Nicolas P. Avdelidis

Comparative Study of Active Thermography Techniques for the Nondestructive Evaluation of Honeycomb Structures

In this article, the theoretical and experimental aspects of three active thermography approaches: pulsed thermography (PT), lock-in thermography (LT), and vibrothermography (VT), are discussed in relation to the nondestructive evaluation (NDE) of honeycomb sandwich structures. For this purpose, two standard specimens with simulated defects (delaminations, core unbonds, excessive adhesive, and crushed core) were tested, and results were processed, examined, and compared. As will be pointed out, the similarities and differences between these active approaches allow conclusions to be made about the most suitable approach for a particular application. In addition, results from NDE inspection by X-rays and c-scan ultrasounds are provided and discussed for reference.

Delamination detection and impact damage assessment of GLARE by active thermography

GLAss REinforced (GLARE) is a fibre metal laminate (FML) consisting of alternating layers of thin aluminium and glass fibre reinforced prepregs, whose improved physical properties confer it an interesting advantage over aluminium and composite materials for a number of aerospace applications. On the other hand, contrary to monolithic structures, GLARE can suffer from internal damage either during fabrication or in-serve stages. Non-destructive testing and evaluation (NDT&E) of GLARE is still a challenge, especially considering that large structures are typically sought (e.g., aircraft fuselage). In this paper, we investigated the use of infrared thermography for the inspection of GLARE. The experimental results presented herein demonstrate that it is possible to detect delamination-type defects and to assess the impact severity on GLARE through active thermography techniques, specifically pulsed thermography and vibrothermography. C-scan ultrasonic testing was performed as well with the intention of providing supplementary results.

An application of thermography for detection of delaminations in airport pavements

Since delaminations in airport pavements can lead to failure, cost effective ways must be suggested to inspect the quality of pavements. One of these cost effective ways is the infrared thermography technique. This research paper covers the determination of delaminations in asphalt pavements situated at one of the busiest airports in Greece, the International Airport of Athens, by means of infrared thermography. The infrared thermographic technique is used efficiently in the detection of cracks, flaws and other imperfections appearing on airport pavements. The outcome of this investigation leads to the proposition of a forecasting non-destructive technique intended for assessment of efficient airport pavements.

Active thermography signal processing techniques for defect detection and characterization on composite materials

Active thermography has been extensively investigated in the past few years for the nondestructive evaluation of different types of materials. Composites in particular have received considerable attention given that active thermography has shown to be well suited for the detection and characterization of most kinds of defects typically found in these materials such as impact damage, delaminations, disbonds and inclusions. Signal processing is a necessary step of the inspection process, especially if defect characterization is required. A wide variety of techniques have been developed from the classical thermal-based techniques to signal transformation algorithms (adapted from the area of machine vision) on which temporal data is transformed to a different domain (frequency, Hough, principal components, Laplace, high-order moments, etc.) with the purpose of simplifying data analysis. In this paper, a review of some of these processing techniques is presented and exemplified using a Kevlar® panel and a GLARE specimen.

Optical and Mechanical Excitation Thermography for Impact Response in Basalt-Carbon Hybrid Fiber-Reinforced Composite Laminates

In this paper, optical and mechanical excitation thermography was used to investigate basalt-fiber-reinforced polymer, carbon-fiber-reinforced polymer, and basalt-carbon fiber hybrid specimens subjected to impact loading. Interestingly, two different hybrid structures including sandwich-like and intercalated stacking sequence were used. Pulsed phase thermography, principal component thermography, and partial least-squares thermography (PLST) were used to process the thermographic data. X-ray computed tomography was used for validation. In addition, signal-to-noise ratio analysis was used as a means of quantitatively comparing the thermographic results. Of particular interest, the depth information linked to Loadings in PLST was estimated for the first time. Finally, a reference was provided for taking advantage of different hybrids in view of special industrial applications.

