An experimental investigation into load relaxation in aluminium (HE30TB) and mild steel (EN1A)

Abstract

Short-time room-temperature tensile load relaxation tests were conducted on aluminium alloy (HE30TB) and mild steel (EN1A) specimens using a closed-loop, electrohydraulic servo-controlled testing machine under strain control by means of an extensometer mounted directly on the parallel section of the tested specimens. The relaxation periods (usually 60 sec.) were interruptions at chosen points in constant strain rate tensile loadings. The loading strain rates in the different tests varied from 5 × 10 −4 to 10 −2 sec. The transient relaxation behaviour was investigated for the purpose of testing the applicability of the most widely assumed viscoplastic constitutive models. This was achieved by comparing the plastic strain rate ϵ ̇ p + just after the beginning of load relaxation at constant total strain to the plastic strain rate ϵ ̇ p − during the tensile loading just before the start of the relaxation interval. All common viscoplastic theories predict that the plastic strain rate ratio ϵ ̇ p + ϵ ̇ p − should be unity. The experimental results for both materials indicate, however, that the plastic strain rate ratio varies from almost zero for relaxation periods early in the loading, to a maximum of around 0·2 for some relaxation periods beginning at relatively high loads and strains. This agrees with previously reported results on pure aluminium, which is not very rate sensitive, but the results for the more rate-sensitive mild steel may be surprising. Only if the actual relaxation rate drops by a factor of about 100 in 0.2 sec, could the findings of this experimental programme be reconciled with predictions of the usual viscoplastic theories. The experimental programme also included constant strain-rate tests at several rates and jump tests, in which the rate was switched back and forth between 10 −4 and 10 −2/sec.