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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.
- 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.
- 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.
- 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.
- On the performance and recyclability of a green composite based on AESO resinPublication . Seabra, Cláudio P.; Sousa, Ana Catarina; Bragança, Ivo; Silva, Carlos; Robalo, M. Paula; Loja, Amélia; Martins, PauloThe aims and objectives of this paper are two-fold: first to investigate the production of a green composite manufactured with renewable materials (i.e., jute fabric as reinforcement, acrylate epoxidase soybean oil (AESO) as matrix and sisal particles (SP) as filler), by the wet layup method; second, to propose a recycling procedure to recover the individual materials and reuse them in the production of a second-life composite. In the first part, di_erent combinations of SP (0, 5 and 10 wt%) and hardener (2, 5 and 10 wt%) were mixed with AESO resin, poured into a mould, cured and submitted to mechanical and physicochemical characterizations to identify the best conditions for the composite production. Virgin green composites with 10 wt% SP, 5 wt% hardener and 5 layers of jute fabric, capable of assuring 91 HA of hardness and 10.6 MPa of tensile strength, were fabricated. The second part describes the recycling process of the composites with acetone, an organic solvente recommended by the safety, health and environmental criteria, to breakdown the resin matrix and recover the jute fabric reinforcement and resin particles, which were then reused to fabricate a second-life composite. Although the hardness values of the second-life composite were smaller (4%) and the tensile strength varied with composition, the absorption of water was considerably reduced (in the range of 22 to 51%). This last result mitigates one of the green composite’s limitations and fosters circular economy by assuring the applications of the second-life composite in the field of transportations, packing and furniture, among others.
- A new joining by forming process to produce lap joints in metal sheetsPublication . Pragana, João; Silva, Carlos; Bragança, Ivo; Alves, Luís; Martins, PauloThis paper proposes a new joining by forming process to produce lap joints in metal sheets. The process combines partial cutting and bending with mechanical interlocking by sheet-bulk compression of tabs in the direction perpendicular to thickness. The lap joints are flat with all the plastically deforming material contained within the thickness of the two sheets partially placed over one another. The design of the lap joints is performed by a simple analytical model and the overall concept is validated by means of numerical modelling and experimentation. Destructive shear tests demonstrate the effectiveness and performance of the new proposed lap joints. (C) 2018 Published by Elsevier Ltd on behalf of CIRP.
- Two-stage joining of sheets perpendicular to one another by sheet-bulk formingPublication . Silva, Carlos; Bragança, Ivo; Alves, Luis; Martins, PauloThis paper proposes a new joining by forming process for fixing longitudinally in position two metal sheets (or plates) perpendicular to one another, at room temperature. The proposed process employs a counterbored variant of the ‘mortise-and-tenon’ joint that eliminates the protrusion of the tenon beyond the mortise after mechanical locking by plastic deformation. The presentation draws from the workability limits and material stress-strain characterization to validation by joining and destructive pull-out testing. Results demonstrate the effectiveness of the new proposed process for producing flat joint surfaces, which are advantageous over typical ‘mortise-and-tenon’ protruded surfaces in most applications.
- A strain acceleration method to identify the onset of diffuse neckingPublication . Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper presents an innovative ‘strain acceleration method’ for determining the onset of diffuse necking in sheet forming tests using data obtained from digital image correlation (DIC). The method identifies the onset time of diffuse necking and provides the corresponding in-plane principal strain values by detecting a local extreme in the second derivative of the minor principal in-plane strain with respect to time at the edges of the sheet surface region where diffuse necking occurs. Results obtained from applying the method to tensile testing on two different materials and comparisons with available methods based on force-time or principal strain rate evolutions confirm its accuracy and validity. The new method was implemented in a computer software to be used for research and education that also enables determination of localized necking and fracture and plotting the strain loading paths in principal strain space.
- Experimental and numerical study of the joinability of sheets by sheet-bulk formingPublication . Bragança, Ivo; Loja, Amélia; Silva, Carlos; Alves, Luís; Martins, PauloThe authors present an innovative mechanical joining process that allows to connect perpendicular sheets to one another. This study is focused on joining similar and dissimilar sheets, based on sheet-bulk forming technology, and it is supported by experimental data and numerical simulation. Destructive tensile tests of different joined materials were performed to determine the maximum force that the joints are capable to withstand without failure. The joining technique should be chosen according to the materials joint combination. The two-stages technique could be a valid option to overcome the clearance between the plastically deformed polycarbonate tenon and aluminium mortise.
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