F. Vander Meulen

Evaluation of the moisture content in phenolic resin via acoustic measurements

Moisture absorption is a water diffusion process known to alter many of the mechanical properties of polymers and polymer-based composites. Ultrasonic waves are sensitive to such changes of properties and allow in situ measurements to be performed. This paper presents linear and nonlinear ultrasonic measurements on 12 phenolic resin plates with water contents ranging from 0% to 5% of their dry mass. Linear parameters are evaluated by an insertion/substitution spectroscopy method yielding to a representation of the evolution, with frequency, of the attenuation and of the phase velocity of acoustic waves propagating in each sample. Nonlinear β coefficient of each sample is determined by a contact phase modulation method. The evolution, with moisture content, of acoustic parameters such as the attenuation and the phase velocity of acoustic waves propagating at a given frequency, the slope of their attenuation with frequency, and the β coefficient of each sample is then studied. Results are discussed in terms...

Moisture content characterization in composite materials based on ultrasonic transmission measurements

Moisture absorption is known to alter many of the mechanical properties of polymer-based composites. This study presents ultrasonic characterization of moisture content based on the transmission of acoustic waves through composite plates. The elastic constants of 14 plates with different controlled water contents are obtained. The redundancy of the results allows to validate the characterization process and the assumption made on the sample symmetry. The sensitivity of the process to moisture content is then discussed.

Acoustic nonlinear parameter measurement in solid with a contact phase modulation method

In this work, a new method to measure in contact the nonlinearity parameter /spl beta/ of solid plates is presented. This method is based on an idea developed by Barriere and Royer (2001) to measure the nonlinearity parameter in liquids. A high frequency (HF) tone-burst signal of 20 MHz is inserted in the material by a contact-transducer (with a suitable coupling). A low frequency (LF) pulse (2.5 MHz) is applied to the other face, in the opposite direction, such that the nonlinear interaction of the two waves takes place during the back propagation toward the HF transducer. This collinear interaction creates a phase modulation of the HF tone-burst which is directly proportional to the /spl beta/ coefficient and the particle velocity of the LF wave. To determine this particle velocity a self reciprocity calibration of the contact LF transducer is used, as proposed by Dace et al. (1991). A numeric phase demodulation has been performed, giving the beta coefficient of the sample. The proposed method is validated by nonlinearity parameter measurements in Fused Silica.

Self calibration method to determine acoustic wave velocity in highly loaded liquids

Acoustic wave velocity determinations performed through time of flight measurement, require a time reference to be defined. Here, a minimum phase method is proposed to obtain a self calibrating measurement. Experimental results on particule-loaded liquids are obtained and found to be in good agrement with previously published results and with those of a theoretical homogenisation model.

Towards a simple acoustic method to evaluate the nonlinear parameter B/A of fluids

The objective of this work is to develop an easy-to-build and robust setup for measuring the nonlinearity parameter B/A in fluids using ultrasound. The method is based on the pulse echo technique, using a single element broadband acoustic transducer, and requires electrical signal measurements. Results obtained in water and denatured alcohol validate the proposed procedure. The choice of a suitable primary wave frequency is discussed with regard to the transducer sensitivity. Further, the influence of the perturbations introduced by the experimental device nonlinearities, and the role of the reflector on the measured second harmonic field amplitude are investigated.

Application of multiple scattering theories to the evaluation of granular distributions based on phase velocity measurements

Using multiple scattering theories, the influence of granular distribution on acoustic phase velocity in particle-loaded liquids was studied. Theoretical results were compared to measurements in suspensions of acrylic spheres in ethylene glycol. It appears that the phase velocity versus frequency shows a minimum, the position and sharpness of which are related to the mean diameter of particles and to the standard deviation of the diameter distribution. Its amplitude is only influenced by the concentration. Therefore, it is shown that granulometric properties of suspensions can be determined by an ultrasonic method, through the behavior of phase velocity versus frequency.

Strong nonlinear harmonic generation in a PZT/Aluminum resonator

In this work, the extentional vibration mode of a coupled PZT/ Aluminum rod resonator is studied experimentally. Geometrical characteristics of the PZT are its 27 mm length and its 4×4 mm2 cross section area. The excitation voltage consists in sinusoidal bursts in the frequency range (20-80 kHz). Velocity measurements are performed at both ends of this system, using a laser probe. Strong harmonic distortions in the mechanical response (up to -20 dB with respect to the primary wave amplitude) have been observed. The corresponding input levels are far lower than those which are necessary to observe quadratic second harmonic generation in a free PZT resonator. The strong nonlinear effect can be explained as a super-harmonic resonance of the system due to a specific ratio between the eigen frequencies of the two parts of the resonator. Evolution of fundamental and harmonic responses are observed as a function of input levels, highlighting hysteretic behavior.

Evaluation of the water content in phenolic resin plates via nonlinear ultrasonic measurements

In this study, we dispose of 12 phenolic resin samples at the diffusion process equilibrium with a water content controlled (from 0% to 5% of the dry mass). In these samples, celerity, attenuation and nonlinear coefficient β behaviour with water content is estimated. Nonlinear coefficient is evaluated with a contact phase modulation method. This method is based on parametric interaction between two waves with very different frequencies. It induces a phase variation of the high frequency toneburst which is proportional to the nonlinear coefficient β. The nonlinear coefficient exhibits a non monotonous behaviour with the water content and a better sensitivity than the linear parameters in a restricted range of water content.Complementary series of experiments have been conducted in order to study non homogeneous water content situations. Linear and nonlinear parameters are measured. The theoritical water profile through the sample thickness is known and used in a pseudo‐spectral numerical code, in wich the ...

Nonlinear Parameter Measurement for Nondestructive Evaluation of Solids : Calibrated Phase Modulation Method

A phase modulation method is used for Nondestructive Evaluation of solid samples. We take advantage of a self reciprocity calibration procedure to achieve quantitative measurements of the nonlinear parameters of homogeneous solids samples made of fused silica, aluminum, steel and glass, as well as inhomogeneous ones (cracked glass and quenched steel).

Thermosetting SMC resin characterization using linear and nonlinear ultrasonics

The material properties of viscous pastes used for the fabrication of thermosetting SMC (Sheet Molding Compound) have been investigated with ultrasonic spectroscopy working in linear and nonlinear modes. Attenuation versus frequency curves exhibit variations due to the time evolution of unsaturated polyester (UP) resin. A power amplifier was needed because of the high attenuation coefficient of resins. Different ultrasonic responses were measured versus the age and the proportion of gas bubbles and clusters in the paste. We present experimental results and show the interest of using nonlinear acoustics for nondestructive characterization of SMC.