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Air-coupled ultrasonic excitation is a relatively poorly studied method of nondestructive testing of polymer and composite materials. The combination of noncontact ultrasonic stimulation in conjunction with laser vibrometry can provide an assessment of the quality of the product without external influence on the tested material. This work is devoted to the study of an ultrasound diagnostic system based on the combined use of a noncontact magnetostrictive transducer and scanning laser Doppler vibrometry for nondestructive testing of polymer and composite materials. We believe that such combination can provide sufficient power for rapid detection of defects, while the optimization of the experimental conditions increased the efficiency of testing results. Measurement of the directional diagram of the magnetostrictive transducer has made it possible to determine the features of the radiation of the magnetostrictor in an assembly with a titanium step-type waveguide.

Abstract The robotic equipment for and the technique of combined thermal nondestructive testing (NDT) of large-sized products by zones with subsequent data synthesis are described. The effectiveness of the combination of two methods, infrared and ultrasonic thermographic testing, is shown by the example of the developed complex-shaped reference sample with 18 simulators of production and operational defects. The developed algorithms for the synthesis of test results, including spatial “matching” of a set of thermograms and automated flaw detection and defect characterization with the use of neural networks have illustrated the effectiveness of the proposed approach for practical application.

The specialized software intended for active thermal nondestructive testing (NDT) is developed. The software is oriented toward both commercial and laboratory thermal NDT systems including heat sources and infrared imagers of various types. A modeling module is an essential part of this software allowing optimization of test procedures. Experimental data is treated by using a processing and defect characterization modules intended for enhancing signal-to-noise ratio and identifying lateral size, depth and thermal resistance of hidden defects.

We propose a combined method for thermal nondestructive testing using optical and ultrasonic stimulation by means of combining individual thermograms obtained at the corresponding time points. The resulting sequences of infrared thermograms provide more efficient identification of defects of various types and can also be processed using well-known algorithms, for example, thermographic signal reconstruction, principal component analysis, etc. We obtained experimental results on a composite CFRP sample imitating aircraft ribs using a robotic manipulator.

Purpose Active infrared (IR) thermography, because of its high productivity and illustrativeness, is a promising technique in nondestructive testing (NDT). The purpose of this paper is to discuss a concept and practical implementation of a portable experimental unit intended for IR thermographic NDT of corrosion in metallic shells. Design/methodology/approach The basic theory relates to the analysis of heat conduction in a plate with rear-surface material loss subjected to pulse, thermal wave or arbitrary heating. Findings The amplitude of temperature anomalies over defects and their characteristic observation times depend on material loss, size and shape of corrosion defects. A flexible architecture of the inspection unit is proposed to include flash tubes, halogen lamps and laser-emitting diode (LED) panels as sources of stimulating thermal radiation. In particular, LED heaters might be perspective due to their narrow spectral band, which is beyond a spectral sensitivity of modern IR imagers. It has been found that the IR thermographic technique is convenient for detecting material loss of up to 15–20 per cent in uniformly painted steel shells with thickness up to 8 mm. The concept of signal-to-noise ratio has been applied to evaluate efficiency of data processing techniques, such as Fourier transform and principal component analysis. Originality/value The developed equipment and inspection guidelines can be used for detecting hidden corrosion in metallic objects, such as above-ground tanks, pipes, containers, etc.

This paper describes research in the field of fiber optic nondestructive testing and structural health monitoring (SHM) of carbon fiber reinforced polymers (CFRP) by the use of integrated optical fiber sensors (OFS) based on fiber Bragg gratings (FBG). Basic mathematical expressions that represent optical SHM of composites are presented. Some new relationships are derived by considering the non-linear character of the FBG-sensor response and the combined effect of temperature and deformation. Both linear and non-linear coefficients of the sensor elements, as well as combined strain–temperature coefficients before and after the sensors are embedded into the composite panels, have been obtained. Experimental results on the strain state of CFRP under static and dynamic loads demonstrate the effectiveness of the proposed non-linear model for evaluating deformation in composites. It is shown that integrated OFS’s allow SHM of composite parts when mechanically loaded to failure, and that they can provide the actual level of strain in the composite parts in real time. SHM improves the operational safety of highly loaded and/or critical aerospace structures by providing real-time stress data, which would permit data-based decisions on overload conditions or imminent failure. Additionally, actual stress data from CFRP samples, or from real parts in use, could show whether the design of the parts should be changed to improve safety margins or to reduce weight.

