Expanding fracture behaviors of single- and double-layer metallic cylindrical shells subjected to detonation of hollow explosives

The detonation-driven expansion and fracture behavior of single-layer and double-layer metallic cylindrical shells (with equal total thickness) were investigated via the DPS (Doppler pins system) array and high-speed photography. The velocity curves and high-speed photography images of the outer surface of the single- and double-layer cylindrical shells were obtained. The velocity curves show that spallation occurs at approximately 40% of the thickness from the outer surface in the single-layer cylindrical shell. In addition, the spallation part was caught up after ~1.89 μs by the fragments of the single-layer cylindrical shell, leading to the secondary impact loading. In the case of double-layer cylindrical shell, the inner and outer shells of the double-layer cylindrical shell separated quickly since they cannot withstand the tensile stress. The outer cylindrical shell expands freely with high velocity at initial stage. It is then caught up by the inner cylindrical shell after 26.13 μs, leading to the secondary impact loading. There is a significant difference in secondary impact loading times the single- and double-layer shell expansion experiments. The premature separation of the double-layer cylindrical shell hinders the crack penetration from the inside to the outside, and delays the crack penetration until the complete fracture of the whole cylindrical shell. In contrast to the widely investigated fracture behavior in a single-layer cylindrical shell, the detonation product leakage time is delayed when the outer layer of the double-layer cylindrical shell expands. The fragment size of a double-layer shell is obviously smaller than that of a single-layer shell. The distribution of the overflowing detonation product is relatively scattered and rare, and the overall crack of the outer cylindrical shell is clearly visible. The results of SPH (smoothed particle hydrodynamics) numerical simulations demonstrate that under certain conditions, the lower the spallation strength in single-layer cylindrical shells, the later the residual fragments catch up with the spalled layer during secondary loading, the less likely the occurrence of penetration fracture. Meanwhile, the interfacial gaps in double-layer cylindrical shells significantly inhibit the penetration and propagation of cracks.