Sheet Components Research Articles

Material characterization for plane and curved sheets using the in-plane torsion test – An overview

The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains beyond 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to truely ideal simple shear during the entire process. The full-field strain and stress analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without loosing the shear state. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves of sheets with curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape sheets were performed. Finally, first results for the determination of the local strength at arbitrary positions of a sheet component are presented.

Structural Pricing of CoCos and Deposit Insurance with Regime Switching and Jumps

In this article, we construct a structural model with jumps and regime switching to price banks' contingent convertible debt (CoCos) and deposit insurance. We use an Esscher transform that is applicable to regime switching double exponential jump diffusions to move from the historical world to the risk-neutral world. Further, we define and implement a matrix Wiener-Hopf factorization associated with the latter processes, allowing us to price the various components of a bank's balance sheet. Thus, we obtain valuation formulas for the bank's equity, debt, deposits, CoCos, and deposit insurance. We also show in an illustration the respective effects of the jump risk and of regime switching on the values of all of a bank's balance sheet components.

Influence of PU foam reinforcement of I-beam on buckling resistance

The paper presents the influence of PU foam on the buckling resistance of an I-beam made with sheets aluminum alloy 6061-T6 sheets and GFRP plates. Four beam types were considered: an aluminum beam, a composite aluminum-fiberglass beam, a composite aluminum-fiberglass-PU beam and a composite aluminum-fiberglass-PU-ribs beam. Each beam was made with two C-profiles connected by webs, with flanges reinforced by flat bars. Channels were made by bending 0.8 mm thick aluminum alloy 6061-T6 sheets. Flat bars were made from aluminum alloy 6061-T6 2.0 mm in thickness. The metal sheets components were connected by Refill Friction Stir Spot Welding RFSSW technology. The composite aluminum-fiberglass beam additionally has GFRP plates glued to the flanges. The 3.0 mm thick GFRP plates were connected by a polyurethane adhesive. The composite aluminum-GFRP-PU and aluminum-GFRP-PU-ribs beams have a rectangular cross-section. The composite aluminum-GFRP-PU beam was made like the composite aluminum-GFRP beam, but with stiffening of the web by polyurethane foam. The aluminum-GFRP-PU-ribs beam was made like the composite aluminum-GFRP beam, but with stiffening of the web by aluminum ribs and polyurethane foam. The PU foam filled up the I-sections in such a way that rectangular cross-sections were obtained. The beams were subjected to three-point bending tests.

Analysis of forming defects in electromagnetic incremental forming of a large-size thin-walled ellipsoid surface part of aluminum alloy

Electromagnetic incremental forming (EMIF) is a novel and flexible technology for forming large-size thin-walled components. However, the practical challenge is how to effectively control the forming defects, e.g., wrinkling, local humps and depressions that are caused by the multi-station discharge and the large-size thin-walled structure of sheet. To this end, the formation mechanism and rules of defects during the EMIF process of a large-size thin-walled ellipsoid surface part of aluminum alloy are studied through numerical simulation and experiment verification. In this study, a numerical model combining finite element method (FEM) with boundary element method (BEM) is established with consideration of two-way sequential coupling of the electromagnetic field and the structure field. In this model, the introduction of BEM for modeling air and insulator avoids FE meshing and thus enhances computational stability; air damping is considered by applying mass damping to the sheet, which reduces the ineffective long-lasting oscillation of the sheet after discharging and thus significantly improves computational efficiency. With this model, the discharge-induced stress wave is found propagating outwards from the center of the coil acting zone in the form of a sine wave; radial stresses on the top and bottom surfaces of the sheet are symmetrically distributed with respect to the axis of zero stress and attenuate along radial distance; circumferential stresses on both surfaces are negative away from the discharge acting zone. Consequently, wrinkling is attributed to circumferential stress caused by the propagation of the stress wave and the weak circumferential rigidity of the sheet, and local hump and depression are attributed to the stress concentration because of the interruption of previously formed structure on the propagation of stress wave. In light of the propagation of stress wave and energy method, a criterion for wrinkling is established and then the relations between the parameters of discharging positions, voltage and geometry of the sheet and the defects are obtained. As the result, aiming at the objective ellipsoid surface part, a well-scheduled eight-station discharging approach was established in this work. The corresponding processing parameters were determined. Then, a qualified production cycle with consistent profile is determined by comparison of simulation and experiment. Therefore, this study provides a way for forming process design of large-size thin-walled sheet components with EMIF.