Natural fibres present ozone-friendly solutions in the field of construction. The attenuation of the sound and heat losses is an important feature in such type of materials above all, when used in non-woven fabrics and fibre-reinforced composites. Hemp fibres show robust insulation performance, this research work should be considered beneficial to the development of a non-destructive thermographic methodology, which can address the thermal barrier (typically applied on multi-layer panel) effects. The intent is to assess the integrity of the sub-surface reinforcing glass fibres, such integrity state will help confer the rigidity and the resistance to mechanical stresses. The testing proposed in this study can be further developed in a laboratory right after the manufacturing process of similar type of components. The testing needs preliminary numerical simulations to help guide the selection of the appropriate pre- and post-processing algorithms combined with or without segmentation operators. A set of numerical and experimental tests were performed through controlled thermal stimulation while recording the thermal responses. The study also highlights the advantages, disadvantages, and future development of the presented technique and methodologies.

Recent developments in electronics have demonstrated some advantages of using carbon fibres in manufacturing flexible PCBs thus requiring to elaborate some new approaches for checking PCB quality. Barely visible impact damage (BVID) represents a typical type of defects in carbon-based composites. In this study, the two inspection techniques, namely active infrared thermography and laser vibrometry, were comparatively used for detecting BVID in composite materials manufactured by three-dimensional printing from kevlar and carbon fibres. The comparative analysis of the composite resistance towards multifold impacting with the total energy of up to 15 J has demonstrated advantages of the hybrid configuration obtained by using the technology of additive manufacturing. The measurement of damping characteristics of the composites in the 50–100 kHz frequency range has also confirmed the ability of the hybrid composite to absorb more energy than classical single component materials.

The algorithm makes it possible to simplify the procedure for processing results of the thermal testing aimed at both revealing latent defects and evaluating their transverse dimensions and shape. Applying this algorithm requires certain participation and experience of the thermography operator, as well as preliminary preparation of initial data by using techniques that increase the signal-to-noise ratio. The algorithm includes selection of defective zones on the thermogram of the test object, automated identification of points with extreme signals, and a pixel-by-pixel threshold analysis of the zones adjacent to these points, culminating in the construction of binary defect maps.

Three nondestructive testing techniques, namely, optically- and ultrasonically stimulated infrared thermography, ultrasonic laser vibrometry and laser ultrasonics, have been comparatively investiga...

The high reliability of modern 3D printing technologies makes it possible to create structural elements for aerospace and rocket industry products. Currently, the team of authors are developing a system for designing and additive manufacturing of composite and polymer structures for high-tech applications. In this paper, we studied the body of a Tomsk-TPU-120 CubSat satellite, the first Russian spacecraft to contain additive manufactured elements. Device quality control was carried out using resonant ultrasonic stimulation in conjunction with the use of scanning Doppler laser vibrometry, as well as experimental modal analysis. The testing results made it possible to reveal a defect at the butt end surface of the satellite body by analyzing the amplitude-frequency characteristic of surface vibrations. Experimental vibration-scanning data were used to verify a mathematical model developed using the ANSYS software.

It is common on space vehicles to have thermal insulation adhesively bonded to a metal structure. A typical defect in such structures is an interlayer disbond, which may occur either between the insulation and the metal substructure or between the layers of multilayer thermal insulation. One-sided thermal nondestructive testing (TNDT) using surface optical heating, such as Xenon flash or quartz tube, may detect disbonds if the thermal insulation thickness does not exceed a few millimeters and disbonds are not very small. In thicker insulation, the effectiveness of the inspection can be improved by using electrical induction to heat the metal base. In both cases, thermal excitation can be areal heating, which is heat projected over an area by a stationary heat source, or scanned linear heating (SLH), which is a linear heater scanned across the test subject. In the latter, either the linear heater is moved across a stationary test subject, or the linear heater is stationary and the test subject is moved. The SLH method usually provides a higher inspection rate (inspected area unit time). In this research, both the theoretical and experimental features of both optical and induction heating have been investigated and compared in the application to non-metallic insulation adhesively bonded to a metal structure. The effectiveness of using neural networks (NN) for characterizing defects has also been studied to demonstrate that optimal NN training should involve 4–5 points selected in defect areas close to non-defect areas, and the NN input data should be prepared by applying the known technique of Thermographic Signal Reconstruction (TSR). Since SLH provides more uniform heating, it provides higher quality IR thermograms than those obtained from areal (flash) heating and this improves the detectability of defects in thermal insulation to a depth of 4–6 mm. Other advantages of SLH for TNDT testing are (1) an inspection rate that is twice as high as an area heating technique and (2) a better potential for fully automated (robotic) testing.

