Active IR Thermography Research Articles

The Application of Two-Dimensional Continuous Wavelet Transform Based on Active Infrared Thermography for Subsurface Defect Detection in Concrete Structures

The early condition-based assessment of civil infrastructures plays an essential role in extending their service life, preventing undesirable sudden failures, and reducing maintenance and rehabilitation costs. One of the most commonly used and fastest nondestructive testing (NDT) techniques is infrared thermography (IRT), which has emerged as a powerful method for assessing general concrete quality and detecting subsurface damage in structural members. Nevertheless, the accurate detection and classification of localized defects is still a challenging task to achieve. The contribution made by enhancing defect detection using two-dimensional (2D) wavelet transformation (WT) as a post-processing method, however, has received little attention within the field of active IR thermography. In this study, we explored the use of continuous wavelet transform (CWT) to visualize how the wavelet function at different frequencies could enhance the damage features of thermal images. A concrete slab under an applied heat flux was tested experimentally by an IR camera with well-controlled excitation sources. The qualitative visualization of thermograms was translated into quantitative results by extracting, processing, and post-processing the values assigned to the pixels in the thermal images. With the assumption of there being no oriented damage features, an isotropic (non-directional) Mexican hat wavelet was utilized as the mother wavelet. The experimental results showed that the 2D-CWT method achieved strong detection performance in extracting discriminatory features (defective areas) from the acquired thermal images. Compared with raw thermograms, the resultant CWT-transformed images were less affected by the non-uniform heating effect, and the boundaries of the defects contrasted more strongly. The 2D-CWT method demonstrates good sensitivity when an appropriate wavelet type and scale factor are chosen. Due to the desire to detect localized defects, adjusting the scale factor of the wavelet is important to improve the efficiency of detection as lower scale factors provide the finer details of thermal images, whereas higher scale factors provide the general outline of internal defects. The findings of this study represent a further step toward improving thermographic data for more precise defect-detection imaging, and principally for large concrete structures, that can be verified easily using other NDT surveys.

Characterising Hidden Defects in GFRP/CFRP Composites by using Laser Vibrometry and Active IR Thermography

ABSTRACT This paper describes the use of laser vibrometry and thermal non-destructive testing for detecting defects in GFRP composite by applying a single test procedure. The laser vibrometry involves the averaging of vibration amplitudes over an entire frequency spectrum accompanied by the frequency-phase analysis. Such procedure may lead to suppression of low signals, produced by smaller and deeper defects, by higher signals conditioned by larger and shallower defects. The fusion of results obtained with both laser vibrometry and thermal NDT was used to enhance defect detectability. The corresponding experimental techniques were applied to detecting four types of defects (air gaps, foam, blank grooves, resin) in GFRP composite, as well as evaluating defect size and depth. Also, laser vibrometry was used to characterise barely visible impact damage in CFRP composite, and test data was compared with C-scan ultrasonic inspection results. It has been shown that the results supplied by three inspection techniques have been reasonably consistent in characterisation of defect lateral area and depth. When analysing the complex structure of impact damage in composites, the detection of both shallow and deep defects can also be enhanced by performing fusion of test results supplied by laser vibrometry and active thermal NDT.

Soft-sensing reconstruction of in-depth defect geometry from active IR-thermography data

This paper proposes a new approach for processing measured data from active Infra Red (IR) thermography, where a soft sensing algorithm is exploited for in depth defect reconstruction. This is achieved by propagating the information gathered at the wall surface to the inner layers. Correlating the experimental 2D measurements to a Finite Element (FE) model of the tested specimen it is possible to update the model with the measured data and change the geometry of the simulated inner defect, until the surface temperature distribution calculated corresponds to the measured one. Following that strategy, the unknown defect geometry can be determined. The method developed and presented in this paper consists of an optimization problem based on the minimization of the difference between the surface temperature distribution measured on the sample subjected to an active thermography test and the one resulting from the FE model. The optimization variables are the geometrical parameters (depth, width, thickness and position) characterizing the defect which will be fully determined at the complete convergence, within a given tolerance, of the optimization problem. The method includes also a preprocessing algorithm, based on the same experimental data and FE model, which allows to determine thermal and mechanical properties of the object under test, like surface emissivity, heat capacity and material conductivity and density, which are often unknown especially in the case of works of art. This soft-sensing procedure has been applied to a virtual experiment to estimate the accuracy of the reconstructed geometry and to a simulacrum of ancient fresco including defects realized on purpose.

