Thermographic Nondestructive Inspection Research Articles

Developing effective thermal signal processing to improve thermographic non-destructive inspection of metallic components

ABSTRACT A broad range of defects can be formed in metallic equipment . The present study adopted active infrared thermography with optical excitation sources to detect the artificial flat-bottom holes in a steel sample. Raw thermal data always have some problems, such as noise and blurred edge. A proposed method, namely TPPT, was adopted to eliminate these problems. In addition to the developed technique, several well-known processes such as PCT were employed. The Contrast Enhancement parameter (CE) was defined to compare preprocessed images. CE data indicate that the TPPT has the best performance in improving the raw data. A post-processing procedure was also proposed to improve the segmentation results. This strategy consisted of two steps, histogram equalisation of preprocessed images and applying 2D wavelet noise-reduction. Ultimately, several segmentation methods were applied to the post-processed images. The outstanding performance of TPPT was proved one more time as the TPPT segmented images revealed the highest number of defects, almost 14 out of 20, on average. Moreover, the performance of segmentation algorithms was compared together regarding the number of detectable defects and estimation of defective regions’ area. FCM was recognised as the most appropriate segmentation algorithm for this study based on the obtained results.

Influence of composition and surface discoloration of concrete on active thermographic nondestructive inspection

The capability of active thermographic nondestructive inspection of concrete is influenced by the concrete testing condition. In this study, the influence of concrete composition and surface discoloration caused by efflorescence is investigated. Experiments performed on the concrete specimens under various conditions indicate that active thermography is barely affected by the variation in the concrete composition. This feature makes active thermography a practical tool for inspecting concrete structures with coarse aggregates. Furthermore, it is observed that the surface discoloration caused by efflorescence enhances the surface reflectance in the wavelength range of a halogen lamp. This leads to a reduction in inspection capability owing to heat input reduction. Based on the results, the active thermography method is unsuitable for concrete with great efflorescence.

Precious walls built in indoor environments inspected numerically and experimentally within long-wave infrared (LWIR) and radio regions

A thermographic non-destructive inspection conducted during the winter season on precious walls confined to an indoor environment (Baiocco room, L’Aquila, Italy) is illustrated herein. The use of a heat conveyor ad hoc selected has produced a particular benefit in terms of defect detection after the application of advanced algorithms because the thermal stimulus coming from an air heater hit the finishing layer under a laminar flow regime. Thermocouples registered both the heat-up and cool-down phases to confirm the numerical simulations conducted via Comsol Multiphysics® software. The results obtained by applying the principal component thermography (PCT) technique to raw data registered in the time domain were a priori studied through thermographic simulations. Defects were modelled on the basis of results already shown in a previous work centred on the Baiocco room, too. A comparison between the results obtained from PCT and the innovative sparse principal component thermography (SPCT) technique was also added to the discussion. The SPCT results show their benefits above all when they are supplemented with ground-penetrating radar results. The experimental analyses were conducted on a multi-layer wall called thereafter rectangular portal, in which the instant thermal conductance was experimentally measured. Subsurface cracks were detected using a long heating-up phase. This procedure is preferred by the Monuments and Fine Arts Department with respect to the use of flash lamps. Indeed, short energy pulses should be avoided on finishing layers having low emissivity values because producing spurious reflections affecting the thermographic results. A discussion on this point is also provided. On the basis of the data analysis, it is possible to say that the thermal waves should arrive at the interface between the masonry and its top layer in order to obtain a clear thermal map of the defects present at different depths. As shown, this goal can be realized through a smart solution based on: (a) the inverse knowledge of the aedicula via thermal analyses, (b) the best mesh to be selected for the numerical model, and (c) the optimal type of thermal stress to be delivered to the object under inspection.

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.

The use of pulse-compression thermography for detecting defects in paintings

Interest in the conservation of paintings grows year by year. Their periodic inspection is essential for their conservation over the time. Thermographic non-destructive inspection is one technique useful for paintings, but it is essential to be able to detect buried defects while minimising the level of thermal stimulus. This paper describes a pulse-compression infrared thermography technique whereby defect detection is optimized while minimising the rise in temperature. To accomplish this task, LED lamps driven by a coded waveform based on a linear frequency modulated chirp signal have been used on paintings on both a wooden panel and a canvas layer. These specimens contained artificially fabricated defects. Although the physical condition of each painting was different, the experimental results show that the proposed signal processing procedure is able to detect defects using a low temperature contrast.

