Plastic instability in A-75 titanium sheet deformed in uniaxial tension at 300–700 K

Abstract

The effects of material parameters on the plastic instability in uniaxial tension of commercial A-75 titanium sheet tested at 300–700 K and strain rates of 10 −4−10 −2s −I were investigated employing gridded specimens. Three plastic instability strains were identified: 1. (a) ϵ 1 ∗ the initiation of strain concentration leading to a diffuse neck, 2. (b) ϵ II ∗ , the restriction of strain to the diffuse neck region and 3. (c) ϵ III ∗ , the initiation of local necking. The effects of material parameters on the critical strains for each of these instability conditions were given reasonably well by the following expressions: ϵ I ∗ = n I + δ In A 0 δ In ϵ ϵ II ∗ = ϵ I ∗ + 3.3m u 2 3 ϵ III ∗ = 2{n u + m u(α III + ϵ u)} where A0 is the original cross-sectional area, n = dln σ dl nϵ , m = ϖInσ ϖ In ϵ ̇ and α = d In ϵ ̇ d Inϵ of the element which eventually failed. The subscripts indicate the strain at which the material parameters were determined. The expressions for ϵ I ∗ and ϵ III ∗ , were derived, whereas that for ϵ II ∗ is purely empirical, n, m and a were found to vary with strain, strain rate and temperature.