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R Willinger

Publications and source records attributed to R Willinger.

8 recordsLinked to original sources
PubMed2005

Shear linear behavior of brain tissue over a large frequency range.

The literature review about the shear linear properties of brain tissue reveals both a large discrepancy in the existing data and a crucial lack of information at high frequencies associated with traffic road and non-penetrating ballistic impacts. The purpose of this study is to clarify and to complement the linear material characterisation of brain tissue. New data at small strains and high frequencies were obtained from oscillatory experiments. The tests were performed on thin porcine white matter samples (corona radiata) using an original custom-designed oscillatory shear testing device. At 37 degrees C, the results showed that the mean storage modulus (G') and the mean loss modulus (G'') increased with the frequency (0.1 to 6310 Hz) from 2.1+/-0.9 kPa to 16.8+/-2.0 kPa and from 0.4+/-0.2 kPa to 18.7+/-2.3 kPa respectively. The reliability of these new dynamic data was checked over a partially common frequency range by conducting similar experiments using a standard rheometer (Bohlin C-VOR 150). Data were also compared in the time field. From these experiments, the relaxation modulus (G(t)) was found to decrease from 24.4+/-2.1 kPa to 1.0+/-0.3 kPa between 10(-5) s and 270 s.

Animals↗
PubMed2004

Human Neck Finite Element Model Development and Validation against Original Experimental Data.

This study proposes a detailed FEM of a human volunteer's neck and proceeds to an original model validation against experimental data recorded with this human volunteer. In order to evaluate the new model against existing data a successful temporal validation of the model was obtained under frontal, lateral, oblique and rear impact. New validation parameters are based on an experimental test proceeded in the frequency domain in order to extract the volunteer's Head-Neck system's modal characteristics. In depth validation of the head neck FEM is then performed by superposing the numerical and experimental frequency response function. Model optimisation in the frequency domain permitted after significant properties modification to reproduced accurately both, the neck extension mode at 1.4 Hz and the head retraction mode at 8.8 Hz. Finally the "frequency domain optimised" FEM response was superimposed with the temporal corridors provided in the literature. It must be mentioned that the model's response in the temporal domain remains inside existing corridors after this model optimisation in the frequency domain illustrating that the temporal validation is not accurate enough. This study proposes a neck model with improved geometry description and biofidelity with special attention paid to the retraction mode, a phenomenon which is often masked in the temporal domain.

Journal Article↗
PubMed1995

Modal and temporal analysis of head mathematical models.

The basic hypotheses used during these investigations were based on the vibration analysis of the head, which demonstrated that the head is not a solid nondeformable body, but a complex structure including deformable elements. Laboratoire des Systemes Biomecanique (LSBM) has recently proposed three mathematical models: a lumped model, a finite element model of the head in its sagittal plane, and a three-dimensional finite element model. These models were validated by their modal behavior and enabled the lesion mechanisms to be distinguished as a function of the spectral characteristics of the shock. The objective of this study is to complete these modal results by temporal analysis of the models by calculating the evolution of the intracranian mechanical parameters under shock conditions. To describe the head's dynamic behavior in the temporal domain, constant energy shocks of variable duration were simulated to evaluate their influence on different quantities as the intracerebral stresses in terms of compression, tensile, and shearing stresses, the relative brain-skull displacement, and the skull deformation. The importance of modal behavior of the head is illustrated by analyzing its temporal response to variable duration impacts, thus exciting very different frequencies. For a triangular shock, the critical duration times are between 10 and 15 x 10(-3) s, which correspond to impacts that excite the first resonance frequency of the head. Taking modal behavior into consideration in developing the finite element model leads to a harmonization of the calculated intracerebral stresses, even for short duration shocks. So, when the head is considered as a complex structure made up of several deformable elements, risk limitation is conditioned by an impact energy reduction for frequencies close to the natural frequencies of the structure. In the time field, the objective will be to avoid a number of impact shapes and durations. Therefore, the aim will not be to dampen the impact at any cost, but to damper it in an "intelligent" manner. In the future, this will allow the reduction of an injury mechanism-related risk, without increasing the risk of an injury generated by another mechanism.

Biomechanical Phenomena↗
PubMed1994

Mechanisms of brain injury related to mathematical modelling and epidemiological data.

Measurements of the frequency response of head impact points on the exterior and the interior of a car were used to characterize the dynamic behavior of the object that was struck. These points were then arranged in a hierarchy of increasing stiffness. Thirty-two cases in which the distribution of injury to the brain had been recorded were grouped according to the stiffness of the object struck and by the location of the impact on the head. The distribution of the brain lesions were determined for each class of stiffness and location of impact. Three probable mechanisms of brain injury were distinguished: relative motion between the brain and the skull, local bone deformation, and intracerebral stresses. Each mechanism was related to a range of stiffness and natural frequency of the structure impacted. These theories of brain injury mechanisms are consistent with observed epidemiological data and with conclusions drawn from mathematical modelling.

Accidents, Traffic↗
PubMed1990

In vivo determination of long bone flexibility--the epiphyses influence.

A lot of studies are devoted to the in vivo vibration analysis of long bones. In general they compare mechanical parameters relative to two instrumented bones or to one bone considered in different configurations. In this study a method of E. I product numerical determination is presented including for the first time the epiphysis influence in the calculation. The calculation of the maximum bending yield limit of the bone is carried out using this parameter. A method is proposed to determine the ultimate bending moment and the bone in vivo rigidity.

Biomechanical Phenomena↗
PubMedDate not supplied

Three-dimensional human head finite-element model validation against two experimental impacts.

The impact response of a three-dimensional human head model has been determined by simulating two cadaver tests. The objective of this study was to validate a finite-element human head model under different impact conditions by considering intracranial compressibility. The current University Louis Pasteur model was subjected initially to a direct head impact, of short (6 ms) duration, and the simulation results were compared with published experimental cadaver tests. The model response closely matched the experimental data. A long duration pulse was chosen for the second impact and this necessitated careful consideration of the head-neck joint in order to replicate the experimental kinematics. The skull was defined as a rigid body and was subjected to six velocities. Output from the model did not accurately match the experimental results and this clearly indicates that it is important to validate a finite-element head model under various impact conditions to define the range of validity. Lack of agreement for the second impact is attributed to the nonlinearity in the dynamic behavior of intracranial stress, a problem that is not reported in the literature.

Acceleration↗