The response of singly curved fibre reinforced sandwich and laminate composite panels subjected to localised blast loads
This report presents results from a study on the response of singly curved fibre reinforced polymer (FRP) sandwich and laminate panels subjected to localised blast loads. The aim of the project was to investigate and compare the blast mitigation potential of each panel type and the influence of curvature on the response. Three radii of curvature were examined for both panel types, namely infinite (flat), 1000mm and 500mm. The FRP laminate panels were designed to consist of 1-5 layers of Eglass fibre reinforced epoxy sheets. The FRP sandwich panels consisted of a PVC foam core with 6 layers of FRP sheets on either side. Vacuum infusion, with the aid of three moulds, was used to manufacture the panels. The average thicknesses and areal densities of the FRP sandwich and laminate panels were 18.7mm and 4.9mm; and 862-8g/1m2 and 8458-g/m2 respectively. Three point quasi-static flexural tests were conducted on FRP sandwich and laminate specimens where the localised compression failure beneath the central loading bar was evident on both types of structures. The presence of the core reduced the damage observed on the back face of the FRP sandwich specimens. Blast tests were conducted on a horizontal ballistic pendulum at the Blast Impact and Survivability Research Unit (BISRU), University of Cape Town. Localised blasts were generated by detonating circular cylinder PE4 plastic explosives, placed at a constant standoff distance of 10mm. The charge mass ranged from 10g to 32.5g across all the panels. The failure modes of the blast loaded panels were identified by a post-test inspection. The failure mode initiation charts for the F RP sandwich panels revealed that failure modes were initially observed on the front face sheet and core material with slight appearance of delamination on the back face sheet. Increasing the charge mass resulted in the rupture of the front face sheet and penetration of the core. Additional failure of the back face sheet was also evident as the charge mass increased. The failure mode initiation charts of the FRP laminate panels exhibited less severe failure modes across a greater charge mass range that eventually lead to complete fibre rupture at higher charge masses. Delamination of the front face sheet of the flat FRP sandwich panels was initially observed in the centre of the panel and spread into the exterior region for increasing charge mass. The failure of the core material initially reduced the delaminated area of the back face sheet, however once the rupture of the front face sheet occurred, the delaminated area of the front face sheet reduced and the delaminated area of the back face sheet increased. This was similar for the curved FRP sandwich panels except that the delaminated area was predominately parallel to the axis of curvature prior to rupture and perpendicular to the axis of curvature subsequent to rupture. Delamination in the flat FRP laminate panels was initially observed in the centre of the panel and along the clamped boundary. Increasing charge mass resulted in the delaminated region spreading across the panel. As with the FRP sandwich panels, the delaminated area of the curved FRP laminate panels was initially observed parallel to the axis of curvature prior to rupture. Debonding of the FRP sandwich panels was initially observed at both of the front and back interfaces. For the front interface, the debonded lengths were observed in the centre and in exterior test area of the panel, but only in exterior test area for the back interface. With the rupture of the front face sheet, the debonded length of the front interface decreased and the back interface increased and spread across the entire test area. The blast rupture threshold of the two panel types were compared in terms of largest charge mass resisted. For each radii category, the FRP laminate panels outperformed the FRP sandwich panels, namely by 5g for the flat panels (25g vs 20g) and 9g for the 1000mm curved panels (27.5g vs 18.5g). However, for the 500mm curved panels the FRP laminate and sandwich panels ruptured at identical charge masses of 27.5g.
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