Intracranial Response in Helmet Oblique Impacts

Transcription

1 Intracranial Response in Helmet Oblique Impacts M. Ghajari 1, U. Galvanetto 2, L. Iannucci 3, R. Willinger 4 Abstract The purpose of this work was to investigate the influence of the presence of the body in helmet oblique impacts on the tissue level response of the human head. The kinematic field of the head obtained from full body and detached head oblique impacts were used to drive a validated Finite Element model of the human head and evaluate predictors of the diffuse axonal injury (Von Mises stress in the brain) and subdural haematoma (internal energy of the Cerebro Spinal Fluid (CSF)). It was found that the presence of the body considerably influences the intracranial response. To take into account this effect in detached head impacts, inertial properties of the head were modified. When the head mass was increased, the Von Mises stress in the brain and the internal energy of the CSF decreased for impacts with relatively high tangential velocities. Modifying the head inertia matrix lowered the Von Mises stress in the brain and the internal energy of the CSF. The best correlation between the modified detached head and full body impact results was found when both the head mass and its inertia matrix were modified. Keywords helmet, impact, intracranial response, standard test I. INTRODUCTION According to current helmet standards (e.g. ECE 22.05), the impact absorption capability of a helmet is assessed by dropping the helmet fitted onto a headform on a rigid anvil. Physical properties of the headforms are similar to those of the human head, which implies that this test method relies on the assumption that the effect of the rest of the body on impact outputs is negligible. However, previous studies have shown that the presence of the body influences the head acceleration. Furthermore, it increases the crushing distance of the liner, which may lead to complete bottoming out of the liner. To include these effects in headform impacts, increasing the mass of the head and modifying its inertia matrix were found to be effective measures [1, 2]. Kinematic head injury predictors are usually based on the assumption that either linear acceleration or rotational acceleration is the main cause of head injury while it has been shown that their combination increases the injury risk [3]. Furthermore, they are not capable of distinguishing between injuries of head tissues, e.g. diffuse axonal injury (DAI) and subdural haematoma (SDH). These drawbacks have formed a tendency among researchers to use head injury predictors which are based on the tissue level response of the head rather than on its kinematics. This can be achieved by using calibrated FE models of the human head. This paper is dedicated to the evaluation of predictors of DAI and SDH in full body and detached head oblique impacts reported in [2], which are outlined in the next section. It investigates the influence of the presence of the body on tissue level head injury predictors. In addition, the suitability of the proposed methods to take into account the effect of the body on the kinematics of the head is examined with respect to the intracranial response. II. METHODS A validated FE human head model available at Strasbourg University (called SUFEHM) was used to evaluate the intracranial response in helmet oblique impacts. This model includes almost all biomechanically important parts of the head, including the scalp, skull, brain, CSF, tentorium and falx. To calibrate the LS DYNA format of the head model, fifty eight well documented real world accidents were simulated [4]. Injury risk curves were 1 Research associate in the Aeronautics Department, Imperial College London, UK (m.ghajari@imperial.ac.uk, tel: , fax: Prof of aircraft structures, Department of Structural and Transportation Engineering, University of Padova, Italy. 3 Reader in advanced structural design, Aeronautics Department, Imperial College London, UK. 4 Prof of biomechanics, CNRS, Strasbourg University, France

