Jean-Pierre Monchalin

Photoacoustic spectroscopy of thick powdered or porous samples at low frequency

This paper sets forth a self‐contained theoretical description of the photoacoustic effect in a thick powdered or porous sample at low modulation frequencies where the sample medium can be considered homogeneous from the point of view of heat diffusion. It is shown that the photoacoustic signal results from two contributions: the heat flux from the sample to the gas plus the expansion of the interstitial gas and that the pressure is essentially uniform throughout the sample. Scattering effects are introduced with a Kubelka–Munk modified model and an expression is first given for the conventional partially filled cell. We discuss of these results and their application to the explanation of several texture effects and to the analysis of experimental data. Using a sample composed of chrysotile asbestos powder, we present experimental data and perform an approximate analysis to retrieve the absorption spectrum. Several experimental results are also presented and thoroughly discussed for a cell completely fill...

Thermal contact probing by photoacoustic effect

The influence on the photoacoustic signal of the thermal contact resistance at the interface sample substrate is investigated using a thin metallic foil (thickness ∼12.5 μm) as sample. The foil is either applied by pressure against the substrate or epoxyed on it. The resistance is determined by interpreting magnitude and phase data obtained by varying the laser source modulation frequency. In the light of the results obtained with the thin foil, we discuss the limitations introduced by thermal contact in the photoacoustic detection of aerosols collected on a surface.

Absolute optical absorption spectra in graphite epoxy by Fourier transform infrared photoacoustic spectroscopy

Optical absorption is obviously of prime interest in the efficiency of laser generation of ultrasound in graphite-epoxy laminates. However, no quantitative spectrum of optical absorption in this composite material has yet been published in the literature. Transmission techniques are inefficient, and other techniques, like attenuated total reflectance or diffusive reflectance, do not give absolute values. The Fourier transform photoacoustic spectroscopy technique seems to be a good alternative that can analyze adequately and quantitatively a graphite-epoxy laminate. We used three different methods to compute the absolute optical absorption from the photoacoustic signal. The three methods are: the saturation of the real part of the photoacoustic spectrum, the comparison of the spectra obtained with two different mirror velocities, and the calibration of the photoacoustic cell with a transmission measurement. The spectra obtained in the IR band of 2.5 to 25 μm are presented, and the problems and limitations of each method are discussed. The results permit a better understanding of the absorption process in the composite laminate, and in this way, will help us enhance the efficiency of laser generation of ultrasound in graphite epoxy.

REMOTE THICKNESS MEASUREMENT OF OIL SLICKS ON WATER BY LASER-ULTRASONICS

ABSTRACT At the National Research Council of Canada Industrial Materials Institute, research is in progress on the application of laser-ultrasonics to remote measurement of the thickness of oil on ...

Laser-Ultrasonic Determination of Elastic Constants at Ambient and Elevated Temperatures

It is well known that the elastic constants of an isotropic solid (the bulk and shear moduli or the Young’s modulus and the Poisson’s ratio) can be determined ultrasonically when both longitudinal and shear wave velocities are measured. At high temperature, traditional ultrasonic techniques are difficult to apply because they require a coupling medium operating in the same temperature range. However, some results have been previously reported using momentary contact1–2, but the application remains difficult, especially for shear wave coupling, and above 1000°C. Obviously, an ultrasonic technique where ultrasound is generated and detected without contact can avoid such problems.

AIRBORNE OIL SPILL SENSOR TESTING: PROGRESS AND RECENT DEVELOPMENTS

ABSTRACT It is now possible to measure the thickness of an oil slick on water by remote sensing. A laboratory sensor has been developed to provide this absolute oil slick thickness measurement. A j...

Magnitude and phase photoacoustic spectra of chrysotile asbestos, a powdered sample

Magnitude and phase photoacoustic spectra of a powdered sample made of chrysotile asbestos have been recorded with a low resolution grating spectrometer at ~16 Hz. A good reproduction of the true absorption spectra is obtained from the recording of the phase variation. The optical, thermal, and acoustical effects occurring within this heterogeneous sample are discussed, and a qualitative explanation of the observations is given.

Determination of residual stresses using laser-generated surface skimming longitudinal waves

A laser-ultrasonic technique is described to non-destructively determine residual stresses in metals such as those produced by shot peening. The method is based on monitoring the small ultrasonic velocity change of the laser-generated surface skimming longitudinal wave (LSSLW) propagating just below the surface. The main advantage of using LSSLW is that the effect of surface roughness induced by shot peening is greatly reduced compared to using surface acoustic waves (SAW). To improve resolution in the measurement of small velocity changes, a cross-correlation technique is used with a reference signal taken on the same but unstressed material in similar conditions. Also, the low-frequency SAW can be used to correct the LSSLW results when affected by minute changes in the path length during the measurements. The validity of the approach is demonstrated by measuring quantitatively the near surface stress in a four-point bending experiment with different levels of surface roughness. Then, scanning results on properly and improperly laser shock peened samples are reported. In particular, the LSSLW velocity variations for the properly peened samples clearly show an increase in the laser-peened area well indicative of a compressive stress.

Investigation of gaseous heat transfer between surfaces by photoacoustic effect

The heat transfer, by the interstitial gas, between two metallic surfaces in contact has been investigated by photoacoustic effect. The thermal conductance is determined by interpreting the phase data obtained by varying the laser source modulation frequency. Our experimental results confirm the hypothesis, implicit in most theoretical formulations of the photoacoustic effect, that the surface thermal resistance is generally negligible.

SAFT data processing applied to laser-ultrasonic inspection

By relying on optics for providing the transduction of ultrasound, laser-ultrasonics brings practical solutions to a variety of nondestructive evaluation problems that cannot be solved by using conventional ultrasonic techniques based on piezoelectric transduction [1,2]. Laser-ultrasonics uses two lasers, one with a short pulse for the generation of ultrasound and another one, long pulse or continuous, coupled to an optical interferometer for detection. Laser-ultrasonics allows for testing at a large standoff distance, inspection of moving parts on production lines and inspection in hostile environments, such as the one encountered in the steel industry. The technique features also a large detection bandwidth, which is important for numerous applications, particularly involving material characterization. Another feature of laser-ultrasonics, particularly useful for inspecting parts of complex shapes, is the generation of an acoustic wave propagating normally to the surface, independently of the shape of the part and of the incidence angle of the optical generation beam. This characteristic feature occurs either when the ablation mechanism is used for generation or when light from the generation laser penetrates sufficiently deep below the surface. This last condition occurs usually with many polymer-based materials and on materials with painted surfaces.