A combined finite element and modal decomposition method to study the interaction of Lamb modes with micro-defects.

This paper presents a combined finite element and modal decomposition method to study the interaction of Lamb waves with damaged area. The finite element mesh is used to describe the region around the defects. On the contrary to other hybrid models already developed, the interaction between Lamb waves and defects is computed in the temporal domain. Then, the modal decomposition method permits to determine the wave reflected and transmitted by the damaged area. Modal analysis allows also identifying the mode conversions induced by the defects. These numerical results agree with previous finite element results concerning the interaction of Lamb modes with a notch. Experiments, carried out with gauged defects on an aluminum plate, are also compared to numerical predictions to validate the simulation. Compared to classical techniques of simulation, this new method allows us to investigate the interaction of Lamb modes generated at high frequency-thickness product with micro-defects as corrosion pitting.

Identification of linear viscoelastic constitutive models.

Accelerations induce in the brain mechanical stresses that may explain the loss of consciousness feared by fighter pilots. In this study, the brain is modelled as a multi-domain structure and a finite element method is used to identify the constitutive law parameters of each domain and then to analyse the stress level in the brain. The loading and observed strain rates induced by hypergravity seem to indicate a quasi-static behaviour of the brain structure. A general procedure has been developed to characterise the behaviour of a structure including several domains. Each of them is assumed to be isotropic and homogeneous with a linear viscoelastic behaviour. These constitutive laws were identified using only the displacements of several nodes on the envelope discarding the displacements between domains at the interaction surfaces. These interfaces may be buried inside the structure and not connected with the external surface. Two validation examples are proposed to show the reliability and effectiveness of the method.

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.

Evaluation of cerebral stresses under acceleration taking into account the lateral ventricles.

In certain flight configurations, fighter pilots are exposed to high Gz acceleration that may induce inflight loss of consciousness (LOC). That LOC is usually preceded by visual prodromes as greyout and blackout. The pathophysiological cause of these phenomena is used to be related to the effects of accelerations on the vascular system (Burton, 1988; Whinnery, 1990). However technological advances have created aircraft generating high accelerations with rapid onset rates (1-6 Gs-1). The symptomatology of inflight LOC has changed and prodromes no longer appear. Pilots also reported a lacunar amnesia of the LOC. In order to evaluate the potentially adverse effect of acceleration on the brain tissue, it was important to study its mechanical behavior under hypergravity. An approximation of the cerebral stresses was obtained by coupling an 'ex vivo' experiment (Guillaume et al., 1997) with a numerical simulation. Firstly, the calculations have been realized considering the brain as homogeneous. Secondly, the cerebral ventricles have been individualized. The results of these two approaches were compared.

Effects of perfusion on the mechanical behavior of the brain-exposed to hypergravity.

In certain flight configurations, fighter pilots are exposed to high Gz acceleration which may induce inflight loss of consciousness (G-LOC). In order to study the mechanical effects induced by these accelerations on the cerebral structures, an experimental model has been developed in vitro. Fresh bovine brains were excised and placed in a transparent mold modeling the inside of the skull. Half of these brains were perfused during the experiment. This assembly was placed into the gondola of a centrifuge, in front of a camera lens. Displacements and deformations of the brains were filmed and recorded at different onset rates. Measurements were made after off-line digitalization of images. Experimental data were incorporated into a finite element calculation code whose mesh represented the brain. The applied behavior law was elastic, the structure being considered as homogeneous and isotropic. The first results concerned the elastic properties of the brains under hypergravity. The mean value of the Young's modulus of the nonperfused brain was 46.8 kPa, which corresponded to the values published in reference literature. For the perfused brains, the mean value of the Young's modulus was higher. The mean value of the equivalent Poisson's ratio was 0.35. In fact, contrary to impacts, the mechanical stimulation is long enough to allow fluid displacements. The mean value of the equivalent Poisson's ratio calculated in the present study should probably be increased since this study was performed post mortem.

Experimental model for studying the mechanical effects of +Gz accelerations on the brain.

NASA: Bovine brains were excised and placed into a transparent mold equipped with a pump and perfusion system. This unit was then centrifuged and changes in brain contour were video recorded. Analysis of the resulting images showed that changes occurred in brain structures as a result of crushing. The effects of perfusion on the amount of deformation and other results are discussed.^ieng