2 plotted for moderate DAI (coma duration < 24 hours) and severe DAI (coma duration > 24 hours) with respect to the maximum Von Mises stress and first principal strain in the brain; the best correlation was found for the Von Mises stress. The values corresponding to 50% risks of mild and severe DAI were 28 kpa and 53 kpa respectively. SDH was best predicted by the internal energy of the CSF, with a 4950 mj threshold for a 50% injury risk. The head model was used to evaluate the predictors of DAI and SDH for some of the oblique impacts simulated in [2]. Oblique impacts were chosen since they induced substantial rotational acceleration and are more probable to occur in real world accidents. For full body impacts, the THUMS human body model equipped with the FE model of a commercially available helmet was employed. The contact defined at the head/helmet interface was validated through simulating helmeted Hybrid II headform oblique impacts and comparing the results with experimental data [5]. Good agreement was found with respect to the rotational acceleration. The impact occurred at the side of the helmet and the body impact angle was 30, as shown in Fig. 1. The normal and tangential impact velocities (respectively V N and V T ) are presented in Table 1 and a code is assigned to them. As reported in [2], the maximum head acceleration ( a max ) varied between 143 g to 230 g and the maximum head rotational acceleration ( α max ) between 8 krad/s 2 to 15.8 krad/s 2. Fig. 1 Oblique impact; body impact angle and impact velocity components. The intracranial parameters were evaluated for full body (F) and detached head (DH) impacts. In addition, three modified versions of the THUMS s detached head were used to simulate the same impacts and calculate the predictors of DAI and SDH. They were: MDH1) the mass of the head was increased by 20% corresponding to a body impact angle of 30. This modification includes the effect of the body on crushing distance of the liner and a max, as explained in [5]. MDH2) the moments of inertia around the x, y, and z ISO axes were respectively multiplied by 1.12, 1.23 and 1.36, corresponding to the side/30 oblique impact; this includes the effect of the body on α max, as explained in [2]. MDH3) both modifications mentioned above were applied to the head. Table 1 Helmet oblique impacts; side/30 V N (m/s) V T (m/s) code To calculate the intracranial parameters, the skull of the SUFEHM was switched to rigid and constrained to an inertia part located at the centre of gravity of the head. Inertial properties of the skull were defined for this part. Three linear and three rotational acceleration components of the THUMS head vs. time, which were known from the oblique impacts, were prescribed for this part. III. RESULTS The maximum values of the evaluated intracranial parameters are plotted in Fig. 2. The maximum Von Mises stress in the brain and the internal energy of the CSF are lower in full body impacts as compared to detachedhead impacts. This is probably related to the difference in head accelerations. When the mass of the head was increased (MDH1), the Von Mises stress decreased but it was still far from the full body impact results for C2 and C4 cases. In these cases, α max was high as compared to C1 and C3. The internal energy of the CSF of MDH1-91 -

3 agrees well with the full body data. Modifying the head inertia matrix (MDH2) lowered the Von Mises stress in the brain and the internal energy of the CSF. The best correlation between the modified detached head and full body results was found when both the head mass and its inertia matrix were modified (MDH3). Von Mises stress in brain (kpa) / 53 kpa F DH MDH1 MDH2 MDH3 Internal energy of CSF (mj) / 4950 mj F DH MDH1 MDH2 MDH Fig. 2 Maximum values of intracranial parameters in the oblique impacts. To explore the influence of linear and rotational accelerations on Von Mises stress in the brain and the internal energy of the CSF, they were calculated when translational, rotational and complete kinematics of the head in full body impacts were used to drive the SUFEHM; the results are shown in Fig. 3. This figure indicates that both linear and rotational accelerations had substantial contributions to DAI. For C2 and C4, the contribution of rotational acceleration was greater as compared to C1 and C3, respectively. This is due to an increase in α max while a max remained constant. When translational and rotational motions were considered together, the probability of DAI increased, which confirms the results of a previous study conducted by Ueno and Melvin [3]. They found that if translational and rotational motions were combined, the limit of DAI had to be decreased trans. rot. trans. and rot trans. rot. trans. and rot. Von Mises stress in brain (kpa) internal energy of CSF (mj) Fig. 3 Intracranial parameters evaluated with translational, rotational and complete kinematics of the head. As shown in Fig. 3, the internal energy of the CSF, which is an indicator of SDH, was 6 to 10 times larger in pure translation than in pure rotation. Increasing V N from 5 m/s to 7.5 m/s caused a 38% increase in a max, which in turn nearly doubled the internal energy of the CSF. Therefore, this parameter was substantially dependent on the translational motion of the head rather than its rotation. This conclusion explains why increasing the head mass was much more effective in reducing the internal energy of the CSF than modifying the head inertia matrix (Fig. 2). IV. DISCUSSION By using the SUFEHM, predictors of DAI and SDH were evaluated for full body and detached head oblique impacts. It was found that the presence of the body has influenced these parameters (up to 25%), which is

