Yves H. Berthelot

Detection of small surface-breaking fatigue cracks in steel using scattering of Rayleigh waves

The long-wavelength (Rayleigh) scattering of a 5 MHz ultrasonic pulse propagating at the surface of a sample under cyclic tensile loading is used to monitor the growth of small surface-breaking cracks. The scattered signals are used to detect the presence of cracks as small as 80 μm in length, in the specimens made of AISI 4130 steel, even in the presence of strong curvature of the surface in the path of the Rayleigh waves. Ultrasonic data is recorded as a function of the number of cycles and compared with crack length measured optically with a microscope. Experiments indicate that the rms amplitude of the scattered signal is proportional to the square of the crack radius measured optically. On specimens with rougher surfaces, ultrasonic detection occurs before optical detection of the cracks. Results also indicate that the short-time Fourier transform (STFT) may be useful to determine whether a single crack (high cycle fatigue) or a distribution of cracks (low cycle fatigue) is present.

The use of optical fibers to enhance the laser generation of ultrasonic waves

It is shown that optical fibers offer a convenient way to control, both spatially and temporally, the laser light used to generate ultrasound in a solid. Proper phasing between the acoustic signals received at the transducer can enhance the ultrasonic signal in a particular direction. The principle is validated experimentally, with two optical fibers guiding the light emitted by a cw argon‐ion laser.

Determination of the complex Young and shear dynamic moduli of viscoelastic materials

The Young and shear dynamic moduli of viscoelastic materials are determined from laser vibrometric measurements of the surface motion of a three-dimensional sample excited by a piezoelectric actuator inside a chamber with controllable temperature and static pressure. The moduli are estimated from an inversion code that minimizes the difference between the data and the predictions from a finite element model in which the elastic moduli are the adjustable parameters. The technique is first used to measure the dynamic properties of homogeneous samples and the results are compared with those obtained by the standard rod resonance technique. Results are then obtained with microvoided samples in the 0.5-3 kHz frequency range, at temperatures ranging from 7 to 40 degrees C, and static pressures ranging from ambient to 34 atm (3.45 MPa or 500 psi). The limitations of the technique are discussed.

Design, microfabrication and testing of a CMOS compatible bistable electromagnetic microvalve with latching/unlatching mechanism on a single wafer

This paper reports our work on the design, microfabrication and testing of a novel bistable electromagnetically actuated microvalve that is fully fabricated by surface micromachining on a single silicon wafer with a potentially CMOS compatible process. The microvalve has an overall diameter of 1600 µm and an overall height of 600 µm that includes the thickness of the silicon wafer. The microvalve consists of four main components: a soft magnetic (NiFe) base, an Au microcoil, a CoNiMnP permanent magnet and a soft magnetic (NiFe) membrane with supported legs. The latching and unlatching mechanism of the microvalve governs the bistable positions of the membrane. The microvalve was tested for its performance to open/unlatch and close/latch a flow of deionized water at flow rates of 10–50 µl min−1. The microvalve operates at a current of 0.43 A, and its power and energy consumption are of the order of 1.6 W and 16 mJ, respectively. The microvalve has a response time of 10 ms and a burst pressure of 7.8 kPa.

Laser detection of sound

A differential laser Doppler velocimeter (LDV) has been assembled and tested to provide noninvasive absolute measurements of acoustic particle displacements of standing waves generated in a water‐filled tube. The principle of the technique [see K. J. Taylor, J. Acoust. Soc. Am. 59, 691–694 (1976)] is to measure the Doppler shift of laser light scattered from colloidal microparticles oscillating under the action of an acoustic field. The system tested is capable of detecting particle displacements of the order of a few nanometers with a bandwidth of several kilohertz. The performances and limitations of the system are discussed. In particular, the effect of Brownian motion is shown to produce only negligible broadening of the spectral density of the signal of interest. The sensitivity of the present LDV system is estimated to be very close to the shot noise limit of the photomultiplier tube used to detect the Doppler shift of the scattered light. Experimental results are obtained under controlled laborator...

Minimum variance guided wave imaging in a quasi-isotropic composite plate

Ultrasonic guided waves are capable of rapidly interrogating large, plate-like structures for both nondestructive evaluation and structural health monitoring (SHM) applications. Distributed sparse arrays of inexpensive piezoelectric transducers offer a cost-effective way to automate the interrogation process. However, the sparse nature of the array limits the amount of information available for performing damage detection and localization. Minimum variance techniques have been incorporated into guided wave imaging to reduce the magnitude of imaging artifacts and improve the imaging performance for sparse array SHM applications. The ability of these techniques to improve imaging performance is related to the accuracy of a priori model assumptions, such as scattering characteristics and dispersion. This paper reports the application of minimum variance imaging under slightly inaccurate model assumptions, such as are expected in realistic environments. Specifically, the imaging algorithm assumes an isotropic, non-dispersive, single mode propagating environment with a scattering field independent of incident angle and frequency. In actuality, the composite material considered here is not only slightly anisotropic and dispersive but also supports multiple propagating modes, and additionally, the scattering field is dependent on the incident angle, scattered angle, and frequency. An isotropic propagation velocity is estimated via calibration prior to imaging to implement the non-dispersive model assumption. Imaging performance is presented under these inaccurate assumptions to demonstrate the robustness of minimum variance imaging to common sources of imaging artifacts.

