Thin-walled truncated conical structures under axial collapse: analysis of crushing parameters

Axially crushed thin-walled tubular structures are extensively used as energy absorbers in various automotive and aerospace applications because of their high energy absorption efficiency and long strokes. Moreover the necessity to reduce weight of structural components to contain gas emissions and reduce pollution impels to use lightweight engineering materials, such as composites. Although significant numerical and experimental works on the collapse of fiber-reinforced composite shells have been carried out, studies on the theoretical modeling of the crushing process are quite limited given the complex and brittle fracture mechanisms of composite materials. A mathematical and finite element approach on the failure mechanisms, pertaining to the stable mode of collapse of thin-walled composite frusta subjected to axial loading, are investigated. The theoretical analysis is conducted from an energetic point of view. The main energy contributions to the absorption (bending, petal formation, circumferential delamination, friction) are identified and then, by the conservation law, the total internal energy is equated to the work done by the external load. The total crushing process can be seen as a succession of deformation states, each responsible for a partial absorption. The minimum configuration for each impact force in the specific state of deformation, function of several variables and dependent on geometric and material parameters, is obtained. Moreover the numerical modeling through a finite element analysis is conducted; all parametric values and cards set in the explicit dynamic code LS-DYNA is described in detail. Finally a comparison between theory, finite element approach, and experiments concerning crushing loads and total displacements is presented, showing how the proposed strategy is able to capture the crushing mechanisms of such composite structures despite the simplification adopted.