Structural health monitoring of fibre metal laminates under mode I and II loading

Delamination is one of the most common defects in FRP and occurs due to low interlaminar strength. Such defects are difficult to detect and lead often to catastrophic failure of the composite. In this work a new approach of a multi-functional FML for structural applications is investigated. The interlaminar fracture toughness is determined and the capacitance change between the metal plies during crack propagation is measured in-situ. The metal sheet component undergoes a chemical pre-treatment by nanoscale sculpturing before manufacturing by resin transfer moulding, to prevent adhesive failure of the interfacial metal/matrix bond.The experimental test results show high potential for detecting defects in FMLs. Delamination, which occurs between the glass fibre/matrix interface, can be electrically detected using the capacitance measurement. The fracture surface of the pre-treated metal sheet demonstrates that the adhesive bond between the cubical hook-like surface structure of the aluminium and the matrix remains intact.

Analysis of Financial Risk Management Strategies of Microfinance Banks

Risk taking is described as an integral part of financial services. For micro-financing in particular, engaging in proactive risk taking is essential to their viability and long term sustainability. Maintaining a good strategy that ensures an optimal mix in risk-return trade-off is much more important for the microfinance banks (MFBs) that operate on a for-profit basis. Having faulted the value-at-risk technique which is common in the asset and liability literature, we introduce the multi-stage stochastic programming using econometric time series model. Specifically, for the scenario generation, we specify a VaR model with the inclusion of dichotomy regime which captures the multi-stage characteristics of assets. We use the liability derived investment (LDI) model to generate the liability series over the period of study. The optimization result showed that MFBs in Nigeria are by far more risk averse than they are profit seeking. This comes with the attendant effect of not being able to achieve the outreach and sustainability objectives to the fullest. MFBs in Nigeria need to look into their investment strategy with a view to structuring the mix and value of the balance sheet components at different periods to meet their stated objectives.

A Structural Model to Assess the Impact of Bank Capitalization Changes Conditional on a Bail-in versus Bail-out Regime

We develop a structural model for valuing bank balance sheet components such as the equity and debt value, the value for the government when the bank is operated by private shareholders including the present value of a possible future bailout, the bailout value incurred by the government following the abandonment of the private shareholders, and, moreover, some price and risk parameters, including the funding cost spread and the banks’ probability of default. The structural model implies an abandonment threshold, at which if total income drops below this threshold, private shareholders abandon the bank. In this case, the shareholders lose part (or all) of the capital that they hold in the bank, the creditors lose part or all of their debt, and the government receives a portion (or all) of the capital and all of the debt that is not recovered by creditors. Hence, we assume that part of the capital can be lost due to financial distress or to cover bankruptcy costs. We use the model framework to assess the impact of capital-based macro-prudential policy measures and focus in particular on assessing the difference that an assumed bail-in as opposed to bail-out regime can make. JEL Classification: G21, G28, H81

Feasibility study of a novel hot stamping process for Ti6Al4V alloy

To investigate the feasibility of a novel hot stamping process for the Ti6Al4V titanium alloy using low temperature forming tools, mechanical properties of the material were studied using hot tensile tests at a temperature range of 600 - 900°C with a constant strain rate of 1s-1. Hot stamping tests were carried out to verify the feasibility of this technology and identify the forming window for the material. Results show that when the deformation temperature was lower than 700°C, the amount of elongation was less than 20%, and it also had little change with the temperature. However, when the temperature was higher than 700°C, a good ductility of the material can be achieved. During the forming tests, parts failed at lower temperatures (600°C) due to the limited formability and also failed at higher temperatures (950°C) due to the phase transformation. The post-form hardness firstly decreased with the temperature increasing due to recovery and then increased due to the phase transformation. Qualified parts were formed successfully between temperatures of 750 - 850°C, which indicates that this new technology has a great potential in forming titanium alloys sheet components.

Model for the Formation of Acoustic Self-Oscillations in a Chamber with a Jet Flowing Through its Nozzle

This paper describes free acoustic oscillations of gas in a chamber with a jet flowing through its nozzle in the case of nonstationary intensity component of vortex sheet flowing down from the edge of the nozzle. There is established feedback between acoustic oscillations and oscillations induced by a corresponding vortex sheet component. It is shown that, in the presence of given feedback, there could be instability of acoustic oscillations, which would result in acoustic self-oscillations in the chamber. The boundaries of the domain in which instability is formed are determined by developing a mathematical model of stable acoustic oscillations in the chamber with account for the influence of the vortex sheet.

Nanoheater Underwater Robotic Welding for Marine Construction and Manufacturing

Underwater manufacturing and maintenance processes such as welding are accompanied by high occupational risks for welder-divers. This paper attempts to eliminate these hazards by introducing a 6-degree of freedom underwater welding robotic system to ignite nanoheater foils for metal joining, thusextending the use of these nanoheaters from soldering, brazing, and joining of components in microchip industry to underwater welding. Ni/Al reactive multilayers are utilized to perform aluminum sheet component joining. These commercially available nanoheaters release large amounts of heat when an exothermic reaction is initiated by an electrical ignition stimulus. Integrity of the welds performed by nanoheater underwater welding is ensured through introducing openings in the nanoheater foil, allowing for weld areas in a lap joint. The generated temperature field is simulated during such welding, establishing the Al sheet and nanoheater thickness, as well as the opening geometry conditions for reaching the melting temperature at the weld interface to generate successful and sound joints in the experiments.