Investigation of laser generation of Lamb waves in copy paper

Abstract Experiments have been conducted to study the generation of ultrasonic Lamb waves in copy paper using a pulsed Nd : YAG laser. The signals were detected with an out-of-plane heterodyne Ar+ laser interferometer. The effect of the generation laser spot size, energy density, and total energy on the transfer of energy from the generation laser to the two lowest Lamb wave modes, a0 and s0, was investigated. As with metals excited in the linear thermoelastic regime, the signal energy increases as the square of the input laser energy. However, unlike in metals, significant statistical variations of the waveforms (peak amplitudes, arrival time) were observed and quantified by means of cross-correlation techniques. Also unlike in metals, it appears that there is an optimum spot radius of about 90 ± 20 μm (i.e. an energy density of about 22.4 J/cm2) for efficient generation of both a0 and s0 mode in the copy paper we studied. The shift in frequency content of the Lamb waves was examined as a function of the generation spot size. Damage at the generation spot was assessed by photographic microscopy.

Dispersion of Guided Circumferential Waves in a Circular Annulus

Fatigue cracks have been found to initiate and grow in the radial direction in many of the annulus shaped components in aging helicopters. Those include some of the most critical components such as the rotor hub, connecting links and pitch shaft, etc. At the present time, detection of such radial fatigue cracks relies mostly on visual inspection. A more systematic, automated, and efficient method to detect these cracks must be developed.

Theory of laser-generated transient Lamb waves in orthotropic plates

This paper presents a theoretical study for modelling the thermoelastic excitation of transient Lamb waves propagating along the principal directions in an orthotropic plate. The normal mode expansion method is employed to express the transient displacement field by a summation of the antisymmetric and symmetric Rayleigh - Lamb wave modes in the surface stress-free orthotropic plate. This method is particularly suitable for waveform analyses of transient Lamb waves in thin sheet materials because one needs only to calculate contributions of the lowest few antisymmetric and symmetric modes. The dispersion characteristics and the transient Lamb waveforms excited by a pulsed laser in machine-made paper are analysed numerically and discussed in detail and attention is focused on the influence of the elastic stiffness constants. This work provides a quantitative analysis for noncontact and nondestructive detection of the elastic stiffness properties of the machine-made paper by the laser-generated Lamb wave technique.

A differential effective medium model for piezoelectret foams

Polymer foams with piezoelectric properties, called piezoelectrets, have recently gained interest in the acoustics and scientific community. These foams are heterogeneous materials consisting of a continuous polymer containing electrically polarized elliptical voids. The macroscopically observable piezoelectric behavior results from the unique combination of the high void-polymer elastic contrast and the void polarization. Existing modeling methods to approximate the macroscopic piezoelectric properties of these foams are elementary one-dimensional noncoupled electrostatic models. In this study, a coupled mean field microelectromechanical model has been developed as a predictive tool of the macroscopically observed piezoelectric material behavior. This technique employs Green’s function solutions of the three-dimensional (3D) stress equilibrium equations and Gauss’ Law. The result is a multiscale differential effective medium model approximating the 3D effective stiffness, dielectric permittivity, and piezoelectric coupling coefficients as a function of the constituent material properties, void shape and orientation, and deposited charge density. The model is employed to study the sensitivity of macroscopic foam behavior to various constituent material and void variables. This approach improves the approximation of the true piezoelectret behavior by capturing the influence of microscopic material structure and properties on macroscopic piezoelectric performance.Polymer foams with piezoelectric properties, called piezoelectrets, have recently gained interest in the acoustics and scientific community. These foams are heterogeneous materials consisting of a continuous polymer containing electrically polarized elliptical voids. The macroscopically observable piezoelectric behavior results from the unique combination of the high void-polymer elastic contrast and the void polarization. Existing modeling methods to approximate the macroscopic piezoelectric properties of these foams are elementary one-dimensional noncoupled electrostatic models. In this study, a coupled mean field microelectromechanical model has been developed as a predictive tool of the macroscopically observed piezoelectric material behavior. This technique employs Green’s function solutions of the three-dimensional (3D) stress equilibrium equations and Gauss’ Law. The result is a multiscale differential effective medium model approximating the 3D effective stiffness, dielectric permittivity, and pie...