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This paper is focused on double-sided self-pierce riveting (DSSPR) of overlapped sheets made from dissimilar materials with very different mechanical strengths. The methodology draws from previous work on sheets made from similar or identical materials and the extension to dissimilar materials is built-upon a combined experimental and numerical investigation that makes use of AA5754-H111 aluminum and polyvinylchloride sheets. Results show that reengineering of conventional DSSPR by modification of the chamfered rivet angles to account for the different resistances to penetration of the two materials is not effective, because it cannot solve the formation of asymmetric mechanical interlockings with very small undercuts in the harder sheet material. The proposal of an innovative two-stroke DSSPR process in which the tubular rivet is first forced through the harder sheet with the help of a dedicated compression tool, and only subsequently pressed through the softer sheet, proves to be effective in obtaining symmetric joints with good undercuts in both sheets. Destructive shear tests confirm the good performance of the joints produced by means of the new proposed two-stroke DSSPR.

Flexible roll forming process is an advanced manufacturing process, for producing complex and lightweight profiles with variable cross-section. One of the major observed defects in this process is the flange wrinkling of the profiles, which has limited its development especially for the automotive industry. In this paper, the deformation mechanics of the flexible roll forming process is revisited, and the methodology for predicting the onset of wrinkling, which had previously been developed based on the wrinkling limit curve in the space of effective strain versus stress triaxiality, is improved. Then, a new criterion based on the wrinkling limit curve is proposed to predict the possibility of wrinkling in finite element analysis. The criterion is applied to analyze the influence of geometric parameters of the variable cross-section profile and material properties of the sheet on the occurrence of wrinkling. The prediction accuracy is evaluated by performing experiments with different profile shapes. Results show that the new proposed criterion can successfully predict the onset of wrinkling in the flexible roll forming process, and its utilization is recommended for predicting wrinkling in the finite element analysis of sheet metal forming processes.

This paper presents a new joining by forming process capable of attaching a sheet to the end of a tube without the need of accessories and extra materials. The process is named as ‘self-pierce riveting of tubes to sheets’ and is an extension to sheet-tube connections of the working principle of double-sided self-pierce riveting utilized for connecting two overlapped sheets. The process requires tubes with chamfered ends to be pushed against the sheets and its ends to flare (curl) in order to create an interlocking that holds the sheets and tubes tightly together. The resulting joints are invisible because the interlockings are hidden inside the sheets. The presentation draws from deformation mechanics and identification of the major parameters to characterization of the interlocking and evaluation of the pull-out destructive strength that the new joints are capable to withstand without failure. Aluminium sheets and carbon steel tubes are utilized to demonstrate the effectiveness of the new joining by forming process and to demonstrate its ability to connect sheets and tubes made from dissimilar materials at room temperature. Finite element modelling using an in-house computer program gives support to the presentation.

This paper presents a new joining by forming process named as “double-sided self-pierce riveting” that makes use of tubular rivets with simple geometry placed in-between two sheets to be joined. The sheets are pushed against each other by flat parallel punches so that the rivets are forced through the sheets and their ends are flared (curled) to create mechanical interlocking’s which holds the sheets tightly together. The methodology draws from deformation mechanics and identification of major process parameters, to fabrication and destructive testing of joints. Experimental work in AA5754-H111 aluminum sheets and AISI 304 stainless steel tubular rivets was supported by finite element modelling. Results allow concluding that the new process incorporates the advantages of conventional self-pierce riveting and successfully overcomes its main disadvantages, namely the material protrusions above and below the sheet surfaces and the difficulties in joining sheets of dissimilar materials with large thicknesses. The new mechanical joints also have the advantage over those produced by clinching and self-pierce riveting of being invisible, because they are hidden inside the cross section of the overlapped sheets.

This paper presents a new joining by forming process to produce lap joints in metal-polymer sandwich composite sheets. The process involves drilling a blind hole in each sheet for removing the upper metal skin and the polymer core layer, and fixing them together by compressing a metal insert placed in-between to obtain a form-fit mechanical nugget. The cross-section of the joint resembles that of resistance spot welding, with the cold formed insert (‘nugget’) hidden inside the sheets. The geometry and size of the inserts are analysed by finite elements and experimentation to determine the effectiveness and performance of the new joints.

