Jshm Jac Wismans

The influence of test conditions on characterization of the mechanical properties of brain tissue.

To understand brain injuries better, the mechanical properties of brain tissue have been studied for 50 years; however, no universally accepted data set exists. The variation in material properties reported may be caused by differences in testing methods and protocols used. An overview of studies on the mechanical properties of brain tissue is given, focusing on testing methods. Moreover, the influence of important test conditions, such as temperature, anisotropy, and precompression was experimentally determined for shear deformation. The results measured at room temperature show a stiffer response than those measured at body temperature. By applying the time-temperature superposition, a horizontal shift factor a(T)=8.5-11 was found, which is in agreement with the values found in literature. Anisotropy of samples from the corona radiata was investigated by measuring the shear resistance for different directions in the sagittal, the coronal, and the transverse plane. The results measured in the coronal and the transverse plane were 1.3 and 1.25 times stiffer than the results obtained from the sagittal plane. The variation caused by anisotropy within the same plane of individual samples was found to range from 25% to 54%. The effect of precompression on shear results was investigated and was found to stiffen the sample response. Combinations of these and other factors (postmortem time, donor age, donor type, etc.) lead to large differences among different studies, depending on the different test conditions.

A finite element model of the human buttocks for prediction of seat pressure distributions.

Seating comfort is becoming increasingly important for the automotive industry. Car manufacturers use seating comfort to distinguish their products from those of competitors. However, the development and design of a new, more comfortable seat is time consuming and costly. The introduction of computer models of human and seat will accelerate this process. The contact interaction between human and seat is an important factor in the comfort sensation of subjects. This paper presents a finite element (FE) model of the human buttocks, able to predict the pressure distribution between human and seating surface by its detailed and realistic geometric description. A validation study based on volunteer experiments shows reasonable correlation in pressure distributions between the buttocks model and the volunteers. Both for simulations on a rigid and a soft cushion, the model predicts realistic seat pressure distributions. A parameter study shows that a pressure distribution at the interface between human and seat strongly depends on variations in human flesh and seat cushion properties.

THE INFLUENCE OF MUSCLE ACTIVITY ON HEAD-NECK RESPONSE DURING IMPACT

In the past, muscle activation has been identified as having an important effect on the head-neck response in dynamic conditions. However, this claim has been largely based on global observations, and not by accurate analysis. In this study, the influence of muscle activation on the head-neck response is investigated by mathematical modeling. The detailed mathematical head-neck model presented by M. de Jager (See IRRD 891656 and 895764) is improved by modeling the neck muscles in more detail. A multi-segment muscle description is applied in which the muscles curve around the vertebrae, resulting in realistic muscle lines of action. The model is validated with human volunteer responses to frontal and lateral impact at several severities. The model response with maximum muscle activation to high severity frontal and lateral impacts agrees well with volunteer responses, whereas a submaximum activation level or a larger reflex delay provides better results for the low severity impacts. The simulations show that muscle contraction has a large influence on the head-neck response. (A) For the covering abstract of the conference see IRRD E201172.

A mathematical human body model for frontal and rearward seated automotive impact loading

Mathematical modelling is widely used for crash-safety research and design. However, most occupant models used in crash simulations are based on crash dummies and thereby inherit their apparent limitations. Several models simulating parts of the real human body have been published, but only few describe the entire human body and these models were developed and validated only for a limited range of conditions. This paper describes a human body model for both frontal and rearward loading. A combination of modelling techniques is applied using rigid bodies for most body segments, but describing the thorax as a flexible structure. The skin is described in detail using an arbitrary surface. Static and dynamic properties of the articulations have been derived from literature. The RAMSIS anthropometric database has been used to define a model representing a 50th percentile male. The model has been validated using volunteer tests performed at NBDL ranging from 3-15 G severity, and using established dummy biofidelity requirements for blunt thoracic impact. A satisfactory prediction has been obtained for chest deflections, head kinematics and accelerations and for kinematics and accelerations at the upper thoracic vertebra. Recommendations are given for further development and validation of the model, and for validation of models of different body sizes.

Characterisation of the mechanical behaviour of brain tissue in compression and shear.

No validated, generally accepted data set on the mechanical properties of brain tissue exists, not even for small strains. Most of the experimental and methodological issues have previously been addressed for linear shear loading. The objective of this work was to obtain a consistent data set for the mechanical response of brain tissue to either compression or shear. Results for these two deformation modes were obtained from the same samples to reduce the effect of inter-sample variation. Since compression tests are not very common, the influence of several experimental conditions for the compression measurements was analysed in detail. Results with and without initial contact of the sample with the loading plate were compared. The influence of a fluid layer surrounding the sample and the effect of friction were examined and were found to play an important role during compression measurements.To validate the non-linear viscoelastic constitutive model of brain tissue that was developed in Hrapko et al. (Biorheology 43 (2006), 623-636) and has shown to provide a good prediction of the shear response, the model has been implemented in the explicit Finite Element code MADYMO. The model predictions were compared to compression relaxation results up to 15% strain of porcine brain tissue samples. Model simulations with boundary conditions varying within the physical ranges of friction, initial contact and compression rate are used to interpret the compression results.