Studie se zabývá kvantitativním stanovením hloubky defektů pomocí flahs pulzní termografie. Nová metoda vyhodnocení navržená v této práci zohledňuje 3D difuzi tepla, konečnou velikost defektů a koeficient tepelné reflexe mezi materiálem a defekty. Metoda je založena na kombinaci známých analytických modelů a procedury nelineárního fittingu (NLF). Algoritmus je ověřen numericky a experimentálně na vzorcích z PLA plastu vyrobených pomocí 3D tisku. Přesnost metody a její efektivcita byla hodnocena porovnáním s referenční metodou založenou rekonstrukci termografického signálu. This study is focused on the quantitative estimation of defect depth by applying pulsed thermal nondestructive testing. The majority of known defect characterization techniques are based on 1D heat conduction solutions, thus being inappropriate for evaluating defects with low aspect ratios. A novel method for estimating defect depth is proposed by taking into account the phenomenon of 3D heat diffusion, finite lateral size of defects and the thermal reflection coefficient at the boundary between a host material and defects. The method is based on the combination of a known analytical model and a non-linear fitting (NLF) procedure. The algorithm was verified both numerically and experimentally on 3D-printed polylactic acid plastic samples. The accuracy of depth prediction using the proposed method was compared with the reference characterization technique based on thermographic signal reconstruction to demonstrate the efficiency of the proposed NLF method

Detecting delaminations in the stringer foot areas of “T -joints” made of carbon fiber reinforced polymer (CFRP) composite is a challenging task for standard nondestructive testing (NDT) techniques. In this study, several methods of thermal NDT (TNDT) have been used to inspect eight CFRP T-Joint specimens with polyolefin film implants which represent subsurface defects. Both one- and two-sided TNDT procedures were used. Thermal modeling was performed to predict the results of TNDT tests. The potential of laser vibrometry was also investigated, and UT (ultrasonic) phased array C-scan was used for verification of test results. Two-sided TNDT was able to detect simulated defects throughout the entire thickness of the test material. Thermal images of defect-free T-Joints clearly establish baseline thermal patterns of “good” stringers, and subsurface defects may create an identifiable distortion to the baseline patterns. The ability of one-sided TNDT to detect defects depends strongly on their depth and size. Ultrasonic (“sonic”) infrared thermography was not successful in detecting the implants, due to the formation of standing waves and complicated thermal patterns observed on the stringers. And laser vibrometry has proven to be ineffective in the detection of the implants. Phased array ultrasonic C-scan testing has also been successful in detecting polymeric implants located outside the ultrasonic “dead zone” but the best sensitivity of UT is achieved in immersion techniques, with water between the ultrasonic transducer and a part to be tested. In all cases, the use of advanced data processing techniques has been indispensable to provide reasonable test results.

Pulsed thermography is a common technique for nondestructive testing (NDT) of materials. This study presents the apparent effusivity method for the quantitative evaluation of coating thickness in a one-sided thermal NDT procedure. The proposed algorithm is based on determining a threshold value of apparent effusivity, which can be found for particular coating-on-substrate structures. It has been found that the square root of the time at which the apparent effusivity curve reaches this threshold is proportional to the coating thickness. The efficiency of the proposed approach is demonstrated by analytical modeling and experimentation performed on thermally-sprayed coatings.

The paper considers the thermo-mechanical mechanism of interaction between the damage agents and armor protection made of polymer fabrics. A simplified mathematical model is proposed to describe the deceleration of a damage agent within an armor fabric due to the dissipation of energy expended on irreversible stretching deformations of fabric fibers, as well as fiber slipping friction and material heating. Woven fabric layers are replaced by solid layers characterized by averaged stiffness and viscosity. A discrete numerical model of a solid material is proposed to reduce a problem with a finite number of degrees of freedom; motion equations are obtained on the basis of the Lagrange equations of the second kind, and for their integration, a stable non-conservative difference scheme is used. The software implementation is based on a functional-object paradigm which allows the modeling of conjugated processes. The parameters of governing equations are identified by using the experimental data. Some illustrative examples of interaction between damage agents and armor barriers with different arrangement of fibers are presented. The proposed model can be used to predict the quality of armor protection with the changing number and location of fibers, as well as to test the armor protection by applying the technique of infrared thermography.