This 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.

This paper is focused on injection by upsetting of sheet-formed side walls. The presentation draws from fundamental aspects of material flow to typical failures caused by buckling and by the development of material flaws. Results show that the utilization of two-piece blank holders in conjunction with an adequate selection of the die cavity thickness and the creation of relief-holes to allow divided material flow are effective and easy-to-implement strategies to increase process formability and ensure the production of sound sheet-bulk parts. The work is carried out in two completely different materials (polyvinylchloride and aluminium AA1050) and its originality stands not only for the investigation on sheet injection but also for the utilization of thermoplastics in sheet-bulk forming. Numerical modelling with finite elements gives support to the overall results and discussion.

This paper is focused on fixing sheets to tubes away from the tube ends by annular sheet squeezing. The main objectives are the investigation of the physics behind material separation at the cross-section recess corner of the punches and the determination of the appropriate squeezing depth values. The methodology combines experimentation and finite element modelling and the overall presentation is illustrated with selected test cases retrieved from the experimental work plan. Results show that material separation is caused by through-thickness shear, that forces and energy required to form the new surfaces are negligible and that appropriate squeezing depth values result from a compromise between the quality of the mechanical interlocking and the pull-out destructive strength of the joints.

This paper proposes a new joining by forming process to secure a sheet to a tube away from the tube end. The process is an alternative to welding, adhesive bonding, mechanical fastening or riveting and consists of squeezing the sheet adjacent to the outer tube radius, instead of applying direct loading on the tube itself as is customary in existing joining by forming processes. This process is cost effective, can be easily used on site and prevents cracking of the plastically deformed beads in tubular materials with low fracture toughness. The presentation combines experimentation and finite element modelling to identity the major process parameters, understand their influence in plastic flow and characterize the different modes of deformation that lead to acceptable and unacceptable joints. Utilization of a deformation-zone geometry parameter Δ , defined as the ratio of the sheet thickness to the cross-section recess length of the punch, is successfully utilized to characterize plastic flow inside the sheet thickness and to establish fundamental design rules. Appropriate values of the deformation-zone parameter in the range Δ = 2 − 3 and placed in-between the limiting values corresponding to inhomogeneous and homogeneous deformation, revealed adequate to obtain sound sheet-tube connections. The maximum pull-out forces are sensitive to the direction of applied loading due to the non-symmetric shape of the joints but no detachments were observed for forces up to 2 kN.

Este trabalho foi financiado pelo Concurso Anual para Projetos de Investigação, Desenvolvimento, Inovação e Criação Artística (IDI&CA) 2016 do Instituto Politécnico de Lisboa. Código de referência IPL/2016/CompSBJ_ISEL Submitted by Isabel Melo (imelo@sa.isel.pt) on 2019-07-29T09:59:38Z No. of bitstreams: 1 Experimental_MARLoja.pdf: 2048657 bytes, checksum: 5c5af5020ac2fe21c79ab3e66ecd0eb8 (MD5) Made available in DSpace on 2019-07-29T09:59:38Z (GMT). No. of bitstreams: 1 Experimental_MARLoja.pdf: 2048657 bytes, checksum: 5c5af5020ac2fe21c79ab3e66ecd0eb8 (MD5) Previous issue date: 2019-07 info:eu-repo/semantics/publishedVersion

This paper proposes a methodology to determine the formability limits of thin-walled tubes and to plot them in principal strain space and in the space of effective strain vs. stress-triaxiality. Digital image correlation (DIC), combined either with time-dependent methodologies or strain-force approaches, is utilized to identify the onset of failure by necking and obtain the corresponding limit strains. Thickness measurements and determination of the gauge length strains across the cracked regions are utilized to characterize the onset of fracture and to evaluate the fracture limit strains. Results show that the utilization of tube expansion with rigid punches and elastomers allow obtaining strain loading paths and fracture loci by necking and fracture across a wide range of tube forming conditions ranging from biaxial stretching in the first quadrant to pure tension in the second quadrant of principal strain space. The fracture forming line (FFL) is the first time ever determined for thin-walled tubes. The forming limit curve (FLC) and the FFL resemble those of sheet and strip materials and their use is of paramount importance in the design and optimization of tube forming processes.

