. Furthermore, the model only requires a small region of interest (ROI) as training samples to achieve effective training and efficient crack identification under the same detection conditions, allowing for rapid deployment in practical inspections.

Research on In-Plane Thermal Conductivity Detection of Fuel Cell Bipolar Plates Based on Laser Thermography.

The thermal properties of bipolar plates, being key elements of polymer electrolyte membrane fuel cells, significantly affect their heat conduction and management. This study employed an innovative approach known as a heat flow loop integral method to experimentally assess the in-plane thermal conductivity of graphite bipolar plates, addressing the constraints of traditional methods that have strict demands for thermal stimulation, boundary or initial conditions, and sample size. This method employs infrared thermal imaging to gather information from the surface temperature field of the sample, which is induced by laser stimulation. An enclosed test loop on the infrared image of the sample's surface, situated between the heat source and the sample's boundary, is utilized to calculate the in-plane heat flow density by integrating the temperature at the sampling locations on the loop and the in-plane thermal conductivity can be determined based on Fourier's law of heat conduction. The numerical simulation analysis of the graphite models and the experimental tests with aluminum have confirmed the precision and practicality of this method. The results of 1060 aluminum and 6061 aluminum samples, each 1 and 2 mm in thickness, show a deviation between the reference and actual measurements of the in-plane thermal conductivity within 4.3% and repeatability within 2.7%. Using the loop integral method, the in-plane thermal conductivities of three graphite bipolar plates with thicknesses of 0.5 mm, 1 mm, and 1.5 mm were tested, resulting in 311.98 W(m·K)-1, 314.41 W(m·K)-1, and 323.48 W(m·K)-1, with repeatabilities of 0.9%, 3.0%, and 2.0%, respectively. A comparison with the reference value from the simulation model for graphite bipolar plates with the same thickness showed a deviation of 4.7%. The test results for three different thicknesses of graphite bipolar plates show a repeatability of 2.6%, indicating the high consistency and reliability of this measurement method. Consequently, as a supplement to existing technology, this method can achieve a rapid and nondestructive measurement of materials such as graphite bipolar plates' in-plane thermal conductivity.

Using Laser Point Scanning Thermography for Quality Monitoring of Products Made of Composite Materials

Introduction. Control of the presence of subsurface defects in products from composite materials is necessary for verification of products after release from production and in the process of operation. Aim of the Study. The purpose of the presented work is to estimate the parameters of subsurface defects using local laser thermography, suitable for quality control of both small objects and suspicious areas of large objects with curved surfaces. Materials and Methods. The laboratory setup on which this work was carried out includes a robotic arm, a COX CG640 thermal imager and a 3 W laser. The method was tested on a fiberglass sample with introduced delamination defect simulations located at different depths below the surface. By means of computer modeling rational parameters of thermographic control were selected, providing reliable detection of the defect at a depth of up to 3 mm under the surface of the composite sample. Results. Numerical modeling of surface temperature field induced by moving focused laser beam was carried out using COMSOL software package. It showed that laser beam with 3 W power moving at 5 mm/s provided the thermal contrast sufficient to detect the defects at the depth up to 3 mm. The obtained experimental data are in satisfactory agreement with numerical modeling both qualitatively and quantitatively. Experimental data were used to construct a regression model for determining defect depth based on the maximal thermal contrast and the time interval between heating and the contrast maximum. Discussion and Conclusion. The results obtained in this work allow us to propose a technique for detecting defects in fiberglass plastics and estimating their depth. The coefficient of determination for the obtained regression model was found to be equal to 0.95, and the mean square error of the metric was no more than 0.016 mm2. The use of a robotic arm to scan objects will make it possible to investigate objects with complex curved surfaces.

