A Local Isotropic/Global Orthotropic Finite Element Technique for Modeling the Crush of Wood in Impact Limiters

Wood is often used as the energy absorbing material in impact limiters, because it begins to crush at low strains, then maintains a. near constant crush stress up to nearly 60 percent volume reduction, and then \"locks up.\" Hill (Hill and Joseph, 197 4) has performed tests that show that wood is a.n excellent absorber. However, wood's orthotropic behavior for large crush is difficult to model. In the past, analysts have used isotropic foam-like material models for modeling wood. A new finite element technique is presented in this paper that gives a. better model of wood crush than the model currently in use. The orthotropic technique is based on locally isotropic, but globally orthotropic (LIGO) (Attaway, 1988) assumptions in which alternating layers of hard and soft crushable material are used. Each layer is isotropic; however, by alternating hard and soft thin layers, the resulting global behavior is orthotropic. In the remainder of this paper, the new technique for modeling orthotropic wood crush will be presented. The model is used to predict the crush behavior for different grain orientations of balsa wood. As an example problem, an impact limiter containing balsa wood as the crushable material is analyzed using both an isotropic model and the LIGO model.