Journal of Materials Processing Technology, 25 (1991) 15-33 15 Elsevier Prediction of the limit strains of sheet steel thermally and mechanically worked in relation to surface integrity changes: A theoretical model A.G. Mamalis, A.P. Karafillis and N.M. Vaxevanidis Department of Mechanical Engineering, National Technical University of Athens, Athens, Greece (Received July 11, 1989; accepted January 24, 1990) Industrial Summary A theoretical model is proposed to predict the limit strains and the forming-limit diagrams both in the first (I) and in the second (II) quadrants of the strain locus of thin sheets subjected to biaxial stretching. Based on the Marciniak-Kuczynski model and employing the non-quadratic m-yield criterion, the proposed model also correlates surface integrity changes resulting from me- chanical processing; either low-speed directional rolling or stochastic ball-drop forming and ther- mal stochastic electro-discharge machining. Theoretical predictions and experimental data from previous work are compared and the suitability of the theoretical model in studying the defor- mation behaviour in sheet metal forming is discussed. Notation a = strain ratio, •2A/•IA B = stress ratio, a2/al dg = grain diameter dp -- punch diameter el = maximum principal strain e2 -- minimum principal strain e3 = thickness strain = generalised strain fi = inhomogeneity factors i=0, s, t, n, k hp = depth of plastic zone H = ball-drop height HV = microhardness M -- accumulated mass dropped m = exponent of m-yield criterion n -- work-hardening exponent 0924-0136/91/$03.50 © 1991--Elsevier Science Publishers B.V.