G. Rousset

Photoacoustic characterization of subsurface defects in plasma-sprayed coatings

Abstract The photoacoustic detection of subsurface defects in metallic plasma coatings on metallic substrates has been achieved. The investigation is performed with samples that contain well-known artificial defects (Teflon, alumina or boron nitride) whose thermal properties are sufficiently different from those of the coatings (aluminium or Ni-Cr alloy) or the metallic substrates. The experimental results are interpreted in terms of a theoretical model for the layered structure of the sample. Moreover, the photoacoustic effects of the defect may be characterized by a thermal resistance at the coating-substrate interface. This study enables us to determine different characteristics of the coatings such as thermal properties, thickness and adherence to the substrate, in spite of the effects of the surface roughness on the photoacoustic signal.

Photoacoustic and photothermal evaluation of stratified materials

Photoacoustic and photothermal nondestructive evaluation techniques are an emerging approach for inspection of several materials of industrial interest. This paper focuses on stratified materials such as aluminum-epoxy or graphite epoxy laminates as well as metallurgically coated materials which are widely used, particularly in the aerospace industry. Such materials are usually inspected for layer-to-layer delaminations after manufacture by ultrasonic or radiographic techniques. Optical techniques provide an attractive alternative to such methods because they require no contact with the inspected surface so that they can be deployed in hostile and remote-located environments, while being easy to scan over large surfaces. This paper describes some applications of photoacoustic and photothermal approaches to the inspection of layered materials. Photoacoustic techniques using both microphone detection in air and interferometric detection of the laser-generated ultrasonic wave in the material are reviewed. Photothermal techniques using either a thermoelastic displacement approach or an IR time-resolved temperature monitoring method are also described, and their applicability to different materials is discussed.

Non-destructive interferometric detection of unbonded layers

Abstract Non-destructively detecting and monitoring delaminated areas in stratified materials is an important industrial problem. The possibility of characterizing such delaminations by interferometric dilatometry is analyzed in this paper. A Cu-Be strip adhesively bonded to a massive substrate is resistively heated and its thermal expansion is monitored by a laser interferometer. Large signals are obtained from the unbonded regions, as expected from the theoretical calculations. The initial curvature of the unbonded layer is shown to have a major effect on the lifting capability of uniformly heated layers.

Phase Analysis Of Infrared Fourier Transform Photoacoustic Spectra

From an analysis of the signal phase of a Fourier Transform Photoacoustic Spectrometer,a procedure is proposed to obtain a quantitative photoacoustic spectrum.

Quantitative Photoacoustic Spectroscopy Of Thick Sample By A New Data Analysis.

A new data analysis is proposed for the calculation of quantitative infrared (I.R.) spectra by the photoacoustic (P.A.) method. The theoretical P.A. signal components inquadrature and in-phase with the modulated I.R. excitation exhibit intrinsic variations in function of the variable Op. The identification of such a behavior in the experimental data permits to avoid the signal amplitude saturation and the determination of an external signal phase reference, two common problems in P.A. spectroscopy. The proposed data manipulation leads to the calculation of a quantitative sample spectrum. This is illustrated by an experimental example.