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1 Introduction WhentheDruckerPrageryieldcriterionisused,theMSC/NASTRANprogram[1]oftenpredictsnoticeableerrors.Thiscanbeobservedusingasimpleexample.Whenasingleelementsubjecttouniaxialtensilestressisconsidered,thepredictedstressstrainrelationdeviatesfromthe