4 consistent with the findings of Tinard et al. [6]. They simulated impact tests of a Hybrid III dummy and its detached head onto a car bonnet and drove the SUFEHM with the outputs. They found a large difference between the results but no method was proposed to include the effect of the body in isolated head impacts. In the current study, by modifying the mass and inertia matrix of the head, very good agreement was achieved between intracranial parameters in detached head and full body impacts; their difference was reduced to less than 8%. One of the limitations of this study is that only one impact configuration was modelled. In future, the proposed modifications should be evaluated for other common configurations. Both translational and rotational motions of the head had a considerable contribution to Von Mises stress in the brain, which disagrees with the hypotheses that either linear or rotational acceleration is the main mechanism of brain injury [7]. This finding seems to disagree as well with the results of a recent study by Kleiven [8]. He applied pure linear and pure rotational accelerations to the KTH head model. He found that maximum principal strain in the brain (the predictor of DAI for the KTHHM) was nearly 10 times higher for the rotational motion, while in the current study rotation and translation made similar contributions to the risk of DAI. In the present study, however, the head impact power (HIP) was not the same in translation and rotation in contrast to the Kleiven s study. For instance for C2, HIP = 40.3 kw for translation, while it was 5.8 kw for rotation. The simulation results indicate that at least in helmet impacts, the power transferred to the head by translation is much higher than the power transferred by rotation but their contributions to intracranial injuries are similar. Investigation of 61 pedestrian, motorcycle and football player accidents also indicated that the effect of rotation on increasing the risk of DAI is comparable to the effect of translation [9]. V. CONCLUSIONS It has been shown that the modifications to current headforms, which were proposed to include the effect of the body on the kinematics of the head, also take into account this effect on intracranial parameters. Hence, such a modified headform can be used to test helmets and subsequently more biofidelic tissue level injury predictors can be evaluated by driving an FE head model with the measured linear and rotational accelerations. VI. ACKNOWLEDGEMENT The work presented in this paper was completed within the research training network MYMOSA funded by a Marie Curie fellowship of the 6th framework programme of the EU under contract no. MRTN CT VII. REFERENCES [1] M. Ghajari, U. Galvanetto, L. Iannucci, and R. Willinger, Influence of the body on the response of the helmeted head during impact, International Journal of Crashworthiness, vol. in press, [2] M. Ghajari, S. Peldschus, U. Galvanetto, Z. Asgharpour, and L. Iannucci, Influence of the body on head rotational acceleration in motorcycle helmet oblique impact tests, Proceedings of IRCOBI Conference, Hanover, Germany, [3] K. Ueno and J. W. Melvin, Finite element model study of head impact based on Hybrid III head acceleration the effects of rotational and translational acceleration, Journal of Biomechanical Engineering Transactions of the ASME, vol. 117, pp , Aug [4] C. Deck and R. Willinger, Head injury prediction tool for protective systems optimisation, in 7th European LS DYNA Conference Salzburg, [5] M. Ghajari, The influence of the body on the response of the helmeted head during impact, Ph.D. dissertation, in Aeronautics Department: Imperial College London, [6] V. Tinard, C. Deck, F. Meyer, N. Bourdet, and R. Willinger, Influence of pedestrian head surrogate and boundary conditions on head injury risk prediction, International Journal of Crashworthiness, vol. 14, pp , [7] A. I. King, K. Yang, L. Zhang, and W. Hardy, Is head injury caused by linear or angular acceleration?, Proceedings of IRCOBI Conference, Lisbon, pp. 1 12, [8] S. Kleiven, Evaluation of head injury criteria using a finite element model validated against experiments on localized brain motion, intracerebral acceleration, and intracranial pressure, International Journal of Crashworthiness, vol. 11, pp , [9] C. Deck, D. Baumgartner, and R. Willinger, Influence of rotational acceleration on intracranial mechanical

5 parameters under accident circumstances, Proceedings of IRCOBI Conference, Netherlands, pp ,