Validation via FEM and plasticine modeling of upper bound criteria of a process-induced side-surface defect in forgings

Finite element analysis and plasticine modeling has been performed for an axisymmetric forging with a double ram action to validate the results of upper bound analysis which provided the geometric and processing conditions that can be used to avoid the development of unacceptable side-surface cracks. The finite element analysis, by using NIKE2D, an implicit finite deformation FEM program, has provided detailed information about the stress- and strain-states that the workpiece experiences and shows that large axial strains develop in the mid-thickness region at small workpiece thicknesses, which correlates to the conditions of surface-crack formation as predicted by the upper bound analysis. Surface cracks obtained in an FEM analysis of two half-billet workpieces have shown excellent agreement with the trends developed through the upper bound analysis. The physical modeling experiments with plasticine have provided further validation of both the upper bound model and the criteria curves for the prevention of side-surface defects. The process of side-surface cracking in a double action axisymmetric forging process was analyzed by the upper bound method. The results of this analysis are presented elsewhere (Y.H. Moon, C.J. Van Tyne, W.A. Gordon, J. Mater. Process. Technol., WTRA, pp. 412, 413). The prime objective of the present paper is to provide validation of the cracking criteria that was developed, this validation being done by both the numerical finite element method and by the use of plasticine modeling material with plexiglass tooling in laboratory experiments.