Spall due to short high-intensity impulses

Abstract

Spallation thresholds for (3)/(16) – (3)/(8) -in. 6061-T6, 5456, and 1100 aluminum and 6A14V titanium alloys were determined for impulses from 5 to 30 ktap with a pulse duration of 0.15 μs. These experiments show that for aluminum alloys the spall fracture stress is nearly a constant but varies between 16 and 20 kbar in different batches of material and shows suggestion of a weak dependence on strain rate with strain rates from 0.04 to 0.4 μs−1. The impulse threshold for spall was 7.5 ktap. For 6A14V titanium the spall fracture stress was 60 kbar and the impulse threshold was 15 ktap. The stress pulses were produced by impacting target samples with a 0.012-in. (0.305 mm) thick by 3-in. (76 mm) square Kapton impactor projectile at velocities ranging from 0.13 to 0.44 cm/μs. Data obtained include detailed records of the impactor and rear (spalled) surface velocity versus time measured with a dual-beam Fabry–Perot interferometer, as well as soft recovery of the target residual and spall fragments. From these we are able to determine the partitioning of impulse between the target residual and the spall fragment(s) and the critical spall fracture stress and strain rate. We have also made microphotographic inspections and measured the stress-strain properties of recovered target. The data show that the amount of impulse (momentum per unit area) absorbed by a structural wall by a short duration pulse is limited by the spall threshold. Increases in applied impulse above this spall threshold result in higher spall fragment velocities but no apparent increase in the impulse delivered to the residual structure. Spall is thus an effective damage limiting mechanism which must be considered when estimating the effectiveness of systems whose primary damage is structural collapse due to impulse generated by short duration pulses.