Active Thermography Technique Research Articles

DBHF: A double fusion backbone and bidirectional feature hybrid fusion detector for high-precision inspection of avionics solder joint defects

The quality of avionics thermistor wire solder joints has a critical impact on the performance of in-orbit spacecraft. However, it is still hard to simultaneously realize a high-precision inspection of the multi-scale internal and external defects of avionics solder joints. Herein, in combination with active infrared thermography technique, an innovative DBHF detector with a double fusion backbone and a learnable weighted multi-scale bidirectional feature hybrid fusion pyramid network (BdFPN) is demonstrated based on RT-DETR framework. Specifically, the double fusion backbone framework comprises with two lightweight convolutional neural network-based backbones. Compared with conventional single backbone, the double-backbone design forms a multi-level Resnet-like structure, helping to enrich the extracted multi-dimension feature information and retain rich detail information of small-scale defects. The complex learnable weighted multi-scale BdFPN is designed for cross fusion of deep features. One or more additional multi-scale inputs are added to each node involved in feature fusion to aggregate more features from multiple scales and strengthen the interaction between low-level details and high-level semantic information, enabling the detector to make full use of multi-level feature information. Meanwhile, a learnable weight coefficient is set on each node involved in feature fusion and the importance of the input to each node is estimated by training, so that the effective information can be deeply aggregated at different levels. Our DBHF detector achieves a 92.7 % mAP@0.5 on a self-made avionics solder joint dataset with a body size of only 27.2 MB and FPS of 149, outperforming all the typical advanced real-time detectors.

Robotization of the IR-Thermographic technique – impact on the visualisation quality and considerations on the data workflow

Quality control automation is becoming increasingly popular in industrial production lines. Active thermography techniques are well-regarded for their adaptability, providing fast, non-contact, and full-field non-destructive evaluation. Automating thermographic evaluation can increase assessment speed and repeatability without sacrificing inspection accuracy. By using a robot arm to manipulate the thermographic setup, it becomes possible to inspect large components and refine scans on suspicious zones, even in parts with complex geometries. In this study, the performance of a new thermographic inspection platform is compared with a conventional setup to showcase the potential improvements. A plastic curved-shaped sample with artificial flat bottom hole defects is used as a benchmark for the comparison. The advantages and disadvantages of robotizing the infrared non-destructive setup are analyzed, and the impact of the data workflow and future research activities are also discussed.

An Active Thermography approach for materials characterisation of thermal management systems for Lithium-ion batteries

The aim of this work is an alternative non destructive technique for estimating the thermal properties of four different Thermal Management System (TMS) materials. More in detail, a thermographic setup realized with the Active Thermography approach (AT) is utilized for the purpose and the data elaboration follows the ISO 18755 Standard.As well known, Phase Changes Materials (PCMs) represent an innovative solution in the Thermal Management System (TMS) of Lithium-Ion batteries and, during the years, many solutions were developed to improve its thermal properties. As a matter of fact, parameters such as the internal temperature or heat exchanges impact on both efficiency and safety of the whole battery system. Consequently, the thermal conductivity was often chosen as a performance indicator of Thermal Management System (TMS) materials.In this work, both thermal diffusivity and thermal conductivity were estimated in two different testing conditions, respectively at room temperature and higher temperature conditions. The Active Thermography (AT) technique proposed in this activity has satisfactory estimated both thermal diffusivity and thermal conductivity of Thermal Management System (TMS) materials.An analytical model was also developed to reproduce the temperature experimental profiles. Finally, results obtained with AT approach were compared to those available from commercial datasheet and literature.

