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- Double-sided injection lap rivetingPublication . Pragana, João; Sampaio, Rui F. V.; Chantreuil, Justin; Bragança, Ivo; Silva, Carlos; Martins, PauloThis article presents a double-sided injection lap riveting process for fixing two overlapped sheets with tubular rivets at room temperature. The rivets are injected by compression into the dovetail ring holes that are previously machined in both sheets, and, in contrast to other joining by plastic deformation processes making use of auxiliary elements, the resulting joints are hidden inside the sheets without material protrusions above or below their surfaces. The new process is applied in the fabrication of aluminum busbar joints for energy distribution systems, and comparisons are made against conventional bolted joints that were fabricated for reference purposes. The work combines experimentation and finite element modelling, and results allow concluding that, in addition to invisibility and savings in assembly space, there are important gains in the thermo-electrical performance of the new joints that are of paramount importance for electric distribution applications.
- Revisiting the fracture forming limits of bulk forming under biaxial tensionPublication . Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThe formability limits of bulk metal forming in principal strain space and in the effective strain vs. stress-triaxiality space are characterized by an uncertainty region in which cracks may be triggered by tension (mode I of fracture mechanics) or by out-of-plane shear (mode III). The problem in obtaining experimental data in this region has been known for a long time and the main objective of this paper is to present a new upset formability test geometry that can effectively contribute to the characterization of the formability limits of bulk metal forming parts subjected to biaxial tension. Alongside with this objective, this paper also presents an analytical expression for converting the fracture forming limit line corresponding to crack opening by mode III in principal strain space into a hyperbolic fracture limit curve in the effective strain vs. stress-triaxiality space. The overall methodology employed by the authors combines experimentation along with analytical and numerical modelling, and the contents of the paper is a step towards diminishing the actual lack of knowledge regarding failure by fracture in bulk metal forming parts subject to stress-triaxiality values beyond uniaxial tension. Results show that a new uncoupled ductile fracture criterion built upon combination of the integrands of the Cockcroft-Latham and McClintock criteria can be successfully used to model the physics of the bulk metal forming limits for the entire range of stress-triaxiality values corresponding to cracking on free surfaces.
- Resistance element welding of sandwich laminates with hidden insertsPublication . Calado, Francisco N.; Pragana, João; Bragança, Ivo; Silva, Carlos M. A.; Martins, Paulo A. F.This paper presents a new resistance element welding process capable of producing invisible lap joints between steel-polymer-steel composite laminates. The process involves pre-drilling a flat-bottom hole in each laminate to remove the polymer core and one of the steel sheets, and positioning a cylindrical insert inside the two adjoining holes for subsequent resistance welding. Finite element modeling is utilized to construct the weldability lobe and to identify the parameters that lead to the formation of acceptable joints. Experimental results confirm the applicability of the process to produce invisible lap joints without signs of material protrusions or local indentations resulting from squeezing the polymer out to create contact between the steel sheets. Destructive peel and shear tests allow determining the maximum forces that the joints can safely withstand and comparing their performance against alternative joined by forming lap joints in which the mechanical interlocking is also hidden inside the laminates.
- Influence of corrosion on the electrical and mechanical performance of hybrid busbarsPublication . Sampaio, Rui F. V.; Bragança, Ivo; Pragana, J. P. M; Silva, C. M. A.; Fernandes, João C. S.; Martins, Paulo A. F.This paper is focused on the electrical and mechanical performance of aluminum-copper hybrid busbars subjected to corrosion over time. Two different types of hybrid busbars with joints produced by conventional fastening with M8 hexagonal socket head bolt-nut pairs made from medium carbon steel and by a new injection lap riveting process with semi-tubular rivets made from the material of the softer conductor are used and subjected to salt spray and electrochemical tests. Electrical resistance measurements performed on hybrid busbars taken from the corrosion testing cabinet at the end of each exposure period allow concluding that the new injection lap riveted hybrid busbars have a better electrical performance over time due to the elimination of fasteners with a higher electrical resistivity than aluminum and copper and to the elimination of the aluminum-steel and copper-steel galvanic pairs. The capability of the injection lap riveted hybrid busbars to withstand shear forces after corrosion testing also revealed to be adequate and like those of the original (uncorroded) hybrid busbars.
- Resistance element welding of sandwich laminates with hidden insertsPublication . Calado, Francisco N.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, Paulo A. F.This paper presents a new resistance element welding process capable of producing invisible lap joints between steel-polymer-steel composite laminates. The process involves pre-drilling a flat-bottom hole in each laminate to remove the polymer core and one of the steel sheets, and positioning a cylindrical insert inside the two adjoining holes for subsequent resistance welding. Finite element modeling is utilized to construct the weldability lobe and to identify the parameters that lead to the formation of acceptable joints. Experimental results confirm the applicability of the process to produce invisible lap joints without signs of material protrusions or local indentations resulting from squeezing the polymer out to create contact between the steel sheets. Destructive peel and shear tests allow determining the maximum forces that the joints can safely withstand and comparing their performance against alternative joined by forming lap joints in which the mechanical interlocking is also hidden inside the laminates.