Ultrasonic infrared thermography is a rapid and informative method of nondestructive testing, used to inspect materials and products in aviation and rocket and space industries. High-power piezoelectric emitters stimulate various materials by ultrasound and help revealing hidden flaws by local temperature changes, which can amount to tens of degrees. The possibility of optimizing the procedure of ultrasonic infrared thermography by choosing the frequency of acoustic waves so as to match the frequencies of local resonances of defects of different sizes has been investigated.

The quality control of composite materials remains an important topic of research in nondestructive testing (NDT) of modern materials, in particular, the ones intended for aerospace industry. Over the last decade, there has been a noticeable increase in interest to the ultrasonic stimulation of tested materials and analysis of dynamic temperature distributions. This method is commonly implemented with two techniques, viz. high-power stimulation at a fixed acoustic-signal frequency and low-power resonant ultrasonic stimulation. Results are provided on comparing the above ultrasonic NDT methods using the example of an impact damage in a graphite epoxy-filled composite. The comparison is made based on the criteria of temperature signals, revealed flawed zone, and productivity of testing.

An approach is proposed to evaluate the depth and thickness of planar defects detected by thermal nondestructive testing (TNDT) methods. A 1D 3-layer thermal problem is solved by using the technique of thermal quadrupoles, and the solution of the corresponding inverse problem is presented in the polynomial form. The inverse solution involves a number of TNDT experimental parameters, in particular, differential temperature signals, dimensionless contrasts and their observation times. The accuracy of defect characterization is in the range of 2–15% in defect depth and from 10 to 40% in defect thickness. It is believed that the proposed defect characterization approach can be easily implemented in existing TNDT systems to provide approximate values of defect parameters.

The paper describes the mechanism of heat release during deformation and main features of passive infrared thermographic diagnostics of steel parts subjected to elastic, elastic–plastic and plastic deformation under quasi-static loading. A basic equation for evaluating temperature signals caused by quasi-static deformation is derived. It is stated that the evaluation of steel structures by passive thermal methods should be performed when the heat is generated by mechanical stresses appearing in structures subjected to mechanical loading. The analysis of the loading kinetics and temperature anomalies has shown that thermal nondestructive testing technique is a useful tool for determining stress state in steel structures during the initiation and evolution of fracture. It is shown that the quasi-static deformation of steel structures is accompanied by the formation of elastic clusters separated by shear bands, and the plastic deformation in shear bands causes heat release resulting in a temperature increase of 4–12.5 °C.

The history of thermal nondestructive testing and infrared thermography is briefly reviewed. The state-of-the art of thermal testing with a focus on its applications to testing of composite materials in aerospace industry is described.

The use of infrared thermography for quantitative evaluation of water ingress in aviation honeycomb cells is discussed. Numerical modelling has been performed by analysing a 3D panel model where water fully or partially occupies honeycomb cells. Calculation of several test cases has allowed better understanding of how the thickness of the water layer affects surface temperature anomalies and times of their appearance in active one-sided thermal tests. Experimental results have been obtained on both reference samples and real honeycomb panels.

Marquetry method is important in the culture of the Italian community as can be witnessed from the large quantity of artworks that have been realized in this way. The monitoring of the integrity of such pieces poses a great challenge given the need for a reliable and nondestructive technique able to detect surface and subsurface defects. In this work, two ancient marquetry samples containing natural defects were inspected thanks to active thermography by using time-tested, safe, and resilient advanced signal processing algorithms (i.e., principal component thermography, correlation contrast, pulsed phase Fourier transform amplitude and phase, cold image subtraction contrast, and polynomial fitting). The latter have been applied to provide a 2D map of the defects. Anyway, in the cultural heritage field, one of the main interests of restorers is the volume of the subsurface defects for structural analyses. The emphasis in this study is placed on the use of dynamic thermal tomography (DTT) as an advanced technique of active thermal nondestructive testing. The main concepts of DTT are illustrated in the manuscript, while a special technique for defect thermal characterization has been used during the second analysis to validate tomographic results. Finally, the position of the main defects retrieved by means of the established techniques applied during the first analysis has been confirmed by DTT.

TNDT deals with the sample surface temperature distribution T(τ) evolving in time. In IR thermographic NDT, it is represented by the dynamic pixel function T(i, j, τ). Two examples of this function are shown in Fig. 3.1a for heating a steel plate with a Dirac and square pulse of duration 0.01 s. The temperature curves are practically identical, thus demonstrating that the calculation results are independent of the duration of heat pulse.

