CAE-Companion-2018-2019

Safety WISSEN CAE

Model Based Head Injury Criteria for Head Protection Optimization - SUFEHM

Introduction The head and more specifically the brain is among themost vital organs of the human body. Over the past forty years, a slant has been put by the biome- chanical research on the understanding of the head injury mechanisms. Nevertheless, an injury is always a consequence of an exceeded tissue tolerance to a specific loading. Even if lo- cal tissue tolerance has very early been investigated, the global acceleration of the impacted head and the impact duration are usually being used as impact severity descriptors. TheWayne State University Tolerance Curve has therefore been proposed since the early Sixties thanks to several works by Lissner et al. (1960) [1] and Gurdjian et al. (1958) [2]. This curve shows the link between the impact of the head described by the head acceleration and the impact duration and, on the other hand the head injury risk. Hence, after thework of Gadd (1966) [3],

Fully documented head impact cases canbe simulated inorder to compute themechanical loadings sustained by the head tissues and to compare it to the real injuries described in the medical reports. It has for example been shown in Zhou et al. (1996) [5], Kang et al. (1997) [6] andmore recently in King et al. (2003) [7], Kleiven et al. (2007) [8] andDeck et al. (2008) [9] that the brain shear stress and strain rates predictedby their Finite El- ement HeadModels agreewith the location and the severity of the axonal injuries described in themedical report. Since these finite element headmodels exist, new injury prediction tools basedon the computed intracranial loadings become available for protective systems design. Human HeadModel Development and Validation The proposed head geometry is based on a human skull which has been digitized externally and internally. Membranes such as falx and tentoriumare based on anatomic atlas and a brain-

Figure 1 : SUFEHM (Strasbourg University FE Head Model): skull, CSF, membranes, brain

the National Highway Traffic Safety Administration (NHTSA) proposed the Head Injury Criterion (HIC) in 1972. This is the tool used nowadays in safety standards for the head protection systems using headforms. Since it is based solely on the global

skull interface of 2mm thickness has been considered in order to represent the CSF. Brain, CSF and scalp aremodeledwith brick elements. As a function of application, three approaches exist for the skull model, i.e. a rigid skull, a frangible and de-

Figure 2 : Illustration of the children head and neck FE models with specific structural and geo- metrical characteristics of the 6 weeks, 6 months, 1,3,6 years old.

linear resultant acceleration of a singlemass headmodel, some limitations of this empiric criterion arewell-known, such as the fact that it is not specific to direction of impact and that it neglects the angular accelerations. Aproposed alternativemethod for assessing head injury risk is to use a humanhead Finite ElementModel (FEM), which can en- able the investigationof the intra-cranial response under impact conditions. Thismethod iswell known since 1975whenone of the first three dimensional modelswas developedbyWard et al [4]. Thismethod thereby leads to addeduseful mechanical observableswhich shouldbe closer to the descriptionof known injurymechanisms. Hence, new injury criteria canbe proposed. In the last decades, more than tendifferent three dimensional finite element headmodels have been reported in the literature.

formable skull modeled by a three layered composite structure with constant thickness and finally a detailed skull description with non-constant thickness and taking into account the anatomical reinforcement beams. Figure 1 illustrates the skull, the CSF, themembranes and the brain structure of Stras- bourg University FE HeadModel (SUFEHM). The constitutive laws implemented under LS-DYNA for the different parts of the human head are reported in Deck et al 2008 [9] for the material supposed to be elastic (scalp, CSF, membranes, face), and visco-elastic brainmaterial for the composite frangible elasto-plastic behavior of the skull. In order to ensure that this mechanical headmodel presents a realistic response under impact it was validated against data reported in the literature as reported in [9]. Validation focused on intra-cerebral pres-

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