Effects of strain rate on plastic flow and fracture in pure tantalum

Abstract

This paper describes the strain rate effects on flow and failure properties of pure tantalum. The strain rate dependent plastic behavior was determined from both quasistatic and split Hopkinson bar (SHB) tests. The competing effect of strain rate and inertia on the onset of necking, post-elongation, and strain to failure (ductility) was investigated. The quasistatic tests were performed at strain rates 0.001, 1, and 10/sec. The strain rates in the SHB tests ranged between 700 and 1600/sec. Under quasistatic loading, the strain hardening in tantalum was found to be rate sensitive. The strain hardening coefficient (n) decreased continually from a maximum value of about 0.28 to almost 0 in the quasistatic loading regime. In the SHB tests, it was not possible to determine the shape of the stress-strain curves for strains less than 5% due to spurious wave reflections. Nonzero positive values forn at high strain rates were obtained through viscoplastic, constitutive modeling. The flow stress at SHB strain rate levels was almost twice the quasistatic value at a strain rate of 0.001/sec, indicating significant strain rate sensitivity in tantalum. Both the ductility and ultimate strain decreased with increasing strain rate under the quasistatic loading regime reaching a minimum of 0.36 and about 0.02, respectively, at 10/sec. While the ductility remained at this level in the SHB (dynamic) loading regime, the ultimate strain (at the onset of necking) increased to values greater than 0.1, indicating deformation stability due to inertia.