Fracture of aluminium foam core sacrificial cladding subjected to air-blast loading G.S. Langdon a, * , D. Karagiozova a, b , M.D. Theobald a , G.N. Nurick a , G. Lu c, d , R.P. Merrett a a Blast Impact and Survivability Research Unit (BISRU), Department of Mechanical Engineering, University of Cape Town, Private Bag, Rondebosch 7701, South Africa b Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Block 4, Sofia 1113, Bulgaria c Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Vic 3122, Australia d School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore article info Article history: Received 8 December 2008 Received in revised form 14 July 2009 Accepted 14 July 2009 Available online 22 July 2009 Keywords: Sacrificial cladding Blast Aluminium foam Structural response Fracture abstract The effect of core density and cover plate thickness on the blast response of sacrificial cladding panels has been investigated through blast loading experiments and finite element modelling on structures with steel cover plates and aluminium foam cores. A range of foam core densities were examined, with 10%, 15% and 20% nominal relative densities. The cover plate thickness greatly influenced the response of the sacrificial cladding. Cover plates that were 2 mm thick exhibited significant permanent deformations and variable percentage crush across the section, whereas the 4 mm thick cover plates were more rigid causing the core to compress uniformly. Considerable fracture of the foam was observed after blast testing, particularly for the lower density foams. The effect of bonding the cover plate to the core was also examined. Numerical simulations of the experiments were performed using ABAQUS/Explicit to provide insight into the response mechanism. It was shown through the finite element simulations that tensile fracture of the foam occurred during the unloading phase of response and that adhesion of the cover plate to the foam caused higher levels of cracking. This was consistent with the experimental observations. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The blast response of sandwich structures has been attracting research interest recently, as it appears sandwich constructions may offer improved resistance compared to equivalent mass monolithic metal structures [1]. A sandwich panel comprises of a deformable core sandwiched between (at least) two faceplates. In cladding structures, the back faceplate does not deform, whereas in sandwich panels, both faceplates are able to deform. Many core topologies are available, including lattices [2–4], polymeric foams [5], aluminium honeycomb [5–7], and metal foams [8–11]. Choice of core material is critical to the performance of the sandwich as the core properties control the energy absorption and magnitude of the force transfer through the structure. Many cellular materials sub- jected to quasi-static compression are characterised by a relatively constant plateau stress region over a large range of plastic strains prior to densification, which is the key to the ability of the cellular material to mitigate a blast load. An idealised model of a foam material is shown in Fig. 1 [5]. The general principle is as follows, for the situation of a core sandwiched between two plates: the front plate (the outer cover plate exposed to the blast) deforms into the cellular core and the core reduces in thickness via a cell collapse mechanism. During quasi-static core compression, the stress transmission through the core to the back plate is limited by the plateau stress up to the relatively high (typically 70–75%) densifi- cation strain. The high pressure loading incident on the structure should be converted into a lower magnitude load with a much longer dura- tion (due to conservation of momentum) and hence the core should provide predictable and constant load transfer up to densification. Once densification occurs, the loads increase and the advantages of the cellular material are lost. Previous work on aluminium foam sacrificial cladding subjected to blast loading was reported by Hanssen et al. [10]. Two sets of tests were performed: one set with the blast loading impinging directly onto the aluminium foam and a second set employing aluminium cover plates in front of the foam. Without the protective cladding extremely large pressures are generated by the blast which were directly experienced by the structure. With the use of foam cladding, the pressure experienced by the protective structure is applied over a longer duration with a smaller magnitude. Aluminium foam was considered to be an ideal structure for absorbing energy under blast loads as it was already readily available in various forms and had properties that should be less directionally dependent than aluminium * Corresponding author. Tel.: þ27 (0)21 650 4810; fax: þ27 (0)21 650 3240. E-mail address: genevieve.langdon@uct.ac.za (G.S. Langdon). Contents lists available at ScienceDirect International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng 0734-743X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2009.07.006 International Journal of Impact Engineering 37 (2010) 638–651