A Fiber-Guided Motorized Rotation Laser Scanning Thermography Technique for Impact Damage Crack Inspection in Composites

Laser Thermography manifests superior sensitivity and compatibility to detect cracks and small subsurface defects. However, the existing related systems have limitations on either inspection efficiency or unknown directional cracks due to the utilization of stationary heat sources. This article reports a Fiber-guided Motorised Rotation Laser-line Scanning Thermography (FMRLST) system aiming to rapidly inspect cracks of impact damage with unknown direction in composite laminates. An optical head with fibre delivery integrated with a rotation motor is designed and developed to generate novel scanning heating in a circumferential rotation manner. A FEM model is first proposed to simulate the principle of FMRLST testing and produce thermograms for the development of post-processing methods. A damage enhancement method based on Curvelet Transform is developed to enhance the visualization of thermal features of cracks, and purify the resulting image by suppressing the laser-line heating pattern and cancelling noise. The validation on three composite specimens with different levels of impact damage suggests the developed FMRLST system can extract unknown impact surface cracks efficiently. The remarkable sensitivity and flexibility of FMRLST to arbitrary cracks, along with the miniaturized probe-like inspection unit, present its potential in on-site thermographic inspection, and its design is promising to push the LST towards.

Frequency-based analysis of active laser thermography for spot weld quality assessment

Frequency-based analysis of active laser thermography for spot weld quality assessment

Quantitative test of delamination defects in CFRP with surface interference by laser thermography

Remote onsite inspection and quantitative evaluation of internal delamination defects in carbon fiber reinforced polymer (CFRP) structures is of great significance in practical engineering applications. In this paper, a new method for quantitative test and 3D imaging of internal delamination defects in CFRP with surface interference by long pulse laser thermography is developed. The variation of surface temperature field in a CFRP plate with both defect and surface interference irradiated by a uniform laser source was analyzed by simulation and experiment. Image processing of the collected laser thermography image sequence was carried out to obtain the size and shape of the possible defects. Meanwhile, a method with using the peak time of the temperature difference curve was developed to precisely evaluate the defect depth and eliminate the surface interference. The information was then combined to achieve 3D visualization imaging of the actual defects.

Shape characterization and depth recognition of metal cracks based on laser infrared thermography and machine learning

Due to the operation of modern industrial equipment under high pressure, high speed and high load conditions, cracks inevitably appear on the surfaces of metal components. Crack detection on metal surfaces is increasingly becoming a focal point in the field of non-destructive testing. In order to locate and quantitatively analyze surface cracks on metallic materials as early as possible to reduce the cost in industrial production, this paper achieves rapid localization of surface cracks by establishing a line laser thermography scanning system. An unsupervised edge detection algorithm of principal component analysis combined with K-means (PK) is proposed to achieve accurate crack edge detection by combining with a point-pulse laser thermography system. The accuracy of the PK algorithm is confirmed by comparison with other edge detection algorithms. After that, accurate identification of crack depth is achieved by selecting temperature features through particle swarm optimization algorithm and optimizing hyper-parameters of support vector machine and k-nearest neighbor through Bayesian search method. The experiments show that the best recall and accuracy of PK for crack detection are 82.9% and 86.2%, respectively; Combining the particle swarm optimization algorithm with the BS method, the mean square error of SVR and KNN for crack depth recognition is 0.092 and 0.089, respectively.

Laser spot thermography and Pulse thermography – comparison of performance for non-destructive testing of composite structures

Laser spot thermography (LST) is a sub-domain method in active thermography that uses a laser as the heat source. The method allows for precise control of the beam shape, pulse duration, and delivered energy. Therefore, this technique can be applied, especially for delicate structures where testing conditions must be maintained precisely. The following work includes the introduction of a laser spot thermography technology demonstrator that was designed and constructed at AGH UST. The paper presents laser thermography laboratory experiments along with the developed procedure for automatic defect identification. The measurements were used to establish an automatic procedure and algorithm for automatic defect identification based on regression methods.

