Yves Jayet

Pulsed eddy current signal analysis: application to the experimental detection and characterization of deep flaws in highly conductive materials

Eddy current tests in sinusoidal mode are of great interest to detect flaws in metallic structures. A limitation of this classical method concerns the detection of deep defects in very conductive materials. Such a drawback can be reduced by operating in the transient mode, using a pulsed eddy current technique. In fact, the pulsed character of the electric excitation allows a high peak value of the density of eddy currents in the material; also the broadband signal contains optimized frequencies to probe the sample over a more extended depth. The analysis of the resulting signals is then different from those issue of the sinusoidal mode. The purpose of this paper is to apply pulsed eddy current techniques to specimens containing cracks of different size and depth. First, an original signal analysis was developed to extract the characteristics of well known defects. Then, this approach has been validated by probing different samples with real flaws.

The influence of bonding defects on the electric impedance of a piezoelectric embedded element

When a piezoelectric element is inserted into a medium, it is now well-established that the viscoelastic properties of this medium can easily be deduced from the measurement of the electrical impedance of this element and by using an appropriate optimization algorithm. This paper essentially deals with the influence of bonding defects at the interface between the piezoelectric sensor and the tested medium. First, a model is proposed to calculate the modifications of the electrical impedance due to defects of this type. The validity of this approach is demonstrated by comparing the theoretical predictions with the results obtained experimentally for different samples with well-defined bonding defects. Then, with this model, it is easy to determine the electrical impedance of an embedded piezoelectric element, whatever the value of the relative area on which the bonding defect occurs. In order to simulate what will happen in practice when we ignore that these defects are present, such simulated electrical impedances are processed with the usual optimization algorithm, assuming perfect bonding. Obviously, the viscoelastic properties of the tested medium are subject to error, as the size of the non-bonded area increases. It is shown that a very efficient tool based on the Kramers - Kronig relationships can be used to check the correctness of the extracted parameters.

Theoretical and experimental responses for a large-aperture broadband spherical transducer probing a liquid–solid boundary

A theoretical analysis of a spherical focusing transducer for broadband acoustic microscopy is proposed. The originality of the present contribution is the particular attention we have paid to describe, as rigorously as possible, the diffraction phenomena. Our analysis starts in the harmonic domain with the well-known angular spectrum method, and then gets into the time domain. A new formulation of the angular spectrum in the focal plane has been obtained and compared to other expressions previously reported. This article is deliberately limited to isotropic semi-infinite plane reflectors in order to carry out the inverse Fourier transform in an analytical way. The analytical approach is helpful for the physical interpretation of particular interesting phenomena observed in the transient analysis. A new kind of contribution to the echographic response has been identified and named “geometrical edge waves.” The weight and the arrival time of each discontinuity of the impulse response is analytically evalua...

Monitoring the hydrolytic degradation of composites by a piezoelectric method

Abstract The electric impedance of a piezoelectric element depends on the physical and geometrical properties of the element and on the viscoelastic characteristics of the media in which this element is inserted. According to this model, an original technique has been developed by inserting a piezoelectric ceramic in the composite structure when processed. The electric signal issued from the sensor, after signal processing and comparison with the model, gives access to the evolution of the viscoelastic properties of the surrounding media. An application of this non-destructive method is the monitoring of the hydrolytic degradation in polyesterbased composites. The evolutions of the electric impedance of the sensor are presented here as a function of water exposition time for the resin and glass reinforced composites. The results are compared to degradation kinetics obtained through micromechanical experiments and classical ultrasonic spectroscopy.

Anisotropic damage evaluation in polymer fiber composites under hygrothermal aging by means of ultrasonic techniques

Ultrasonics is a suitable technique for nondestructive evaluation of structural materials degradation. Although the most common objective of ultrasonic testing is the detection and location of overt flaws, ultrasonics can be used to identify microstructural factors that alter strength and performance. However, while single large defects can be individually detected and characterized, widely dispersed discontinuities are impossible to resolve and only their global effects on bulk properties can be observed and measured. The aim of this paper is, using an immersion technique, to characterize damage progression of a glass fiber epoxy matrix composite during hygrothermal aging by means of attenuation frequency-dependence measured in normal incidence and longitudinal and transversal ultrasonic velocities measured as function of the propagation direction. The anisotropic elastic constants of the material are recovered from the ultrasonic velocities using an optimization process. Effects of hygrothermal aging on...

