Percorrer por autor "Martins, Paulo A. F."
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- Advanced double-flush riveting for multistage forming toolsPublication . Silva, Carlos M. A.; Pragana, João P. M.; Sampaio, Rui F. V.; Bragança, Ivo M. F.; Martins, Paulo A. F.; Bragança, IvoThis paper presents a novel double-flush riveting process for creating lap joints in sheet metal parts. The process involves punching, forging, and shaving to produce countersunk holes with precise geometry, followed by compression of cylindrical rivets into the holes of two overlapped sheet parts to create form-closed mechanical interlocks. Experimental and numerical analyses examine the influence of the primary process parameters on interlock quality and required forming and destructive forces. The investigation is a step forward in joining technology by offering a simple alternative to welding, bonding, and mechanical joining, with seamless integration into multistage forming tools.
- Characterization of sheet formability limits using a novel diagonal-cruciform test specimenPublication . Sampaio, Rui F. V.; Dias, Eduardo B. G.; Viegas, João M. A.; Pragana, João P. M.; Bragança, Ivo M. F.; Silva, Carlos M. A.; Martins, Paulo A. F.; Bragança, IvoThis paper introduces an innovative diagonal-cruciform test specimen that significantly enhances the characterization of formability limits in sheet metal forming. The specimen's unique design features a reticular two-dimensional geometric structure, with four triangular arms connecting at the center, which effectively induce biaxial tension stress states when subjected to uniaxial loading. Furthermore, the incorporation of machined spherical cups at its center to locally reduce thickness ensures that damage accumulates in this region. Experimental strain loading paths are captured using digital image correlation (DIC) and analyzed with in-house software developed specifically for research and education on material formability. The software identifies and plots the onsets of necking and fracture in principal strain space, and results prove that the diagonal-cruciform specimen is highly effective in generating stable biaxial tension strain loading paths in C11000 copper sheets, operating under friction-independent conditions without the necessity for specialized testing equipment. The fracture limits are subsequently validated by comparing them against the strain loading paths obtained from a single-point incremental sheet-formed part up to failure. The investigation confirms the versatility and robustness of the novel diagonal-cruciform test specimen for evaluating the formability of C11000 copper sheets and provides valuable insights into its potential application across the broader field of sheet formability characterization.
- A complete characterization of the fracture forming limits in bulk forming by means of an upset geometry sequencePublication . Sampaio, Rui F. V.; Pragana, João P. M; Bragança, Ivo M. F.; Bragança, I.M.F.; Silva, Carlos M. A.; Martins, Paulo A. F.This paper presents an upset geometry sequence to determine the fracture forming limits in a wide variety of strain loading paths ranging from uniaxial compression to equibiaxial tension. The strains at fracture in principal strain space are obtained by combination of digital image correlation and experimental evolutions of the compression force versus time, and their representation in the effective strain versus stress triaxiality space is accomplished by means of a new analytical framework that uses the instantaneous slope of the strain loading paths. Modeling of the experimental strains at fracture by means of an uncoupled ductile fracture criterion built upon combination of the Cockcroft-Latham and McClintock criteria and fractography analysis using a scanning electron microscope allow understanding and characterizing the crack opening modes by shear and tension as well as the uncertainty region inside which mixed crack opening modes are observed. Results confirm that the overall philosophy and objectives underlying the new upset geometry sequence for determining the fracture forming limits in bulk forming resemble those of the Nakajima test that is commonly used in sheet forming.
- A dieless Nakajima test for additively deposited materialsPublication . Sampaio, Rui F. V.; Rosado, Pedro M. S.; Pragana, João P. M.; Bragança, Ivo M. F.; Nielsen, Chris V.; Silva, Carlos M. A.; Martins, Paulo A. F.; Bragança, IvoThis paper focuses on a novel Nakajima test for characterizing the formability of additively deposited materials without extracting sheet blanks. Material is deposited by wire-arc additive manufacturing in the form of ‘Π-shaped’ vertical walls, and the testing regions are then machined to the desired thickness and shapes for subsequent deformation with a hemispherical punch in a press. Digital image correlation is used to obtain the strain loading paths of the deposited material up to failure. The novel test eliminates the need for the special-purpose tool setup of conventional Nakajima tests, simplifying the overall procedure and reducing costs.
- Double-Flush Riveting for Hybrid Busbar AssemblyPublication . Sampaio, Rui F. V.; Pragana, João P. M.; Figueiredo, Miguel P.; Bragança, Ivo M. F.; Silva, Carlos M. A.; Martins, Paulo A. F.
- 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 the hybridization of wire-arc additive manufacturing and resistance spot weldingPublication . Barros, Maria R.F.; Rosado, Pedro M. S.; Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo M. F.; Silva, Carlos M. A.; Martins, Paulo A. F.
- 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.
- Formability assessment of additively manufactured materials via dieless Nakajima testingPublication . Sampaio, Rui F. V.; Rosado, Pedro M. S.; Pragana, João P. M.; Bragança, Ivo M. F.; Silva, Carlos M. A.; Rosa, Luís G.; Martins, Paulo A. F.; Bragança, Ivo; MDPIThis paper delves into the formability of material deposited by wire arc additive manufacturing. It presents a novel dieless Nakajima testing procedure that offers a practical solution for obtaining strain loading paths up to failure directly from the deposited material without the need for extracting sheet blanks. The procedure involved machining a region of the deposited material to the desired shape and thickness and using a press to drive and control the movement of a hemispherical punch. The test was designed using finite element modeling, and its effectiveness in obtaining the required strain loading paths directly from the deposited material was verified through experimentation with digital image correlation. Importantly, this novel test eliminates the need for the special-purpose tool setup required in conventional Nakajima sheet formability tests, thereby simplifying the overall testing process.
- Hybrid additive manufacturing of silver collector coins: an experimental and numerical case studyPublication . Sampaio, Rui F. V. Pragana, João P. M.; Alexandrino, Paulo; Aráujo, Andreia; Bragança, Ivo M. F.; Silva, Carlos M. A.; Martins, Paulo A. F.
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