Enhanced ice sheet melting driven by volcanic eruptions during the last deglaciation

Volcanic eruptions can impact the mass balance of ice sheets through changes in climate and the radiative properties of the ice. Yet, empirical evidence highlighting the sensitivity of ancient ice sheets to volcanism is scarce. Here we present an exceptionally well-dated annual glacial varve chronology recording the melting history of the Fennoscandian Ice Sheet at the end of the last deglaciation (∼13,200–12,000 years ago). Our data indicate that abrupt ice melting events coincide with volcanogenic aerosol emissions recorded in Greenland ice cores. We suggest that enhanced ice sheet runoff is primarily associated with albedo effects due to deposition of ash sourced from high-latitude volcanic eruptions. Climate and snowpack mass-balance simulations show evidence for enhanced ice sheet runoff under volcanically forced conditions despite atmospheric cooling. The sensitivity of past ice sheets to volcanic ashfall highlights the need for an accurate coupling between atmosphere and ice sheet components in climate models.

Resolving discrepancies between field and modelled relative sea‐level data: lessons from western Ireland

ABSTRACTAccurate reconstruction of Lateglacial and Holocene relative sea‐level (RSL) histories is complicated where mismatches exist between geological data and RSL curves generated by models of glacio‐isostatic adjustment (GIA). In Ireland, such discrepancies have profound implications for interpreting the glacial history of the British Isles and for the use of glacial rebound models to predict future sea‐level changes. To address this issue we present new RSL data from four sites along the western coast of Ireland, including 17 data points from the critical period before 5000 14C a BP for which very few data are available. We generate new RSL simulations from an existing GIA model, incorporating a thickened Irish Ice sheet component. Simulated curves from Co. Mayo and Co. Donegal accommodate the higher than present Lateglacial RSL inferred from glaciomarine muds while still meeting the requirement for below present RSL indicated by the new terrestrial limiting data points. Relaxation of trimline constraints on maximum ice sheet thickness provides considerable scope for improved GIA performance. These results demonstrate inferences about RSL drawn from GIA modelling and glacio‐sedimentary data are not mutually exclusive, and represent a significant step towards resolving a long‐standing debate between the field‐based and modelling communities.

Microstructural characterization and simulation of damage for geared sheet components

The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution.

Research on residual stresses during hot stamping with flat and local-thickened plates

Residual stresses generated during hot stamping have a significant effect on the mechanical properties and dimensional stability of aluminum alloy sheet components. What is more, few studies have paid attentions to the effects of hot stamping parameters on residual stresses and the methods of reducing residual stresses during hot stamping of aluminum alloy plates with thickness around 5 mm. Hence, in this study, the effects of forming temperature, blank holder force, die entrance radius, die corner radius, and local thickening on the residual stresses of square cups were studied, by conducting both finite element simulations and experiment investigations of the hot stamping of 2024 aluminum alloy plates. The results showed that the residual stress distribution is not uniform through the thickness. Within the range of process parameters investigated in this study, increasing the forming temperature, blank holder force, and die corner radius or decreasing the die entrance radius all lead to lower values of residual stresses. Compared to flat plates, local-thickened plates can significantly reduce the residual stresses in hot stamped square cups, which is attributed to the metal supplement mechanism at the thickened locations. When the side length of the square-ring-shaped convex rib of the plate is equal to the punch width and the convex rib facing downward, lower residual stresses in square cups are obtained. The established residual stress calculation model is capable of describing the residual stress distribution in the bottom circular arc of hot stamped square cups.

Case Studies on Local Reinforcement of Sheet Metal Components by Laser Additive Manufacturing

This paper details two case studies that make use of laser metal deposition for local reinforcement of sheet metal components. Two benchmark scenarios are investigated, both using aluminum alloys: (i) using laser cladding to increase the stiffness of a pre-formed component, and (ii) applying a local cladding on sheet metal for increasing the thickness prior to a hole-flanging operation. The results show that both routes are viable. Applying claddings onto sheet metal before a metal forming operation must ensure suitable formability, which may be limited by the layer material and undesired changes in the microstructure of the sheet. The limited formability has to be taken into account in the design of the forming operation. Cladding onto already formed components has to cope with inevitable distortion of the component. Nevertheless, introducing additive manufacturing into the field of sheet metal forming opens the possibility to produce new products such as tailored laser-cladded blanks, combinations of sheet and bulk components and to develop new methods such as stiffness management in lightweight design.