This chapter summarizes applications of both passive and active IR thermography and TNDT, including power generation, civil engineering and buildings, smokestacks, electrical installations, aerospace, petrochemical, automotive industry, art objects, medicine and many others. The discussions are supported by numerous illustrations and an extensive bibliography.

In this chapter, professional certifications for TNDT are summarized. An extensive list of certification documents, including national and international standards, inspection guidelines and other documents, is presented.

Thanks to its good strength/mass ratio, a glass fibre reinforced plastic (GFRP) composite is a common material widely used in aviation, power production, automotive and other industries. In its turn, active infrared (IR) nondestructive testing (NDT) is a common inspection technique for detecting and characterizing structural defects in GFRP. Materials to be tested are typically subjected to optical heating which is supposed to occur on the material surface. However, GFRP composite is semi-transparent for optical radiation of both visual and IR spectral bands. Correspondingly, the inspection process represents a certain combination of both optical and thermal phenomena. Therefore, the known characterization algorithms based on pure heat diffusion cannot be applied to semi-transparent materials. In this study, the phenomenon of GFRP semi-transparency has been investigated numerically and experimentally in application to thermal NDT. Both Xenon flash tubes and a laser have been used for thermal stimulation of opaque and semi-transparent test objects. It has been shown that the penetration of optical heating radiation into composite reduces detectability of shallower defects, and the signal-to-noise ratio can be enhanced by applying the technique of thermographic signal reconstruction (TSR). In the inspection of the semi-transparent GFRP composite, the most efficient has been the laser heating followed by the TSR data processing. The perspectives of defect characterization of semi-transparent materials by using laser heating are discussed. A neural network has been used as a candidate tool for evaluating defect depth in composite materials, but its training should be performed in identical with testing conditions.

International audience; This study has illustrated the potentials of optical and THz infrared thermography in the identification of inserts of different nature in glass fiber reinforced polymer. The inspection efficiency has been comparatively evaluated by applying the Tanimoto criterion. The best test procedure has proven to be one-sided stationary thermal nondestructive testing (TNDT) implementing optical heating with Xenon lamps (Tanimoto criterion 87%). Close values of the Tanimoto criterion have been achieved by using the optical and THz line-scanning procedures. A more subjective evaluation of test procedures has been performed by comparing eight parameters of test performance. The line-scanning TNDT procedure implementing optical heating with halogen lamps seems to be optimal in practical applications.

This review paper describes in short the application areas of infrared thermography and thermal testing, such as civil and electrical/thermal engineering, petrochemistry, military, aerospace and others.

The paper describes the concept of an automated defect characterization procedure by using infrared nondestructive testing of glass fiber reinforced composite. The proposed algorithms have allowed determination of defect depth, lateral dimensions and area, as well as coordinates of defect centers. The algorithms are based on the use of the neural network trained on both experimental and theoretical temperature profiles. An acceptable for practice accuracy of defect characterization has been obtained on the experimental data (0–15% by defect depth and 26–139% by defect area).

Results of using powerful LED matrices to inspect metal articles for hidden corrosion by active thermal-wave method are described. The effect of the color of paint-and-lacquer coating on the efficiency of heating with LED and halogen emitters is analyzed. The possibility of using powerful LED panels as thermal stimulation sources in portable flaw detectors is described.

IR NDT (Infrared Nondestructive Testing) is a popular method for detecting defects in composite, ceramic, and metallic structures. The effectiveness of IR NDT depends on various thermal and optical properties of the material being tested. The thermal properties, including thermal conductivity, thermal diffusivity, specific heat and density are important and have been discussed extensively in many treatises on IR NDT. However the optical properties of the surface are equally important and while the thermal properties cannot be changed, sometimes the optical properties can be. Bare metal surfaces have high reflectivities and low emissivities, and as a result, they are generally not good candidates for IR NDT. Painted, coated, anodized, and oxidized metal surfaces can, in some cases, be successfully tested with IR NDT, but the effectiveness depends on the optical properties of the surface. It is well known by IR NDT practitioners that the easy solution to the testing of reflective materials is to paint the surface black. However, this is not always practical and it may not be permitted by the “owner” of the part. This paper demonstrates a process of analyzing the interaction of spectral curves that are relevant to the IR NDT process. This process can be used to evaluate the effectiveness of an IR NDT process for use on real parts with specific coatings and can help select a coating that may improve the effectiveness. This paper shows examples of optical properties for some typical paints and coatings that may be used on aluminum aircraft structures. It shows the spectrum of a generic incandescent radiant heat source and how the energy from this source is absorbed by several of these paints. Further, it shows the interaction between an IR camera detector response curve and the other curves. And finally, it shows how these three can be combined to produce an “IR NDT” efficiency rating for several examples.

