Keywords: ultra-lightweight concrete, sandwich composite panel, J-hook shear connectors, drop-weight, low velocity impact, finite element simulation.

A recent study on steel-ultra-lightweight-concrete-steel (SLCS) sandwich composite panels has demonstrated a significant potential of combining ultra-lightweight concrete with a density less than 1400 kg/m³ and structural steel to develop composite structure that exhibits excellent impact performance. Novel J-hook shear connectors [1] were used in the sandwich panels to enhance composite action between concrete and steel. The main advantage of the SLCS sandwich system is its high stiffness at low weight, making it viable for marine and offshore applications.

Feasible use of ultra-lightweight concrete as a core material and the effectiveness of J-hook connectors in maintaining the composite action under impact loading were demonstrated in the impact test conducted by Liew et al. [2] on 1200 mm x 1200 mm SLCS sandwich panels. As physical impact tests on the sandwich composites are limited, time-consuming and costly, numerical solutions are needed to complement the experimental program. Moreover, an experimentally-verified numerical model can be further extended for the analysis of larger or more complicated SLCS sandwich structures. Hence, the LS-DYNA explicit nonlinear finite element (FE) code was utilized in the present study to simulate the response of SLCS sandwich panels subjected to a 1.2 ton drop-weight impact.

To model the actual geometry of the J-hook connectors and the surrounding concrete material, fine solid elements of irregular shapes are inevitable in the FE simulation. Such an irregular mesh often leads to numerical instabilities particularly for contact problems and is associated with long computational times that can considerably reduce the efficiency of FE analysis. Thus, a simplified FE model that is able to simulate the local and global deformations of the SLCS sandwich panels with reasonable accuracy is introduced in this paper.

Each pair of the J-hook connectors was simplified in the FE model to two straight solid connectors joined by a discrete beam element with zero initial length. Tensile tests on a pair of J-hook connectors embedded in plain and fibre-reinforced ultra-lightweight concretes were conducted to determine the tensile load-displacement relationship of the discrete beam, taking into consideration the effect of the surrounding concrete material.

From the FE simulation, it was found that all J-hook connectors were under tension during impact except for the connectors located directly below the projectile. The tensile separations between top and bottom connectors were noticeably small, and hence, the connectors were effective in resisting the separation of steel plates and maintaining the integrity of the sandwich panels under the drop-weight impact. Despite using a simplified model for the J-hook connectors and the lack of triaxial test data for the characterization of the ultra-lightweight concrete, it was demonstrated that the simplified FE model is capable of describing the permanent deformation and central displacement time-history of the sandwich panels with reasonable accuracy when compared with impact test data.

1

J.Y.R. Liew, T.Y. Wang, K.M.A. Sohel, "Separation Prevention Shear Connectors for Sandwich Composites Structures", US Provisional Patent 61/047130, 23 April, 2008.

2

J.Y.R. Liew, K.S. Chia, K.M.A. Sohel, D.X. Xiong, "Innovation in Composite Construction - Towards the Extreme of High Strength and Lightweight", in "Proceedings of the Fifth International Conference on Coupled Instabilities in Metal Structures", Sydney, Australia, 23-25 June, 2008.

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