Quantitative detection of surface crack width of aluminum alloy based on laser thermography

Quantitative detection of surface crack width of aluminum alloy based on laser thermography

Weld crack detection and quantification using laser thermography, mask R-CNN, and CycleGAN

Steel members are susceptible to cracking, and a number of non-destructive testing (NDT) techniques are used for crack detection. However, these NDT techniques are not only labor intensive but also time consuming. In particular, inspection of welded areas requires surface treatment, and data must be interpreted by experienced engineers to differentiate cracks from weld patterns. In this study, an automated weld crack detection and quantification system was developed by integrating laser thermography, Mask R-CNN, and CycleGAN. The developed system comprises a laser heat source, an IR camera, and a control unit. The laser applied heating to the target surface, and the resulting thermal radiation emitted from the surface was measured using an IR camera. Subsequently, the thermal images were processed for crack detection using Mask R-CNN, and for crack quantification using medial axis transform. The detection and quantification performance of the developed system were validated through laboratory and field tests.

Pulsed thermography with laser beam homogenizing for thickness prediction of thin semi-transparent thermal barrier coatings

A pulsed thermography system with laser beam homogenizing is developed to measure the thickness of thermal barrier coatings (TBCs) by numerical fitting. A simplified model is proposed to avoid the complexity of deriving an analytical solution of thermal diffusion in TBC systems that include a thin semi-transparent top coating. A characteristic parameter is identified that captures the system response to fast heating by a short laser pulse and to optothermal effects due to volume heating, bulk emission and multiple reflections in the thin coating. It is demonstrated that small thickness changes can be measured when the unpainted ultra-thin TBC is thermally excited using short laser pulses with flat-top spatial profile. Experimental results show that the proposed laser thermography reconstruction method is fast and reliable when using laser beam shaping with a homogenizer.

Contactless inspection of CFRP artificial disbonds using combined laser thermography and laser ultrasonics with optical microphone

Precise and efficient inspection of disband defect in sub-mm layers of composite structures is a big challenge for the non-destructive and testing industry. In this work, 16 artificial disbands (flat bottom holes), with different sizes and depths, in carbon fiber reinforced polymer (CFRP) composites were contactless detected using combined laser thermography and laser ultrasonics with optical microphone. The presented laser ultrasonic technique employed a membrane free optical microphone is a contactless and nondestructive technique and shows high detectability for flat bottom holes with deep depth in this preliminary investigation. Sizes and depths of the flat-bottom holes were determined by the presented laser ultrasonics technique. This investigation combined the high detection efficiency of laser thermography and high detection sensitivity of laser ultrasonics with the optical microphone together for the contactless inspection of artificial disbonds.

Defect Detection Method for CFRP Based on Line Laser Thermography.

A continuous line laser scanning inspection technique for tracing load-bearing structures was developed and applied to defect detection of unidirectional carbon-fiber-reinforced polymers for aero engines. The heat transfer model of the material was analyzed using the finite element software COMSOL. Meanwhile, a laser platform was built and an image algorithm was used to verify the feasibility of the method. The potential of this technique for detecting defects and providing information on the location of defects in carbon fiber composites was analyzed. Results indicate line laser thermal imaging can successfully determine the size, location, and crack angle of surface damage with extremely high accuracy. The positioning accuracy error for impact and fracture defects is less than 20%, and the detection rate can reach 100% if the defect is in the special position of just leaving the heating area. The angle detection of fracture cracks can be accurate within 10°.

