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

Abstract

Almost complete rate independence or negative rate sensitivityand an increase in strength with a concurrent decrease in ductilitytogether with jerky flow are reportedly observed when dynamic strainaging occurs in metals and alloys. These phenomena are sometimesreferred to as the ‘Portevin Le Chatelier’ effect. Most engineeringalloys experience dynamic strain aging in some temperature region andnormal behavior (positive rate sensitivity and a decrease in strengthwith an increase in temperature) is observed outside the strain-agingregion. The ‘unified’ viscoplasticity theory based on overstress (VBO),which models positive rate sensitivity, is modified to reproduce thezero and negative rate sensitivity and the increase in strength withtemperature. This is accomplished through the introduction of a singlenew term in the growth law of one state variable. In the initialquasi-elastic region the new term has no influence on the modelpredictions but reaches full influence when the flow stress is reached.During dynamic strain aging, creep and relaxation behaviors arepredicted to be normal. While there are very few experimental resultsavailable, they are all in agreement with these predictions. An analysisof the properties of this new term and numerical experiments illustratethe capability of the extended VBO model.