Thermographic Nondestructive Testing Research Articles

Time- and Phase-Domain Thermal Tomography of Composites

Active infrared (IR) thermographic nondestructive testing (NDT) has become a valuable inspection method for composite materials due to its high sensitivity to particular types of defect and high inspection rate. The computer-implemented thermal tomography, based on the analysis of heat diffusion in solids, involves a specialized treatment of the data obtained by means of active IR thermographic NDT, thus allowing for the “slicing” of materials under testing for a few layers where discontinuity-like defects can be underlined on the noise-free background (binary thermal tomograms). The time-domain thermal tomography is based on the fact that, in a one-sided test, temperature “footprints” of deeper defects appear later in regard to shallower defects. The phase-domain tomography can be applied to collected IR data in a direct way, for instance, by using the Fourier transform, but quantification of results is more difficult because the relationships between phase and defect depth depend on experimental parameters, and the corresponding “phase vs. defect depth” calibration functions are ambiguous. In this study, the time- and phase-domain thermal tomography techniques have been compared on simulated IR thermograms and experimentally applied to the evaluation of carbon fiber reinforced plastic composite containing impact damage defects characterized by impact energy 10, 18, and 63 J. Both tomographic techniques have demonstrated similar results in the reconstruction of thermal tomograms and, in some cases, supplied complementary information about the distribution of single defect zones within impacted areas.

Investigation of heat source reconstruction of thickness-through fatigue crack using lock-in vibrothermography

Lock-in vibrothermography (LVT) is an active thermography nondestructive testing technique, which utilizes thermal contrast induced by lock-in modulated vibration to detect surface or subsurface defects. During the LVT testing, only the surface thermal distribution of the specimen can be directly captured. However, in most cases special cares need to be taken about the subsurface thermal distribution and the related heat source reconstruction. In this paper, a direct and inverse problem for vertical heat source reconstruction by using LVT is investigated. Specifically, a 3D transient analytical model is proposed to investigate heat source distribution of a metallic plate with a fatigue crack. The Tikhonov regularization is used to reconstruct the heat source from the analytical/experimental data. Further, H-index is introduced to quantitatively evaluate the reconstructed results. The shapes of reconstructed heat source from both analytical and experimental data are similar to a ‘half-penny’. Additionally, the integral operator has a significant influence on the experimentally reconstructed result.

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.

Термографический способ неразрушающего контроля циклической прочности в производственных условиях

Термографический способ неразрушающего контроля циклической прочности в производственных условиях

Sparse ensemble matrix factorization for debond detection in CFRP composites using optical thermography

With the increasing use of CFRP in aircraft and automotive industry, it has become necessary to monitor the health of the composite structure during the manufacture and service process from flaws such as debond or delamination by using optical pulsed thermography non-destructive testing (OPTNDT) techniques. However, current OPTNDT methods cannot efficiently tackle the influence from the strong noise, and the resolution enhancement of the defects detection remains as a critical challenge. To alleviate this problem, this paper proposes the sparse ensemble matrix factorization approach to remove the noise and enhance the resolution for the defects detection. Specifically, the algorithm is based on the sparse representation and noise is modeled as a mixture of Gaussian (MoG) distribution. It provides a projection from the raw data onto the sparse and low dimensional sub-space while the defects information is significantly enhanced with the layer decomposed approach in the low dimensional space. Not withstand above, a Gaussian low pass filtering and non-linear enhancement is conducted for further enhancement. The proposed method is coupled with several comparison studies to verify the efficacy.

Advances in RGB Projection Technique for Thermographic NDT: Channels Selection Criteria and Visualization Improvement

Infrared thermographic nondestructive testing is a proven technology for material inspection and characterization that is based on the analysis of the dynamic thermal behavior of specimens induced by an intentional and controlled external stimulation. The analysis and review of the image sequences resulting from the inspections is time consuming, which cancels out the advantage provided by the quick inspection process. The RGB projection method has been recently proposed to overcome this limitation. This method synthesizes long sequences into only one true color image, selecting the three most representative images from the original sequence. This study advances the development of this methodology by automating the selection of the channels to be included in the RGB image, making the process objective. Seven different criteria based on different metrics are proposed to select the channels. They are analyzed and evaluated by theoretical and real thermographic tests obtaining RGB images that contain up to 99.6 % of the defects detected in the original thermographic sequences. Visualization improvement is also provided to the RGB image by image processing algorithms.

