Carosena Meola

Recent advances in the use of infrared thermography

Infrared thermography transforms the thermal energy, emitted by objects in the infrared band of the electromagnetic spectrum, into a visible image. This feature represents a great potentiality to be exploited in many fields, but this technique is still not adequately enclosed in industrial instrumentation because of a lack of adequate knowledge; at first sight, it seems too expensive and difficult to use. The aim of the present paper is to shortly overview existing work and to describe the most relevant experiences devoted to the use of infrared thermography in three main fields, i.e. thermo-fluid dynamics, technology and cultural heritage, which have been performed in the department the authors belong to. Results may be regarded from two points of view, either as validating infrared thermography as a full measurement instrument, or as presenting infrared thermography as a novel technique able to deal with several requirements, which are difficult to perform with other techniques. This study is also an attempt to give indications for a synergic use of the different thermographic methods and sharing experiences in the different fields.

A survey on infrared thermography for convective heat transfer measurements

During the past several years infrared thermography has evolved into a powerful investigative means of thermo-fluid-dynamic analysis to measure convective heat fluxes as well as to investigate the surface flow field behaviour over complicated body shapes. The basic concepts that govern this innovative measurement technique together with some particular aspects linked to its use are herein reviewed. Different operating methods together with their implementations are also discussed. Finally, the capability of infrared thermography to deal with several simple, or complex, fluid flow configurations is analysed.

Non-destructive control of industrial materials by means of lock-in thermography

Lock-in thermography is employed for non-destructive control to evaluate several aspects of industrial interest: inclusions of spurious materials in both carbon-epoxy and glass-epoxy, impact damage and delaminations occurring around holes during drilling in carbon-epoxy, bonding improvements in Certran® after plasma treatments and steel modifications after welding. Phase images are analysed to find quantitative information for industrial characterization.

Comparison between pulsed and modulated thermography in glass–epoxy laminates

Abstract Infrared (IR) thermography is a two-dimensional, non-contact technique which can be usefully employed in non-destructive evaluation of materials. Basically, two different approaches are possible: traditional pulse thermography (PT) and modulated (or lock-in) thermography (MT). The attention of the present work is focused on the peculiar aspects, which characterise the two different techniques. Tests are carried out by considering glass–epoxy specimens and the results obtained, by employing either PT or MT, are compared. The capability of each technique, to detect a defect and give precise information about size, depth and thermal resistance of the defect, is analysed. The advantages and disadvantages of using these techniques are discussed in order to assess the fundamental requirements for the most appropriate choice in quality control processes.

Comparison between thermographic techniques for frescoes NDT

The use of infrared thermography in the architectural restoration field is examined. Three samples, made of a support of marble, brick, or tuff, covered with a layer of plaster with inclusions to simulate detachments or cracks in frescoes, are considered. Different techniques: pulse thermography, lateral heating thermography, lock-in or modulated thermography and pulse phase thermography are employed to detect the flaws artificially created; advantages and disadvantages of each technique are discussed. It is found that pulse thermography is easy and fast to use for information about the state of the art treasures, but data may be affected by non-uniform heating and local variation of thermal emission; the lateral heating can help to overcome interference effects due to non-uniform heating but it is more troublesome to use. When the evaluation regards rare art treasures the lock-in technique seems to be the only response since it is able to operate within very low increase of surface temperature; this technique is also able to give information about the material composition. The pulse phase thermography may be used to detect more in depth flaws but it needs higher temperature increase with respect to the ambient temperature and so it is recommended to control, before testing, the temperature sensitivity of the artefact.

Influence of Shear Layer Dynamics on Impingement Heat Transfer

Measurements of convective heat transfer coefficients on a flat plate with an air jet impinging on it perpendicularly are made to investigate the influence of some governing parameters. Particular attention is focused on the effects of the shear layer dynamics. For certain flow conditions and/or test arrangements, coherent structures and/or recirculation currents are observed affecting the distribution of the heat transfer coefficients. Measurements of wall temperature as well as of adiabatic wall temperature of the stream are made by means of an infrared scanning radiometer, and the heat transfer coefficients are calculated by means of the so-called heated thin foil technique. The data are reduced in dimensionless form as Nusselt numbers and compared with data from the available literature. Both spatial distributions and averaged values of the Nusselt number are discussed. A new explanation for the second peak in the local Nusselt number is proposed.