Integration study among flying spot laser thermography and terahertz technique for the inspection of panel paintings

Integration study among flying spot laser thermography and terahertz technique for the inspection of panel paintings

The Wavelet-Based Self-similarity Analysis for the Detection of Fatigue Microcrack by the Joint Scanning Laser Thermography

The Wavelet-Based Self-similarity Analysis for the Detection of Fatigue Microcrack by the Joint Scanning Laser Thermography

Shape Characterization and Depth Identification of Cracks on Metal Surface Based on Combined Point and Line Laser Thermography

Shape Characterization and Depth Identification of Cracks on Metal Surface Based on Combined Point and Line Laser Thermography

The curve cluster analyses for the characterizations of material defects by long-pulsed laser thermography

Laser thermography is a novel non-destructive testing method enabling the detections of defects on vertical surfaces (e.g., cracks) and those on parallel surfaces (e.g., delaminations). The thermal response of defects near the surface of the specimen in the temperature field is the basis for defect detection work; however, the thermal response characteristics of defects under direct laser irradiation have not been systematically investigated. In this paper, we investigated the thermal response characteristics of five materials under long-pulsed laser excitations: copper, 45 steel, aluminum alloy, carbon fiber reinforced polymer plate, and ceramic resin composite coating. These specimens exhibited significantly different thermal conductivities. An important characterization parameter, characteristic temperature, was found, which can characterize the difference in thermal conductivity of different specimens. The surface of the 45 steel was coated with 3Cr13 coating, and cracks on the coating surfaces generated significantly different thermal response characteristics. The findings of this study illustrated the feasibility of long-pulsed laser thermography for demonstrating the heat entrapment effect due to cracks and provide a method for defect detection.

A portable fiber laser thermography system with beam homogenizing for CFRP inspection

In this paper, an improved laser thermography method and system with laser beam homogenizing is developed to remote inspect delamination defect in carbon fiber reinforced polymer (CFRP) laminates. A fiber laser beam shaper is designed to transform the high-power laser beam with uneven intensity into large area square beam with uniform distribution. A numerical model of the optical system is developed to calculate and optimize the light intensity distribution of the output laser from the beam shaper. To investigate the performance of the laser thermography system with beam shaping, the temperature field in CFRP with a delamination defect, excited by different laser illumination, is simulated. Finally, an experimental comparison between conventional laser thermography without beam shaping and the proposed one is reported with two CFRP samples with different artificial defects. Both the simulation and experiment results show that the defect detection capability in the CFRP is significantly improved by the proposed laser thermography method with laser beam shaping.

Influence of absorptivity of the material surface in crack detection using laser spot thermography

Laser thermography (LT) is one of the novel NDT methods for detecting surface-breaking cracks in metals. LT technique is well known for its non-contact nature and relatively fast-detection of surface defects for objects that are at room temperature. In LT, a continuous-wave (CW) laser is used to make a laser spot/line/area, which excites the sample under inspection to create local heating. Using optical mirrors, the spot is then rapidly made to move over the surface of the test sample. The temperature distribution over the sample due to the laser excitation is then monitored using an infrared (IR) thermal camera. This paper investigates the influence of the absorptivity of the material surface during defect detection using LT and its influence in detecting surface-breaking cracks. Six different materials were used under this study, i.e., Aluminum, Brass, Copper, mild Steel, Stainless Steel, and Titanium and with two surface conditions i.e., with and without black paint. 3D FEM numerical models were developed and validated with experiments for the different case studies for surface crack detection. The term Crack Thermal Contrast (CTC) is introduced to indicate the detectability of the surface crack and found to increase linearly with absorptivity of the material.

Laser thermography NDT for the inspection of debonding in titanium based Fibre Metal Laminates

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. The evaluation of bonding integrity in multilayered primary structures is of great importance, as is generally the case with the specific performance requirements of aeronautical applications. This work describes the successful implementation of an Infrared-NDT approach to evaluate debonds in FMLs panel made of Titanium/Graphite (Ti-Gr). The proposed approach uses a flying laser heat-source, raster scanning the surface, in search of defect signatures arising on the temperature distribution profile left by the moving laser, and acquired from the component surface by an IR camera. A numerical simulation of the above NDT approach is proposed and used to investigate the mechanisms allowing to correlate defects signatures to defect features and material thermal response. The outcomes of the model are also evaluated in order to optimize the experimental setup.