Sheet-bulk forming (SBF) processes based on a combination of deep drawing and upsetting are being increasingly used to produce new sheet metal parts with significant changes in wall thickness to accommodate three-dimensional functional features such as solid bosses, teeth and ribs. The utilisation of sheet-bulk forming processes introduces additional complexity in the design, setup and costs of tooling due to the inherent high forces and complex material flow that easily lead to underfilling and failure in critical die regions. This paper is concerned with these issues and is focused on the development of a sheet-bulk forming demonstrator capable of replicating material flow and failure in sheet thickening, local sheet thickening and sheet injection operating conditions. The demonstrator combines sheet bending and upsetting operations with a flexible design concept based on the utilisation of active modular tool parts to allow different materials, geometries and tool concepts to be tested under laboratory conditions and rapidly transferred to industrial SBF tools.

This article presents a new type of bi-material coin with a polymer centre and a metal ring. The polymer-metal coin is fabricated by combination of coin minting and joining by forming in a single die stroke. The design of the coin is based on an analytical model built upon plasticity theory and plastic instability of circular plates under uniform radial edge compression. The analytical model and the bi-material coin concept are supported and validated by means of finite element modelling and experimentation. Destructive tests for pushing the polymer centre out of the metal ring demonstrate the effectiveness of the mechanical joint resulting from the interface contact pressure between the polymer and the metal. The new bi-material coin was developed to obtain new aesthetic effects for the collection market, and its polymer core can additionally be used for other applications such as the incorporation of advanced security features in high-denomination coins.

This article describes a methodology to characterize the failure limits by necking and fracture, and to determine the critical value of ductile damage in tube forming. The methodology makes use of digital image correlation, thickness measurements and force–displacement evolutions to obtain the strain loading paths and the strain values at the onsets of failure by necking and fracture. The onset of failure by necking is determined by a new technique that combines the strain and force–displacement evolutions whereas the fracture strains at the cracked regions of the tubes are obtained by a similar technique utilized by the authors in sheet metal forming. Transformation of the strain loading paths from principal strain space into the space of effective strain versus stress-triaxiality allows determining the critical experimental value of ductile damage at the onset of failure by necking. The methodology is applied to tube expansion with circular, elliptical and square cross-section punches and results confirm its importance and helpfulness for researchers and engineers involved in the development and optimization of industrial tube forming processes.

This paper proposes a new joining by plastic deformation process to connect sheets to rods that extends a previous idea of the authors to connect sheets to thin-walled tubes. The process makes use of boss forming for compressing the outer radius of the rods and pile-up material into annular flanges with controlled cross-section geometry and size. Special emphasis is put on the identification of the major operating parameters and on understanding their influence on material flow. Two different modes of deformation are identified and a distinction is made between unacceptable chip formation and successfully material pile-up to form the annular flanges that are needed to connect the sheets to the rods. Special design of the upper die to include a pressure ring is introduced as an efficient solution to prevent cracking of the new freshly formed surfaces of the rods during material pile-up. The presentation is supported by experimentation and finite element modelling and the overall performance of the new proposed joint is evaluated by means of a destructive pull-out test.

This paper presents a new joining method by a forming process for attaching sheets to tube ends. The process consists of two different forming stages carried out sequentially in a single stroke. Firstly, the free tube end is flared by compression with a contoured die, then is squeezed (indented) against the sheet surface to create a mechanical interlocking. The new process is carried out at an ambient temperature and, in contrast to existing joining by forming operations based on tube expansion, it avoids seal welds, tube protrusions above the sheet surfaces, and machining of grooves on the sheet holes to obtain the form-fit joints. The paper starts by analyzing the process deformation mechanics and its main operating variables and finishes by presenting examples that demonstrate its effectiveness for attaching sheets to tube ends made from polyvinylchloride and aluminum. Experimental and numerical simulation work provides support to the presentation.