- Coin minting by additive manufacturing and formingPublication . Pragana, João; Rosenthal, Stephan; Alexandrino, Paulo; Araújo, Andreia; Bragança, Ivo; Silva, Carlos; Leitão, Paulo J.; Tekkaya, A. Erman; Martins, PauloAdditive manufacturing is proposed as a novel alternative to coin blank's production routes based on rolling, blanking and edge rimming. The presentation draws from laser powder bed fusion of cylinders, slicing into individual coin blanks by electro discharge machining and surface preparation by polishing, to coin minting in a laboratory press-tool system. Special emphasis is given to material deposition and coin minting due to the originality of producing coin blanks with complex intricate contoured holes and to the necessity of subjecting the additive manufactured coin blanks to extreme compressive stresses that are typical of coin minting. Numerical and experimental results confirm the excellent performance of the additive manufactured coin blanks. The new design layouts included in the additive manufactured coin blanks open the way to produce high value-added singular collector coins, which are disruptively different from those available in the market nowadays.
- Integration of tube end forming in wire arc additive manufacturing: An experimental and numerical investigationPublication . Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloIntegration of tube end forming operations in metal additive manufacturing routes has a great potential for the fabrication of customized features in additively deposited hollow parts. This paper is focused on the integration of tube expansion with rigid tapered conical mandrels to highlight the advantages in the construction of overhanging flares derived from the elimination of support structures and prevention of humping. The work draws from the mechanical and formability characterization of stainless steel AISI 316L tubes produced by wire arc additive manufacturing (WAAM) to the experimental and numerical simulation of the construction of over hanging flares by tube expansion. Strain loading paths obtained from digital image correlation and finite element analysis combined with the strain values at the onset of necking and fracture allow determining the critical ductile damage that additively deposited tubes can safely withstand. Results show that despite formability of additively deposited tubes being influenced by a dendritic based microstructure, their performance is adequate for tube end forming operations, such as tube expansion, to be successfully integrated in metal additive manufacturing without the need of using expensive hardware and complex deposition strategies.
- Hybrid manufacturing of stiffening grooves in additive deposited thin partsPublication . Cristino, Valentino A. M.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper is focused on the hybridization of additive manufacturing with single-point incremental forming to produce stiffening grooves in thin metal parts. An analytical model built upon in-plane stretching of a membrane is provided to determine the tool force as a function of the required groove depth and to estimate the maximum allowable groove depth that can be formed without tearing. The results for additively deposited stainless-steel sheets show that the proposed analytical model can replicate incremental plastic deformation of the stiffening grooves in good agreement with experimental observations and measurements. Anisotropy and lower formability caused by the dendritic-based microstructure of the additively deposited stainless-steel sheets justifies the reason why the maximum allowable depth of the stiffening grooves is approximately 27% smaller than that obtained for the wrought commercial sheets of the same material that are used for comparison purposes.
- Revisiting the ring hoop test in additively manufactured metal tubesPublication . Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, P. A. F.This paper is focussed on the mechanical and formability characterisation of wire-arc additive manufactured (WAAM) AISI 316-L stainless-steel tubes. The methodology to be presented involved carrying out tension and ring hoop tension tests on specimens extracted from the tube longitudinal, transverse and inclined directions. The force evolutions, acquired from the load cells, and the strain measurements, retrieved from digital image correlation and from thickness measurements along the cracks, allowed obtaining the stress-strain curves, the strain paths and the onset of failure by fracture for the three different tube directions. Special attention was paid to the ring hoop test, which was revisited to determine the appropriateness of using ring specimens with one or two dumbbell geometries. The originality of using the ring hoop tension test in WAAM tubes with strong anisotropic behaviour allowed obtaining strain loading paths that range from plane strain to pure shear deformation conditions. Resort to commercial AISI 316-L stainless-steel tubes during the presentation is included for reference purposes.
- New self-clinching fasteners for electric conductive connectionsPublication . Sampaio, Rui F. V.; Martins, Paulo; Pragana, J. P. M; Bragança, Ivo; Silva, C. M. A.This paper presents new rotational and longitudinal symmetric self-clinching fasteners to fabricate reliable connections in busbars with low electrical resistance for energy distribution systems. Connections consist of form-closed joints that are hidden inside regions where two busbars overlap. The investigation into the fabrication and performance of the new self-clinched joints involved finite element modelling and experimentation to determine the required forces and to evaluate the electric current flow and the electrical resistance at different service temperatures. The original design of the joints that was proposed in a previous work was modified to account for busbar strips of copper and/or aluminum with similar or dissimilar thicknesses, connected by means of self-clinching fasteners made from the same materials of the busbars, instead of steel. The effectiveness of the new self-clinched joints was compared to that of conventional bolted joints that are included in the paper for reference purposes. The results show that rotational symmetric self-clinching fasteners yield lighter fabrication and more compact joints with a similar electrical resistance to that of bolted joints. They also show that longitudinal symmetric self-clinching fasteners aimed at replicating the resistance-seam-welding contact conditions yield a reduction in electrical resistance to values close to that of ideal joints, consisting of two strips in perfect contact and without contaminant or oxide films along their overlapped surfaces.