Application of infrared thermography to adhesion science

Infrared thermography (IRT) is a non-contact, non-intrusive technique which converts the invisible thermal energy, radiated from the surface of an object in the infrared band of the electromagnetic spectrum, into a video signal, each energy level being generally represented by a color or a gray level. IRT has been considered an exciting scientific breakthrough since its introduction in the early 1960s. Indeed, the new generation of fully-computerized infrared imaging systems can provide both qualitative and quantitative measurements which are useful in many industrial and research fields. Nevertheless, infrared thermography is still not completely exploited. The reason for this lies, in part, in the lack of knowledge, since at first sight IRT seems to be too expensive and difficult to use and, in part, in the industrial inertia to change the routine procedures. The aim of this review article is to provide the reader with a background to infrared theory and with an overview of the most relevant applications of IRT to the adhesion field. The use of IRT as a non-destructive evaluation technique with the two different approaches of pulse thermography (PT) and lock-in thermography (LT) is discussed. Many applications are described which involve several different materials (metals, plastics, plaster, composites, hybrid composites and sandwiches) and different types of bonds (coatings, sandwiches and joints). The results show that both PT and LT are able to detect material modifications caused by surface treatments, presence of inhomogeneities in bulk materials, as well as disbonding, delamination, and cracks and slag inclusions in bonded structures. The LT is also capable of evaluating materials characteristics (e.g., variations in density, porosity, hardness, etc., which induce variations in the phase angle), the dimension of the heat affected zone in welded joints, coating thickness, bondline thickness, the effects of adhesive thickness, the effects induced in bonded structures by substrate surface treatments, and the effects of crosslinking in polymers. The LT technique is particularly advantageous in the evaluation of frescoes, mosaics and antique artworks. The reported applications provide also information which is useful for decision making about the use of IRT alone, or combined with other techniques.

A New Correlation of Nusselt Number for Impinging Jets

Jet impingement heat transfer has received a remarkable amount of attention by researchers throughout the world for many years due to widespread interest from both academic and industrial points of view. However, the fluid dynamics involved with arrays of jets are very complex and still not completely understood. Therefore, a functional relationship of the Nusselt number, which involves thermophysical and dynamic quantities as well as geometric parameters, is still not univocally and generally established. In this research analysis, some empirical equations, which are available in the literature for arrays of round impinging jets, are reviewed, and a new, simpler correlation is developed that better represents experimental data published by several researchers.

Azimuthal instability in an impinging jet: adiabatic wall temperature distribution

The present work has been aimed at gaining some new insights into instability phenomena arising when an air jet impinges on a flat plate under certain conditions. At a critical Mach number, depending on the impingement distance, the jet loses its circumferential appearance with the formation of evenly equidistant azimuthal structures, whose number and location depend on the nozzle geometry and on the flow conditions. The instability is investigated in terms of pressure and adiabatic wall temperature; the latter is measured by means of an infrared scanning radiometer. Entrainment effects are found to play a key role in the priming and evolution of the instability.

Experimental Characterization of an Innovative Glare® Fiber Reinforced Metal Laminate in Pin Bearing

An experimental investigation is performed on an innovative Glare® Fiber Reinforced Metal Laminate (FRML), which is produced at the Alenia Aerospazio (Italy), with the aim to characterize its strength and behaviour in the case of mechanical joints. Several specimens are fabricated by varying width and hole-to-edge distance and tested in pin-bearing way without lateral restraints, which is the most critical testing procedure in the simulation of mechanical joints. Specimens, after bearing stress, are analysed in both non-destructive and destructive ways. Non-destructive evaluation is performed by means of lock-in thermography; for a validation of this technique, phase images are compared to photomicrographs. Results prove that a remote infrared imaging system may be a valuable tool to monitor the material behaviour either during the manufacturing processes, or in service.