This paper presents an injection lap riveting process to connect two sheets placed one on top of the other. The process is carried out at ambient temperature and differs from self-pierce riveting because its joining principle is based on plasticity and friction without fracture and formation of new surfaces. The working principle is based on two consecutive operations - first, a dovetail ring hole is machined in the lower sheet and then a semi tubular rivet is injected through the upper sheet into the dovetail ring hole of the lower sheet, by compression with a punch. The presentation is based on a combined experimental and numerical investigation and special emphasis is put on the influence of the dovetail ring hole geometry in material flow, riveting force, and pull-out and shear destructive forces. The last past of the paper includes details of a prototype cutting tool that was developed by the authors for producing the dovetail ring holes in-site and fostering the portability and applicability of the process.

The objective of this paper is to present a new hybrid additive manufacturing route for fabricating collector coins with complex, intricate contoured holes. The new manufacturing route combines metal deposition by additive manufacturing with metal cutting and forming, and its application is illustrated with an example consisting of a prototype coin made from stainless steel AISI 316L. Experimentation and finite element analysis of the coin minting operation with the in-house computer program i-form show that the blanks produced by additive manufacturing and metal cutting can withstand the high compressive pressures that are attained during the embossing and impressing of lettering and other reliefs on the coin surfaces. The presentation allows concluding that hybrid additive manufacturing opens the way to the production of innovative collector coins with geometric features that are radically different from those that are currently available in the market.

Due to the Covid-19 crisis all around the world in 2020, the face-to-face meeting has been canceled. However, the paper selection process was completed. The accepted papers have been included in the ARITH-2020 proceedings and will soon be published on IEEE Xplore. They are also posted in the Programme section of this web site: http://arith2020.arithsymposium.org/; International audience; State-of-the-art implementations of homomorphic encryption exploit the Fan and Vercauteren (FV) scheme and the Residue Number System (RNS). While the RNS breaks down large integer arithmetic into smaller independent channels, its non-positional nature makes operations such as division and rounding hard to implement, and makes the representation of small values inefficient. In this work, we propose the application of the Hybrid Position-Residues Number System representation to the FV scheme. This is a positional representation of large radix where the digits are represented in RNS. It inherits the benefits from RNS and allows to accelerate the critical division and rounding operations while also making the representation of smaller values more compact. This directly benefits the decryp-tion and the homomorphic multiplication procedures, reducing their asymptotic complexity, in dimension n, from O(n 2 log n) to O(n log n) and from O(n 3 log n) to O(n 3), respectively and has resulted in noticeable speedups when experimentally compared to related art RNS implementations.

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.

This article presents new joining-by-forming processes to assemble longitudinally two metal–polymer sandwich composite panels perpendicular to one another. Process design draws from an earlier development of the authors for metal sheets to new concepts based on the combination of sheet-bulk forming with mortise-and-tenon joints. Selected examples obtained from experimentation and finite element modelling give support to the presentation. A new three-stage joining by the forming process is capable of producing mechanically locked joints with larger and stiffer flat-shaped heads than those fabricated by alternative single- or two-stage solutions. Failure in the new three-stage joining by the forming process is found to take place by cracking instead of disassembling after unbending the flat-shaped head of the joint back to its original shape. The required forming forces to produce the new metal–polymer joints are below 15 kN, allowing them to be an effective, easy-to-implement alternative to existing solutions based on adhesive bonding, welding and mechanical fastening.

This paper draws from the existing processes and applications of joining by plastic deformation to a comprehensive overview of a new set of processes that have been recently developed by the authors. The presentation includes solutions for connecting tubes, sheets and tubes to sheets and provides information on the tooling systems, operating variables, deformation mechanics and workability limits. Results from analytical modelling, finite element analysis and experimentation give support to the presentation and prove the feasibility of the new joining by plastic deformation processes for connecting tubes, sheets and tubes to sheets made from dissimilar materials, at room temperature, without having to use addition materials or adhesives. The resulting joints are easy to disassembly at the end of live, thereby allowing recyclability of the individual parts.

This paper presents a new joining by forming process for connecting tubes to sheets. The process consists of forming an annular flange with rectangular cross section by partial sheet-bulk of the tube wall thickness and performing the mechanical interlock by upsetting the free tube end against a flat-bottomed (counterbored) sheet hole. The presentation identifies the variables and the workability limits of the process and includes an analytical model to assist readers in the design of the new joints. The new proposed joining by forming process and the corresponding analytical model are validated by experimentation and numerical simulation using finite element analysis. The process allows connecting tubes to sheets made from dissimilar materials at room temperature, avoids the utilization of addition materials or adhesives and produces joints that are easy to disassembly at the end of live, allowing recyclability of the tubes and sheets.