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.

Active infrared (IR) thermography is a non-contact, fast and wide-area nondestructive testing (NDT) technique that has been increasingly used in aerospace applications to detect both manufacturing and in-service environment-induced defects. The classical pulsed IR thermographic testing suffers from the problem of false indications due to surface clutter conditioned by uneven optical properties across a test sample surface. To some extent, this problem can be relaxed by using a technique called ‘normalization’. A simple normalization algorithm, conventionally called 1D, involves the division of all images in a sequence by a chosen single image, often captured immediately after a flash. The novel technique of 3D normalization is implemented in this study to overcome the problems arising due to non-uniform heating and lateral heat diffusion in the case of pulsed IR thermographic NDT. In this case, normalization is carried out by dividing an experimental IR image sequence by the synthetic sequence which is calculated by solving the corresponding 3D problem of heat conduction. As a result of 3D normalization, the artefacts appearing due to uneven heating and absorption can be subdued through data normalization (division). Furthermore, fully automatic defect detection becomes possible as defining a reference point for sound area is not required as opposed to thermal contrast methods. In this work, the effectiveness of proposed 3D normalization approach is demonstrated for different heating conditions applied to glass and carbon fiber reinforced composites.

The efficiency of eight algorithms of defect depth characterization (pulse phase thermography – PPT, thermographic signal reconstruction by analyzing the first and second derivatives– TSR, early observation – EO, apparent thermal inertia – ATI, thermal quadrupoles - TQ, non-linear fitting - NLF and neural networks – NN) has been comparatively analyzed on both theoretical and experimental IR image sequences obtained in the inspection of CFRP composite. Synthetic noise-free image sequences have been calculated by means of the ThermoCalc-3D software, while experimental results have been obtained by applying a one-sided procedure of pulsed thermal NDT to the inspection of artificial defects in CFRP. A relative error in the evaluation of defect depth has been chosen as a figure of merit. It has been demonstrated that a simple and robust processing technique is the use of the Fourier transform resulting in phase-domain data (PPT). The technique of TSR ensures maximal values of signal-to-noise ratio and is less susceptible to uneven heating and lateral heat diffusion. The calculation of ATI has allowed the characterization of defects at depths up to 1.5 mm, but it is sensitive to uneven heating thus requiring to carefully choose a non-defect area. The EO method, as well as the technique of TQ, have revealed inferior results in defect depth identification because of a noisy character of raw signals. Non-linear fitting is a convenient processing technique allowing simultaneous characterization of some test parameters, such as material thermal properties, defect depth and thickness, etc., but this technique is time-consuming and can hardly be applied to full-format images. In the whole defect depth range, minimal characterization errors have been ensured by the use of the NN that is a promising tool for automated identification of hidden defects.

Contemporary composite materials are continuously being modified and improved in accordance with growing quality requirements of aerospace systems. New manufacturing technologies stimulate development of appropriate (novel) nondestructive testing (NDT) methods and respective hardware for detecting and characterizing hidden defects. One of the recent improvements in this area is related to combining acoustic stimulation of test objects and scanning laser vibrometry. This paper describes an ultrasonic spectroscopic approach to laser vibrometry applied to hybrid composite materials subjected to impact damage. The meander-shaped signal for resonance stimulation was applied to enhance the detection of multi-component defects. It has been shown that wide-band frequency stimulation provides some advantages in regard to mono-frequency excitation because it activates more zones of multi-component defects thus allowing more accurate estimation of defect size.

This paper presents a brief history and the current status of research at the Thermal Nondestructive Testing Laboratory of Tomsk Polytechnic University (TPU), Russia. The emphasis of the laboratory research is advanced modelling (analytical and numerical) and data processing, including Fourier and wavelet transform, neural networks, correlation analysis, dynamic thermal tomography, and other methods of thermal NDT. This paper describes experimental tests, practical applications in aviation and energy audits, as well as areas of future research.

