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- Characterization of sheet formability limits using a novel diagonal-cruciform test specimenPublication . Sampaio, Rui F. V.; Dias, Eduardo B. G.; Viegas, João M. A.; Pragana, João P. M.; Bragança, Ivo M. F.; Silva, Carlos M. A.; Martins, Paulo A. F.; Bragança, IvoThis paper introduces an innovative diagonal-cruciform test specimen that significantly enhances the characterization of formability limits in sheet metal forming. The specimen's unique design features a reticular two-dimensional geometric structure, with four triangular arms connecting at the center, which effectively induce biaxial tension stress states when subjected to uniaxial loading. Furthermore, the incorporation of machined spherical cups at its center to locally reduce thickness ensures that damage accumulates in this region. Experimental strain loading paths are captured using digital image correlation (DIC) and analyzed with in-house software developed specifically for research and education on material formability. The software identifies and plots the onsets of necking and fracture in principal strain space, and results prove that the diagonal-cruciform specimen is highly effective in generating stable biaxial tension strain loading paths in C11000 copper sheets, operating under friction-independent conditions without the necessity for specialized testing equipment. The fracture limits are subsequently validated by comparing them against the strain loading paths obtained from a single-point incremental sheet-formed part up to failure. The investigation confirms the versatility and robustness of the novel diagonal-cruciform test specimen for evaluating the formability of C11000 copper sheets and provides valuable insights into its potential application across the broader field of sheet formability characterization.
- Double-flush riveting by punching and compressionPublication . Sampaio, R. F. V.; Larue, P.; Pragana, J. P. M.; Bragança, I. M. F.; Silva, C. M. A.; Martins, P. A. F.; Bragança, IvoThis paper presents a new double-flush riveting process to fabricate flat lap joints that can be easily integrated in progressive or transfer multistage tool systems. The process is based on a punching-compression sequence and the design methodology combines experimentation and finite element modeling followed by destructive peel testing. Special emphasis is placed on the identification and selection of punch-to-die clearances to produce tapered cut surfaces that are appropriate for subsequent compression of the solid rivets into double-flush riveted lap joints. Results confirm that rivet materials with lower mechanical resistance than the sheet materials allow fabricating double-flush riveted lap joints with flat surfaces on both sides in which the mechanical interlocking in the undercut area is obtained by plastic deformation of the rivets instead of the sheets. The utilization of aluminum sheets and copper rivets allows extending the process scope of application from structural to energy distribution parts in which the electrical resistance of joints is a critical design parameter.