This article is focused on the formability limits by fracture obtained from standard bulk metal forming tests performed with cylindrical, tapered and flanged specimens. A total of two novel features are presented: the use of digital image correlation to determine strain paths and immersion of steel specimens in liquid nitrogen after the onset of crack formation to reveal the mode of fracture. A new methodology to determine the fracture loci in principal strain space is proposed based on the combination of experimental force–displacement evolutions with in-plane strain measurements. The experimental work is performed in cold-drawn steel AISI 1045 and two new formability tests with different values of stress triaxiality are proposed for obtaining strains at fracture in regions of principal strain space that are not sufficiently well covered by standard bulk metal forming tests.

This 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.

This paper draws from new developments in boss forming of tubes to the proposal of a new joining by forming process for fixing tubes to sheets at room temperature. The process takes place in two stages and involves the production of an annular flange by partial compression of the tube wall thickness along the longitudinal direction and the upsetting of the free tube end against a sheet with a bevelled hole in order to lock the two parts together. The presentation includes an analytical model for designing the joints and the validation of the overall joining concept by means of finite element modelling and experimentation. Destructive tests demonstrate the effectiveness of the new proposed joining process.

This 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.

This paper presents a new joining by forming process for connecting tubes to sheets that is carried out in two stages; (i) forming of an annular flange with rectangular cross section by partial sheet-bulk of the tube wall thickness and (ii) mechanical interlock by upsetting of the free tube end against a counterbored (flat-bottomed) sheet hole. The process allows connecting tubes to sheets made from dissimilar materials without protrusion of the tube end above the sheet surface, avoids the utilization of addition materials or adhesives, and produces joints that are easy to disassembly at the end of live, allowing recyclability of the tubes and sheets. The paper identifies the main variables and workability limits of the process and presents an analytical model to assist readers in the design of the new proposed type of joint. The analytical model is built upon volume incompressibility and fundamental concepts of plastic instability (buckling) and cracking. Experimentation and finite element modelling are utilized to validate the process and the corresponding analytical model.

This paper introduces a new boss forming process for producing annular flanges in thin-walled tubes by partial compression of the tube wall thickness along the longitudinal direction. The presentation combines experimentation and finite element modelling and uses different arrangements of process variables for characterizing typical failures and defects, for understanding the mechanics of material pile-up and for determining the overall force requirements. The new boss forming process results from improving a solution previously developed by the authors through the use of a die cavity and the change of kinematics to allow the upper and lower dies to move together. Results show that the direct dependency between the height and the radius of the annular flanges that is typical of boss forming can be effectively eliminated in order to produce sound annular flanges with custom geometries and sizes in thin-walled tubes.

This paper presents a new deformation assisted process for attaching tubes to tubesheets. The process is named ‘joining by flaring and indentation’ and consists in forming a single-lap flange by compressing the free tube end with a contoured die and indenting the tubesheet by pressing the tube flange against its surface. The process is carried out in a single stroke at ambient temperature and requires no surface preparation nor preliminary machining of grooves in the tubesheet holes. The presentation draws from the working principle and deformation mechanics of the new process to the identification of the operating parameters, typical deformation modes and required forming forces. Destructive pull-out tests are performed to determine the maximum force that the new joints are capable to withstand without failure by detachment. Numerical simulation with a finite element computer program gives support to the presentation.

This 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.

The modulo $2^{n} + 1$ multiplier is the bottleneck of a wide range of applications from residue number system arithmetic to cryptography. Recently, with demand for low-power and energy-efficient designs, the radix-8 Booth recoding has been considered to derive modulo $2^{n} + 1$ multipliers. This brief presents two novel methods to increase the performance and improve the efficiency of radix-8 modulo $2^{n} + 1$ multipliers. The first technique is a method to significantly reduce the amount of bias terms that need to be handled. The second technique is a new hard multiple generator based on a parallel-prefix structure that computes only for odd positions; it results in a lightweight parallel-prefix adder for the computation of the triple of a number with significant area-saving and improved fan-out. The implementation results based on the TSMC 65-nm technology show improvements of at least 27% and up to 57% in the area–time2 product when compared with the recently proposed radix-8 multiplier.