Double active thermographic inspection of additive manufacturing composites: numerical modelling and validation

This study investigates the Non-Destructive Testing (NDT) inspection of 3D-printed fibre-reinforced polymers, comparing the conventional Active Transient Thermography (ATT) technique with a novel variant known as Double Active Transient Thermography (DATT). Finite element models for the simulation of both inspection techniques are validated by evaluating two statistical measures to correlate the numerical results with the experimental responses. The thermal contrast obtained with DATT was approximately twice the one obtained with ATT, for all tested samples. Numerical models allowed an insight evaluation of the heat dissipation along the thickness of the specimens, specifically around the fibres and the defects. These validated numerical models evidenced a relevant tool to predict the results of thermal contrast and to optimize the inspection parameters.

Analysing the Probability of Detection of Shallow Spherical Defects by Means of Pulsed Thermography

The capability of Active Thermography (AT) techniques in detecting shallow defects has been proved by many works in the last years, both on metals and composites. However, there are few works in which these techniques have been used adopting simulated defects more representative of the real ones. The aim of this work is to investigate the capability of Pulsed Thermography of detecting shallow spherical defects in metal specimens produced with laser powder bed fusion (L-PBF) process and characterized by a thermal behaviour very far from the flat bottom hole and so near to the real one. In particular, the quantitative characterization of defects has been carried out to obtain the Probability of Detection (PoD) curves. In fact, it is very common in non-destructive controls to define the limits of defect detectability by referring to PoD curves based on the analysis of flat bottom holes with a more generous estimation and therefore not true to real defect conditions. For this purpose, a series of specimens, made by means of Laser-Powder Bed Fusion technology (L-PBF) in AISI 316L, were inspected using Pulsed Thermography (PT), adopting two flash lamps and a cooled infrared sensor. To improve the quality of the raw thermal data, different post-processing algorithms were adopted. The results provide indications about the advantages and limitations of Active Thermography (AT) for the non-destructive offline controls of the structural integrity of metallic components.

Dynamic Inspection of Surface-Breaking Defects using Induction Thermography

In comparison to other active thermography techniques, induction thermography can provide instantaneous results, is not affected by material emissivity/reflectivity and is indifferent to surface geometry. These advantages make it a promising technique to be applied for in-line/dynamic inspections. In this work, the thermal signatures from a titanium sample under different excitation parameters and moving speeds will be studied. To simulate surface-breaking cracks, electrical discharge machining (EDM) notches of varying lengths, widths and depths were made on the titanium sample. As with most applications, only a single side is accessible for inspection. Therefore, induction thermography in the reflection mode will be utilized in this work. After evaluating the signal-to-noise ratio (SNR) under various conditions and inspection speeds, the optimized configuration will be presented. The findings will be beneficial to researchers interested in implementing induction thermography in an in-line/dynamic inspection setting.

Fine Alignment of Thermographic Images for Robotic Inspection of Parts with Complex Geometries.

Increasing the efficiency of the quality control phase in industrial production lines through automation is a rapidly growing trend. In non-destructive testing, active thermography techniques are known for their suitability to allow rapid non-contact and full-field inspections. The robotic manipulation of the thermographic instrumentation enables the possibility of performing inspections of large components with complex geometries by collecting multiple thermographic images from optimal positions. The robotisation of the thermographic inspection is highly desirable to improve assessment speed and repeatability without compromising inspection accuracy. Although integrating a robotic setup for thermographic data capture is not challenging, the application of robotic thermography has not grown significantly to date due to the absence of a suitable approach for merging multiple thermographic images into a single presentation. Indeed, such an approach must guarantee accurate alignment and consistent pixel blending, which is crucial to facilitate defect detection and sizing. In this work, an innovative inspection platform was conceptualised and implemented, consisting of a pulsed thermography setup, a six-axis robotic manipulator and an algorithm for image alignment, correction and blending. The performance of the inspection platform is tested on a convex-shaped specimen with artificial defects, which highlights the potential of the new combined approach. This work bridges a technology gap, making thermographic inspections more deployable in industrial environments. The proposed fine image alignment approach can find applicability beyond thermographic non-destructive testing.

