Franck Enguehard

Effects of optical penetration and laser pulse duration on laser generated longitudinal acoustic waves

In this article, we concentrate on the optical penetration and laser pulse duration effects on the features of the ultrasonic waves generated in a solid by a laser impact. We consider the simple case of a uniform irradiation of the sample, which we describe with the help of a simple analytical one-dimensional model. In spite of its simplicity, this model clearly highlights the competition that occurs between two temporal convolution sources, related to the optical penetration and the laser pulse duration, respectively, to produce the longitudinal arrivals on the rear side of the sample. The model is also used for an accurate description of the features of the longitudinal precursor.

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.

Temporal deconvolution of laser-generated longitudinal acoustic waves for optical characterization and precise longitudinal acoustic velocity evaluation

The laser thermoelastic generation of ultrasound is a promising technique with many potential applications, but it is also a complicated process with many physical phenomena involved. Contrary to a conventional piezoelectric transducer generation, which is a surface phenomenon, a laser generation can activate acoustic sources within the material by optical penetration of the excitation wavelength, resulting in asynchronous wave arrivals at a given point. More generally, in the ideal case of a nondispersive isotropic material, the laser-ultrasonics displacement signals result from temporal convolutions between optical penetration, laser pulse duration, and laser spot extension effects. In this paper, a deconvolution technique is presented that extracts the laser pulse duration contribution from the experimental displacement signals. This deconvolution scheme applied to one-dimensional experiments, in which the laser excitation is spread over a sufficiently large area on the front side of the sample, allows...

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.

Modeling of time-resolved coupled radiative and conductive heat transfer in multilayer semitransparent materials up to very high temperatures

This paper presents an original modeling approach that enables the calculation of the temperature field within multilayer materials submitted to the flash method. The model takes into account the time-resolved coupled conducto-radiative heat transfer and the temperature of experiments. The compound can be subdivided into as many layers as desired, and their thicknesses and relevant physical properties can be chosen arbitrarily. Unconventional experimental thermograms can be reproduced faithfully by the calculations. This model, thus, makes it possible to correctly estimate the effective thermal diffusivity of semitransparent materials, thereby providing a deeper insight into the analysis of the physical phenomena involved.

Phase optimization for quantitative analysis using phase Fourier-transform photoacoustic spectroscopy

The phase of the photoacoustic signal is of prime importance for obtaining accurate optical absorption coefficients using the photoacoustic technique. Variations in the spectrometer or the photoacoustic cell parameters between the measurement of the sample spectrum and the carbon black reference spectrum are the main source of phase shifts. We reconsider a simple model that provides an accurate description of the photoacoustic effect—including photoacoustic saturation—for thermally thick, homogeneous samples. The model includes absorption from a thin layer at the sample surface. We propose a method for optimizing the photoacoustic phase for this model. The method is based on the internal calibration at the onset of the photoacoustic saturation, and on a simple analysis of the shape of the calculated surface absorption spectrum. Optimization is illustrated with a simulated spectrum and an experimental spectrum of ethylene-propylene-rubber.

A Two-Layer Model for the Laser Generation of Ultrasound in Graphite-Epoxy Laminates

We previously reported the performances of a numerical simulation model [1] that calculates the mechanical displacements induced within a sample by the absorption of a laser pulse. This model solves the heat diffusion and acoustic wave propagation equations over an orthotropic slab of finite thickness with the help of temporal Laplace and spatial 2D Fourier transformations. The parallel and normal displacements predicted by the model were found to be in generally very good agreement with experimental data obtained on various samples in various excitation conditions. Among these experiments, one consisted in the CO2 laser excitation of a graphite-epoxy sample. We performed an optical study of the graphite-epoxy composite using FTIR photoacoustic spectroscopy [2] to determine the optical penetration depth spectrum of this material. This study revealed that a thin (≈ 30 μm thick) epoxy layer covered the top graphite fiber sheet of the composite, and that the optical penetration depth of the CO2 radiation in the epoxy was about 20 μm. Consequently, when a CO2 laser pulse impinges on the composite, all the radiation is absorbed in the epoxy layer, and it is easy to simulate this situation with the model, using the rigidity-expansion tensor [λ] of the epoxy for the generation and the rigidity tensor [C] of the composite for the propagation (see [1]).

Laser-Ultrasonic Optical Characterization of Nonmetals

There is now a growing interest in the use of laser-generated ultrasound for nondestructive evaluation of materials [1,2]. In the case of nonmetals, the laser-ultrasonic displacement signals result from temporal convolution between the optical penetration, the laser pulse duration and the laser spot extension [3,4]. The temporal information present in the first longitudinal arrival called “the precursor” depends on this three parameters [4,5 ]. A temporal broadening of the precursor with increasing the optical penetration have been already observed experimentally [6]. It has been shown that a measurement of the full width at half maximum (FWHM) of the precursor allows an evaluation of the optical absorption coefficient of the material at the excitation wavelength [7]. To evaluate this coefficient with a good reliability one needs to take into consideration the two other parameter effects (the extension spot size effect and the pulse duration effect). In this paper, first we present experimental and theoretical studies of the optical penetration on the acoustic waveforms. As the laser wavelength was fixed (1064 nm), we have used Schott glasses with identical thermomecanic properties and different optical absorption coefficients. Characteristic curves (in the 1-d and the 2-d regimes) that relates the FWHM of the precursor to the optical absorption coefficient have been plotted. Llinear variation regions have been identified on these curves. We have shown that it is possible to deduce the optical absorption coefficient in these regions by fitting the curves. Optical absorption coefficients of the Schott glasses have been deduced using this approach. Results were in good agreement with data given by the manufacturer and data calculated using a theoretical approach [5].

Analysis and Correction of the Source Parameter Effects for Optimizing the Laser Ultrasonic Mechanical Characterization

The laser-ultrasonic technique is a promising tool in the field of the mechanical characterization of materials [1,2]. The use of a laser is associated to three temporal convolutions related to the source parameters (the spot size, the pulse duration and the wavelength of the excitation) [3,4]. For a given wavelength, the evaluation of the mechanical properties of low-damage threshold materials in the thermoelastic regime, requires a compromise between the space distribution and the pulse duration of the excitation. An enlargement of the spot or an increase of the pulse duration introduces delays on the arriving times of the acoustic waves and consequently affects the accuracy of the velocity measurement. In the case of a non-metallic material, a third effect related to the optical penetration might be considered.