Jan Vychytil

The simple model of cell prestress maintained by cell incompressibility

Living cells are reinforced by polymer fibers (the so-called cytoskeleton) which are responsible for their mechanical behaviour. There are many evidences that these fibres are prestressed without an external load. To include this prestress into mechanical models of living tissues is not an easy task. We propose an approach in which the intracellular prestress is maintained by the incompressibility of cells. A simple illustrative structure is studied in order to determine the dependence of stiffness on the level of prestress. Some macroscopic models of living tissues with prestressed cells are formulated. The results show a clear dependence of the macroscopic mechanical response on the level of prestress at microscale. The model exhibits some features of living cells (prestress-induced stiffening, strain hardening).

Novel Approach in Vehicle Front-End Modeling for Numerical Analyses of Pedestrian Impact Scenarios

In this paper a novel approach in developing a simplified model of a vehicle front-end is presented. Its surface is segmented to form an MBS model with hundreds of rigid bodies connected via translational joints to a base body. Local stiffness of each joint is calibrated using a headform or a legform impactor corresponding to the EuroNCAP mapping. Hence, the distribution of stiffness of the front-end is taken into account. The model of the front-end is embedded in a whole model of a small car in a simulation of a real accident. The VIRTHUMAN model is scaled in height, weight and age to represent precisely the pedestrian involved. Injury risk predicted by simulation is in correlation with data from real accident. Namely, injuries of head, chest and lower extremities are confirmed. Finally, mechanical response of developed vehicle model is compared to an FE model of the same vehicle in a pedestrian impact scenario. VIRTHUMAN model of a 13-year-old boy (150 cm, 40 kg) is chosen to represent the pedestrian and the lateral impact at 45 km/h is considered for various initial positions of pedestrian. While local deformation of the MBS bonnet leads to the HIC value corresponding to the EuroNCAP assessment, prediction obtained in the case of FE model may differ. This reflects the fact that deformation of the bonnet caused by an impact of the torso may influence the shape and the stiffness of the bonnet at the location of head strike

GRID CONTINUUM DESCRIPTION: A NOVEL APPROACH IN TWO-SCALE HYPERELASTICITY

In this paper, a generalization of the standard continuum theory is proposed in order to describe materials with a more complex microstructure. The key idea consists in dividing the studied body onto a set of small but finite disjunct cells whose boundaries form what is called the grid. The state of the grid is described by macroscopically smooth functions. The interior of each cell, on the other hand, is described by an additional field that may be discontinuous or highly oscillating. Such an approach allows us to include non-trivial effects of the microstructure in resulting material models of macroscopic bodies. The theory is illustrated on two examples of two-scale hyperelastic models. The first one represents a crystalline material with a failure of the Cauchy-Born rule. The second one, motivated by an arrangement of soft tissues, includes prestress at the reference state as well as non-trivial response caused by a collapse of micro-constituents. Transparent physical meaning of several material parameters of the latter model provides for their direct identification instead of the least-squares fitting.