F. Cohen-Tenoudji

Study of critical behavior in concrete during curing by application of dynamic linear and nonlinear means

The monitoring of both linear and nonlinear elastic properties of a high performance concrete during curing is presented by application of compressional and shear waves. To follow the linear elastic behavior, both compressional and shear waves are used in wide band pulse echo mode. Through the value of the complex reflection coefficient between the cell material (Lucite) and the concrete within the cell, the elastic moduli are calculated. Simultaneously, the transmission of a continuous compressional sine wave at progressively increasing drive levels permits us to calculate the nonlinear properties by extracting the harmonics amplitudes of the signal. Information regarding the chemical evolution of the concrete based upon the reaction of hydration of cement is obtained by monitoring the temperature inside the sample. These different types of measurements are linked together to interpret the critical behavior.

A simple approach to estimate the size of small surface cracks with the use of acoustic surface waves

The scattered radiation patterns of surface cracks irradiated by acoustic surface waves are interpreted to provide estimates of crack size. The technique is demonstrated for cracks as small as 100 μm in radius with an accuracy of about 10%.The key features are the positions and spacings of nulls in the angular dependence of the backward scattered surface‐wave intensity. A simple model based on optical diffraction theory is presented and demonstrated on cracks (made with the identation technique) in commercial hot‐pressed silicon nitride studied at 100 MHz and on cracklike flaws in commercial aluminum studied at 2–10 MHz.

The Possibility of Assigning a Signature to Rough Surfaces Using Ultrasonic Backscattering Diagrams

Summary It is suggested that ultrasonic waves can be used in NDE as a reliable tool for quantitative flaw characterization. A model is developed in which a flaw in a metal diffracts (scatters) incident ultrasonic waves. An electromagnetic type of diffraction theory was used, and expressions for the diffracted fields were obtained for circular and elliptical crack-like flaws. Corresponding broadband scattering experiments were carried out using as targets metal shims in water, flat-bottomed holes in metals, and cavities embedded in diffusion-bonded samples. The scattered shear and longitudinal waves could be separated due to their differing velocities and were separately analyzed. Experiments agree well with theoretical predictions for the case of crack-like flaws. Significant information can be obtained concerning the shape of the cavities using both shear and longitudinal waves. With further experimental and theoretical developments, these model studies might develop into quantitative characterization of real flaws.

A Shear Wave Rheology Sensor

A rheology sensor for monitoring the dynamic viscosity of a thermally curing resin is described. The complex shear modulus of the resin at 1 MHz is derived from the measured reflection coefficient of shear wave pulses at the resin surface. A special transducer-buffer assembly has been developed that operates at high temperature (T = 145°C) and provides a reference calibration signal. With this assem- bly, absolute determinations are made throughout the cure cycle of the real and imaginary components of the shear modulus. From the latter high-frequency dynamic viscosity is calculated. The results are com- pared with data obtained at low shear rates with a 10 Hz torque vis- cometer.

Ultrasonic evaluation of geometrical and surface parameters of rough defects in solids

Ultrasonic measurements from rough cracks were carried out using both broad-band and narrow-band methods. An analysis is suggested to determine parameters of the crack quantitatively such as size, shape, rms surface roughness, and distribution function of the surface roughness. Ultrasonic measurements of the parameters compare very well with the actual parameters of the defect.

Attenuation and Grain Noise Parameters in Ni-Base Alloys

The frequency dependent ultrasonic attenuation and grain noise were determined for samples of three alloys often used in jet aircraft engine turbine discs: Waspalloy, IN-100, and Ti-6246. In addition to propagation of longitudinal waves, also shear waves were considered. The frequency dependence was extracted from broadband echos received through a low attenuation buffer. A key feature of the results for IN-100 is the presence of a low concentration of micropores which appear to influence the scattering of ultrasound and therefore the attenuation and material noise values. Of the three alloys, Waspalloy was found to have the highest attenuation value and Ti-6246 the lowest.

Elastic Wave Inversion Transformation

This paper presents results of a study on an inversion transformation ℐA for backscattering of ultrasonic waves from an obstacle embedded in a solid. For a rigid sphere the resulting function A(t) is derived which is related to the cross-sectional area intercepted at any given time by a transverse plane moving across the obstacle in the same direction as the incident wave. The maximum value of A(t) is the total backscattering cross section. From this study emerges a proposed inversion algorithm for cavities. The use of the inversion algorithm is demonstrated for “exact” theoretical data obtained from Opsal as well as experimental data collected on an ellipsoidal cavity and a double cavity. Associated with the inversion of the experimental data is an iterative technique which optimizes the construction of A(t) and therefore the estimates for the size and shape of the scatterer.

Ultrasonic Wave Dispersion and Attenuation in Fluid Filled Porous Media

The study of ultrasonic wave propagation in granular materials can lead to a better understanding of wave interaction with such materials as uncured cement and concrete. The measured parameters can then be used to investigate the curing process in particular the time required for a given mixture to consolidate. The cohesionless granular materials having loose contact between the constituent grains form a matrix that has negligible shear modulus. Sediment, sandy ground and concrete before solidification can be considered as examples of cohesionless granular materials. The shear and rigidity moduli of these materials can differ greatly from the values obtained by effective medium theories. In particular these differences could affect the ultrasonic wave propagation in such a material. In the case of cohesionless granular material the complete description of mechanical properties requires the consideration of discrete nature of the solid frame and the contact areas between the grains. Therefore wave interaction with such a material should also include the above mentioned effects. The goal of this work is to investigate the ultrasonic wave dispersion and attenuation in cohesionless granular materials the results can be to applied to the monitoring of cement and concrete during the curing process.

Effective Area-Function Approach To Inverse Problems In The Scattering Of Elastic And Electromagnetic Waves

This paper presents work using the effective area-function approach to inverse problems in the scattering of elastic and electromagnetic waves. The emphasis in this paper is on the technique of inventing experimental elastic wave scattering data to obtain size and shape information. The procedure of elastic wave data acquisition and some results of the inversion technique are described for a small (~ 1000 μm) ellipsoidal cavity. The inversion is demonstrated for 1 < ka ~ 4 were k is the elastic wave number and a is the effective radius of the scatterer. The scatterers are embedded in the interior of a Ti-alloy sphere.

Acoustic monitoring of the curing process in cement and concrete

Acoustic wave velocity and attenuation measurements are used to monitor cement and concrete during hardening. The measured acoustic parameters can then be used to investigate the curing process. The goal of this work is to determine the time required for a given mixture of concrete to be hard enough for the removal of the molds, in order to speed up the construction process. Currently this removal time is unpredictable. In addition concrete, mortar, and cement, before hardening, can be considered as examples of granular materials. The shear, rigidity moduli, and acoustic wave velocity of these materials can differ from the values obtained by applying effective‐medium theories and Biots theory. The complete description of the mechanical properties of these materials requires the consideration of discrete nature of the solid constituents and the effects of contacts between the grains. These effects are included in the theoretical description of the process. The methods of acoustic microscopy are also applie...