Active Thermographic Inspection Research Articles

Numerical Modelling of the Heat Source and the Thermal Response of an Additively Manufactured Composite during an Active Thermographic Inspection.

This paper deals with the numerical modelling of non-destructive testing of composite parts using active thermography. This method has emerged as a new approach for performing non-destructive testing (NDT) on continuous carbon fibre reinforced thermoplastic polymer (CCFRTP) components, particularly in view of detecting porosity or delamination. In this context, our numerical model has been developed around references containing internal defects of various shapes and sizes. The first novelty lies in the fact that the heat source used in the experimental setup is modelled exhaustively to accurately model the radiation emitted by the lamp, as well as the convection and conduction around the bulb. A second novelty concerns the modelling of the CCFRTP making up the benchmark used. Indeed, its thermal properties vary as a function of the sample temperature. Therefore, the actual thermal properties have been experimentally measured and were later used in our model. The latter then captures the material dependency on temperature. The results obtained by our model proved to be in close agreement with the experimental results on real reference points, paving the way for future use of the model to optimise experimental configurations and, in particular, the heating parameters.

Self-stimulating sandwich structure with resistive heating honeycomb core for quick thermographic inspection

The paper presents the novel concept of a honeycomb sandwich structure with carbon fibre reinforced plastics face sheets capable of its own internal thermal stimulation for rapid active thermographic inspection, where an adapted electrically powered honeycomb core serves as a heat source. The proposed sandwich structure effectively reduces the equipment necessary for external thermal stimulation, while improving the controllability of the thermal pulse. A new design of the modified aluminum honeycomb core, required for resistance heating with sufficient homogeneity, is proposed. Numerical modeling was used to test the concept’s viability and to predict its efficiency for defect detection. The altered honeycomb was then manufactured and its heating characteristics were measured. This structure was then used as a core in a sandwich specimen with carbon fibre reinforced plastics face sheets, which also contained artificial defects embedded in the face sheets and in the face sheet/core interface. The applicability of the proposed modified honeycomb structure for thermal stimulation for active infrared thermography was experimentally confirmed, demonstrating the ability to visualize even very small defects simulating disbonding, or delamination. The impact of water ingress on the operation of the concept was tested as well. The proposed method has the potential to simplify and expedite the non-destructive inspection of aviation-grade sandwich structures in service via active infrared thermography.

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.

Development of a thermal excitation source used in an active thermographic UAV platform

ABSTRACT This work aims to address the effectiveness and challenges of using active infrared thermography (IRT) onboard an unmanned aerial vehicle (UAV) platform. The work seeks to assess the performance of small low-powered forms of excitation which are suitable for active thermography and the ability to locate subsurface defects on composites. An excitation source in multiple 250 W lamps is mounted onto a UAV and is solely battery powered with a remote trigger to power cycle them. Multiple experiments address the interference from the UAV whilst performing an active IRT inspection. The optimal distances and time required for a UAV inspection using IRT are calculated. Multiple signal processing techniques are used to analyse the composites which help locate the sub-surface defects. It was observed that a UAV can successfully carry the required sensors and equipment for an Active thermographic NDT inspection which can provide access to difficult areas. Most active thermographic inspection equipment is large, heavy, and expensive. Furthermore, using such equipment for the inspection of complex structures is time-consuming. For example, a cherry picker would be required to inspect the tail of an aircraft. This solution looks to assist engineers in inspecting complex composite structures and could potentially significantly reduce the time and cost of a routine inspection.

Solving inverse geometry heat conduction problems by postprocessing steady thermograms

This paper deals with the use of steady active thermographic inspection as a structural health monitoring method to detect defects and inclusions in a heat conductive medium. The proposed postprocessing method has the advantage of taking into account the whole physics of the problem instead of a reduced set of physical information, as most of the traditional methods do. In comparison with other non-destructive testing techniques like ultrasonics, thermographic inspection is non-contact, more non-intrusive, and safer. However, processing thermographic data is a fairly demanding problem because heat conduction is short range and exhibits a poor signal-to-noise ratio. Numerical experiments illustrate that the present postprocessing technique detects damage fairly well even in very challenging scenarios where the size of the defects is rather small and/or thermograms are highly noisy.

Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating

In this study, inspection capability of active thermographic nondestructive inspection using the laser scanning method was examined. Carbon fiber reinforced plastic (CFRP) specimens were heated using a CO2 laser scanning device and their inspection capability was investigated. The experimental results indicate that the temperature and phase behavior after laser scanning heating were similar to those after conventional instantaneous flash heating. We also investigated the effect of the relationship between the scanning direction and fiber direction of the tested CFRPs on defect detection. The numerical and experimental results showed that the observed temperature contrast appearing on the defective surface varied depending on the scanning direction, and the temperature contrast increased when the scanning direction was the same as the fiber direction of the laminates. In addition, it was found that defects in a CFRP specimen located 10 m from the heat source could be detected using the laser scanning heating. These results suggest that using the laser scanning heating method is effective to inspect large objects located at far distances.

Influence of Composite Ply Layup on Active Thermographic Non-destructive Inspection of Carbon Fiber-Reinforced Plastic Laminates

This paper examines the effect of the composite ply layup on defect detection using non-destructive active thermographic inspection of carbon fiber-reinforced plastic (CFRP) laminates. Numerical calculations using simplified models were performed and verified experimentally. The results indicated that the local temperature contrast ( $$\Delta T$$ ) observed on the surface of the laminates, generally caused by existing internal defects, depended on two characteristics of the laminates: the difference between the orientation angles of neighboring layers $$(\theta _{\mathrm{d}})$$ and the number of different orientation angles in the layers $$(N_{\mathrm{a}})$$ . The $$\Delta T$$ was higher when $$\theta _{\mathrm{d}}$$ and $$N_{\mathrm{a}}$$ were small. These effects were more clearly observed when inspecting CFRPs reinforced with carbon fibers with higher thermal conductivity (such as pitch-based fibers), and when the number of layers is larger.

A quantitative comparison of stimulation and post-processing thermographic inspection methods applied to aeronautical carbon fibre reinforced polymer

Active thermographic inspection is a very powerful tool for many areas of science. However, there is no agreement about the proper stimulation and post-processing method for a particular material and defect type, which forces technicians to carry out trial and error tests. This study presents a comparison of three stimulation and six post-processing thermographic inspection methods applied to aeronautical carbon fibre reinforced polymer with different kind of simulated defects. The obtained results have been quantitatively assessed using the signal-to-noise ratio metric. Conclusions provide recommendations for technicians interested in thermographic inspection of this type of material.

Automatic detection of impact damage in carbon fiber composites using active thermography

Accidental impacts can severely reduce the structural strength and stability of composite materials, which can lead to severe consequences due to the degradation of the mechanical properties of components designed to perform for decades. Because accidental impacts are difficult to avoid, robust and reliable inspection methods to detect impact damage are required. Many methods have been proposed recently. However, most of them require an experienced technician to analyze the data, which leads to a significant decrease in manufacturing productivity. This work proposes a method to automatically detect impact damage in carbon fiber composites using active thermography. The proposed system detects defects caused by impact damage in the infrared images without human intervention. Impact damage detection is performed using a robust method based on an active thermographic inspection. Thermographic data is preprocessed to improve signal-to-noise ratio and to remove non-uniform background caused by non-uniform heating. Then, peaks and edges are identified and clustered, and regions corresponding to impact damage are detected. The proposed procedure has been applied to three specimens that contain 6 and 12 plies, different types of cores, and damage caused by energies from 6J to 50J. All defects are detected correctly.

Principal component thermography for flaw contrast enhancement and flaw depth characterisation in composite structures

This study reports on the application of principal component thermography to the nondestructive inspection of composite structures. The technique is based on a singular value decomposition of the measured response to pulsed thermal excitation as applied in the course of a normal flash thermographic inspection. Trials on synthetically generated data show evidence of excellent noise-rejection qualities which manifest high levels of flaw contrast relative to that present in the unprocessed data. A simple analytical expression is then derived that relates a characteristic time furnished by the decomposition to the flaw depth, providing a basis for flaw depth estimation. This framework is shown to provide flaw-depth estimates with good levels of precision and robustness. Finally, application to experimental data is considered. An AS4/3501 composite laminate sample containing an assortment of teflon inserts is subject to active thermographic inspection and the response data analysed using the proposed approach. The results conclusively demonstrate the practical efficacy of the approach, and confirm the attractive properties outlined in relation to the numerical tests.

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

This paper presents an approach to the inverse geometry problem pertaining to the characterization of material loss due to corrosion damage in metallic structures. It is based on a simplified geometric representation which admits, in the context of a restricted time frame, a simple analytical form for the contrast response evolution. The inverse problem is then solved by means of a deterministic optimisation scheme applied to a suitably defined objective function. Experimental trials on specimens containing simulated corrosion damage have shown that estimates for residual thickness obtained on the basis of this approach are both precise and robust.