Comparison between pulsed and modulated thermography in glass–epoxy laminates
Introduction
During the last few years, the industrial interest has been oriented towards the development of new materials, which are generally manufactured by superposition of carbon–epoxy or glass–epoxy, resin-preimpregnated substrates in order to achieve high strength performance with low weight. At the same time, there has been an increasing demand for safety, which, in turn, involves an increasing demand for quality. Most of the methods [1], generally in use for metallic materials, are based on electromagnetic techniques and are not applicable in the case of composites because of the worse electrical conductivity and permeability of the former.
Infrared (IR) thermography is a two-dimensional, non-contact technique for measuring the surface temperature maps which can be usefully employed in non-destructive evaluations of all kinds of materials. Analysis of subsurface features in solid objects, by means of an IR scanning radiometer, generally requires heat energy to be transferred to the object in the active mode and mapping of surface temperatures in the transient heating phase. Indeed, analysis of IR images may give information about most of the requirements of non-destructive evaluations such as size, depth and thermal resistance of defects.
Cielo et al. [2] describe several applications for the evaluation of industrial materials and relate the thermal propagation time to the depth and thickness of defects. Vavilov [3] highlights the image processing techniques which relate the temperature variation to the defective properties (thermal resistance, thickness, depth, etc.). Giorleo and Meola [4] consider defective glass–epoxy laminates and propose a method to measure depth and size of defects, by analysing the thermograms, acquired in time sequence in the transient heating phase, in terms of temperature gradients, between damaged and undamaged zones, over the surface of the specimen.
The development of MT [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] has added novel interest. This technique uses thermal waves, instead of pulses, and the resulting phase image gives a direct indication (without troublesome post-processing image procedures) not only of the size but also of the depth of the damaged zone, and so it seems promising in terms of reduction of waste of time and costs. Indeed, MT also gives indications about the thickness and the nature of defects [16].
MT was mostly used to acquire information about the fibre orientation [9], to visualise delaminations of veneered wood [12], to measure thickness, and/or density, and/or porosity of ceramic coatings [14].
Maldague et al. [15] perform a new technique, the so-called pulse phase thermography (PPT), which has the advantages of both conventional PT and MT. The specimen is pulse-heated as in PT and the mix of frequencies of the thermal wave launched into the specimen is unscrambled by performing the Fourier transform of the temperature decay, which enables computation of phase images.
The intention of the present work is to compare results obtained by the two thermographic techniques, PT and MT, on specimens made of glass–epoxy in order to acquire information about the capability of each technique to deal with the fundamental requirements of non-destructive evaluations. The choice of glass–epoxy is justified by the wide industrial applications especially in the manufacture of electronic components.
Section snippets
Test specimens and experimental procedure
The material under test is a glass–epoxy laminate obtained by a staking sequence of (M10/50%759-120CM) resin-preimpregnated substrates (0.1 mm thick) positioned at 0 and 90° to have a total thickness of 5 mm. To simulate delaminations and/or non-homogeneity, diskettes of aluminium, cork and teflon of five different diameters, φ, ranging from 2 to 8 mm, and thicknesses, s, of 1 and 2 mm are manufactured, from plates of given thickness, by means of punch knives and positioned at different depths, p,
Analysis of results
The thermal (pulse measurement) and phase images relative to the specimen without defects are shown in Fig. 1(a) and (b). As can be seen, the phase image displays areas characterised by different phase angles (different colours), which are due to a non-uniform distribution of resin and are not appreciable in the thermal image.
The phase images, relative to samples with defects made of cork and aluminium of are reported in Fig. 2 for Defects made of aluminium appear as
Conclusions
IR thermography can be considered as a powerful tool for non-destructive evaluation of materials. The choice of the most appropriate technique, pulse or lock-in, mainly depends on the specific application with particular regard to the properties of the materials involved and the specific requirements of the quality control process.
Within the PT, a defect is recognised through temperature differences between contiguous points over the object surface. From the material quality control point of
Acknowledgements
The authors thank Prof. G.M. Carlomagno (DETEC) for permitting the use of the infrared scanning system.
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