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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
- 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.
- Hybrid metal additive manufacturing: A state–of–the-art reviewPublication . Pragana, J. P. M.; Sampaio, Rui F. V.; Bragança, Ivo; Silva, C. M. A.; Martins, P. A. F.This paper starts from the early developments and working principles of the additive manufacturing of polymers, continues with a glimpse on the extension to metals with identification and characterization of the two most widespread technologies, and ends with an overview of the recent developments in hybrid metal additive manufacturing. Earlier classifications of hybrid manufacturing with roots on the utilization of primarily processed raw materials in the form of ingots, sheets, rods, tubes, profiles, powders and pellets are revisited in the light of the emergence of a new type of hybridization resulting from the combination of additive manufacturing with traditional manufacturing processes. Special emphasis is given to the combination of additive manufacturing with forming processes with the two-fold objective of (i) increasing the applicability domain of metal additive manufacturing and overcoming its limitations related to low productivity, metallurgical defects, rough surface quality and lack of dimensional precision, and (ii) adding flexibility and fostering new applications of traditional forming processes.
- Injection lap riveting of aluminum busbars — a thermo-electro-mechanical investigationPublication . Pragana, J.P.M; Sampaio, Rui F. V.; Bragança, Ivo; Martins, P. A. F.This paper presents a new mechanical joining process to assemble aluminum busbars in energy distribution systems. The process is based on the extension of injection lap riveting to the connection of busbars made from the same material as the rivets and requires redesigning the joints to ensure complete filling with good mechanical interlocking and appropriate contact pressures on the overlapping area. The experimental work was carried out in unit cells and involved the fabrication of the riveted joints and the evaluation of their electrical resistance at different service temperatures. Comparisons with the bolted joints that were fabricated and tested for reference purposes show that injection riveted joints provide lower values of electrical resistance and require much less space for assembly due to the absence of material protrusions above and below their surfaces. Numerical simulation with finite elements allows the relating of the reduction in electrical resistance with the changes in the electric current flow when the bolts are replaced by the new type of rivets. The experimental and numerical predictions revealed that the new type of rivets experience an increase in electrical resistance of up to 6 μΩ (30%) when the service temperature approaches 105 °C. Still, the resistance at this temperature (26.2 μΩ) is more than 3 times smaller than that of the bolted joints (80.5 μΩ).
- Form-fit joining of hybrid busbars using a flexible tool demonstratorPublication . Reichel, A.; Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper is focused on hybrid busbars made from copper and aluminum strips and presents a flexible tool demonstrator capable of replicating material flow in the lancing, bending and compression stages of a new joining by forming process without auxiliary elements. The flexible tool demonstrator is defined by its modular concept that allows the active tool components to be easily interchanged for testing and exploring different materials and thickness combinations, surface conditions and cross-section areas of the strips under laboratory conditions. Experimental and numerical simulation with a selected hybrid busbar geometry validates the overall concept and fabrication of the demonstrator and shows that the new joining by forming process can produce permanent form-fit joints with smooth upper and lower surfaces containing all the plastically deformed material within the thickness of the two strips. Complete filling of the free volume left in-between the thickness of the two strips allows obtaining an electric resistance lower than that of fastened hybrid busbars.
- 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.
- Joining of hybrid busbars for e-mobility: an economic and environmental studyPublication . Pragana, João P. M.; Sapage, Miguel S. T.; Sampaio, Rui F.V.; Bragança, Ivo; Ribeiro, Inês; Silva, Carlos M. A.; Martins, PauloThis paper presents a model to evaluate and analyze the costs of joining hybrid (copper-aluminum) busbars when different production processes are deployed. The process-based cost model (PBCM) is built upon the subdivision of the production cycle in three different stages related with the fabrication or purchase of auxiliary joining elements, preparation of the individual copper and aluminum conductors, and final joining of the hybrid busbars. The total cost per hybrid busbar is obtained by converting the major physical, human, and financial resources associated with the production cycle into itemized costs that make use of the expenses in materials, labor working hours, number and usage time of machines and tools, among other production costs. Application of the PBCM is illustrated with three different joining processes and enriched with a life cycle assessment (LCA) focused on the environmental performance of hybrid busbars throughout its fabrication, service use and end of life. The combined economic and environmental sustainability analysis of joining hybrid busbars allows concluding that despite conventional fastening being the cheaper process it has the highest environmental impact due to the use of bolts, nuts and washers made from galvanized medium carbon steel. Injection lap riveting arises to be the most well-balanced process in terms of production cost and environmental impact.
- Multi-planar injection lap rivetingPublication . Sapage, Miguel S. T.; Pragana, João P. M.; Sampaio, Rui F.V.; Bragança, Ivo; Silva, Carlos M. A.; Martins, PauloThis paper is focused on multi-planar hybrid busbars made from copper and aluminum for electric energy distribution systems. The objective is to provide an overview of its assembly by injection lap riveting in multidirectional tools and to compare the electrical performance of its joints against that of conventional (in-plane) busbars. The injected lap riveted joints require a dovetail ring hole and a countersunk hole to be first machined in the overlapped copper and aluminum sheets and then to inject the semi-tubular rivets by compression through the lined-up holes in order to fix the sheets in position. In this work, the injection of the semi-tubular rivets was carried out in a laboratory multidirectional tool set that converts the vertical press stroke into two-orthogonal horizontal movements by means of cam slide units consisting of compression punch holders and sliding wedge actuators attached to the upper bolster. Experimental results obtained for a multi-planar, three-conductor, rake-shaped elbow of a hybrid busbar system allow concluding that while the required compression force is proportional to the number of injected lap riveted joints, the electrical performance is non-proportional due to changes in the distribution of electric current density. Numerical simulation with finite elements gives support to the discussion and allows readers to recognize the pitfalls of designing busbar joints exclusively based on mechanical requirements.
- A software for research and education in ductile damagePublication . Sampaio, Rui F.V.; Alexandre, N. S. M.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper gives insight into the development and utilization of a computer software that uses raw experimental data from the load cells and DIC systems to obtain the instant of time at fracture tf, the loading paths in principal strain space & epsilon;1 = f(& epsilon;2), and their conversion into the space of effective strain vs. stress triaxiality & epsilon; = f (& eta;). Special emphasis is given to the different assumptions and stress triaxiality measures that can be used to convert the loading paths from principal strain space into the space of effective strain vs. stress triaxiality. Results for double-action radial extrusion show the differences of treating the loading paths as linear or non-linear from beginning until the onset of failure by fracture. Results also allow concluding on the importance of accounting for the stress triaxiality derived from individual experimental measurements in an average sense over the entire loading paths, to avoid overestimation and mislocation of the fracture forming limits. The applicability of the software for education and training of students in formability is also discussed.