Keywords: cellular structures, mechanical properties, finite element modelling, non-linear response, static compression, low velocity impact.

This paper focuses on the static compressive and low-velocity impact failure behaviour of metallic open-lattice cellular materials, fabricated by the selective laser melting (SLM) process, as well as sandwich structures based on this type of core material, which is critical for the successful development of improved sandwich structures with tailored properties. A generic three-dimensional structure, which shows regular geometry repeated periodically in all principal directions, is selected for analysis. The structure was manufactured by the University of Liverpool [1], with stainless steel 316L as the powder material.

From theoretical point of view, research addressing the analysis and optimisation of lattice structures in the context of core materials included in sandwich construction is currently underway. The research focuses mainly on the elastic properties of cellular materials and static non-linear response of metallic open-cell [2,3]. The response of the truss-core sandwich specimens to dynamic loading is also analyzed, however in reference [4] the SLM open-cell lattice core sandwich structures are only analyzed for low velocity impact.

For modelling the static response of the cellular core of the sandwich panel, a simple beam-type finite element (FE) model is developed in an implicit FE code. The static linear and non-linear behaviour of the cellular structure is estimated for the 'global' engineering stress-strain response. The effective elasticity modulus of the cellular material is estimated as the ratio of the computed 'global' engineering stress over the 'global' engineering strain obtained from a linear analysis. For the non-linear response, a 'global' engineering stress-strain diagram is obtained from a non-linear analysis that includes material failure mechanisms.

The prediction of the low-velocity impact response of a sandwich structure with cellular core is performed by developing an explicit dynamic FE model. The material models developed for the core material and composite skin, as well as the self-impacting contact interface defined to simulate the core densification under impact loading are validated using the experimental results.

The modelling methodology developed for both static and dynamic response is proven to have good predictive capabilities and will be a useful tool for the design and optimization of cellular structures.

1

W.K. Brooks, S. Tsopanos, R. Stamp, C.J. Sutcliffe, W.J. Cantwell, P. Fox, J. Todd, "The production of open cellular lattice structures using selective laser melting", 6th National Conference on Rapid Design, Prototyping, and Manufacturing, Buckinghamshire Chilterns University College, 2005.

2

M.H. Luxner, J. Stampfl, H.E. Pettermann, "Numerical simulations of 3D open cell structures - influence of structural irregularities on elasto-plasticity and deformation localization", International Journal of Solids and Structures, 44, pp. 2990-3003, 2007. doi:10.1016/j.ijsolstr.2006.08.039

3

V.S. Deshpande, N.A. Fleck, M.F. Ashby, "Effective properties of the octet-truss lattice material", Journal of the Mechanics and Physics of Solids, 49, pp. 1747-1769, 2001. doi:10.1016/S0022-5096(01)00010-2

4

S. McKown, R.A.W. Mines, "Impact behaviour of metal foam cored sandwich beams", 16th European Conference on Fracture, Alexandroupolis, Greece, Paper No. 254, 2006.

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