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.

Thermographic Non-Destructive Evaluation for Natural Fiber-Reinforced Composite Laminates

Natural fibers, including mineral and plant fibers, are increasingly used for polymer composite materials due to their low environmental impact. In this paper, thermographic non-destructive inspection techniques were used to evaluate and characterize basalt, jute/hemp and bagasse fibers composite panels. Different defects were analyzed in terms of impact damage, delaminations and resin abnormalities. Of particular interest, homogeneous particleboards of sugarcane bagasse, a new plant fiber material, were studied. Pulsed phase thermography and principal component thermography were used as the post-processing methods. In addition, ultrasonic C-scan and continuous wave terahertz imaging were also carried out on the mineral fiber laminates for comparative purposes. Finally, an analytical comparison of different methods was given.

Thermographic non-destructive inspection of wind turbine blades using unmanned aerial systems

Wind turbines are gaining importance in the last years because of their high efficiency during energy production without greenhouse gas emission. Furthermore, they can be installed both on- and off-shore. However, wind blade maintenance operations have become difficult and expensive due basically to size aspects and operative costs related with their location and out-off service period. A non-destructive technique capable of detecting most significant in service defects in composite wind blade is infrared thermography (IRT). However, this method can only be applied by developing cost efficient in situ inspection strategies. This work presents a feasibility study for defect detection during maintenance operations in wind blades using unmanned aerial systems (UASs). IRT inspections are performed by means of passive IRT methodology first (i) for specimen located at the ground level with artificial defects (like delaminations, cracks, impact damage and debondings) and later (ii) during flight operation using an rotorcraft UAS. The developed inspection strategies, test results and several features of the developed acquisition system are reported.

A novel algorithm for fast compression and reconstruction of infrared thermographic sequence based on image segmentation

High resolution in space and time, which is associated with massive data processing and storage, is becoming the new trend of thermographic non-destructive inspection application. Therefore, it is demanding to develop a fast and precise data post-processing technique for high resolution thermal images. Through analyzing the attenuation of thermal wave temperature and the morphology feature of thermograph, the authors propose a novel algorithm, which provides an effective way to improve the precision and computing speed for post-processing of thermal image data, in this paper. Firstly, the algorithm will sort the thermographic sequence data in space by using image segmentation method. Secondly, the algorithm will employ classical fitting calculation to fit all the temperature decay curves. At last, the algorithm will use the fitting parameters of the curves as the parameters for compression and reconstruction of thermographic sequences. The proposed algorithm manages to compress thermographic sequences in time and space simultaneously. To validate the proposed algorithm, the authors used two defective specimens which were made of different materials to conduct the experiment. The experimental results showed that the proposed infrared thermographic sequence compression and reconstruction algorithm is an effective solution with high speed and high precision. Compared to the conventional method, this algorithm is not only noise resistant in time domain but also can increase the computing speed by hundreds of times.

NDE OF ADVANCED AUTOMOTIVE COMPOSITE MATERIALS THAT APPLY ULTRASOUND INFRARED THERMOGRAPHY TECHNIQUE

The infrared thermographic nondestructive inspection technique is a quality inspection and stability assessment method used to diagnose the physical characteristics and defects by detecting the infrared ray radiated from the object without destructing it. Recently, the nondestructive inspection and assessment that use the ultrasound-infrared thermography technique are widely adopted in diverse areas. The ultrasound-infrared thermography technique uses the phenomenon that the ultrasound wave incidence to an object with cracks or defects on its mating surface generates local heat on the surface. The car industry increasingly uses composite materials for their lightweight, strength, and environmental resistance. In this study, the car piston passed through the ultrasound-infrared thermography technique for nondestructive testing, among the composite material car parts. This study also examined the effects of the frequency and power to optimize the nondestructive inspection.