Nicolas Bourdet

Experimental in vitro mechanical characterization of porcine Glisson's capsule and hepatic veins.

Understanding the mechanical properties of human liver is the most critical aspect of numerical modeling for medical applications and impact biomechanics. Many researchers work on identifying mechanical properties of the liver both in vivo and in vitro considering the high liver injury percentage in abdominal trauma and for easy detection of fatal liver diseases such as viral hepatitis, cirrhosis, etc. This study is performed to characterize mechanical properties of individual parts of the liver, namely Glissons capsule and hepatic veins, as these parts are rarely characterized separately. The long term objective of this study is to develop a realistic liver model by characterizing individual parts and later integrating them. In vitro uniaxial quasi-static tensile tests are done on fresh unfrozen porcine hepatic parts for large deformations at the rate of 0.1mm/s with a Bose Electroforce 3200 biomaterials test instrument. Results show that mean values of small strain and large strain elastic moduli are 8.22 ± 3.42 and 48.15 ± 4.5 MPa for Glissons capsule (30 samples) and 0.62 ± 0.41 and 2.81 ± 2.23 MPa for veins (20 samples), respectively, and are found to be in good agreement with data in the literature. Finally, a non-linear hyper-elastic constitutive law is proposed for the two separate liver constituents under study.

Head impact conditions in the case of cyclist falls

In 2009 approximately half of the French population owned a bicycle. However, the cyclist’s accident rate is the highest of all road users. Hence, it is necessary to set up a protection system for cyclists, especially for the cephalic segment. Currently, relatively little literature has dealt with the head impact condition for this kind of accident, especially for cases of cyclist falls. Therefore, the objective of this work was to identify the initial conditions for head impact in cases of cycling fall accidents. The present paper proposes a parametric study by simulating a lot of accident scenarios. A total of 1024 simulations have been automatically carried out using Madymo®’s software and a specially designed program. Two situations of cycling falls have been investigated according to real accident configuration: cyclist falls due to skidding and after hitting a curb. The TNO (Dutch Organization for Applied Scientific Research, Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek) pedestrian 50th percentile human model was coupled to a city bicycle model. The analysed outputs are head impact area and head velocity before impact. The work has provided a solid information base for future work on cyclist accidents. The parametric analysis was also used to study the effects of poorly known environmental parameters, such as speed or torso inclination. The results provided estimates of the impact area and speed of the head, which helps to improve the design of safer helmets and of helmet certification standards tests.

Development and validation of a coupled head-neck FEM - application to whiplash injury criteria investigation

The objective of the present study is to improve the understanding of whiplash injury mechanisms based on the extensive numerical simulation of real-world rear impact accidents with a detailed head–neck finite-element model (FEM). Based on an existing FE neck model and on an existing whiplash accident database including crash pulse recording, the present study proposes an in-depth investigation of the neck response at tissue level with the objective to extract pertinent ‘intra-cervical’ parameters presenting high correlation with the occurrence of whiplash injury.

Bicycle helmet modelling and validation under linear and tangential impacts

In the context of head protection against traumatic brain injuries, this work attempts to understand the degree of protection offered by a commercial bicycle helmet under both linear and oblique impact conditions. In accordance with EN 1078 standard, an experimental program has been carried out on an existing helmet by performing 90 normative impacts. In parallel, a finite element model (FEM) of this helmet has been developed and implemented under the LS-DYNA® crash code to numerically reproduce the 90 experimental standard impact tests. Experimental oblique impacts on this commercial helmet have also been performed in order to validate it and to be able to assess its protection capability in case of tangential impact. Finally, the bicycle helmet FEM has been coupled to a Hybrid III dummy head FEM and oblique impacts have been reproduced numerically leading to a realistic behaviour of the helmet model in terms of rotational accelerations.

Behaviour of helmets during head impact in real accident cases of motorcyclists

A motorcyclist is a vulnerable road user and the most often injured body segment is the head. The present study deals with motorcyclists head protection and is focused on the behaviour of the helmet during simulated head impact in real accident cases. The analysis of real-world motorcyclist head trauma can be divided into three steps, i.e. accidentology to collect several real accident cases, kinematics reconstruction to obtain the initial conditions of the head just before the impact and helmeted head impact simulation to evaluate the behaviour of the helmet during impact and to assess the head injury risk. For each accident case, body kinematics has been simulated using MADYMO software. The initial condition, in terms of velocity and impact location, of the head just before the impact was then extracted and implemented in the finite element model of the human head coupled with a validated finite element model of a helmet in order to reconstruct the head impact. This impact analysis on the helmeted head allowed us to highlight the injury mechanisms specific to motorcyclists especially in terms of helmet and head loading. In this study, the relative velocities between head and impacted structure vary from 25 to 60 km/h with significant tangential components. The deformations of the constituting helmets material vary from 17% to 90% for this rather low-energy impacts, demonstrating that improvement in material behaviour is needed in order to increase survival chances in the case of impact speeds above 30 km/h.

