Open Access

ARTICLE

Modeling of Nonlinear Rate Sensitivity by Using an Overstress Model

KwangsooHo¹

1 Department of Mechanical Engineering, Yeungnam University, 214-1, Daedong, Kyongsan, Kyongbuk, 712-749, Korea. Email: hok@ynucc.yeungnam.ac.kr

Computer Modeling in Engineering & Sciences 2001, 2(3), 351-364. https://doi.org/10.3970/cmes.2001.002.351

Abstract

Negative, zero or positive rate sensitivity of the flow stress can be observed in metals and alloys over a certain range of strain, strain rate and temperature. It is believed that negative rate sensitivity is an essential feature of dynamic strain aging, of which the Portevin-Le Chatelier effect is one other manifestation. The viscoplasticity theory based on overstress (VBO), one of the unified state variable theories, is generalized to model zero (rate independence) and negative as well as positive rate sensitivity in a consistent way. The present model does not have the stress rate term in the evolution law for the state variable equilibrium stress that has been included in previous versions of VBO. The three types of rate sensitivity are classified on the basis of a certain constant of an augmentation function, which is introduced in the evolution law for the equilibrium stress. Based on the augmentation function, the model reproduces the dependence of relaxation rate on prior strain rate. When the augmentation function is selected to depend on the accumulated inelastic strain or the effective inelastic strain rate, a change of rate sensitivity type that depends on strain or strain rate can be modeled. It is also shown that the augmentation function can be used to reproduce the observed dramatic increase of strain hardening at strain rates exceeding 10³ s^-1.

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