INFLUENCE OF THE YIELD SURFACE CURVATURE ON THE FORMING LIMIT DIAGRAMS PREDICTED BY CRYSTAL PLASTICITY THEORY

Abstract

THE AIM OF THIS PAPER IS TO INVESTIGATE THE IMPACT OF THE MICRO-SCOPIC YIELD SURFACE (I.E., AT THE SINGLE CRYSTAL SCALE) ON THE FORMING LIMIT DIAGRAMS (FLDS) OF FACE CENTRED CUBIC (FCC) MATERIALS. TO PREDICT THESE FLDS, THE BIFURCATION APPROACH IS USED WITHIN THE FRAMEWORK OF RATE-INDEPENDENT CRYSTAL PLASTICITY THEORY. FOR THIS PURPOSE, TWO MICROMECHANICAL MODELS ARE DEVELOPED AND IMPLEMENTED. THE FIRST ONE USES THE CLASSICAL SCHMID LAW, WHICH RESULTS IN THE FORMATION OF VERTICES (OR CORNERS) AT THE YIELD SURFACE, WHILE THE SECOND IS BASED ON REGULARIZATION OF THE SCHMID LAW, WHICH INDUCES ROUNDED CORNERS AT THE YIELD SURFACE. IN BOTH CASES, THE OVERALL MACROSCOPIC BEHAVIOR IS DERIVED FROM THE BEHAVIOR OF THE MICROSCOPIC CONSTITUENTS (THE SINGLE CRYSTALS) BY USING TWO DIFFERENT SCALE-TRANSITION SCHEMES: THE SELF-CONSISTENT APPROACH AND THE TAYLOR MODEL. THE SIMULATION RESULTS SHOW THAT THE USE OF THE CLASSICAL SCHMID LAW ALLOWS PREDICTING LOCALIZED NECKING AT REALISTIC STRAIN LEVELS FOR THE WHOLE RANGE OF STRAIN PATHS THAT SPAN THE FLD. HOWEVER, THE APPLICATION OF A REGULARIZED SCHMID LAW RESULTS IN MUCH HIGHER LIMIT STRAINS IN THE RANGE OF NEGATIVE STRAIN PATHS. MOREOVER, ROUNDING THE YIELD SURFACE VERTICES THROUGH REGULARIZATION OF THE SCHMID LAW LEADS TO UNREALISTICALLY HIGH LIMIT STRAINS IN THE RANGE OF POSITIVE STRAIN PATHS.