Health monitoring of smart composite structures using ultrasonic guided waves

The health of a structure depends on both the homogeneously distributed degradation of its mechanical properties during its life cycle and the presence of localised defects such as cracks or delaminations. The proposed non-destructive health monitoring method allows to recover both kinds of information using ultrasonic waves. To avoid traditional techniques limitations, such as coupling reproducibility for instance, we propose here to integrate a piezoelectric element into the plate-like composite structure. The element dimensions are determined in order to uncouple the frequency ranges of the thickness and radial vibration modes. The thickness mode is used to monitor the homogeneous ageing of the structure through electrical impedance measurement. As for the radial vibrations, they are used to generate and detect Lamb waves, which have the advantage of propagating over long distances and offering specific sensitivity of various modes to different kinds of defects. The present work focuses on this last application and studies the ability of the proposed technique to detect and identify defects such as low speed impact-induced delaminations and cracks incomposite plate-like structures.

Nondestructive evaluation of materials using an inserted piezoelectric sensor-correlation with hardness measurements

Using a one-dimensional model, we show that the analytical expression of the electric impedance of a piezoelectric ceramic is function of the acoustic properties of the medium surrounding it. The validity of this model is displayed by comparing simulated and experimental data. Then, numerical treatments are developed for processing the ceramic electric impedance in order to determine the acoustic properties of the medium surrounding the ceramic. This method is applied to monitor polymerization processes and the validity of the technique is shown from the comparison of the results it gives with those resulting from traditional Barcol hardness measurements.<<ETX>>

Theoretical and Experimental Investigation of a Piezoelectric Transducer with a Nonparallel-Faced Wearplate

Abstmet-Using a theoretical model, we establish an analytical result giving the average pressure emitted by an ultrasonic piezoelectric transmitter, the wearplate of which has a lack of parallelism between its two faces. Verification of the model’s validity is carried out hy recording the simulated and experimental responses of piezoelectric transmitters with different degrees of parallelism defect for the wearplate. This study is carried out for a quarter-wave wearplate at the resonance frequency of the piezoelectric element.

Nondestructive evaluation of graphite by ultrasonic velocity measurement using cross-correlation and Hilbert transform methods

The suitability of ultrasonics for materials evaluation of graphite components is investigated. The ultrasonic velocity is measured using cross-correlation techniques and a method based on the Hilbert transform of the pulse response function. The measurements are made on different grades of graphite with different particle sizes and porosities. Comparison of the two methods shows similar results for the graphites having small particle size and low attenuation. For large particle size and highly attenuating graphites, the measured quantities are different. The velocities and some identified elastic constants are correlated with the physical properties of the material (grain size and porosity) on specific examples in order to show the ability and the limits of ultrasonics to evaluate large grain size graphite.<<ETX>>

In-situ multidetection: application for composite cure monitoring

This paper reports on a comparative study carried out for different type of sensors which are able to monitor in-situ curing of thermosets resin. In fact, the thermoset matrix composites are increasingly used as structural parts in complex technological structures. So, monitoring the matrix microstructural evolution and the development of internal stresses during the cure is now of the utmost importance. Taking in account their inhomogeneous structures and processing methods, mesoscopic sized sensors may be embedded into the composite materials. Until now different characterisation techniques have been applied independently in order to monitor essentially the cure process: Frequency dependent dielectric measurements provide a sensitive in situ sensor able to give access to the electrical conductivity and complex permitivity of the surrounding medium. The conductivity parameter related to the ionic mobility is linked to the polymerisation advancement. The ultrasonic waves are generally used for global characterisation of mechanical properties.For in situ applications, a piezoelectric element is embedded in the structure during its processing and the rheological properties of such a material-system can be monitored. Refractive index measurement is carried out with a fibre-optic sensor. This optical parameter allows to determine the density of the thermoset resin during the cure process and to access the extent of cure. The multidetection measurement seems to be a powerful tool to understand the chemorheological mechanisms occurring during the thermosets resin cure process.