Capability to detect and localize typical defects of laser powder bed fusion (L-PBF) process: an experimental investigation with different non-destructive techniques

Additive manufacturing (AM) technologies, generally called 3D printing, are widely used because their use provides a high added value in manufacturing complex-shaped components and objects. Defects may occur within the components at different time of manufacturing, and in this regard, non-destructive techniques (NDT) represent a key tool for the quality control of AM components in many industrial fields, such as aerospace, oil and gas, and power industries. In this work, the capability of active thermography and eddy current techniques to detect real imposed defects that are representative of the laser powder bed fusion process has been investigated. A 3D complex shape of defects was revealed by a µCT investigation used as reference results for the other NDT methods. The study was focused on two different types of defects: porosities generated in keyhole mode as well as in lack of fusion mode. Different thermographic and eddy current measurements were carried out on AM samples, providing the capability to detect volumetric irregularly shaped defects using non-destructive methods.

Phase-transition-induced giant Thomson effect for thermoelectric cooling

The Seebeck and Peltier effects have been widely studied and used in various thermoelectric technologies, including thermal energy harvesting and solid-state heat pumps. However, basic and applied studies on the Thomson effect, another fundamental thermoelectric effect in conductors, are limited despite the fact that the Thomson effect allows electronic cooling through the application of a temperature gradient bias rather than the construction of junction structures. In this article, we report the observation of a giant Thomson effect that appears owing to magnetic phase transitions. The Thomson coefficient of FeRh-based alloys reaches large values approaching –1000 μV K−1 around room temperature because of the steep temperature dependence of the Seebeck coefficient associated with the antiferromagnetic–ferromagnetic phase transition. The Thomson coefficient is several orders of magnitude larger than the Seebeck coefficient of the alloys. Using the active thermography technique, we demonstrate that the Thomson cooling can be much larger than Joule heating in the same material even in a nearly steady state. The operation temperature of the giant Thomson effect in the FeRh-based alloys can be tuned over a wide range by applying an external magnetic field or by slightly changing the composition. Our findings provide a new direction in the materials science of thermoelectrics and pave the way for thermal management applications using the Thomson effect.

Active thermography technique for barrier coatings characterization

Aim of this paper is to define and set up an experimental procedure, based on active thermography, for the characterization of thermal barrier coatings for industrial applications, above all in aerospace field. The developed procedure is intended to be a fast and reliable method, alternative to the consolidated one described in International Standards as ISO18755 and ISO18555. In particular, this approach consists of a pulsed active thermography, in transmission configuration, obtained by means of a laser excitation. Temperature data processing, according to and adapting Standard procedures, allowed us to determine thermal parameters as conductivity and diffusivity. Obtained results were compared in terms of thermal properties variation with respect to base and coated materials, and in term of different coating procedures. These results were also compared to those available in literature.

ARTIFICIAL SPECIMENS MANUFACTURING OF GLASS FIBRE REINFORCED COMPOSITE FOR SUB-SURFACE DEFECTS INSPECTION BY ACTIVE LONG-PULSE THERMOGRAPHY

The detection and analysis of sub-surface defects in composite materials specimens have been problems of interest in recent years in different industries. For instance, in the wind industry, such materials are used for manufacturing wind turbine blades. A variety of defects can occur inside composite materials, e.g., porosities, fibre rupture, and inclusions. This work presents the design and manufacturing of artificial test specimens of composite material, to which specific defects were induced in a non-destructive way. The used manufacturing process was VARTM. In addition, a method to localise and characterise induced defects is proposed. Such method works by applying active long-pulse thermography to the test specimens. Further, we show that both the proposed low-cost optical system and the long-pulse active thermography technique allow the non-destructive thermal analysis of parts to detect sub-surface defects in composite materials based on fiberglass reinforced epoxy resin

Numerical and experimental validation of SMArt thermography for the inspection of wind blade composite laminate

An innovative active thermography technique is proposed for the inspection of typical wind blade material. The proposed technique is based on the use of a multifunctional material obtained adding a grid of Shape Memory Alloy wires, which would serve also as a protection against lightning, to a traditional glass fibre composite panel. This technique, called SMArt thermography, which exploits the SMA wires as internal heat sources, has been compared to a traditional pulsed thermography in the case of a representative panel of unidirectional glass fibre and epoxy matrix with embedded SMA wires and artificial defects. The experimental results of the two techniques are reported and compared to the result of a numerical FEM transient model, in order to establish the reliability and the detectability limit of the proposed technique. The FEM model has been proven to be a useful tool for the definition of the multifunctional material at a design stage.

