Browsing by Author "Martins, Paulo A. F."
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- Enhancing the performance of double-flush riveted joints through hybridization with adhesive bondingPublication . Alpendre, João M. B.; Rosado, Pedro M. S.; Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos M. A.; Martins, Paulo A. F.This paper explores the potential to enhance the mechanical performance of joints created through a new joining-by-forming technique called hybrid double-flush riveting. To achieve this, adhesive bonding is used to form hybrid lap joints with superior mechanical properties. The study focuses on high-strength steel sheets and starts by identifying the appropriate surface conditions necessary for producing strong adhesive-bonded joints. A similar strategy is applied to construct double-flush riveted joints, focusing on the geometric variables involved in the process. Hybrid joints are then created by integrating adhesive bonding with double-flush riveting, with the second carried out before or after curing is completed. The experimental development is supported by finite element analysis conducted with an in-house computer program. The mechanical performance of the hybrid joints is compared to that of purely adhesive-bonded and conventionally double-flush riveted joints through shear and peel destructive testing. Results demonstrate that hybrid joints ultimately provide greater joint strength for both solicitations. This allows showcasing the hybridization of double-flush riveting with adhesive bonding as an effective solution for applications where joint strength and continuity are essential.
- Exploring wire-arc additive manufactured rivets for joining hybrid electrical busbarsPublication . Rosado, Pedro M. S.; Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos M. A.; Martins, Paulo A. F.This paper presents a joining by plastic deformation process for fabricating hybrid electrical busbars made from copper and aluminum sheets. The process comprises the innovative use of wire-arc additive manufacturing to deposit copper rivets at specific locations on the copper sheets, the machining of slots with the required geometry in the aluminum sheets, and the compression of the copper sheets to force the rivets into the pre-machined slots of the aluminum sheets, creating form-closed mechanical interlocks. The work combines experimentation and finite element modeling to analyze the influence of the most significant process parameters, and the electrical performance of the hybrid busbar joints is evaluated and compared to that of conventional fastened joints at different service temperatures. Results demonstrate the effectiveness and potential advantages of the new joining by plastic deformation process for fabricating hybrid electrical busbars.
- 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.
- A joining by plastic deformation process to fabricate butt joints in copper-aluminium busbarsPublication . Pragana, João P. M.; Sampaio, Rui F. V.; Bragança, Ivo; Silva, Carlos M. A.; Martins, Paulo A. F.This paper presents a feasibility study on the fabrication of butt joints in copper-aluminium (hybrid) busbars by means of an innovative joining by plastic deformation process. The joints are obtained by cutting complementary fixing pairs consisting of fingers and grooves in the busbars and by subsequently upsetting the fingers, in position, to create a permanent interlocking with the grooves of an edge bent busbar. The presentation draws from the process working principle to the mechanical and thermo-electrical performance of the joints. Experimentation and numerical modelling give support to the presentation and demonstrate the effectiveness of the process to produce butt joints in hybrid busbars for power distribution systems.
- 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.
- 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.
- An upset geometry sequence for determining the formability limits in bulk formingPublication . Silva, Carlos M. A.; Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo M. F.; Bragança, Ivo; Martins, Paulo A. F.This paper presents an upset geometry sequence capable of providing strain loading paths from uniaxial compression to biaxial stretching. Combination of digital image correlation and experimental force vs. time evolutions allows determining the fracture strains and the instantaneous slope of the strain loading paths at the instant of cracking in principal strain space. Fractography using scanning electron microscopy helps identifying the crack opening modes corresponding to the different fracture strains. Results show that the proposed upset geometry sequence allows creating a new testing methodology for bulk forming similar to that of sheet forming with the widely used Nakajima test.
- An upset geometry sequence for determining the formability limits in bulk formingPublication . Silva, Carlos M. A.; Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo; Martins, Paulo A. F.This paper presents an upset geometry sequence capable of providing strain loading paths from uniaxial compression to biaxial stretching. Combination of digital image correlation and experimental force vs. time evolutions allows determining the fracture strains and the instantaneous slope of the strain loading paths at the instant of cracking in principal strain space. Fractography using scanning electron microscopy helps identifying the crack opening modes corresponding to the different fracture strains. Results show that the proposed upset geometry sequence allows creating a new testing methodology for bulk forming similar to that of sheet forming with the widely used Nakajima test.