Quantitative inspection of non-planar composite specimens by pulsed phase thermography

In pulsed phase thermography (PPT), amplitude and phase delay signatures are available following data acquisition; carried out in a similar way as in classical pulsed thermography, by applying a transformation algorithm such as the Fourier transform (FT) on thermal profiles. An extended review on PPT theory, including a new inversion technique for depth retrieval by correlating the depth with the blind frequency fb (frequency at which a defect produces enough phase contrast to be detected), has already been proposed confirming PPTs capabilities as a practical inversion technique. In this work, planar and non-planar CFRP composites were evaluated by quantitative PPT. Experimental results showed that, for the geometries studied here, surface shape has little impact on depth inversion results.

Infrared thermographic assessment of materials and techniques for the protection of cultural heritage

In this work, infrared thermography was applied and investigated as a non-destructive tool in the assessment of materials and techniques for the protection of cultural heritage. Diagnostic studies on monuments and historic buildings, situated in Greece, were performed. Long wave infrared thermography was used on restoration and traditional - historic materials concerning architectural surfaces and historic structures for research purposes such as: the assessment of moisture impact to porous stone masonries and the evaluation of conservation interventions (materials and techniques) regarding, consolidation interventions on porous stone masonries, restoration of masonries by repair mortars, and cleaning of facades. The results of this work indicate that thermography can be considered as a powerful diagnostic nondestructive tool for the preservation and protection of cultural heritage.

A comparative investigation for the nondestructive testing of honeycomb structures by holographic interferometry and infrared thermography

The nondestructive testing (NDT) of honeycomb sandwich structures has been the subject of several studies. Classical techniques such as ultrasound testing and x-rays are commonly used to inspect these structures. Holographic interferometry (HI) and infrared thermography (IT) have shown to be interesting alternatives. Holography has been successfully used to detect debonding between the skin and the honeycomb core on honeycomb panels under a controlled environment. Active thermography has proven to effectively identify the most common types of defects (water ingress, debonding, crushed core, surface impacts) normally present in aeronautical honeycomb parts while inspecting large surfaces in a fast manner. This is very attractive for both the inspection during the manufacturing process and for in situ regular NDT assessment. A comparative experimental investigation is discussed herein to evaluate the performance of HI and IT for the NDT on a honeycomb panel with fabricated defects. The main advantages and limitations of both techniques are enumerated and discussed.

Emissivity measurements on historic building materials using dual-wavelength infrared thermography

The most reliable method to obtain correct emissivity values for the infrared thermographic systems and applications is to determine the emissivity of the targets to be tested. Although this approach is not possible during in situ applications, samples of the targets can be collected and measured, as in this work, in the laboratory. In the present work, the emissivity values of selected historic building materials were measured at a variety of temperatures, in the 3-5.4 micrometers and 8-12 micrometers regions of the infrared spectrum. Porous stones from the Mediterranean area and marbles, used as historic building materials, were investigated. The examined materials presented different emissivity values, caused by their surface state and microstructure. In addition, the effect of temperature and wavelength on the emissivity values of such historic building materials was also considered.

Thermographic, ultrasonic and optical methods: A new dimension in veneered wood diagnostics

Modern production techniques in the wood-based industry reached a high quality standard at high output rates. While the speed of the production machines increases, it is necessary to introduce modern and faster working online inspection methods to supervise constantly the material for defects. For example, thermographic cameras are able to detect not only invisible defects within wood-based materials like laminated particle and fiberboards, but can be used also to detect defects in lumber and veneered wood [1–4]. In the latter case, there is the need to inspect more accurately the final pieces, given the exponential growth in worldwide sales. Therefore, in order to minimize adhesion problems [5], detecting surface and sub-surface cracks, define the geometry of the sub-surface detachment, in the veneered wood products, an integrated non-destructive test method is needed both during the production process that after to it [6]. Our system can provide a continuous control of the process and the product. In fact, this study compares the performance of transient thermography, three optical methods and ultrasonic testing applied together on a veneered wood sample with real and fabricated defects. The use of phase-shifting holography correlated to Double-Exposure HI and the wavelet transform applied as fusion of images between Thermographic Signal Reconstruction and Double-Exposure HI, are explored in this work.