Detection of dis-bond between honeycomb and composite facesheet of an Inner Fixed Structure bond panel of a jet engine nacelle using infrared thermographic techniques

ABSTRACT The Inner Fixed Structure (IFS) bond panel is a honeycomb sandwich panel with CFRP facesheet and a heat shield on one side, and a perforated CFRP facesheet on the other side, of a jet engine nacelle. It is subjected to extreme temperature on both sides which damages the inner epoxy adhesive bond between the facesheet and the honeycomb core. Accessibility to this layer for non-destructive evaluation is extremely challenging using conventional methods. This work proposes active thermography techniques such as flash thermography and induction thermography for accessing the inner layer. The infrared camera utilises the perforations in the facesheet of the IFS bond panel, which is used for attenuating the engine noise, for imaging the defects. However, flash thermography requires the removal of the thermal insulation layer for the inspection, whereas induction thermography can be performed without any modifications to the structure. The minimum detectable dis-bond size using these techniques is restricted to the spacing between the perforations on the facesheet. A numerical model has developed for induction thermography to optimise the excitation frequency that can produce reasonable thermal contrast at the inner facesheet and minimum temperature rise on the intermediate stainless-steel thin sheet that covers the thermal insulation layer.

Development of a non-destructive technique for measuring thermal conductivity of material with small anisotropy based on application of the reduced order technique

Abstract A new technique for measuring thermal conductivity of isotropic and anisotropic solid body is proposed. This method is associated with active thermography technique. Discussed approach, thanks to application of numerical model as the direct solver reduced to surrogate model can be easily extended for sample with arbitrary shapes. Such operation gives possibility to expand the range of developed approach application. By replacing standard direct model combined with the Levenberg-Marquardt technique to measure components of thermal conductivity tensor of anisotropy material by reduced order model, it is possible to reduce calculation time required for retrieving unknown model parameters from hours to seconds. The accuracy of the model was determined by comparing numerical results to those obtained using a commercial Parker flash method apparatus.

Strain-induced switching of heat current direction generated by magneto-thermoelectric effects

Since the charge current plays a major role in information processing and Joule heating is inevitable in electronic devices, thermal management, i.e., designing heat flows, is required. Here, we report that strain application can change a direction of a heat current generated by magneto-thermoelectric effects. For demonstration, we used metallic magnets in a thin-film form, wherein the anomalous Ettingshausen effect mainly determines the direction of the heat flow. Strain application can alter the magnetization direction owing to the magnetoelastic effect. As a result, the heat current, which is in the direction of the cross product of the charge current and the magnetization vector, can be switched or rotated simply by applying a tensile strain to the metallic magnets. We demonstrate 180° switching and 90° rotation of the heat currents in an in-plane magnetized Ni sample on a rigid sapphire substrate and a perpendicularly magnetized TbFeCo film on a flexible substrate, respectively. An active thermography technique was used to capture the strain-induced change in the heat current direction. The method presented here provides a novel method for controlling thermal energy in electronic devices.

