Impact behaviour and design optimization of a ductile scale-cellular composite structure for protection against localized impact

A good protective structure must be effective in dissipating impact energy as well as minimizing stresses transferred to a protected object. A novel composite protective structure which combines an assembly of ductile scales as an outer layer for energy dissipation and a cellular material with foam-like properties as an inner layer for stress minimization was examined in this study, focusing on its mechanical behaviour and design optimization. Finite element simulations were adopted to investigate the mechanical behaviour and impact performance of specimens with different geometrical and material properties of the assembly of scales and cellular layer. Results from the numerical analyses indicate that the mechanical behaviour and impact performance of the composite structure are governed by the overall stiffness of the assembly of scales relative to the underlying layer. The desired impact performance could be achieved when the assembly of scales are neither too weak such that they collapse easily when subject to impact, nor too stiff such that they tend to puncture into the underlying layer. When the stiffness of the assembly of scales is optimum, the scales are effective in dissipating a significant proportion of the impact energy as they deform. This results in reduced compression on the underlying cellular layer and low peak stress transferred by the composite structure. The following key outcomes were established in this study in order to achieve this desired impact performance: (a) recommended bounds for the Young's modulus and yield strength of the scales relative to those of the underlying layer; (b) a geometric stiffness parameter to account for the combined effects of aspect ratio, curvature, degree of overlapping, and size of the scales on the impact performance of the composite system, and the optimum range for this parameter; and (c) recommended bounds for the volume of the scales relative to that of the underlying layer. These findings can be used to develop an approach to determine the optimum design configurations of the composite structure for various applications.