Automatic seeded region growing for thermography debonding detection of CFRP

The carbon fiber reinforced polymer (CFRP) has been widely used in aerospace, automobile and sports industries. In laminated composite materials, cyclic stresses and impact will cause internal defects such as delamination and debonding. In order to guarantee internal quality and safety, optical pulsed thermography (OPT) nondestructive testing has been used to detect the internal defects. However, current OPT methods cannot efficiently tackle the influence from uneven illumination, and the resolution enhancement of the defects detection remains as a critical challenge. In this paper, a hybrid of thermographic signal reconstruction (TSR) and automatic seeded region growing (ASRG) algorithm is proposed to deal with the thermography processing of CFRP. The proposed method has the capability to significantly minimize uneven illumination and enhance the detection rate. In addition, it has the capacity to automate segmentation of defects. It also overcomes the crux issues of seeded region growing (SRG) which can automatically select the growth of image, seed points and thresholds. The probability of detection (POD) has been derived to measure the detection results and this is coupled with comparison studies to verify the efficacy of the proposed method.

Periodic Pulsed Thermography for Inner Defects Detection of Lead-Steel Bonded Structure

Multilayer materials with metal-metal bonded structure have been widely applied in aviation, aerospace, and nuclear industry. Disbond is prone to exist in lead-steel bonded structure, which degrades the load capacity and mechanical behaviors. Thermography nondestructive testing is a potential candidate for sub-layer defect detection. However, lead-steel bonded structure is unbearable when undertaken with over-heating of instantaneous temperature, which will lead to subsequent damage or generation of more unpredictable disbond. In addition, detection sensitivity of the deeper defects requires to be enhanced. In this paper, the mathematical derivation and the implementation of the periodic pulsed thermography have been established for detecting inner defects of lead-steel structure. This has been especially conducted for detecting small and deep defects that require high energy to increase detectability. Validation of the proposed method has been undertaken on both inductive thermography and optical thermography. The obtained results have demonstrated that periodic pulsed thermography is highly efficient for deep inner defect inspection of metal-metal bonded structure.

Infra-Red Thermography based Inspection of Hybrid Composite Laminates under Flexure Loading

Generally, the aircraft structural parts are economically high in cost so the materials need to be inspected for defects or damages using various non-destructive testing (NDT) methods like ultrasonic, thermography and acoustic emission. The aim of this project is to characterize the defects in composite laminates before and after the flexural loading using infra-red thermography NDT method. GFRP and hybrid (GFRP+CFRP) composite laminates are fabricated with different orientation such as uni-directional, cross ply, anti-symmetric and angle ply and then tested under flexural loading according to ASTM D790 standard. The volume fraction of the fibre and matrix needs to be found out to know the void content and the mixing ratio of reinforcement and binder.

Improved image processing of road pavement defect by infrared thermography

Abstract This paper intends to achieve improved image processing for the clear identification of defects in damaged road pavement structure using infrared thermography non-destructive testing (NDT). To that goal, 4 types of pavement specimen including internal defects were fabricated to exploit the results obtained by heating the specimens by natural light. The results showed that defects located down to a depth of 3 cm could be detected by infrared thermography NDT using the improved image processing method.

A Review of Microwave Thermography Nondestructive Testing and Evaluation.

Microwave thermography (MWT) has many advantages including strong penetrability, selective heating, volumetric heating, significant energy savings, uniform heating, and good thermal efficiency. MWT has received growing interest due to its potential to overcome some of the limitations of microwave nondestructive testing (NDT) and thermal NDT. Moreover, during the last few decades MWT has attracted growing interest in materials assessment. In this paper, a comprehensive review of MWT techniques for materials evaluation is conducted based on a detailed literature survey. First, the basic principles of MWT are described. Different types of MWT, including microwave pulsed thermography, microwave step thermography, microwave pulsed phase thermography, and microwave lock-in thermography are defined and introduced. Then, MWT case studies are discussed. Next, comparisons with other thermography and NDT methods are conducted. Finally, the trends in MWT research are outlined, including new theoretical studies, simulations and modelling, signal processing algorithms, internal properties characterization, automatic separation and inspection systems. This work provides a summary of MWT, which can be utilized for material failures prevention and quality control.

