Coupling of Deformable Rigid Bodies with Finite Elements to Simulate FMVSS Head Impact

Modal methods in linear dynamic analysis are classical techniques to compute the response of structures. The mechanical idea behind is to find a particularly useful problem dependent basis (e.g. so-called eigenfunctions or eigenmodes) which can be separated into some of major importance for the behavior of the structure and others of minor interest (and subsequently disregarded). Unfortunately, this concept is basically a completely linear one. More recent versions of LS-DYNA offer the possibility to combine shape functions from a previously computed modal basis with parts modeled directly with finite element methods. This allows taking into account at least some of the overall elastic behavior of a body with fairly little effort. Other parts of the model which are of specific interest are then computed with standard explicit FE analysis. Hence, the combination allows on one side a substantial reduction of the number of degrees of freedom and on the other side parts of the system can be still computed in a fully nonlinear manner. The implementation of the method into LSDYNA has been described in [1] and was subsequently applied to two different practical problems, head impact and a deep drawing, in [2]. In the present paper, we focus on more realistic head impact problems, modeled with several different parts. We examine how to treat the connections between these parts in a coupled eigenmode/explicit analysis. In addition, we consider the usage of so-called constraint modes as an alternative modal basis, too.