ONSET OF FAILURE IN ALUMINUM HONEYCOMBS UNDER GENERAL INPLANE LOADING

Of interest here is the theoretical prediction of the onset of failure in aluminum honeycombs under arbitrary macroscopic loading conditions . A failure surface is defined in macroscopic stress space by the onse t of the first buckling-type instability encountered along proportiona l load paths, where each load path is defined by a fixed macroscopic l oad orientation and a fixed ratio of principal macroscopic stresses. T he influence of specimen size (i.e., geometric scale effects), and the influence of geometric microstructural imperfections on these failure surfaces, are investigated through a combination of analytical (i.e., Bloch wave) and numerical (i.e., finite element) techniques. All of t he analyses presented here are carried out for commercially available honeycombs, and the results show an extreme sensitivity of the onset o f failure in these materials to the macroscopic load orientation and t he principal macroscopic stress ratio. In addition, the failure surfac e for a perfectly periodic honeycomb of infinite extent. is found to b e an upper bound for the failure surfaces of the corresponding finite honeycomb specimens with microstructural imperfections. Moreover, the construction of the failure surfaces for the imperfect specimens requi res the numerical solution for large, multicell models, while the fail ure surface for the finite, perfectly periodic model is obtained with less computational effort, since calculations involving only the unit cell are required. The methodology proposed in this investigation, the refore, provides a useful predictive tool for the design of these mate rials. (C) 1998 Elsevier Science Ltd. All rights reserved.