This paper summarizes the basics of pulsed thermal nondestructive testing (TNDT) including theoretical solutions, data processing algorithms and practical implementation. Typical defects are discussed along with 1D analytical and multi-dimensional numerical solutions. Special emphasis is focused on defect characterization by the use of inverse solutions. A list of TNDT terms is provided. Applications of active TNDT, mainly in the aerospace industry, are discussed briefly, and some trends in the further development of this technique are described.

One-sided thermal non-destructive testing (TNDT) is of vital importance in aerospace composites to monitor structural degradation, as well as to detect subsurface defects. The objective of this study is to evaluate simultaneously the thermal diffusivity and to characterize the defects within carbon fiber reinforced polymer (CFRP) composites. In this paper, a novel one sided TNDT technique is presented which relies on creating an artificial inflection point by multiplying front-surface temperature evolution by n th power of time, where 0 n n . An analytical solution of the diffusivity calculation and defect characterization in the case of CFRP composite plate is discussed. The characterization of low velocity impact damage is also investigated with the help of theoretical and experimental results. Some key research points which need further exploration are also discussed.

A thermo mechanical problem of internal friction in zero-thickness defects is numerically solved, and the theoretical predictions are qualitatively compared with experimental data. Some examples of impact damage characterization by applying both ultrasonic and optical material stimulation are presented.

The concept of “dynamic thermal tomography” (DTT) was suggested in the 1980s. At that time, there was a wave of interest in the tomographic analysis of materials by active thermal nondestructive testing (TNDT). Unlike particles and quanta of electromagnetic radiation, thermal energy propagates in solids by diffusion. Therefore, a purely geometrical approach, that is characteristic of computed X-ray tomography, is replaced in DTT with the analysis of the evolution of temperature versus time. DTT is based on the fact that, in one-sided TNDT, deeper material layers are characterized by longer time delays of the thermal response. The DTT algorithm is relatively stable when used in the inspection of certain materials. Thermal waves experience damping by amplitude and retardation in time. This limits the detection depth and produces certain artifacts that can be suppressed by thresholding maxigrams. DTT can also be considered as a specific way of data presentation that has proven to be useful in many practical cases, including surface and volumetric thermal stimulation of both metals and non-metals. Thermal tomograms appear similar to binary maps of defects, thus enabling more reliable defect detection in comparison to conventional IR thermograms. In this paper, a “reference-free” approach to DTT is proposed being based on some mathematical manipulations with a front-surface temperature response. Also, the possibility of using the DTT principles for processing the results of ultrasonic infrared thermography is demonstrated.

Many composite parts, such as laminated panels and grid-like shells, operate under high mechanical loading. Evaluation of their structural integrity is crucial to ensure the long-lasting operation of critical components. Since testing a structure under full or “proof” load might be dangerous for personnel, it would be preferable to use a remote, rapid inspection technique. This paper describes a practical application of IR thermography to the inspection of large composite parts used in the aerospace industry. This work has used just one cycle of increasing load from zero load to failure, and this was done for both for tensile and compressive loads. It is shown that, during the formation of micro-defects in polymeric composites, about 40 % of the total dissipated energy is expended for material heating, while about 60 % is related to material damage accompanied by an increase in the defect concentration. Non-uniform composite deformation causes temperature anomalies, whose amplitude may reach 1.5–2.5 °C at a load of about 50–60 % of the limit load.

Deformation of steel parts is accompanied by either heating or cooling of particular zones depending on deformation mechanism. The use of infrared thermographic equipment allows analyzing spatial/temporal temperature distributions on the surface of steel parts thus allowing the evaluation of heat release caused by deformation in bulk material. Determination of stressed state in critical parts by analyzing infrared thermograms can be most simply conducted for components subjected to uniaxial tension–compression. The paper describes some potentials and problems of nondestructive testing of steel parts and constructions based on the analysis of heat release caused by deformation. By analyzing this methodology, it is possible to better evaluate the life expectancy of critical parts in steel structures (components of offshore oil platforms, seismic-resistant buildings, frames of large mining trucks, etc.).

The results of applying three nondestructive testing techniques to the inspection of parts of a new Russian TVS-2DTS airplane made of carbon fiber reinforced plastic are presented. A basic technique implemented in workshop conditions implements optical stimulation of inspected parts. The usefulness of ultrasonic infrared thermography combined with laser vibrometry in the evaluation of parts with complicated geometry is illustrated. Samples with artificial and real defects have been tested in workshop conditions.