This paper draws from the fundamentals of sheet-bulk forming of thin-walled tubes to the proposal of a new joining process for fixing tubes to sheets by means of plastic deformation at room temperature. The work on sheet-bulk forming of tubes is focused on local thickening (boss forming) by partial compression of the wall thickness along the longitudinal direction. The main goals are the understanding of the deformation mechanics of sheet-bulk forming of thin-walled tubes and the characterization of its formability limits in terms of the major process parameters. The work on joining by forming investigates how local thickening can be successfully utilized for fixing tubes to sheets by mechanical locking with a flaring die. The overall presentation is supported by finite element analysis and experimentation, and includes examples of joints made from dissimilar materials.

This paper presents an investigation on different tube forming processes by plastic deformation, including expansion, reduction and inversion of tube ends, using circle grid analysis in order to obtain strain-loading paths in different regions of the principal strain space. The experimental work was carried out on commercial AA6063-T6 tubes and includes independent material characterization by means of tensile, stack compression and instability tests. Finite element modelling of the tube forming processes by means of an in-house computer program gives support to the presentation and allows assessing the overall accuracy, validity and reliability of using circle grid analysis to investigate formability and failure in tube forming.

Three different ductile damage criteria, Ayada, normalized Cockcroft and Latham and a new shear stress based criterion taking into account hydrostatic tension, are utilized for predicting the onset of cracks in various deformation processes. It is found that the Ayada criterion predicts well the onset of cracks when they originate from hydrostatic tension. The shear based criterion predicts cracks triggered by shear and the normalized Cockcroft and Latham criterion indicates the overall area of onset of cracks caused by either hydrostatic or shear stresses. However the prediction is not as accurate as the Ayada criterion for cracks caused by hydrostatic tension.

Laboratory testing of tribo-systems for sheet metal forming applications must ensure similar conditions with the tribo-parameters that are commonly utilized in real production in order to generate data that is meaningful for industry. The main parameters to consider are the tool and workpiece materials, surface roughnesses, normal pressure, sliding length, sliding speed, interface temperature and lubrication. This paper proposes a new Strip Reduction Test (SRT) for industrial ironing processes that is capable of replicating the highly severe tribological conditions that are experienced during both the forward stroke and the backward retraction of the punch. The new SRT tool design is implemented in a new Universal Sheet Tribo-Tester (UST), which can run multiple tests continuously from a coil. The test is capable of simulating various process parameters such as reduction, drawing speed, tool temperature, sliding length and quantifying the onset of breakdown of the lubricant film and subsequent galling after several strokes not only when emulating the forward strokes but also the backward strokes. Preliminary tests disclose promising results as regards the identification of lubricant film breakdown by detecting changes in measured force, surface roughness and/or torque values.

This paper provides a simple and effective experimental procedure to determine the fracture strains and the critical values of ductile damage at the onset of failure by fracture in tube forming. The investigation is limited to crack opening in mode I (by tension) and combines experimentation in tube expansion with analytical modelling using membrane analysis, circle grid analysis and finite element modelling. The presentation is a step towards the characterization of fracture loci in tube forming and in understanding the conditions upon which necking does, or does not, precede failure by fracture in thin-walled tubes. It is showed that good engineering estimation of the fracture strains and of the critical ductile damage at the onset of fracture can be easily obtained from application of the proposed analytical model with experimental measurements of the expanded tube end radius at fracture.

This paper presents a new sheet-bulk forming process to locally pile-up material in thin sheets for subsequent forming or joining operations. An analytical solution is proposed for explaining the influence of the major process parameters, estimating the thickening of the pile-up material and determining the normal and thrust forces applied by the tools. The approach is built upon the slip-line field theory under plane strain assumptions and results are compared against finite element predictions and experimental results using Aluminium EN AW-1050A (EN 573-3) sheets with 3 mm thickness. The last part of the paper introduces a modified tool geometry that is able to control the pile-up material for subsequent mechanical fastening operations.

