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- Aare, Magnus, et al.
(författare)
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Evaluation of head response to ballistic helmet impacts using the finite element method
- 2007
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 34:3, s. 596-608
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Tidskriftsartikel (refereegranskat)abstract
- Injuries to the head caused by ballistic impacts are not well understood. Ballistic helmets provide good protection, but still, injuries to both the skull and brain occur. Today there is a lack of relevant test procedure to evaluate the efficiency of a ballistic helmet. The purpose of this project was (1) to study how different helmet shell stiffness affects the load levels in the human head during an impact, and (2) to study how different impact angles affects the load levels in the human head. A detailed finite element (FE) model of the human head, in combination with an FE model of a ballistic helmet (the US Personal Armour System Ground Troops' (PASGT) geometry) was used. The head model has previously been validated against several impact tests on cadavers. The helmet model was validated against data from shooting tests. Focus was aimed on getting a realistic response of the coupling between the helmet and the head and not on modeling the helmet in detail. The studied data from the FE simulations were stress in the cranial bone, strain in the brain tissue, pressure in the brain, change in rotational velocity and translational and rotational acceleration. A parametric study was performed to see the influence of a variation in helmet shell stiffness on the outputs from the model. The effect of different impact angles was also studied. Dynamic helmet shell deflections larger than the initial distance between the shell and the skull should be avoided in order to protect the head from the most injurious threat levels. It is more likely that a fracture of the skull bone occurs if the inside of the helmet shell strikes the skull. Oblique ballistic impacts may in some cases cause higher strains in the brain tissue than pure radial ones.
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| 3. |
- Aare, Magnus, et al.
(författare)
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Injury tolerances for oblique impact helmet testing
- 2004
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Ingår i: International Journal of Crashworthiness. - : Informa UK Limited. - 1358-8265 .- 1754-2111. ; 9:1, s. 15-23
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Tidskriftsartikel (refereegranskat)abstract
- The most frequently sustained severe injuries in motorcycle crashes are injuries to the head, and many of these are caused by rotational force. Rotational force is most commonly the result of oblique impacts to the head. Good testing methods for evaluating the effects of such impacts are currently lacking. There is also a need for improving our understanding of the effects of oblique impacts on the human head. Helmet standards currently in use today do not measure rotational effects in test dummy heads. However rotational force to the head results in large shear strains arising in the brain, which has been proposed as a cause of traumatic brain injuries like diffuse axonal injuries (DAI). This paper investigates a number of well-defined impacts, simulated using a detailed finite element (FE) model of the human head, an FE model of the Hybrid III dummy head and an FE model of a helmet. The same simulations were performed on both the FE human head model and the FE Hybrid III head model, both fitted with helmets. Simulations on both these heads were performed to describe the relationship between load levels in the FE Hybrid III head model and strains in the brain tissue in the FE human head model. In this study, the change in rotational velocity and the head injury criterion (HIC) value were chosen as appropriate measurements. It was concluded that both rotational and translational effects are important when predicting the strain levels in the human brain.
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| 5. |
- Alvarez, Victor, et al.
(författare)
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Importance of neck muscle tonus in head kinematics during pedestrian accidents
- 2013
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Ingår i: 2013 IRCOBI Conference Proceedings - International Research Council on the Biomechanics of Injury. ; , s. 747-761
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Konferensbidrag (refereegranskat)abstract
- Unprotected pedestrians are an exposed group in the rural traffic and the most vulnerable human body region is the head which is the source of many fatal injuries. This study was performed to gain a better understanding of the influence that the neck muscle tonus has on head kinematics during pedestrian accidents. This was done using a detailed whole body FE model and a detailed FE vehicle model. To determine the influence of the muscle tonus a series of simulations were performed where the vehicle speed, pedestrian posture and muscle tonus were varied. Since the human reaction time for muscle activation is in the order of the collision time, the pedestrian was assumed to be prepared for the oncoming vehicle in order to augment the possible influence of muscle tonus. From the simulations performed, kinematic data such as head rotations, trajectory and velocities were extracted for the whole collision event, as well as velocity and accelerations at head impact. These results show that muscle tonus can influence the head rotation during a vehicle collision and therefore alter the head impact orientation. The level of influence on head rotation was in general lower than when altering the struck leg forward and backward, but in the same order of magnitude for some cases. The influence on head accelerations was higher due to muscle tonus than posture in all cases.