Composite materials are being increasingly used in high-tech industries, such as aerospace, automotive manufacture and building inspection. Thermal nondestructive testing (TNDT) has become an accepted method for composite inspection. However, the majority of investigations have dealt with flat or slightly-curved composite components with a thickness of up to 5 mm. Particular studies have been devoted either to NDT modeling with an emphasis on some theoretical issues, or they have been based exclusively on experimental results. There has been some recent interest in the use of composite materials in the nuclear industry. Some critical parts, including centrifuge components, have been made of carbon fiber reinforced polymer (CFRP) composites. The working conditions in a centrifuge include radioactivity and high rotational speed, and the composites used in centrifuges must have very uniform thermal properties and must be free of defects. This paper describes a complex approach to the TNDT of cylindrical parts made of CFRP by starting from thermal properties measurement, theoretical modeling and preliminary experiments, and finishing with the technical requirements for the development of practical equipment capable of operating in both laboratory and industrial conditions. The objects tested were CFRP cylinders with a diameter of 150 mm and a wall thickness of 4–6 mm, and they contained some artificial defects of varying size and depth. Both one- and two-sided test procedures have been analyzed for spot, line and uniform heating. Ultrasonic excitation has also been used as an alternative stimulation technique. In a one-sided test, the depth detection limit has been about 4 mm. Similar results have been observed in the case of ultrasonic stimulation, but the practical implementation of ultrasonic IR thermography to the inspection of cylindrical parts requires further exploration. In a two-sided test, even fairly mild heating resulted in the reliable detection of all defects independent of their size and depth. In all test cases, the highest signal-to-noise ratio occurred after applying the technique of principal component analysis.

The problem of moisture accumulation in airplane honeycomb panels is so serious that perspective aviation constructions could become monolithic or filled in with special foam. However, the number of airplanes with plentiful honeycombs under exploitation will keep very high in the few next decades. Therefore, quantitative water detection remains an actual task in aviation. The qualitative aspect of this problem can be solved by using the remote and fast technique of infrared thermography. Hidden water can be detected for a certain period of time after landing, or some stimulation heat sources can be used to enhance water visibility in honeycomb panels. However, quantitative evaluation of moisture content is typically achieved by applying a point-by-point ultrasonic technique which allows measuring the height of the water bar in single cells thus compiling maps of water distribution. This technique is contact and can be enough informative when applied to the water which is in contact with the panel skin because of gravitation. The use of solely infrared thermography for evaluating accumulated water mass based on the analysis of temperature patterns is difficult. Recently we found that there is a certain promise in the thermographic determination of water content, but the question is how precise (or how approximate) can be such estimates. The paper contains modeling and experimental results obtained in this direction.

Thermal/infrared non-destructive testing (T/I NDT) is a particular application of IR thermography. T/I NDT is typically classified for passive and active, as well as for steady-state (stationary) and transient (non-stationary, or dynamic). Active T/I NDT can be classified by: (1) the type of thermal stimulation, (2) the arrangement of a sample and a thermal stimulation source, and (3) the size and shape of stimu l ated area. T/I NDT has proven to be a convenient technique for the detection of impact damage in composite materials due to the following: (1) graphite-based composites are similar to a blackbody by absorption/radiation properties in the infrared (IR) wavelength band, (2) their thermal conductivity is lower than that of metals but higher than of many non-metals thus ensuring reasonable temperature signals at convenient observation times, (3) impact damage leads to thin but laterally-extended air-filled defects which produce considerable thermal resistance to the in-depth heat flux, and (4) T/I NDT is a fast, remote and illustrative technique which, unlike ultrasonic inspection, does not require immersing a sample into water. This paper describes some approaches to thermal detection and characterization of impact damage in carbon fiber reinforced plastic (CFRP) of whose inspection is an important issue in several industrial areas, first of all, in aero space where subsurface defects might lead to catastrophic consequences. Realistic solutions of T/I NDT theoretical problems can be obtained by using 3D numerical models of heat conduction. Direct solutions allow better understanding of heat propagation in defect areas while inverse solutions ensure the evaluation of defect parameters, such as defect depth, size and thickness. Several characterization algorithms are available, with a one-sided T/I NDT procedure being better suited for the characterization of defect depth, while defect thickness is best evaluated in a two-sided procedure. In the case of CFRP composites, the defect characterization approaches are well developed, including the technique of dynamic thermal tomography, which enables a considerable reduction of surface clutter and allows the imaging of separate layers of a composite test sample.