This paper is built upon the deformation-assisted joining of sheets to tubes, away from the tube ends, by means of a new process developed by the authors. The process is based on mechanical joining by means of form-fit joints that are obtained by annular squeezing (compression) of the sheet surfaces adjacent to the tubes. The concept is different from the fixing of sheets to tubes by applying direct loading on the tubes, as is currently done in existing deformation-assisted joining solutions. The process is carried out at room temperature and its development is a contribution towards ecological and sustainable manufacturing practices due to savings in material and energy consumption and to easier end-of-life disassembly and recycling when compared to alternative processes based on fastening, riveting, welding and adhesive bonding. The paper is focused on the main process parameters and special emphasis is put on sheet thickness, squeezing depth, and cross-section recess length of the punches. The presentation is supported by experimentation and finite element modelling, and results show that appropriate process parameters should ensure a compromise between the geometry of the mechanical interlocking and the pull-out strength of the new sheet&ndash, tube connections.

This paper investigates the upsetting of bimetallic cylinders with an aluminum alloy center and a brass ring. The influence of the center-ring shape factor and type of assembly fit (interference and clearance), and the effect of friction on the compression force and ductile damage are comprehensively analyzed by means of a combined numerical-experimental approach. Results showed that the higher the shape factor, the lower the forces required, whereas the effect of friction is especially important for cylinders with the lowest shape factors. The type of assembly fit does not influence the compression force. The accumulated ductile damage in the compression of bimetallic cylinders is higher than in single-material cylinders, and the higher the shape factor, the lower the damage for the same amount of stroke. The highest values of damaged were found to occur at the middle plane, and typically in the ring. Results also showed that an interference fit was more favorable for preventing fracture of the ring than a clearance fit. Microstructural analysis by scanning electron microscopy revealed a good agreement with the finite element predicted distribution of ductile damage.

This paper is focused on the utilisation of double edge notched tension, staggered and shear tests to determine fracture toughness and the formability limits by fracture in principal strain space. The experiments were performed in test specimens with different geometries and ligament angles, and the influence of strain hardening was taken into consideration by selecting two materials (aluminium AA1050-H111 and pure copper), with very different strain hardening exponents. Results are plotted in principal strain space, and the discussion is focused on the link between formability limits, fracture toughness and macroscopic fractography characteristics of the specimens that fail by mode I, mode II or mixed-mode.

This paper presents a new joining by forming process to assemble longitudinally two metal-polymer sandwich composite panels perpendicular to one another. The process combines sheet-bulk forming with mortise-and-tenon joints to produce mechanically interlocked joints with large and stiff flat-shaped heads. Experimentation and finite element modelling with representative unit cells give support to the presentation and special emphasis is placed on the application of the process to the fabrication of lightweight composite panels for structural applications. Failure of the joints takes place by cracking and not by disassembling after unbending the flat-shaped head of the joint back to its original shape. The required forces to produce the new type of joints are below 15 kN, allowing them to be an easy to implement alternative to existing solutions based on adhesives or fasteners.

Knowledge of the deformation mechanics in single point incremental forming of tubes is of great importance to understand the physics behind failure and the workability limits in thin-walled tube expansion. Which are the states of stress and strain in the small localized plastic deformation region between the tube and the single point forming tool, and how damage accumulates until fracture are important questions that this paper seeks to address by means of an analytical model based on membrane analysis. Comparisons are made against conventional tube expansion with a rigid punch to explore the main differences between the two processes regarding the states of stress and strain, the accumulation of damage and the likely mode of failure. The investigation is supported by experimentation performed in aluminium AA6063-T6 and shows that differences in deformation mechanics allow incremental tube expansion to withstand slightly higher values of damage before cracking than conventional tube expansion.

This paper proposes a link between plastic flow, void coalescence and growth, ductile damage, crack opening modes and fracture toughness in sheet metal forming. This new integrated view is based on an analytical framework that allows estimating the location of the fracture loci in the principal strain space directly from material stress–strain response and from fracture toughness and thickness at fracture obtained from double-notched test specimens loaded in tension and plane torsion (in-plane shear). Experiments in AA1050-H111 aluminium sheets with 1 mm thickness give support to the proposed analytical framework.