Investigations for inclination angle characterization of angular defects using eddy current pulsed thermography

As a typical branch of active thermography techniques, eddy current pulsed thermography (ECPT) has been used to detect surface and subsurface defects in many scenarios. Among different applications, localization and characterization of rolling contact fatigue cracking (a common angular defect) in rail tracks has gained more attentions. This paper investigates three features for the inclination angle characterization of artificial angular defects. Specifically, one pixel-level and two spatial-level features, i.e., max pixel response, skewness- and kurtosis-based spatial features, are proposed to build the relations to the inclination angle. Based on the 3D FEM, the simulation results show that the relation between the max pixel response and the inclination angle is non-monotonic under different heating pulses. In contrast, the skewness- and kurtosis-based spatial features can give overall monotonic relations to the inclination angle under the condition that a longer pocket length (≥0.38 mm) occurs and a longer heating pulse (≥100 ms) is used. Further, the above simulation results were experimentally verified.

Study on the limit detection of defects by pulsed thermography in adhesive composite joints through computational simulation

In this paper the detection limit of lack of adhesive defects in GFRP adhesive composite joints was evaluated through a computational simulation model of active pulsed thermography technique. In this study, several simulations were carried out in order to evaluate the influence of the thickness of the collar, heating time and excitation energy on the detection limit of defects. In addition to these analyzes, it was also evaluated the reduction of the thermal contrast of the defects as a function of the thickness of the collar (depth of the defects) and thus it was possible to estimate mathematically the limit value of the thickness of the collar that allows the detection of the defects for each heating time. The results obtained showed that the pulsed thermography technique would be able to detect defects of lack of adhesive that are located up to 8.5 mm deep in this type of composite joint.

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.

Application of IR Thermography with Thermal Diffusivity Analysis for Detection of Plies Displacement in CFRP Composites

This paper presents an efficient nondestructive testing (NDT) method for detection of plies displacement in carbon fiber reinforced plastic (CFRP) composites using the active thermography technique. To assess the capability of the proposed technique for detection of plies displacement, the CFRP composite specimens with simulated fault were designed and manufactured. The IR thermography measurements were conducted using two-sided experimental arrangement. From recorded sequences of thermal images, the temperature variations versus time plots were extracted to obtain the maximum thermal contrasts. For quantitative interpretation of the results, the thermal diffusivity values for each composite material were determined and related to the fiber volume fraction. The experimental results showed that the thermal diffusivity increases with an increase of fiber volume fraction; further, the values of thermal diffusivity are specimen thickness dependent. The obtained values of thermal diffusivity varied from 1.5 × 10−7 m2/s (for 3.2 mm composite with 8.5 vol.% carbon fiber) to 2.6 × 10−7 m2/s (for 7.7 mm composite with 34.1 vol.% carbon fiber). On the basis of these results, it was concluded that the active thermography combined with thermal diffusivity analysis can be considered as a reliable NDT technique for detection of plies displacement in CFRP composites.

Detectability of Subsurface Defects with Different Width-to-Depth Ratios in Concrete Structures Using Pulsed Thermography

The active thermography technique is one of the most effective nondestructive tests for evaluating subsurface delaminations in concrete structures. The limitation of this method, which has been studied for some time, is that the width of the smallest detectable defect should be at least two times larger than its depth. However, controversy on this matter remains for concrete material with largely uncertain homogeneity, although the development of the infrared (IR) detector technology improved the above-mentioned limitation. In this study, the pulsed thermography (PT) technique is therefore conducted in the laboratory to investigate the detectability of delaminations with the width-to-depth ratio (w2d) ranging from 1.0 to 7.9 by using a long IR wavelength detector with a focal plane array of 640 $$\times $$ 480 pixels. The study focuses on the w2d ratio lower than 2.0. A concrete specimen was made with 12 embedded simulated delaminations having different sizes and depths. The results showed that a combination of PT and pulsed phase thermography can be used to detect delaminations with a w2d ratio equal or greater than 1.25. In addition, the absolute contrast above the delamination increases with the higher w2d ratio, indicating that even for a relatively deep delamination, it is still detectable if a delamination is provided by appropriate heat energy and its size is sufficiently large. Finally, the study also indicates that as the amount of heating energy provided is increased, the greater accuracy in predicting the depth can be obtained.