Active IR Thermography Evaluation of Coating Thickness by Determining Apparent Thermal Effusivity.

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.

Evaluating quality of marquetries by applying active IR thermography and advanced signal processing

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.

Comparative analysis of scarf-type adhesively bonded and hand lay-up repairs of impacted GFRP composites

In this paper two types of repair methods were considered: adhesively bonded and hand lay-up scarf-type patches. The performance of both repair techniques applied to impacted/damaged glass fibre reinforced plastic (GFRP) laminates was assessed under tensile and three-point bending tests. In the hand lay-up patches, two types of resins, namely unmodified and nano-modified resins were used to evaluate their effects on the flexural strength and failure modes of the repaired composite specimens. The damage was assessed using visual observation, active IR thermography, ultrasonic testing, and optical transmittance scanning techniques at different stages. Results indicate that, depending on the type of repair and nano-filler used, the flexural properties of the repaired composite specimen exhibited significant improvements. The choice of repair method would vary depending on the desired application along with accessibility to the damaged area. Overall, repair of composites allows considerable economic benefits over the replacement of an entire structural component.

Comparative analysis of scarf-type adhesively bonded and hand lay-up repairs of impacted GFRP composites

In this paper two types of repair methods were considered: adhesively bonded and hand lay-up scarf-type patches. The performance of both repair techniques applied to impacted/damaged glass fibre reinforced plastic (GFRP) laminates was assessed under tensile and three-point bending tests. In the hand lay-up patches, two types of resins, namely unmodified and nano-modified resins were used to evaluate their effects on the flexural strength and failure modes of the repaired composite specimens. The damage was assessed using visual observation, active IR thermography, ultrasonic testing, and optical transmittance scanning techniques at different stages. Results indicate that, depending on the type of repair and nano-filler used, the flexural properties of the repaired composite specimen exhibited significant improvements. The choice of repair method would vary depending on the desired application along with accessibility to the damaged area. Overall, repair of composites allows considerable economic benefits over the replacement of an entire structural component.

Thermal modelling and screening method for skin pathologies using active thermography

This paper presents a novel screening approach of human skin pathologies using Active IR Thermography. The inputs of the proposed algorithm are the values of the physical parameters of the skin. Parameters are estimated based on dynamic thermographic measurements of human skin and the developed thermal model of the tissue. The calculations were based on the inverse thermal modelling. Classification was done using Support Vector Machine, Linear Discriminant Analysis and k-Nearest Neighbours classifiers. As an example, one presented the results of screening for psoriasis.

Recognition of wall materials through active thermography coupled with numerical simulations.

In the framework of historical buildings, wall thickness as well as wall constituents are not often known a priori, and active IR thermography can be exploited as a nonintrusive method for detecting what kind of material lies beneath the external plaster layer. In the present work, the wall of a historical building is subjected to a heating stimulus, and the surface temperature temporal trend is recorded by an IR camera. A hybrid numerical model is developed in order to simulate the transient thermal response of a wall made of different known materials underneath the plaster layer. When the numerical thermal contrast and the appearance time match with the experimental thermal images, the material underneath the plaster can be qualitatively identified.

