Numerical modelling of foam-cored sandwich plates under high-velocity impact I. Ivañez, C. Santiuste ⇑ , E. Barbero, S. Sanchez-Saez Department of Continuum Mechanics and Structural Analysis, University Carlos III of Madrid, Avda. De la Universidad 30, 28911 Leganés, Madrid, Spain Keywords: Sandwich plates Finite-element analysis High-velocity impact Foam core abstract This paper studies the high velocity impact response of sandwich plates, with E glass fibre/polyester face sheets and foam core, using finite element models developed in ABAQUS/explicit code. The failure of the face sheets was predicted by implementing Hou failure criteria and a procedure to degrade mate rial properties in a user subroutine (VUMAT). The foam core was modelled as a crushable foam material. The numerical models were validated with experimental data obtained from scientific literature. The contribution of the foam core on the impact behaviour was evaluated by the analysis of the residual velocity, ballistic limit, and damaged area. 1. Introduction Many structural components in several industrial sectors, mainly transport industry, are designed with requirements of high specific strength and stiffness, and damage tolerance. Com posite sandwich structures with polymer foam core can be used in these applications due to their superior performance in terms of strength and stiffness to weight ratios, ease of manufacturing, and flexibility in design. However, these structures are susceptible to be damaged by impact loading, thus the design process must consider their dynamic and impact behaviour. The impact damage could significantly diminish their strength [1], leading to a limita tion of the use of laminate type composite structures [2]. There is an extensive research on the behaviour of sandwich structures subjected to low velocity impact including the analysis of the influence of the foam core; in contrast, there is a lack of studies about their behaviour under high velocity impacts of low mass fragments, thus the influence of the foam core on the high velocity impact behaviour is still not fully understood [3]. High velocity im pact behaviour differs from the low velocity one; according to the comprehensive review of Abrate [4] high velocity impacts are de fined as those where the ratio between impact velocity and the velocity of compressive waves propagating through the thickness is larger than the maximum strain to failure in that direction. This implies that damage is generated during the first few travels of the compressive wave through the thickness when overall plate mo tion is not yet established. Thus, high velocity impact is a phenom enon controlled by wave propagation, and is essentially independent of boundary conditions, whereas a low velocity im pact is highly influenced by the boundary conditions. Consequently the conclusions drawn in studies on static or low velocity impacts are not applicable to high velocity cases. Most studies on high velocity impact behaviour of sandwich structures are based on experimental tests [5 8]. Although exper imental studies provide essential information, since impact phe nomena depends on numerous parameters, a comprehensive knowledge of its influence on ballistic behaviour requires a broad test programme, which is time consuming and expensive. There fore the use of theoretical models, analytical [9] and numerical [10], to analyse the perforation of sandwich structures is critical to reduce cost and time in design processes. The main advantage of analytical models is the quick analysis of the influence of differ ent parameters on the high velocity impact behaviour of sandwich structures. However, with these simplified models, it is not possi ble to study in depth the perforation process of a composite sand wich panel with foam core. A finite element (FE) analysis provides with the possibility to model high velocity impact processes, acquiring information about the contribution of the different ele ments of the sandwich panel to the projectile energy absorption process. An accurate FE analysis of a sandwich structures requires including complex models for the mechanical behaviour of the face sheets as well as the core. The behaviour of laminated com posite materials can be considered lineal elastic until the laminate begins to fail. The damage inflicted on a composite laminate is a complex phenomenon due to the different damage mechanisms that could appear: matrix cracking, tensile and compressive fibre breakage, delamination, etc., which depend on many parameters (fibre and matrix properties, characteristic of the fibre matrix interface, manufacturing process, etc.). The failure of composite laminates can be predicted using three different approaches: frac ture mechanics, failure criteria, and damage mechanics, although ⇑ Corresponding author. Tel.: +34 91 624 99 20; fax: +34 91 624 83 31. E-mail address: csantius@ing.uc3m.es (C. Santiuste). URL: http://www.uc3m.es/mma/amm (C. Santiuste). 1