Showing papers on "Viscoplasticity published in 1969"

The effect of time and temperature on the mechanical behavior of a 'plasticized' epoxy resin under different loading modes

Abstract: Epoxy–Versamid specimens were loaded in tension, compression, and flexure at different strain rates and temperatures to determine mode of failure, yield stress and strain, and tangent and relaxation moduli. Stress-strain curves were used to define brittle, ductile, ductile-rubbery, and rubbery modes of behavior which prevailed in different temperature-strain rate regions. The time-temperature superposition principle was applied to yield stress, initial tangent moduli, and relaxation moduli data for all three types of loading. The transition regions, tangent and relaxation moduli, and shift factors were the same in tension, compression, and flexure. Thus the most convenient mode of loading can be used to determine the general time-temperature dependence. The ratio of compressive-to-tensile yield stress was almost constant over the entire ductile region. Flexural yielding data were used to predict yield stress in tension and compression, and stress relaxation master curves were shown to be related to elastic modulus vs. strain rate curves. The yielding phenomenon was interpreted using Eyring's theory of non-Newtonian viscoplastic flow. The apparent activation energy and activation volume were larger for tension than compression. A theory is offered to explain why yielding can occur in a cross-linked system.

On thermal effects in viscoplasticity

Abstract: The basic object of the present paper is the description of the behaviour of an elastic/viscoplastic material during thermodynamic process within the framework of thermodynamics with internal state variables1.

The effect of temperature on the delayed yield and failure of “plasticized” epoxy resin

Abstract: : Epoxy-versamid specimens were loaded in tension up to failure at different constant strain-rates and temperatures. Results revealed three modes of behavior prevailing at different temperature-strain-rate regions and associated with brittle, ductile and rubbery failure modes. The ductile region was found to be confined within a narrow band on the temperature-strain-rate plane, and is characterized by a yield plateau in the stress-strain curve and by linear dependence of yield stress on log strain rate and temperature. Yield strain seems to be almost unaffected by strain-rate, but decreases slightly with temperature rise. Analysis indicated that experimental data within the ductile region are consistent with Eyring's formulation for non-Newtonian viscoplastic flow. It leads to the evaluation of the 'apparent activation energy' and activation volume for the two epoxy systems tested. Comparison with previous work indicates that the above parameters as well as yield stress and elastic modulus tend to increase with the decrease of versamid content in the resin.

Propagation of two-dimensional stress waves in an elastic/viscoplastic material

Abstract: In recent years much attention has been devoted to problems of wave propagation for loading conditions which produce plastic deformation. Many dynamic boundary value problems have been solved assuming plane, cylindrical or spherical symmetry of the body and of the pressure applied to its boundary. All these problems may be treated using a one-dimensional theory. Mathematically, the governing equations form a system of hyperbolic partial differential equations of the first order in two independent variables; a good review of these equations and their possible methods of solution is given in [1] and [2].

Harmonic Dispersion Analysis of Incremental Waves in Uniaxially Prestressed Plastic and Viscoplastic Bars, Plates, and Unbounded Media

Abstract: The equations governing the propagation of infinitesimal incremental loading stress waves in a medium prestressed uniaxially into the plastic region are derived for a strain-rate-independent and a strain-rate-dependent constitutive equation. These equations, applied to a circular rod, a flat plate, and an unbounded medium, are solved using harmonic analysis to give the phase-velocity dispersion curves for component waves as a function of frequency. The results of this analysis are compared to the corresponding elastic dispersion curves.

On stability of viscoplastic systems with thermo-mechanical coupling†

Abstract: : The stability of equilibrium configurations of thermo -mechanically coupled systems in the plastic and the viscoplastic range is examined through simple examples and by dimensional analysis. A uniform fully-insulated thin-walled tube twisted into the inelastic range is studied first. The material is idealized as linear work-hardening viscoplastic with a current yield stress which decreases linearly with increase in temperature. Any annealing or age-hardening or similar influence of time at temperature is supposed negligible. A local stability criterion beta > 1 governs, where beta is the ratio of the heat stored in the material to the heat generated by inelastic deformation with a given temperature rise. Neither viscosity nor heat conduction enters into the stability or instability of this homogeneous system. They both play a role in successive modifications of the simple model which demonstrate the stabilizing effect of heat transfer to the surroundings and of the heat capacity in regions of material below yield. (Author)

Group classification and invariant solutions for the equations of flow and heat transfer of a viscoplastic medium

Abstract: Exact solutions without major restrictions on the properties of the material are needed in research on the flow (especially viscosity) of metals at high temperatures under nonisothermal conditions. Often the shear resistance is governed mainly by the temperature and the deformation rate. Here are examined the group properties of the equations of flow and heat transfer of a medium whose shear resistance is a function of temperature and rate of shear deformation. The properties specific to metals are not used, so the results are applicable to a variety of media.