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| 6. |
- Alvarez, Victor S, et al.
(författare)
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Effect of pediatric growth on cervical spine kinematics and deformations in automotive crashes
- 2018
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Ingår i: Journal of Biomechanics. - : Elsevier. - 0021-9290 .- 1873-2380. ; 71, s. 76-83
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Tidskriftsartikel (refereegranskat)abstract
- Finite element (FE) models are a powerful tool that can be used to understand injury mechanisms and develop better safety systems. This study aims to extend the understanding of pediatric spine biomechanics, where there is a paucity of studies available. A newly developed and continuously scalable FE model was validated and scaled to 1.5-, 3-, 6-, 10-, 14- and 18-year-old using a non-linear scaling technique, accounting for local topological changes. The oldest and youngest ages were also scaled using homogeneous geometric scaling. To study the effect of pediatric spinal growth on head kinematics and intervertebral disc strain, the models were exerted to 3.5 g acceleration pulse at the T1 vertebra to simulate frontal, rear and side impacts. It was shown that the head rotation increases with age, but is over predicted when geometrically scaling down from 18- to 1.5-year-old and under predicted when geometrically scaling up from 1.5- to 18-year-old. The strain in the disc, however, showed a clear decrease with age in side impact and for the upper cervical spine in rear impact, indicating a higher susceptibility for neck injury at younger ages. In the frontal impact, no clear age dependence could be seen, suggesting a large contribution from changed facet joint angles, and lower levels of strain, suggesting a lower risk of injury. The results also highlight the benefit of rearward facing children in a seat limiting head lateral motion.
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| 7. |
- Alvarez, Victor S, et al.
(författare)
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Importance of Windscreen Modelling Approach for Head Injury Prediction
- 2016
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Ingår i: 2016 IRCOBI Conference Proceedings - International Research Council on the Biomechanics of Injury.
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Konferensbidrag (refereegranskat)abstract
- The objective of this study is to evaluate the capability of two modelling approaches in capturing both accelerations and deformations from head impacts, and to evaluate the effect of modelling approach on brain injury prediction. The first approach is a so‐called smeared technique, in which the properties of the two glass sheets and the intermediate polyvinyl butyral (PVB) are combined and divided into two coinciding shell layers, of which one can fracture. The second approach consists of three shell layers, representing the glass and PVB, separated by the distance of their thickness, using a non‐local failure criterion to initiate fracture in the glass. The two modelling approaches are compared to impact experiments of flat circular windscreens, measuring deformations and accelerations as well as accelerations from impacts against full vehicle windscreens. They are also used to study head‐to‐windscreen impacts using a detailed Finite Element (FE) model, varying velocity, impact direction and impact point. Only the non‐local failure model is able to adequately capture both the accelerations and deformations of an impactor. The FE head model simulations also reveal that the choice of modelling approach has a large effect on the both localisation of the strain in the brain and the characteristics of the strain‐time curve, with a difference in peak strain between 8% and 40%.
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| 8. |
- Alvarez, Victor, et al.
(författare)
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The Influence of Neck Muscle Tonus and Posture on Brain Tissue Strain in Pedestrian Head Impacts
- 2014
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191. ; 58, s. 63-101
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Tidskriftsartikel (refereegranskat)abstract
- Pedestrians are one of the least protected groups in urban traffic and frequently suffer fatal head injuries. An important boundary condition for the head is the cervical spine, and it has previously been demonstrated that neck muscle activation is important for head kinematics during inertial loading. It has also been shown in a recent numerical study that a tensed neck musculature also has some influence on head kinematics during a pedestrian impact situation. The aim of this study was to analyze the influence on head kinematics and injury metrics during the isolated time of head impact by comparing a pedestrian with relaxed neck and a pedestrian with increased tonus. The human body Finite Element model THUMS Version 1.4 was connected to head and neck models developed at KTH and used in pedestrian-to-vehicle impact simulations with a generalized hood, so that the head would impact a surface with an identical impact response in all simulations. In order to isolate the influence of muscle tonus, the model was activated shortly before head impact so the head would have the same initial position prior to impact among different tonus. A symmetric and asymmetric muscle activation scheme that used high level of activation was used in order to create two extremes to investigate. It was found that for the muscle tones used in this study, the influence on the strain in the brain was very minor, in general about 1-14% change. A relatively large increase was observed in a secondary peak in maximum strains in only one of the simulated cases.
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