The computer simulation of an explosive test rig to determine the spall strength of metals

Spall fracture is produced by a tensile wave after reflection of a compressive shock wave at a free surface. The original shock wave may be initiated by the detonation of explosive in contact with the material or by the impact of a high velocity projectile. An explosive test rig has been devised where a shock wave produced by explosive detonation is attenuated by propagation along a bar. The degree of spall fracture in a sample on the other end of the bar depends on the bar length. A computer simulation, using the EPIC-2 code, has been made and compared to published values of the free surface velocities for various bar lengths. Good agreement was obtained by careful choice of input parameters used in the γ-law burn of the explosive, and when correction of the experimental values was made for the change in free surface velocity that occurs when the material has a finite spall strenght. The computer simulation enabled extrapolation of the pressure values to be made to longer bar lengths, to obtain the limit of incipient spall fracture. Also, information was obtained on the shock pulse shape and the pressure distribution across the bar. The spall strength of a 6061-T6 aluminium alloy was found to be 1400 MPa, in good agreement with published values for a similar pulse duration.