An active thermography approach for thermal and electrical characterization of thermoelectric materials

The enhancement of figure of merit (ZT) of thermoelectrics is becoming extremely important for an efficient conversion of thermal energy into electrical energy. In this respect, reliable measurements of thermal and electrical parameters are of paramount importance in order to characterize thermoelectric materials in terms of their efficiency.In this work, a combined theoretical-experimental active thermography approach is presented. The method consists of selecting the right sequential interdependence between the excitation frequency and the sampling rate of the infrared camera, by computing a temporal Fourier analysis of each pixel of the recorded IR image. The method is validated by using a reference sample which is then applied to a recent synthesized titanium trisulphide thermoelectric material (TiS3). By combining AC and steady-state experiments, one can obtain information on both thermal and electrical parameters of TE materials (namely thermal diffusivity, Seebeck coefficient). The thermal diffusivity and thermal conductivity of TiS3 are also measured using photothermal radiometry technique (PTR) and the resulting values of these parameters are α = 9.7*10−7 m2 s−1 and k = 2.2 W m−1 K, respectively. The results obtained with the two techniques are in good agreement. In the case of TE materials, the main benefit of the proposed method is related to its non-contact nature and the possibility of obtaining the electric potential and temperature at the same probes. The Seebeck coefficient obtained by active IR thermography (S = −554 μV K−1) is consistent with the one obtained using an ULVAC-ZEM3 system (S = −570 μV K−1). For a large number of users of thermographic cameras, which are not equipped with a lock-in thermography module, the present approach provides an affordable and cheaper solution.

Review of Active IR Thermography for Detection and Characterization of Defects in Reinforced Concrete

Active thermography methods enable structural investigations of reinforced concrete elements taking into account many different testing problems. The goal of this review is to provide an overview on the state-of-the-art regarding the use of active infrared thermography (IRT) for detection and characterization of defects in reinforced concrete. The paper will provide the physical background, equipment being used, as well as post-processing methods that are used to analyse sequences of thermograms. This work also presents the fields of applicability of IRT with a focus on the aspects related to reinforced concrete structures, as well as the advantages, limitations and potential sources of errors of IRT employment. Additionally previous non-destructive testing (NDT) studies that employed thermography techniques with natural excitation are briefly presented. A review of the future trends of thermal imaging are also included in this work. It can be concluded that while IRT is a useful tool for the characterisation of defects in the building sector, there is great prospect for the development of more advanced, effective and accurate approaches that will employ a combination of thermography approaches.

Infrared image processing devoted to thermal non-contact characterization-Applications to Non-Destructive Evaluation, Microfluidics and 2D source term distribution for multispectral tomography

The cost of IR cameras is more and more decreasing. Beyond the preliminary calibration step and the global instrumentation, the infrared image processing is then one of the key step for achieving in very broad domains.Generally the IR images are coming from the transient temperature field related to the emission of a black surface in response to an external or internal heating (active IR thermography). The first applications were devoted to the so called thermal Non-Destructive Evaluation methods by considering a thin sample and 1D transient heat diffusion through the sample (transverse diffusion). With simplified assumptions related to the transverse diffusion, the in-plane diffusion and transport phenomena can be also considered.A general equation can be applied in order to balance the heat transfer at the pixel scale or between groups of pixels in order to estimate several fields of thermophysical properties (heterogeneous field of in-plane diffusivity, flow distributions, source terms).There is a lot of possible strategies to process the space and time distributed big amount of data (previous integral transformation of the images, compression, elimination of the non useful areas...), generally based on the necessity to analyse the derivative versus space and time of the temperature field. Several illustrative examples related to the Non-Destructive Evaluation of heterogeneous solids, the thermal characterization of chemical reactions in microfluidic channels and the design of systems for multispectral tomography, will be presented.

