The Mechanism of Strength Enhancement under Impact Loading of Cellular Materials

Abstract This paper presents a multiscale analysis of the macroscopic rate sensitivity of metallic cellular materials. It begins with a review of likely causes responsible for the strength enhancement of cellular materials, which include the rate sensitivity of the base material, the effects of the pressure of the air entrapped in the cell, the inertia effect and the shock enhancement. A testing method using 60 mm diameter Nylon Hopkinson pressure bars with suitable signal processing is used to investigate the rate sensitivity of various metallic cellular materials, i.e. 5056, 5052 aluminium honeycombs of different cell sizes and wall thickness, IFAM and Cymat aluminium foams. In order to identify the factor responsible for the strength enhancement of those materials, a multiscale analysis is performed on a model structure which is a square tube made of rate insensitive materials. At the macroscopic level, significant enhancement is experimentally observed under impact loading, whereas the crushing mode is nearly the same under both static and impact loading. At mesoscopic level, post mortem microhardess measurement provides a map of residual plastic strain everywhere in the cell wall. Numeric simulations and theoretical models give a satisfactory explanation of the role of the lateral inertia.