PREDICTION OF DUCTILE FRACTURE STRAINS IN UPSETTING PROCESSES.pdf
Creators: name: Hassan, Mostafa
Publisher: name: figshare
Year: 2020
Subjects: subject: 90203 Automotive Mechatronics
subject: FOS: Mechanical engineering
subject: FOS: Mechanical engineering
subject: Mechanical Engineering
subject: 91207 Metals and Alloy Materials
subject: FOS: Materials engineering
subject: FOS: Materials engineering

description: Forgeability and workability are measures of the plastic deformation limit that workpieces can undergo under bulk-forming processes until the formation of cracks. Crack prediction helps to prevent damage occurrence in products during the production sequence. Ductile fracture (DF) is the major failure mode in metal forming and it is commonly characterized by the mechanism of void nucleation, growth with plastic deformation until coalescence. Under some forming conditions, which have a dominant compression loading as in the upsetting and extrusion process, void growth does not contribute to the failure mechanism where coalescence takes place due to shear localization. In upsetting processes, the presence of friction between the specimen and the die causes a barrel shape and with the increase of deformation, the specimen cracks at the equatorial surface. Upsetting tests for different axisymmetric specimens are usually used to construct the forming limit diagram at fracture (FLD F) for different stress states found in bulk-forming. Literature related to upsetting failure prediction shows that there is no pure analytical or numerical reliable prediction approaches, and, in most cases, the experimental investigation is required to complete the analysis. The objective of the present work is to predict the FLDF for upsetting processes using coupled DF approach and different coalescence criteria. Uncoupled DF approach is also used in predicting the limiting strain using energy integrals. A parametric study is conducted to relate the friction conditions and specimen dimensions to the critical height reduction of blocks (of square or rectangular cross-sections) under cold upsetting conditions. Numerical simulations are performed using finite element software ABAQUS™ to determine the stress and strain distribution at the locations of crack formation. Two FE models are adopted after validating the results against published data; one considers voided materials while the other does not. The effect of initial void fraction, initial void aspect ratio, material type, friction conditions, and specimen configuration on stress and strain states at the equator are studied. It has been found that the coupled DF approach is in principle capable of predicting analytically the failure in upsetting processes. The models considering void shape evolution need modifications even after adding the shearing effect to obtain better FLDF predictions. For uncoupled DF approach, an average value of the critical parameter (Cavg) in the out-of-plane shear and hydrostatic integrals has to be assigned from experiments to predict a semi-empirically bilinear forming limit diagram at fracture. The parametric study showed that the limiting strain in rectangular upsetting is highly affected by the friction conditions while the specimen dimensions have minor effect. The fitting expression developed from the parametric study gives results close to the FE numerical results obtained numerically with an error less than 9.69 %.