The aim and objectives of this article are to provide an analytical model for the incremental forming of gears along the direction perpendicular to the sheet thickness. The model allows determining the influence of the major process parameters in the indentation force and in the material volume undergoing plastic deformation during indentation by means of double-wedge gear tooth punches. Special emphasis is placed on the influence of superimposing tension stresses along the in-plane direction. The analytical model is built upon the slip-line theory under plane strain deformation conditions, and results are compared against those obtained from experiments in DC04 mild steel and from numerical simulations performed with the finite element method. Results show that the indentation force can be significantly reduced by stress superposition, and that a minimum distance from previous indentations is necessary to produce a new gear tooth in a material free from residual strains and stresses.

Number comparison, sign identification and overflow detection are important operations, especially for digital signal processing, but hard to perform using the residue number system (RNS). In this paper, a new method is proposed for sign identification and number comparison based on an optimized version of the mixed radix conversion for the augmented 3-moduli sets $$\{2^n+1, 2^n-1, 2^{n+x}\} (0 \le x \le n)$${2n+1,2n-1,2n+x}(0≤x≤n). Notably, most of the computations are directly performed on the moduli channels, thus allowing to easily adapt this new method to any RNS processor. Accordingly, this paper proposes an efficient unified very large scale integration architecture based on the presented methodology, which can be used not only to design application specific integrated circuits (ASICs) but also to configure field-programmable gate arrays (FPGAs). The implementation results that were obtained using $$65\,\hbox {nm}$$65nm CMOS technologies show that the proposed architecture provided comparators that are more efficient than the related state of the art, by considering as a figure of merit the area time product. More specifically, the considered ASIC and FPGA implementations provide relative improvements in the efficiency of up to 57 and $$38\,\%$$38%, respectively. The experimental assessment also shows that the power consumption of the proposed circuits is significantly lower than the related state of the art, with relative reductions of up to $$50\,\%$$50%.

This article investigates the possibility of failure by crack-opening mode III (out-of-plane shearing) in sheet–bulk metal forming processes. The investigation makes use of experimentally and theoretically determined fracture-forming limits of aluminium AA1050-H111 sheets with 1 mm thickness, experimental tests in incremental ploughing with a roll-tipped tool and numerical simulation using a commercial finite element programme. Results show that incremental ploughing of thin sheets with a roll-tipped tool under large indentation depths gives rise to transverse cracks that are triggered at the upper groove surface and propagate downward across thickness along an inclined direction to the sheet surface. In contrast to sheet–metal forming processes that only fail by fracture in crack-opening modes I and II, sheet–bulk metal forming processes present the unique ability of failing in all three possible crack-opening modes, namely, in mode III that is typical of bulk metal–forming processes.

This paper is focused on the development of innovative testing machines and methodologies for the characterization of materials in working conditions similar to those found in mechanical processing of materials. Special emphasis is given to the design, fabrication, and instrumentation of a flexible drop weight testing machine, an electromagnetic compressive Hopkinson bar, and an electromagnetic cam-driven compression testing machine that are capable of performing the mechanical characterization of materials under mediumand high rates of loading. The fundamentals of plastic flow in selected forming and cutting processes are utilized to assess the validity and reliability of the proposed testing machines and associated experimental methodologies. Experimental data with technical pure Lead allows understanding the combined influence of strain, strain rate, and strain rate versus strain loading paths in the overall stress response of the materials. Results also show that the new proposed electromagnetic cam-driven compression testing machine equipped with a root type or logistic cam profile can successfully replicate the operating conditions of the two other testing machines.

This paper analyses the differences between incipient and repeatable material flow in the incremental sheet-bulk metal forming (SBMF) of gears produced by indentation along the direction perpendicular to the sheet thickness. The underfilling of the punch cavity during the first indentation, which prevents the production of sound disk gears, is explained on the basis of constrained material flow under material strain hardening. A solution based on the utilization of a tailored disk blank is proposed to overcome this defect. The geometry of the tailored disk blank is determined by means of finite element analysis, and the overall methodology involved material characterization and experimentation with DC04 mild steel. The discussion on the extent of the plastic deformation region under constrained and free-material flow during indentation is complemented by experimental results obtained with a flat punch in rectangular sheet blanks of aluminium EN AW-1050A.

The 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, different 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 solvent 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&rsquo, 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.