Experimental and finite element analysis for prediction of kidney injury under blunt impact

Kidneys are third most injured organs in abdominal trauma after liver and spleen; this study therefore is an attempt to understand the behaviour of kidneys under blunt trauma. Dynamic impact tests were performed on 20 fresh porcine kidneys to study the injury propagation in the organ, and the acceleration of the impactor was measured. A kidney model was developed with structural details like capsule and cortex. The kidney cortex was modelled with solid hexahedral elements and the capsule was modelled with quadratic shell elements. The material models for the capsule and cortex were used from the experimental data reported in our previous study. The developed model was calibrated using previous and current experimental results to reproduce the injuries of the organ in terms of acceleration of the impactor, and the injuries sustained by the organ during the experiments. The developed kidney model is observed to be robust and can be integrated with the available human body finite element models to simulate accidents and to predict or simulate injuries.

Improvement of Q0 dummy restraint in lateral sled impacts regarding R129 criteria

ABSTRACT Newborns are one of the most vulnerable car occupants in road accident. Collisions lead to serious head and neck injuries especially in side impact. This work consists in improving child restraint systems (CRS) efficiency in a regulatory context. Two consecutive Design of Experiments (DoEs) have been conducted in which Q0 dummy head, shoulder and pelvis were independently restrained by calibrated absorbers. The first study aims to understand interactions relative to Q0 dummy in side impact. A localised impact wall constituted of instrumented columns has been developed prior to experiments. The second DoE attempts to numerically design the best energy absorption materials to integrate in CRS lateral wings. For that purpose, a simplified finite element model of the experimental device has been developed, and then validated under Radioss® explicit code. Column stiffnesses were changed for all the 250 simulations with the objective to minimise both Head Performance Criterion (HPC) and head acceleration criteria.

Comparison of two child seats subjected to a regulatory side impact regarding biomechanical criteria associated to the newborn

ABSTRACT The new Regulation 129 is a keystone in child safety as child restraint systems are now evaluated in side impact tests with Q-dummies. Research works have, nevertheless, shown that regulatory assessment criteria are not well correlated with head injuries. As a consequence, the present study aims to compare two child seat versions regarding biomechanical criteria associated to the anatomical head–neck system of the newborn. The main results show that the enhanced child seat leads to significative lower brain loading under regulatory test configuration.

Whiplash Injury Criteria Investigation with a Global Head-neck Torso Model

In this study, a minimum complexity head-neck-thorax model validated in the frequency domain was coupled to a car seat-head rest complex on Madymo software to establish more accurate neck injury criteria and associated tolerance limits. A total of 87 accident cases were then simulated using three seats. Several injury criteria, such as Neck Fx, Neck Fz, T1 acceleration, Neck Injury Criterionmax, Nkm and Neck Displacement Criterion, were calculated and correlated to the risk of AIS1 neck injury. A similar work has been done with the BioRID II model to compare the predictive risk curves obtained on both models. This comparison was expressed in terms of Nagelkerke R² values obtained with these analyses. It appears that the minimum complexity head-neck-thorax model gives a higher correlation than the BioRID II model for all parameters and that the lower neck axial force is shown as the best candidate to correlate with the neck injury.

Computation of axonal elongations: towards a new brain injury criterion

Finite element head models are a well-known way to describe the intracerebral mechanical behaviour during a head impact and, hence, to develop diffuse axonal injury (DAI) prediction tools. Owing to the lack of experimental data, the brain model is usually considered as homogeneous and isotropic. This hypothesis is acceptable from a mechanical point of view, but the classical computed metrics are not adequate according to the varying vulnerability of the neurological cells in the brain. The present study aims to show that the brain axonal orientations map provided by diffusion MRI can be combined with the computed strains in order to evaluate axonal elongations. This new metric seems to be more representative of the DAI mechanisms, which are known to be due to axon elongation.