Modeling of positive, negative and zero rate sensitivity by using the viscoplasticity theory based on overstress (VBO)

Almost complete rate independence or negative rate sensitivity and an increase in strength with a concurrent decrease in ductility together with jerky flow are reportedly observed when dynamic strain aging occurs in metals and alloys. These phenomena are sometimes referred to as the 'Portevin Le Chatelier' effect. Most engineering alloys experience dynamic strain aging in some temperature region and normal behavior (positive rate sensitivity and a decrease in strength with an increase in temperature) is observed outside the strain-aging region. The 'unified' viscoplasticity theory based on overstress (VBO), which models positive rate sensitivity, is modified to reproduce the zero and negative rate sensitivity and the increase in strength with temperature. This is accomplished through the introduction of a single new term in the growth law of one stare variable. In the initial quasi-elastic region the new term has no influence on the model predictions but reaches full influence when the flow stress is reached. During dynamic strain aging, creep and relaxation behaviors are predicted to be normal. While there are very few experimental results available, they are all in agreement with these predictions. An analysis of the properties of this new term and numerical experiments illustrate the capability of the extended VBO model.