PREDICTION OF CENTRAL BURSTING IN EXTRUSION

Central burst defects which are also called chevron cracks, in extrude d products are analyzed. The criterion based on plastic instability wa s used in order to predict central bursting. It satisfies sufficient c ondition (s) over dot(i)(epsilon) over dot(i) = 0 for the multiplicity of the solution, which is derived from the necessary condition Delta( s) over dot(i)Delta(epsilon) over dot(i) = 0, where (s) over dot(i) de notes the nominal stress rate, and Delta the difference of any two mul tiple solutions. The criterion gives the limit strain identical to tha t derived from Swift's criterion (diffuse necking) under plane stress condition, but conceptually it is entirely different from Swift's. Str ess and strain on the specimen were evaluated using the finite element method (FEM). We proposed a measure y(M) which represents the risk pr obability of cracking and calculated it using the stress and strain on the central axis of the product. When y(M) less than or equal to 0, b ursting occurs. The value of y(M) is shown in the range of combination s of the die half angle and the reduction area. Boundary curve that se parates the defect zone from the non-defect zone in the relationship w ith respect to the above combinations takes a nose-like shape, which i s quite different from that based on Avitzur's prediction. (C) 1998 El sevier Science S.A. All rights reserved.