IR THERMOGRAPHY IN NDE OF CARBON-GLASS LAMINATE

The article presents a comparison of the effects of using active IR Thermography technique for the tests of composite aircraft structures. The most important question arising together with an increased use of composites in aviation industry is to work out a method, which would allow identification of the technological defects, and damages appeared in the process of the aircraft use. Non-destructive testing which has been used so far to identify defects in the aircraft structure made from metallic materials has a quite limited application in case of composite structures. With the growing use of composites in the aviation industry, a necessity appears to work out a method allowing early identification of technological defects and damages appeared in the process of the aircraft operation. It could be the active thermography techniques used for the thin walled elements tests, such as wing and fuselage skin, with the focus on fractures and delamination. Light composite aircraft have been built for a many of years, however, the annual flight hours of these aircraft are incomparably lower than those of the aircraft used in air transport. In group of light aircraft, the less important elements were made of composites regarding the strength of the whole construction. When the newest aircraft were introduced, with most important construction elements such as the fuselage, wings, horizontal and vertical tails, doors and the interior made of composite materials, it was expected that the increased amount of the annual flight hours (more than 3000 hours) and the expected airliner lifetime (about 30 000 hours) would soon cause degradation of the mechanical properties of the composites following from the progress of ageing of the composite structures. The obtained research results bring closer the problem of selection of the well-fitted non-destructive testing method depending on the kind of the diagnosed construction.

Research on thermal wave processing of lock-in thermography based on analyzing image sequences for NDT

Lock-in thermography, an active IR thermography technique for NDT, is based on propagation and reflection of thermal waves which are launched from the surface into the inspected component by absorption of modulated radiation. In this paper, thermal wave image sequences were sampled by a Cedip JADE MWIR 550 FPA infrared camera. Thermal wave signal processing algorithms are investigated to obtain information on subsurface defects. The Fourier transform, four-point correlation, and digital lock-in correlation algorithms are applied to extract the amplitude and phase of thermal wave’s harmonic component. A novel method called the time constant method (TCM) is proposed to analyze subsurface defects by using lock-in thermography. The experimental results confirm the thermal wave signal processing algorithms’ efficiency on subsurface defect detection.

LOCALIZING HISTORICAL CLUES USING IRT AND PETROGRAPHIC ANALYSES AT VILLA MIRABELLO, MONZA (ITALY)*

The rebuilding of Villa Mirabello started in 1666. Built on designs of Gerolamo Quadrio, the project refurbished and enlarged a smaller 16th century country house (‘casa da nobile e da massaro’). Archival documents report on the remodelling, which consisted of selective demolition of portions of the structure, re‐using most of the masonry as the core of the new building, and the reinforcement of retained masonry by insertion of ‘ceppo’ stone quoins to update new exteriors. The aim of Quadrio was to create an elegant and sumptuous mansion, suitable for noble owners such as Giuseppe Durini and his family. The plan and a small part of original finishing (‘marmorino’ plaster decoration) are recognizable nowadays. Nevertheless, there are many questions in interpretation when one compares the documents and the building. That is, Cardinal Angelo Durini made a major modification in the 18th century, and further minor changes were made in the 19th century. Although these latter modifications did not alter the plan, they changed the distribution of rooms and masked both decoration and finishing. After over 20 years of neglect, Villa Mirabello is now close to being restored. Architects in charge ordered a preliminary set of diagnostics to assess damage and study the historical evolution of the building. Integration of IR thermography (IRT) and endoscopy allowed restorers to detect the structure's texture underneath the plaster, and to detect openings filled with masonry. Moreover, archive documents confirmed these results and, above all, defined a date for the use of specific building techniques. This research constitutes a reference for buildings set in the same time and location that do not have significant archival documentation. Active IRT provided useful information for structural assessment (such as location of arches, chimney stacks, different thickness of wall, wooden elements, voids, beams etc.) and for crack pattern evaluation. This information directs design professionals working on this conservation project, and helps define the costs of intervention.

Unsteady-state thermophysical models for detection of physiological and pathological conditions of the kidney by methods of active dynamic IR thermography

Unsteady-state processes of biological heat transfer are simulated and their numberical analysis is made to find out whether it is possible to determine thermal properties of the kidney by methods of active dynamic IR thermography. Criteria are developed for interpretation of dynamic IR thermal charts for medical diagnosis that noticeably increase its accuracy and reliability in comparison with the present potentialities of static IR thermography. The analysis performed allows thermal diagnostic interpretation of the dynamics of the functioning of the kidney in normalcy and in a particular pathology.