Post Processing of Thermographic NDT Data Using Dirac Comb

In the present study, a new method is proposed for the post processing of the thermographic non-destructive testing (TNDT) data. In this method, the temperature data of the thermograms is first converted into a pulse train using the Dirac comb. The pulse train is then transformed to the frequency domain using the Discrete Fourier Transform (DFT). The real part of the transformed data has a number of cycles; showing the shifting and duality property of the DFT. At the beginning as well as at the end of the cycles the contrast of the data is sharp enough to form images showing distinctly the defect and the sound areas of the sample. The method is named as DFT of Comb Data, in short DCD. The technique showed promising results even for small (O5mm) defects located at the depth of 10 mm below the surface. Results from the DCD method are compared with the two of the established post processing techniques, namely the Pulsed Phase Thermography (PPT) and the Thermographic Signal Reconstruction (TSR). An industrial grade long wave infrared (LWIR) camera, FLIRTM T440, is used in the present study. Three samples with synthetic defects are tested here that include, two samples of glass fibre reinforced polymer (GFRP) and one sample of honeycomb. The GFRP samples are 18 and 8.5 mm thick and have defects in the form of flat bottomed holes. The honeycomb sample has three square shaped debonding defects between its aluminium core and one of the face-sheets (1.5 mm thick Carbon Fibre Reinforced Polymer - CFRP).

Numerical simulation method for IR thermography NDE of delamination defect in multilayered plate

In this paper, a new efficient numerical method based on the finite element method (FEM) and the frequency domain summation (FDS) strategy is proposed for the simulation of the pulsed infrared thermography non-destructive testing. With use of the FDS strategy, the temperature distribution due to a pulsed thermal source is simulated based on the thermal response of series of single-frequency sinusoidal heat sources. An interpolation strategy is also adopted to reduce the necessary number of harmonic frequency responses to be calculated, and consequently the simulation time is saved. A comparison of the FDS strategy and the direct time domain integration method is also presented, which indicates that the FDS strategy can simulate pulsed thermography problem with high precision and can promote simulation efficiency.

3D thermography in non-destructive testing of composite structures

The combination of 3D scanners and infrared cameras has lead to the introduction of 3D thermography. Such analysis produces results in the form of three-dimensional thermograms, where the temperatures are mapped on a 3D model reconstruction of the inspected object. All work in the field of 3D thermography focused on its utility in passive thermography inspections. The authors propose a new real-time 3D temperature mapping method, which for the first time can be applied to active thermography analyses. All steps required to utilise 3D thermography are discussed, starting from acquisition of three-dimensional and infrared data, going through image processing and scene reconstruction, finishing with thermal projection and ray-tracing visualisation techniques. The application of the developed method was tested during diagnosis of several industrial composite structures—boats, planes and wind turbine blades.

Autonomous mobile lock-in thermography system for detecting and quantifying voids in liquefied natural gas cargo tank second barrier

In this study, an autonomous mobile inspection system that can detect and quantify hidden voids in a secondary barrier (triplex layer) of a liquefied natural gas cargo tank is developed using lock-in thermography. The triplex layer is the secondary barrier to prevent gas leaks from a Mark III–type membrane liquefied natural gas carrier cargo tank and consists of three sub-layers: a flexible secondary barrier, a bonding layer, and a rigid secondary barrier. The proposed mobile inspection system consists of a lock-in thermography measurement unit, a mobile inspection unit, and image processing algorithms. First, thermal images are obtained using the lock-in thermography unit as the mobile inspection system maneuvers over triplex layers. Second, the raw thermal images are processed by several image processing techniques to compensate for non-uniform heating, eliminate noise components, and disregard trivial voids in accordance with the current inspection guideline. Third, the void size is more precisely quantified using an empirical mapping function that relates the void size estimated in the previous step to that measured by an independent X-ray test. The contributions of this study include the following: (1) an autonomous mobile inspection system is developed for real-time inspection of the triplex during its installation, significantly saving the inspection cost and time; (2) a suite of image processing techniques is developed, overcoming shortcomings of the existing thermography non-destructive testing techniques; and (3) the sizes as well as the locations of the hidden voids are quantified with high accuracy, reliability, and fast inspection speed.

