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The rate-dependent properties of AISI Type 304 stainless steel and of the high-strength Ti-7Al-2Cb-1Ta alloy obtained with tension specimens using a servocontrolled testing machine at room temperature are discussed. Tests involve strain rates between 10−8 and 10−2 s−1, strain rate changes, relaxation periods, stress rates between 10−1 and 102 MPa s−1, and creep periods.
The viscoplasticity model based on total strain and overstress is introduced and shown to qualitatively reproduce the properties observed under strain control. The theory is then examined for its qualitative prediction under stress control, and it is found that the average creep rate need not increase with an increase in stress owing to the overstress (effective stress) dependence of the creep rate. Numerical experiments illustrate this property, which is then confirmed by real experiments.
The theory has two material functions representing work hardening and rate dependence. A method of determining the function representing rate dependence from strain rate change tests is discussed. For Type 304 stainless steel this function is represented by two equivalent expressions which can be used from static (10−8 s−1) to dynamic strain rates (103 s−1). A rate sensitivity factor M (akin to the m-factor used in materials science) is introduced. It is shown to vary with overstress or strain rate.
stainless steel, Ti-alloy, rate dependence, creep, relaxation, servocontrolled testing, constitutive equations, viscoelasticity, viscoplasticity
Professor of Mechanics, Rensselaer Polytechnic Institute, Troy, N.Y.