Effect of the Stiffness of the Testing System on the Discontinuous Deformation of 7075-Al Alloy

Abstract

In the previous studies of the discontinuous deformation, little attention has been paid to the effect of the stiffness of testing machines and specimens. The purpose of this experiment is to study the effect of the stiffness of the testing system on the discontinuous deformation. In paticular, the stress drop and the time interval which characterize the discontinuous deformation are taken into consideration.
The experiment was carried out at room temperature using the 7075-Al alloy as the specimen.
The main results obtained are as follows:
(1) The plastic elongation due to load drops increases with increasing elastic potential energy of the testing system, while the average plasic strain increases depending on the ratio α of the spring constant of the specimen to that of the machine.
(2) The stress drop and the time interval depend on the spring constant ratio, which may be expressed as Δσ=Δσ₀(1+k(ε,\dotε)·α) and Δt=Δt₀(1+k′(ε,\dotε)·α).
(3) The follwing relation holds when the stress drop and the time interval are extrapolated to α→0, Δσ₀=f₀\dotεΔt₀, where f₀ means a tangential hardening rate of the stress strain curve and its value is approximately equal to 0.4E.