HUMAN HEAD-NECK RESPONSE DURING LOW-SPEED REAR END IMPACTS

Neck injuries resulting from rear-end collisions rank among the top car safety problems and have serious implications for society. Many rear impact sled experiments with volunteers and PMHSs have been performed in the past. However, in most of these studies, T1 kinematics were not obtained so that the kinematic behavior of the neck could not be separated from the motion of the rest of the spine. Also, to the best knowledge of the authors, the effect of anthropometric parameters on the head-neck kinematics was not studied before. The objective of this study is to describe the kinematic response of the head-neck system during low severity rear end impacts. In addition, the effect of anthropometric parameters such as height, weight and neck circumference was investigated. For this purpose, a total of 43 tests with 19 subjects was performed. Values for (delta)V ranged between 6,5 and 9.5 km/h. Linear accelerations of the head-CG and the first thoracic vertebra (T1) and angular accelerations of the head were obtained. Head angle and head-CG trajectories were obtained from film targets. Finally, head restraint impact forces were measured using a strain gauge attached to the support rods of the head restraint. Trajectories of the occipital condyles (OC trajectories) as well as upper neck forces and moments were calculated. All measured and calculated kinematic data were presented in response corridors representing the mean +/- one standard deviation. Although only three females participated in this study, a marked increase in head x-acceleration was observed for the females compared to the males. Also, neck circumference correlated well with peak x-accelerations: a thinner neck resulted in higher values for the x-accelerations. The results of this study can be used for evaluation of biofidelity of crash dummy necks, and for validation of mathematical head-neck models. Also, our finding that thinner necks result in higher head peak accelerations may be a partial answer to the question why women are at higher risk for whiplash injuries compared to men.

A Global and a Detailed Mathematical Model for Head-Neck Dynamics

A detailed three-dimensional mathematical model describing the dynamic behavior of the human head and neck in automotive accidents is developed. The strategy is to proceed from a relatively simple model (the global model) for gaining insight into head-neck dynamics toward a complex model (the detailed model) providing the loads and deformations of the tissues of the neck. The models have been implemented in the integrated multibody/finite-element package MADYMO, version 5.1.1 of the TNO Crash Safety Research Centre. Language: en

Estimation of spinal loading in vertical vibrations by numerical simulation

OBJECTIVE This paper describes the prediction of spinal forces in car occupants during vertical vibrations using a numerical multi-body occupant model. BACKGROUND An increasing part of the population is exposed to whole body vibrations in vehicles. In literature, vertical vibrations and low back pain are often related to each other. The cause of these low back pains is not well understood. A numerical human model, predicting intervertebral forces, can help to understand the mechanics of the human spine during vertical vibrations. METHODS Numerical human and seat models have been used. Human model responses have been validated for vertical vibrations (rigid and standard car seat condition): simulated and experimental seat-to-human frequency response functions have been compared. The spinal shear and compressive forces have been investigated with the model. RESULTS The human model seat-to-pelvis and seat-to-T1 frequency response functions in the rigid seat condition and all seat-to-human frequency response functions in the standard car seat condition approach the experimental results reasonably. The lumbar and the lower thoracic spine are subjected to the largest shear and compressive forces. CONCLUSIONS The human model responses correlate reasonable with the volunteer responses. The predicted spinal forces could be used as a basis for derivation of hypothetical mechanisms and better understanding of low back pain disorders. RELEVANCE In order to solve the problem of whole body vibration related injuries, knowledge about the interaction between human spinal vertebrae in vertical vibrations is required. This interaction cannot be measured in volunteer experiments. This paper describes the application of a numerical human model for prediction of spinal forces, that could be used as a basis for derivation of hypotheses regarding low back pain disorders.

Optical characterization of acceleration-induced strain fields in inhomogeneous brain slices

The aim of this study was to measure high-resolution strain fields in planar sections of brain tissue during translational acceleration to obtain validation data for numerical simulations. Slices were made from fresh, porcine brain tissue, and contained both grey and white matter as well as the complex folding structure of the cortex. The brain slices were immersed in artificial cerebrospinal fluid (aCSF) and were encapsulated in a rigid cavity representing the actual shape of the skull. The rigid cavity sustained an acceleration of about 900m/s(2) to a velocity of 4m/s followed by a deceleration of more than 2000m/s(2). During the experiment, images were taken using a high-speed video camera and Von Mises strains were calculated using a digital image correlation technique. The acceleration of the sampleholder was determined using the same digital image correlation technique. A rotational motion of the brain slice relative to the sampleholder was observed, which may have been caused by a thicker posterior part of the slice. Local variations in the displacement field were found, which were related to the sulci and the grey and white matter composition of the slice. Furthermore, higher Von Mises strains were seen in the areas around the sulci.

Computational Human Body Models

Computational human body models are widely used for automotive crashsafety research and design and as such have significantly contributed to a reduction of traffic injuries and fatalities. Currently crash simulations are mainly performed using models based on crash-dummies. However crash dummies differ significantly from the real human body and moreover crash dummies are only available for a limited set of body sizes. Models of the real human body offer some promising advantages including the prediction of injury mechanisms and injury criteria. In this paper, a review will be given of a number of developments in the field of occupant crash simulations in the past 40 years. Topics presented include history of occupant crash simulation codes, human body geometry, human body material modelling and model quality rating. A discussion on foreseen future developments in this field will conclude this paper.