The Tearing of Petalled Holes in 2014–T651 Aluminium Alloy Produced by Explosions and its Possible Contribution to the Loss of Aircraft by Sabotage

Abstract

Petalled holes were formed in thin 2014–T651 aluminium alloy sheet by blast, explosively formed projectiles and shaped charges, as well as by explosives placed directly on the sheet. Tensile testing of the sheets containing the various holes showed no macroscopic plasticity. Analysis of the lengths of petalled cracks produced by 1.26 g charges of an RDX-wax explosive in direct contact with a sheet, acting as a diaphragm in a canister, showed that these were increased by 50 per cent if the canister was pressurized to 0.6 MPa (6 bar), indicating that the length of petalled cracks produced by an explosion was affected by the stress applied to the thin sheet. Calculations showed that the strain energy in the diaphragm was 17.5 J, while the energy required to produce the extra cracking was estimated to be 57 J, assuming that the work required to extend the crack is 0.1 MJ/m2. This suggested that the energy stored in the gas, as well as that elastically stored in the diaphragm, was used to extend the cracks. The lengths of petalled cracks produced by 55 g charges of the RDX-wax explosive at 400 mm stand-off were substantially less than those produced by the 1.26 g charges placed in contact with the sheet, indicating the beneficial effect of the attenuation of the blast wave by stand-off and the greater damage caused by the presence of explosives in contact with or very close to the sheet.