M. de Billy

Surface wave scattering from elliptical cracks for failure prediction

The scattered radiation patterns from surface cracks irradiated by acoustic surface waves are interpreted to provide estimates of crack length and aspect ratio, geometric crack parameters needed to enable failure prediction. The technique is demonstrated for circular and elliptical cracks as small as 100 μm in depth with an accuracy of about 10%. The key features are the positions and spacing of peaks and nulls in angular frequency dependence of scattered surface wave intensity. A simple model based on optical diffraction theory is demonstrated on cracks in commercial hot‐pressed silicon nitride studied at 100 MHz and on spark‐eroded slots in commercial aluminum studied at 2–10 MHz. The results are used to calculate the stress intensity factors and to describe the direction of crack propagation for a variety of real and simulated cracks. Implications of the technique with respect to crack closure and effects of stress and time are also discussed.

The effect of imperfections on acoustic wave propagation along a wedge waveguide

This is a note which describes the experimental results concerning the influence of defect of acuteness, such as edge milling burrs and truncation, on the dispersive behavior of acoustical wave trains transmitted through a right angle wedge. It is observed that high frequency components propagate more slowly or more quickly than low frequency components according to the kind of defects which affect the edge of the wedge. The observed dispersive behavior may be explained qualitatively by considering that the sharp providing burrs act as a wedge with a depth-dependent apex angle. The other explanation for the dispersion effect involves modeling the imperfections as rectangular waveguides on a half semi-infinite medium. However this theoretical approach does not agree with the experimental results qualitatively.

Acoustic technique applied to the measurement of the free edge wave velocity

Abstract In this paper, the influence is examined of the apex angle on the velocity of the acoustic edge waves propagating in free linear elastic wedges. The guided antisymmetric waves (ASF modes) are generated and detected with a technique which uses two shear transducers, the directions of polarization of which are perpendicular to the bisecting plane of the wedge. This method, Suggested by Pajewski, is analysed and discussed. The experimental measurements of the variation of the free edge wave velocity with respect to the apex angle reveal a very good agreement with the theoretical computations.

Acoustic wave localization in one-dimensional Fibonacci phononic structures with mirror symmetry

This paper reports on numerical and experimental results of acoustic transmission spectra of bead chains with symmetric and asymmetric Fibonacci-like structures. As a matter of comparison, perfect periodic acoustic waveguide structures are also examined. This study shows that Fibonacci structures with mirror symmetry can exhibit localized modes with higher amplitude, due to resonant transmission induced by the presence of dimers inside the 1D structure. A good agreement is observed between the theoretical predictions and the experimental power spectra.

Ultrasonic inspection of defects in welded plates and tubes

The characterization of defects, their size and orientation is of primary interest in quantitative NDE. A model for wave propagation through a weld between two plates is presented. An analytic method based on orthogonality relationship for Lamb modes is used to determine the reflected and transmitted fields for two welded plates. Some interesting experimental results have been obtained using guided waves inspection of hollow cylinders. Some frequencies are more sensitive to specific shapes and positions of defects.

Measurement of the periodicity of internal surfaces by ultrasonic testing

When an ultrasonic wave insonifies a corrugated surface it is diffracted in agreement with the optical laws. Diffraction peaks are observed and their positions are directly related to the periodicity of the grating. This property has been exploited to estimate the periodicity of an internal periodic surface. Experiments have been performed at 5 and 16 MHz for gratings engraved on different samples (plates and half cylinders) of various materials (copper, Perspex).

Influence of the witting and the angle of immersion on the generation of a Scholte wave: Experimental investigation

Abstract The Rayleigh wave propagation on a partially immersed plane surface gives rise to two surface waves: a generalized Rayleigh wave and a Scholte wave which propagate in solid and liquid half-spaces respectively. Our experimental results show that the energy carried by the Scholte wave is strongly dependent on the wetting and the angle of immersion.

Experimental investigation of reflection coefficients for lossy liquid-solid-liquid systems

Abstract The reflection coefficient for a bounded ultrasonic beam incident on a liquid-solid-liquid system (LSL) is studied. Systematic measurements were carried out to investigate the variations of the amplitude of the reflection modulus ∣RLSL∣ with incident angle and parameter Fd (frequency-thickness of the plate). The experimental plots confirm the theoretical works of Claeys et al1 and Bogy and Gracewski2. It is shown that the attenuation of the media has a great influence on the variations of this coefficient as has been already mentioned in the literature, for liquid-solid configurations.

A new simple method for measuring the velocities of sound in isotropic, nondispersive solids

A simple method of simultaneously measuring the longitudinal and shear acoustic velocities in isotropic solids has been developed by exploiting the behaviour of ultrasonic waves upon refraction at a water-solid interface and using a semi-cylindrical sample shape. The technique needs only one broad-band transducer, no mode-conversion wedge is required and the immersion coupling ensures excellent reproducibility. Velocity measurements have been made on steel, aluminium and brass samples (with a radius greater than 4 mm) immersed in water and a precision of about 0.6% for the shear velocity and 0.3% for the longitudinal velocity has been attained.

Methods of analysis for backscattering from tissues

Two techniques for tissue characterization by ultrasonic analysis are described. In the first, using narrow-band pulses, the angular variation I (theta) of the ultrasonic intensity back-scattered by vegetal or animal targets is analysed. We examine how it is possible to estimate the surface roughness and the average spatial periodicity d characteristic of the tissues, by calculating the Fourier transform or the autocorrelation function of I (theta). In the second technique the sample is insonified with a broadband pulse at a fixed angle. A value of the mean periodicity can be obtained directly from the frequency spectrum of the echo for tissues of fairly regular structure.