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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.

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.

The influence of the average size of the structure elements on the deformation and thermal behavior of the Zr-1 wt.% Nb (Zr1-Nb) alloy under quasi-static tension was investigated using the digital image correlation and infrared thermography methods. It is shown that with increasing average size of the structural elements in the range 0.2–2.0 µm the physico-mechanical properties, such as yield strength, microhardness, maximal true strain, and maximal temperature increment during deformation decrease, while longitudinal and transverse strain increase. According to the obtained results, correlations between the mentioned deformation characteristics and the average size of the structural element d–1/2 can be described by linear functions.

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.

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.

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 study is devoted to the analysis of impact damage in composite samples by applying techniques of IR and Terahertz imaging. The samples made of high-density homogeneous particleboards of sugarcane bagasse and castor oil polyurethane resin were subjected to impacts with the energy from 5 to 30 J. The impact damage defects were evaluated by applying one- and two-sided thermal non-destructive testing. Both the apparent thermal effusivity and diffusivity were determined to analyse their relationship to impact energy. In the above-mentioned range of impact energies, the thermal effusivity varied from 5 to 18%, while the variations in thermal diffusivity were from 4 to 24%. The algorithm of dynamic thermal tomography was used to demonstrate that predominant subsurface cracking occurred at depths up to 1 mm. In addition, the defect areas were tested by applying the Terahertz technique, whose results qualitatively matched the IR thermographic ones to show that the damaged areas were larger than they appeared visually. However, both inspection techniques have revealed no significant dependence of analysed parameters on impact energy. This is believed to prove that non-uniform composite structure is the decisive factor in producing structural defects under impacting.

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.

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.).

Contemporary thermoset composites using natural fibres offer a wide range of strength performance. Recently, the combination of flax and carbon fibres has received an increasing attention, mainly dictated by the possibility of merging in a single material high damping properties of flax fibres and the well-known high mechanical properties of carbon fibres. Evaluation of low energy impact damage defects has received a little coverage even if these composites are well known to be susceptible to impact damage. In this study, the use of ultrasonic resonance vibrometry is proposed as an effective nondestructive tool to detect the extent of impact damage in natural/synthetic fibre reinforced composites with different stacking sequences. The results for impacts at 10 and 40 J highlighted the role played by the different stacking sequences with damaged areas being twice smaller in composites with flax skins and carbon core compared to carbon-flax-carbon sandwiches.

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.

The use of wide frequency band piezoelectric transducers in ultrasonic infrared thermography allows analyzing material structural defects under low power ultrasonic stimulation compared to single-frequency stimulation which is performed, for example, by means of powerful magnetostrictive stimulation. Defect resonance frequencies can be determined through the detailed analysis of material surface vibrations by using a technique of laser vibrometry in a wide range of frequencies. This paper describes the approach to analyze ultrasonic resonances in samples with hidden defects by using resonant piezoelectric transducers. The effectiveness of the method is assessed by discussing some key examples of impact damaged graphite/epoxy composite samples hybridized with flax fibers. Optical and powerful ultrasonic stimulation have been also used as alternative inspection techniques.

The usage of terahertz infrared thermography for assessing the humidity of wooden samples is described. Terahertz radiation transmitted through a sample is imaged by means of a tera-thermoconverter. The temperature field of the converter is analyzed with an infrared imager. The possibility of using the technique for testing the nonuniformity of drying of timber is illustrated.

Thermal nondestructive testing (TNDT) may be considered to be a more widely applicable method than many traditional techniques, such as X ray, ultrasonic, eddy current, liquid penetrant, etc. It can be applied to both metals and non-metals containing subsurface defects such as cracks, foreign inclusions, disbonds, delaminations, variations in thermal properties, etc. This is especially true for composite materials, and TNDT is very appropriate for screening purposes. TNDT test results may be analyzed by advanced image processing algorithms. This paper provides a concise review of composite NDT using TNDT in combination with other inspection techniques, providing an opportunity for data fusion.

We have used infrared thermography to study water trapped in aviation honeycomb panels, namely: 1) possibility of quantitative evaluation of trapped water, 2) influence of honeycomb panel orientation on the efficiency of water detection, and 3) discrimination between honeycomb cells filled with water and adhesive. It is shown, both theoretically and experimentally, that it is difficult for TNDT to discriminate between the presence of water and epoxy trapped in honeycomb cells, by analyzing only the surface temperature patterns. It has been demonstrated that water trapped in honeycomb cells can be reliably detected by an increase in effusivity starting from the inflection time. This process is referred to as the “concept of apparent effusivity”.

Several reviews and summary papers describing the history and the current status of pulsed thermal nondestructive testing (TNDT have been published recently. However, some of the theoretical possibilities cannot easily be implemented in practical applications. This paper contains useful tables and formulas that are illustrated with typical IR thermograms to provide a general overlook of pulsed TNDT.

Solid rocket motors (SRMs) are key components of missiles, space rockets and shuttles. In order to prevent their combustion chamber cases from being damaged by high-temperature and high-pressure gas, they must be protected by special thermal insulation layers. Disbonds, or poor bonding in the adhesion interfaces often appear during manufacture because of technological reasons, thus possibly causing catastrophic failure of space vehicles when in operation. Pulsed IR thermography (PIRT) has been used successfully for detecting defects in aero space components due to its high productivity, one-sided access and safe exploitation. This paper describes some results of applying PIRT to the inspection of the thermal insulation of SRM cases. Particular emphasis is laid on the development of some novel approaches based on the correlation technique.

Algorithms for determining the thermal resistance under the conditions of a time-dependent thermal regime, namely, a moving average, a black box, and nonlinear fitting, are studied on the basis of analysis of theoretical and experimental data.

The thermal inspection of thermal-barrier coatings of turbine buckets is briefly reviewed. An inspecting and data-processing technique is proposed and illustrated by experimental results.

Corrosion in metals is simulated with variations in plate thickness which are laterally infinite in a 1D model and are represented by milled flat-bottom holes in a 2D model. Temperature contrast over corroded areas is chosen as an informative parameter, quite independent of absorbed thermal energy in the infra-red thermographic test. It is shown that, due to lower sensitivity to rear-side effects at the beginning of the thermal process and increasing 3D heat diffusion effects at the end of the process, there is an optimum time to detect corrosion. A robust inversion function is proposed and its stability against variations in tested material, heat pulse duration and observation time is analysed using numerical modelling. Corrosion in a steel specimen of 1.3 mm thickness is experimentally studied, having proved the validity of the inversion algorithm with an average accuracy of 17% for material loss ranging from 74 to 14%.