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- A self-clinching fastener for hidden lap jointsPublication . Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo M. F; Silva, Carlos M. A.; Martins, P. A. F.This paper presents a new self-clinching fastener to connect two sheets (or plates), made from similar or dissimilar materials, placed over one another by means of a mechanical form-closed joint that is hidden inside the sheets. The development of the fastener, the definition of its main design variables and the identification of its workability limits are carried out by means of a combined experimental and numerical simulation work based on finite elements. It is shown that self-clinching by pressing the two overlapped sheets against each other to displace material around the annular groove of the fastener shank and create undercuts in both sheets requires an appropriate choice of the design variables. Wrong values of the design variables resulting in lack or excess of material displaced by plastic flow gives rise to inappropriate lap joints that cannot be used in production. The new proposed fastener allows, for the first time ever, joining by forming with the use of auxiliary elements that are harder than the sheet materials to fabricate invisible joints with no material protrusions in applications requiring minimum installation space
- Formability of wire-arc deposited AISI 316L sheets for hybrid additive manufacturing applicationsPublication . Pragana, João P. M.; Bragança, Ivo; Reis, L.; Silva, Carlos M. A.; Martins, P. A. F.This paper is focused on the formability of wire-arc additively manufactured AISI 316L stainless steel sheets with the purpose of analysing the feasibility of including this material in hybrid additive manufacturing chains involving sheet metal forming operations. Conventional tensile tests performed in specimens obtained from different orientations to the building direction were carried out to characterise the mechanical properties, the strain loading paths and the limiting strains at fracture. Microstructure observations combined with fractography analysis add insight to the results by establishing a link between the forming limits by fracture and the crack opening mechanisms. Results obtained for wrought commercial AISI 316L stainless steel sheets are included for comparison purposes and reveal that the additively manufactured sheets have a much stronger anisotropic behaviour and poorer formability due to their dendritic-based microstructure. Still, the forming limits obtained from the experiments allow concluding that the additively manufactured sheets can withstand large plastic deformations and, therefore, can be used in hybrid additive manufacturing routes.
- 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.
- Integration of forming operations on hybrid additive manufacturing systems based on fusion weldingPublication . Pragana, João; Cristino, Valentino A. M.; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper is focused on the integration of metal forming operations in hybrid systems that combine additive manufacturing (AM) by gas metal wire arc and subtractive manufacturing by machining. The investigation is carried out in AISI 316L stainless steel wire and draws from tensile testing to incremental sheet forming of truncated conical shapes. Commercial sheets from the same material are utilized for comparison purposes. Thickness measurements, digital image correlation (DIC), circle grid analysis (CGA) and microstructural and scanning electron microscopy (SEM) observations are carried out to understand how different is the mechanical behaviour of the deposited metal from that of commercial metal sheets and how significant is the influence of the deposited metal microstructure on its overall formability. Results confirm that integration of metal forming operations in hybrid AM routes is feasible despite the formability of deposited metal being smaller than that of the commercial metal sheets due to the strong anisotropy induced by the dendritic based microstructure of the deposited metal. Incremental forming of two deposited parts also allows concluding that integration of metal forming operations in hybrid AM systems is a step towards green and sustainable manufacturing by extending their field of applicability to the fabrication of complex ready-to-use parts requiring combination of different processes.
- 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.
- Groove stiffening of sheets by single point incremental formingPublication . Cristino, Valentino A. M.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos M. A.; Martins, PauloThis paper investigates the applicability of single point incremental forming to fabricate stiffening grooves in thin metallic panels. The work combines experimentation, finite element and analytical modelling to determine the required forces and the maximum allowable groove depths that can be produced without tearing. The analytical modelling is based on a framework that was previously developed by the authors and combines in-plane membrane stretching and fracture forming limits. Comparison of the results obtained by the analytical framework against experimental and finite element data proves its effectiveness to replicate the deformation mechanics of groove stiffening by single point incremental forming. Results also prove that groove stiffening by single point incremental forming is an easy and effective alternative to conventional reinforcement of thin metallic panels by welding or fastening of stringers.
- Electric performance of fastened hybrid busbars: An experimental and numerical studyPublication . Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos M. A .; Nielsen, Chris Valentin; Martins, P.A.F.This paper is focused on fastened hybrid busbars made from copper and aluminium with the purpose of analysing the influence of the steel bolts, of their tightening torque and of the surface condition of the sheets on the electric current flow and electric resistance of the joints. The methodology combines experimentation with unit cells that are representative of the joints and electro-mechanical numerical simulation using a finite element computer program developed by the authors. Results are a step forward in understanding the combined influence of bolts, contact pressure and surface roughness on the electric performance of fastened hybrid busbars. Design guidelines for dimensioning the cross sections of the copper and aluminium sheets and for effectively distributing bolts across the contacting surfaces are also provided.
- 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.
- Joining by forming of additive manufactured 'mortise-and-tenon' jointsPublication . Silva, Diogo F. M.; Bragança, Ivo; Silva, Carlos; Alves, Luís; Martins, PauloThis article is aimed at extending the 'mortise-and-tenon' joining concept commonly utilized in corner or tee joints to lap joints in which one sheet is partially placed over another without any change in their shape. The approach makes use of wire arc additive manufacturing to fabricate the tenons and allows various shapes and thicknesses to be made from a wide range of metallic materials. Upset compression of the tenons is utilized to mechanically lock the two sheets being joined. Experimental and finite element simulation works performed with monolithic (aluminium-aluminium) and hybrid (aluminium-polymer) 'unit cells' consisting of a single lap joint are utilized to investigate the deformation mechanics and the feasibility of the new proposed joining process. Tensile-shear loading tests were carried out to determine the maximum force that the new proposed joints are capable to withstand without failure. Pull-out forces of approximately 8 and 6 kN for the monolithic and hybrid joints allow concluding on the potential of additive manufactured 'mortise-and-tenon' lap joints to connect sheets made from similar and dissimilar materials.