An Experimental and Numerical Study on Forming Force, Fracture Behavior, and Strain States in Two Point Incremental Forming Process

Two-point incremental sheet forming process (TPIF) is an emerging and promising manufacturing process for the production of complex geometries or customized functional sheet components. In this study, the single-pass TPIF process is investigated using experimental and numerical approaches to study the forming force evolution, fracture behavior and strain states with a varied wall angle hemisphere shape. It can be concluded that both the peak force and fracture depth increases with tool diameter and incremental depth in TPIF process. It seems the deformation mechanism or the failure mechanism is strongly dependent on particular forming conditions based on a failure parts morphology observation. FEM simulation results indicated that the major plastic strain is positive while the minor plastic strain is negative in the TPIF process on a hemiphere shape. it can be concluded that the strain increment and total equivalent plastic strain is affected by both tool diameter and incremental depth.

A thermal model of resistance spot welding

An analytical model of spot welding of flat components is proposed. An equation is derived for calculating the non-stationary temperature field in the three-dimensional expression. The main geometrical and thermal physical parameters of the welded components are taken into account. It is shown that when developing the welding conditions, it is necessary to take into account not only the type of welded material and the thickness of sheets components, but also the overall dimensions of the component and the spacing of the spot welds.

Tailored boron steel sheet component properties by selective laser heat treatment

This investigation is focused on the stamping behaviour of boron steel, the properties of which are modified by selective laser heat treatment. Both CO2 and fibre lasers are tested. By using different laser processing parameters, the hardening depth in the 1 mm thick boron steel sheet Boloc 02 is varied. Four routes are tested and verified. The forming operation (in which a so-called flexrail beam is produced) in all four routes is conducted at ambient (room) temperature. The Reference route comprises stamping of the sheet. The GridBlank route starts with selective laser heat treatment of the blank, after which the blank is allowed to cool down, moved to a hydraulic press and stamped. In the GridTube route, the blank is first stamped, after which the part is moved to a laser cell and selectively laser heat treated. The fourth route, the RapidLaser route, is similar to the GridBlank route, but a higher laser speed is used to promote higher total productivity. The GridBlank route results in the highest hardness values and the best shape accuracy. The initial sheet material exhibits a hardness of 200 HV, while the parts produced in the GridBlank route exhibit a hardness of 700 HV.

Strength evaluation of beam made of the aluminum 6061-T6 and titanium grade 5 alloys sheets joined by RFSSW and RSW

The paper presents the strength evaluation of the beam made of the sheets from the aluminum alloy 6061-T6 and titanium alloy grade 5. The beam was made of two C-profiles connected by the webs, with the flanges reinforced by the flat bars. Channels were made by bending alloy aluminum 6061-T6 sheets with a thickness of 0.8mm. Flat bars made from titanium alloy grade 5 having a thickness of 0.8mm were used to reinforce the flanges. The metal sheets components were connected by Refill Friction Stir Spot Welding RFSSW and Resistance Spot Welding RSW technology. The welding parameters of RFSSW and RSW were selected on the basis of the strength of the joints with a single RFSSW and RSW spot. The joints made of two aluminum alloy 6061-T6 sheets, as in the case of the beams webs, and the joints made of aluminum alloy 6061-T6 and titanium alloy grade 5 sheets, as in the case of beams flanges, were analysed. The beam was subjected to three-point bending using the optical deformation system analysis Aramis. The load-capacity of the beam, the distribution of plastic strains on the reference surfaces and the method of beam cracking were analysed. The results of experimental studies were compared with the results of numerical analysis performed using the ADINA System based on the Finite Element Method.

Microstructural Influence on Mechanical Properties in Plasma Microwelding of Ti6Al4V Alloy

The complexity of joining Ti6Al4V alloy enhances with reduction in sheet thickness. The present work puts emphasis on microplasma arc welding (MPAW) of 500-μm-thick Ti6Al4V alloy in butt joint configuration. Using controlled and regulated arc current, the MPAW process is specifically designed to use in joining of thin sheet components over a wide range of process parameters. The weld quality is assessed by carefully controlling the process parameters and by reducing the formation of oxides. The combined effect of welding speed and current on the weld joint properties is evaluated for joining of Ti6Al4V alloy. The macro- and microstructural characterizations of the weldment by optical microscopy as well as the analysis of mechanical properties by microtensile and microhardness test have been performed. The weld joint quality is affected by specifically designed fixture that controls the oxidation of the joint and introduces high cooling rate. Hence, the solidified microstructure of welded specimen influences the mechanical properties of the joint. The butt joint of titanium alloy by MPAW at optimal process parameters is of very high quality, without any internal defects and with minimum residual distortion.