Biomedical subjects

J Assaad

Publications and source records attributed to J Assaad.

6 recordsLinked to original sources

PubMed2004

Application of the FEM and the BEM to compute the field of a transducer mounted in a rigid baffle (3D case).

A three-dimensional finite element model has been developed which allows the harmonic analysis of a piezoelectric structure mounted on a rigid baffle and radiating into water. The solution of this problem consists of coupling a finite element method to a boundary element method. The first one enables the modelling of the vibrating structure and the second one the modelling of propagating waves in the semi-infinite fluid medium surrounding the structure. In this way, the near-field and the far-field pressures are calculated as well as the displacement field of the piezoelectric structure taking into account the acoustical interaction. Numerical and experimental results are provided which validate the numerical procedure. The good agreement obtained indicates that this three-dimensional model is a very useful tool to optimise the design of transducer arrays used in medical imaging.

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PubMed2004

Health monitoring of a composite wingbox structure.

This work was devoted to the development of a health monitoring system assigned to aerospace applications. Those applications concerned the detection of damaging impacts and debonding between stiffeners and composite skins, since they are the major causes of in-service damage of aircraft structures. The chosen health monitoring system was first based on the excitation and reception of Lamb waves along the structure by using thin piezoelectric transducers (active mode) and secondly on a continuous monitoring taking the same transducers used as acoustic emission sensors (passive mode). The composite specimen used was consistent with aircraft wingbox in terms of structure and loading. Several impacts with increasing energy increments were applied on the composite specimen. In passive mode, the study showed the ability of using the acoustic signature of an impact to detect possible damage. Moreover, the damage emergence in the case of damaging impact was confirmed in active mode. Further measurements during fatigue testing were performed. The aim was to demonstrate the ability of the system to monitor disbond growth between the stiffener and the composite skin. The sensitivity of the health monitoring system to the disbond growth was further demonstrated.

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PubMed2004

Signal processing for damage detection using two different array transducers.

This work describes an investigation into the development of a new health monitoring system for aeronautical applications. The health monitoring system is based on the emission and reception of Lamb waves by multi-element piezoelectric transducers (i.e., arrays) bonded to the structure. The emitter array consists of three different elementary bar transducers. These transducers have the same thickness and length but different widths. The receiver array has 32 same elements. This system offers the possibility to understand the nature of the generated waves and to determine the sensitivity of each mode to possible damage. It presents two principal advantages: Firstly, by exciting all elements in phase, it is possible to generate several Lamb modes in the same time. Secondly, the two-dimensional fourier transform (2D-FT) of the received signal can be easily computed. Experimental results concerning an aluminum plate with different hole sizes will be shown. The A0-, S0-, A1-, S1- and S2-modes are generated at the same time. This study shows that the A0 mode seems particularly interesting to detect flaws of this geometrical type.

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PubMed2002

Numerical technique to reduce cross-coupling in acoustical arrays.

Applicability of finite element method to optimize the far-field directivity pattern of an individual element in a piezoelectric transducer array is reported. The FEM algorithm calculates the electrical potentials needed to be applied to the two elements which are immediate neighbours to the element being activated in order to minimize acoustic cross-coupling. The feasibility of the algorithm was successfully tested using a simple five elements array made of conventional PZT-5H piezoelectric ceramic material.

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PubMed2000

Modeling of integrated Lamb waves generation systems using a coupled finite element-normal modes expansion method

As part of the research work on Smart Materials and Structures, the development of self-monitoring materials is an emerging issue. In the case of plate-shaped structures, Lamb waves can be used for their relevant properties: long-range propagation, sensitivity to internal flaws and whole-thickness interrogation. This concept requires the use of thin piezoelectric transducers integrated to the structure. Since it is of primary importance to be able to control the generated modes, a suitable modeling technique of this kind of system has been tested on different cases of practical interest. The model uses a coupled finite element-normal modes expansion approach, which allows one to consider either the case of bonded or embedded transducers. The results presented deal with examples of multi-element transducers integrated to composite materials. The influence of parameters such as the dimensions, positions and relative excitation delays of the transducers is studied.

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PubMed2000

Modeling of lamb waves generated by integrated transducers in composite plates using a coupled finite element-normal modes expansion method

Thin piezoelectric transducers attached to or embedded within composite structures could be used for in situ structural health monitoring. For plate-shaped structures, the useful ultrasonic vibration modes are Lamb waves. Preliminary testing has already demonstrated the suitability and practical feasibility of such integrated transducers, but better control of the generation of Lamb modes seems to be necessary. Therefore, an original modeling approach has been developed, which can be used to design and optimize these "sensitive materials." This modeling technique allows the determination of the amplitude of each Lamb mode excited in a composite plate with surface-bonded or bulk-embedded piezoelectric elements. The method consists of a coupling of the finite element method (FEM) and the normal modes expansion method. The limited finite element mesh of the transducer and its vicinity enables the computation of the mechanical field created by the transducer, which is then introduced as a forcing function into the normal modes equations. The adequacy and accuracy of this modeling method have been numerically and experimentally verified.

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