Delamination and Crack Detection by the Synchronous Heating Method: Theoretical Aspects

In recent years a variety of NDT techniques utilizing an IR camera have been developed (for example [1–4]). In the synchronous heating method [3] an IR camera is used for detecting the surface temperature rise caused by a scanning laser beam. The movement of the heating beam is synchronized with the deflection mirror of the IR camera so the distance between the object point of the camera and the heating point remains constant. This measurement set-up allows rapid inspection of thin coatings that are otherwise problematic from the NDT point of view. In this paper we have computed numerically the temperature profile produced by a laser beam when it is scanned over the surface of a defective sample

Measurement of opaque film thickness

The theoretical and experimental framework for thickness measurements of thin metal films by low frequency thermal waves is described. Although it is assumed that the films are opaque and the substrates are comparatively poor thermal conductors, the theory is easily extended to other cases of technological interest. A brief description is given of the thermal waves and the experimental arrangement and parameters. The usefulness of the technique is illustrated for making absolute measurements of the thermal diffusivities of isotropic substrate materials. This measurement on pure elemental solids provides a check on the three dimensional theory in the limiting case of zero film thickness. The theoretical framework is then presented, along with numerical calculations and corresponding experimental results for the case of copper films on a glass substrate

Parallel Box-Car Imaging of Adhesion Defects in Plasma-Sprayed Coatings

Thermal wave techniques have been successfully used for the characterization of adhesion defects of plasma-sprayed coatings on metal substrates. [1–3] With the advent of thermal wave infrared video imaging,[4–6] it is now possible to image large surface areas at video frame rates using an IR video camera. In this work we describe a novel parallel box-car imaging system, using an IR video camera and WSU-designed pixel by pixel time-gating and averaging and we demonstrate the ability of this system to detect adhesion defects in plasma-sprayed coatings on metal substrates. A particular advantage of our system is that the entire image can be obtain in parallel, thus making it a much faster technique than the conventional cw thermal wave imaging techniques

Thermal Wave Characterization of Coated Surfaces

The experimental techniques and theory for utilizing the mirage effect, or optical probe beam detection, of thermal waves in opaque solids for determining their thermal diffusivities have been described in detail elsewhere. [1–4] An application to a coated nickel-based alloy has also been described elsewhere. [1] In previous papers [5,6] we presented a theoretical expression which describes the mirage effect signal in a three-layer medium (gas-coating-sample system), taking into consideration the effects of the sizes of the heating and probe beams. In this paper we extend the results of numerical calculations from that expression to the case of films which are thermally very thin (thicknesses of the order of 10-3 thermal diffusion lengths). A model system of 100–500 nm thick Cu films on glass substrates was studied experimentally at thermal wave frequencies below 1kHz, and in this paper we compare the results of those measurements to the numerical calculations

Delamination and Crack Detection by the Synchronous Heating Method: Theoretical Aspects

In recent years a variety of NDT techniques utilizing an IR camera have been developed (for example [1–4]). In the synchronous heating method [3] an IR camera is used for detecting the surface temperature rise caused by a scanning laser beam. The movement of the heating beam is synchronized with the deflection mirror of the IR camera so the distance between the object point of the camera and the heating point remains constant. This measurement set-up allows rapid inspection of thin coatings that are otherwise problematic from the NDT point of view. In this paper we have computed numerically the temperature profile produced by a laser beam when it is scanned over the surface of a defective sample.</p

Quantitative Thermal Wave Characterization of Coating Adhesion Defects

Throughout the development of thermal wave NDE techniques the most common application has been the imaging of subsurface structure of coated and bulk materials [1]. The use of thermal wave imaging for the detection of coating adhesion defects was first proposed by Luukkala and Penttinen [2], and Busse and Ograbek presented the first experimental results on the detection of artificial adhesion defects of a graphite coating on aluminum [3].</p

A 5-competitive on-line scheduler for merging video streams

link_to_subscribed_fulltex

A Thermal Wave Technique to Determine Thermal Diffusivities of Polymer Foils

The increasing use of polymers sets more demands on nondestructive characterization and testing of both raw materials and ready products. Thus far, plastics have been considered to be poor electric and thermal conductors but manipulation of electric conductivity has already become an important factor for many applications. Also the more profound knowledge of thermal conductivity mechanisms and the measurement of thermal characteristics is gaining increasing importance in the future.</p

Reflection-Mirage Measurements of Thermal Diffusivity

The experimental technique for the measurement of thermal diffusivity using the mirage effect, or optical probe beam detection of thermal waves in opaque solids has been described elsewhere. [1–3] This is carried out by scanning the probe beam relative to the heating beam with a constant height, h. The separation, x0, of the two points on either side of the center of such a scan where the phase of the transverse deflection signal reaches ninety decrees effectively measures the thermal wavelength, λ = 2(πα /f)1/2 in the solid. The determination of the thermal diffusivity, a, is accomplished by plotting this separation versus the inverse square root of the frequency. It has been shown theoretically [4,5] that the ratios of the slopes of such plots correspond, in the low frequency limit, to the ratios of the actual diffusivities of the solid. The numerical constant which relates the thermal diffusivity to the slope depends on the value of h. The previous measurements [1–3] of α, carried out using the “skimming” optical probe beam technique (see Fig. 1), were found to be in reasonable agreement with nominal values calculated from handbook data, provided that the slopes of the plots were set equal (heuristically) to (1.0 πα)1/2. Careful examination of the theory [5], however, shows that when h is negligibly small compared to the other characteristics lengths in the experiment, the slope should be (1.4 πα )1/2.</p

The Use of Optical beam Deflection (OBD) Technique in the Thermal Diffusivity Characterization of Polymer Foils

The rapid progress in development of new materials has also created more demands on their testing and characterization methods. In spite of the wide use of different plastics, there is a lack of methods to measure thermal diffusivity of polymers and other low-diffusivity materials. The diffusivity determination of oriented polymers is of particular interest: the drawing of a polymer foil orients the molecular chains, which strengthens the foil mechanically and also causes anisotropy to thermal conductivity making it higher parallel to the drawing direction than perpendicular to it. Anisotropy ratios as high as 100 have been reported [1]. The diffusivity through the foil could be measured by standard methods, like the flash method [2], but this way the diffusivity parallel to the foil surface can not be obtained. However, this can be measured by the optical beam deflection (OBD, mirage) technique [3]. Already the method has been applied to higher diffusivity samples than polymers, covering the range between 20 - 0.02 cm2/s [4, 5, 6, 7].</p