Percorrer por autor "Martins, P. A. F."
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- Formability limits in sheet-bulk formingPublication . Leonardo, Pedro N. C.; Magrinho, João P.; Bragança, Ivo; Silva, M. Beatriz; Silva, Carlos; Martins, P. A. F.This paper is focused on the characterization of the fracture limits in sheet-bulk forming. The approach extends to crack opening in mode III (out-of-plane shearing), a digital image correlation-based methodology for determining the fracture forming limits in mode I (tension) and mode II (in-plane shearing). For this purpose, a sheet lengthwise compression test with different end constraints is developed and utilized to obtain the strain loading paths up to fracture in mode III, for the first time directly from sheets. The three fracture forming limits of sheet-bulk forming are first characterized in principal strain space and then transformed into the space of effective strain vs. stress triaxiality by means of an analytical procedure based on anisotropic plastic deformation under proportional loading. A new uncoupled ductile damage criterion is introduced and successfully implemented in an in-house finite element computer program to predict the location where the out-of-plane shearing cracks are triggered. The overall results point out to the difficulty in merging the three different fracture forming limits into a single-branched fracture locus covering the plane stress deformation conditions and the three-dimensional states of stress that are likely to be found in sheet-bulk forming processes.
- Formability of a wire arc deposited aluminium alloyPublication . Silva, C. M. A.; Bragança, Ivo; Cabrita, A.; Quintino, L.; Martins, P. A. F.This paper is focused on the formability of the aluminium alloy AA5083 deposited by wire arc additive manufacturing (WAAM). The presentation draws from metal deposition with a robotic welding system to mechanical and formability characterization by means of standard test specimens. Finite element analysis using porous metal plasticity is utilized to model strain hardening and the changes in porosity due to plastic deformation. Results show that the deposited aluminium alloy has excellent ductility and that its final stress response can significantly improve as a result of strain hardening. Voids resulting from metal deposition are closed by negative values of stress-triaxiality resulting from compression forming. The investigation is also a step towards understanding the potential of including intermediate forming operations in conventional wire arc additive manufacturing (WAAM), consisting of metal deposition and machining.
- 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.
- 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.
- Hybrid wire‑arc additive manufacturing of conformal cooling channels: a feasibility studyPublication . Pragana, João; Bragança, I. M. F; Silva, C. M. A.; Martins, P. A. F.This paper investigates the feasibility of hybridizing wire-arc additive manufacturing with metal forming to create conformal cooling channels in moulds for hot material processing. An axisymmetric mould is used as a prototype test case and plastic deformation by flaring of the additively deposited inner tubular element is utilized to create the overhangs for connecting with the outer tubular element that are needed to fabricate the cooling channels. Finite element modelling of plastic deformation by flaring of the additively deposited material is carried out to identify the different modes of deformation and to determine the maximum allowable radius of the overhangs. Results show that the proposed hybridization of wire-arc additive manufacturing with plastic deformation by flaring adds flexibility to the design and fabrication of conformal channels in the as-built mould by eliminating the use of support structures and avoiding the use of complex deposition strategies. Finishing of the functional surfaces of the as-built mould by turning ensures the required accuracy and surface quality.
- 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).
- Invisible mechanical lap joints for metal–polymer laminatesPublication . Batista, R. J. S.; Bragança, I. M. F; Silva, C. M. A.; Pragana, J. P. M; Martins, P. A. F.Joining by compression of metallic inserts has been recently developed by the authors to create invisible lap joints between metal and polymer laminates. This paper revisits the process with the objective of proposing a new type of bi-material (polymer–metal) cylindrical insert for lightweight construction applications that is capable of ensuring complete filling of the joint at the end of stroke without increasing the forming force and the risk of sheet bending. The presentation is built upon a combined experimental and finite element research work focused on the modes of deformation, formability limits, forming forces, and resistance strength that lap joints produced with the new bi-material cylindrical inserts are able to withstand before failing. Results allowed designing a simple and easy to fabricate bi-material cylindrical insert that overcomes the main problems that have been pointed out to the metallic inserts earlier proposed by the authors.
- Joining of sheets by sheet-bulk forming: a numerical and experimental studyPublication . Bragança, Ivo; Loja, Amélia; Silva, C. M. A.; Alves, L. M.; Martins, P. A. F.The authors present a new mechanical joining process that allow to connect two sheets perpendicular to one another, a variant of the traditional ‘mortise-and-tenon’ joint. This investigation is focused on joining similar and dissimilar sheets based on sheet-bulk forming and it is supported by experimental data and numerical simulation with an in-house finite element program, I-form. Results show that is possible to join materials by sheet-bulk forming and is also presented the joinability window as a function of the main operating parameters. The divided flux technique could be a valid option to eliminate the observed small clearance related to the elastic recovery of polymers. Furthermore, this technique could also be applied whether a lower initial punch force is required or even a different head morphology is desired. Destructive tensile tests were performed to determine the maximum force that the joints are capable to withstand without failure and to identify the failure mechanism.
- 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.
- 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