Ultrasonic infrared thermography in non-destructive testing: A review

Potentials of a novel non-destructive testing technique called ultrasonic infrared thermography (UIT) have been widely recognized for the last decade. This technique is promising for many practical industrial applications being of interest for academic researchers who deal with thermomechanical problems. Significant improvements in the performance of infrared imager have also contributed to increasing popularity of this inspection technique. This paper presents an introduction to the use of UIT in NDT, combining a review of earlier and later research with some experimental illustrations.

The detection and quantification of the defects in adhesive bonded joints of the piezoelectric sensors by infrared thermographic nondestructive testing

In this paper, efficacy of pulsed thermography technique has been explored for the first time for the detection and quantification of the subsurface defects present in the rubber-encapsulated piezoelectric sensors. Initial experiments were performed on adhesively bonded joints of the rubber/Al or rubber/PZT control samples to find out an optimum acquisition time for the 3-mm rubber encapsulants. Thermographic measurements were performed in the reflection mode and acquired thermal images were analysed and processed images were described in terms of the phase images. The defective regions are identified as delamination of the adhesive joints at the interface of rubber and PZT stacks, and presence of porosity in the encapsulation in the inspected hydrophone. The defect depths of the observed anomalies were calculated empirically from the plots of the peak time of thermal contrast (tmax) maximum and thermal contrast maximum (Cmax) for a particular defect. The estimated defect depths of the prominent porosity observed in the PZT hydrophone are found nearly 1 mm from the surface.

A comparison between finite element modeling and various thermographic non-destructive testing techniques for the quantification of the thermal integrity of macro-brush plasma facing components used in a tokamak.

The plasma facing components (PFCs) inside a tokamak are typically exposed to extremely high heat flux of the order of MW/m(2). The brazing quality between the plasma facing materials (PFMs) and the heat sink will determine the structural integrity and hence the effective service life of these PFCs. Suitable non-destructive testing (NDT) techniques for the pre-qualification of these components are thus essential to evaluate their structural integrity at various stages of their service life. Macro-brush type mockups of prototype PFCs with graphite as PFM have been inspected for their brazing quality using different active Infrared (IR)-thermographic NDT techniques. The results obtained from these techniques are compared and discussed. The brazing quality was quantified by establishing a comparison between the experimental results and the results from Finite Element Analysis (FEA). The percentage of contact between the PFM and the substrate was varied in FEA. FEA results when compared with experiments shows that tiles have different amounts of contact with the substrate ranging between 10% and 80%.

遠隔加熱装置を利用した構造物のアクティブサーモグラフィ検査システムの開発

This study focuses on active thermographic non-destructive testing using remote heating equipments. Inspection capability using two different heating systems were examined, one is halogen lamp with light collecting mirror and another is CO2 laser scanning system. Experimental results showed that defect in a concrete specimen located at 20-m distance from heater was detected by using the focused halogen lamp, and that defects in a CFRP specimen were detected in thermal images obtained after laser scanning heating. These results imply that inspection system with these heating equipments is effective to inspect objects located far from observers.

Defect size measurement and far distance infrared detection in CFRP-concrete and CFRP-steel systems

Knowledge of the precise size of unbond, debond areas and delaminations aids the assessment and evaluation of the integrity of structures retrofitted with carbon fibre-reinforced polymer (CFRP) systems. This assessment and monitoring can lead to reduced stress from over loading and keep the structure well beyond the serviceability limit. At the same time, reading the size of defects accurately can help radically with repair and maintenance. This paper investigates the ability of infrared thermography (IRT) non-destructive testing to determine subsurface defect sizes with high accuracy. Different designed defects in several specimens were measured using active pulse thermography technique IRT. These defects were located under different CFRP materials. Far distance detection and measurement of bond defects were also investigated with different pulse intervals applied to different specimens.