Comparing the blast tolerance of different composite structures

Abstract

Large surface ships have traditionally used steel for their construction, which provides good blast resistance and predictable behaviour in use. This research project, however, considered the use of polymeric foam core sandwich panels for the construction of ship hulls, with the intention of reducing the radar signature of the vessel; increasing the maximum speed; reducing fuel consumption; and providing control over desired mechanical properties for specific applications. This project specifically considered the resistance of the sandwich composites to non-contact explosives, specifically sea mines and areal blast.

This research project firstly presents air blast testing performed on polymeric foam core sandwich panels with glass fibre face-sheets. The foam polymer type, the effect of the face-sheet material and the effect of using a graded density foam core for blast wave attenuation were all investigated. When subjected to air blast loading it was found that by grading the core density a smoother back face-sheet displacement was witnessed. This was a significant discovery as protecting the back face-sheet is key to maintaining structural integrity of the ship hulls. In a comparison of foam polymers in the cores of the sandwich panel it was concluded that styrene acrylonitrile offers optimum fracture and adhesion properties. Furthermore, it was found that by interleaving high modulus polypropylene fibres between the glass fibre front face-sheets, front face-sheet cracking and delamination can be prevented, restricting water ingress into the sandwich panel if it were subjected to blast loading. Advances in using 3D high speed imaging for digital image correlation were also achieved, whereby the results were used to estimate core shear strain during deformation, and the kinetic energy and strain energy in the sandwich panel were estimated by comparison with blast wave pressure simulations.

The residual flexural and edgewise compression properties of the air blasted sandwich panels were then determined to calculate the flexural stiffness and strength and edgewise stiffness and strength with varying damage. It was found that the construction of the sandwich panel properties were unaffected by the construction, and that debonding of the face-sheets and the core was the key performance reducing damage mechanism. Residual properties were determined for the sandwich materials, with the intention that these could be used in simulations to predict performance of the ship hulls after successfully withstanding a blast.

In underwater blast scenarios it was discovered that using a graded density foam core reduced the central deflection of the sandwich panel due to the increase in energy absorption due to crushing of the stepwise increase in foam core density. This effect was greater in carbon fibre sandwich panels than in glass fibre panels, due to the increased stiffness of the face-sheets. The central deflection was calculated by averaging the strains measured on the two face-sheets with electronic strain gauges, which was a novel technique and took advantage of the symmetry present in the square sandwich panels.

As the focus of this research project was on foam core properties, the polymeric foam materials were characterised in quasi-static and dynamic tension, and quasi-static and dynamic compression. Dynamic compression tests were performed using a split-Hopkinson pressure bar which was designed specifically for testing low density foam materials. Stress equilibrium was achieved in these tests using a textured polypropylene pulse shaper. Stress equilibrium was checked for by performing high speed digital image correlation during deformation, to ensure that core crushing did not begin at the incident bar.

The final stage of the research project was to construct finite element simulations of the air blast tests, using the polymeric foam material properties measured in the quasi-static and dynamic tests. A large number of simulations were performed with varying charge sizes and stand-off distances, to determine a combination of blast peak pressure and impulse values at which the sandwich panels failed. Developments were achieved in the use of a brittle cracking material model and the effect of the boundary conditions on the simulations was also studied.

This research project furthered the understanding of sandwich composite materials to air and underwater blast loading. Advancements were made into the use of stepwise graded density foam cores and in characterising foams at dynamic tensile and compressive rates. A major conclusion of the project is that the use of graded density cores can be utilised to prevent damage of the sandwich panel on the rear side and can be used to absorb blast energy due to core crushing. The results of the project aid in simulating the response of composite ship hulls to blast loading and better predict the usability of the ships after being subjected to a blast event.