Initial Sheet Thickness Research Articles

The influence of roll bonding parameters on the bond strength of Al-3003/Zn soldering sheets

Al-3003/Zn bi-layer soldering sheets were produced using roll bonding process at different reductions in both cold and hot conditions. The bond strengths of the bi-layer sheets were measured by peel test before and after supplemental annealing treatment. The peeled surfaces were examined using optical and scanning electron microscopy. The results indicate that by increasing the reduction in thickness during rolling and/or the annealing treatment the bond strength improves. Additionally, the decrease of initial sheet thickness and increase of rolling temperature can effectively improve the bond strength. Microscopic examinations show that the fracture type of Al-3003/Zn sheets is predominantly brittle.

Quick triggering of magnetic reconnection in magnetotail

Recently, quick triggering of magnetic reconnection (QMRT) even in an ion-scale current sheet is found to be possible with the help of the nonlinear evolution of the lower hybrid drift instability (LHDI). The details of the QMRT mechanism are reviewed mostly based on three-dimensional full-particle simulation results of our group. QMRT is mediated by LHDI and its time scale is comparable to the saturation time scale of LHDI. Depending on the initial current sheet thickness, two types of QMRT, so-called Type-I and Type-II QMRT, are demonstrated.

Influence of low-strain deformation characteristics of high strength sheet steel on curl and springback in bend-under-tension tests

The influence of low-strain deformation behavior on curl and springback in advanced high strength steels (AHSS) was assessed using a bend-under-tension test. The effect of yielding behavior on curl and springback was examined by heat-treating two dual-phase steels to induce yield point elongation, while keeping a relatively constant tensile strength and a constant sheet thickness. A dual-phase and TRIP steel with similar initial thickness and tensile strengths were also examined to investigate the effect of work-hardening on curl and springback. It is shown that while current understanding limits prediction of curl and springback in bending under tension using only the initial sheet thickness and tensile strength, both the yielding and work-hardening behavior can affect the results. Explanations for these effects are proposed in terms of the discontinuous yielding and flow stress in the materials.

Geospace Environment Modeling (GEM) magnetic reconnection challenge: Resistive tearing, anisotropic pressure and Hall effects

The nonlinear evolution of resistive tearing is studied in various regimes, including one‐fluid isotropic MHD, anisotropic (gyrotropic) MHD with terms approximating the effects of anisotropy driven instabilities, and HaIl‐MHD. Both uniform resistivity and spatially localized resistivity are investigated. The initial state is a plane one‐dimensional current sheet with an initial perturbation representing a periodic structure with magnetic islands and x type neutral points, chosen for the “GEM magnetic reconnection challenge” [Birn et al., this issue]. In the absence of dissipation, within ideal MHD, the initially imposed x type configuration collapses into a thin current sheet of finite length, while the magnetic islands contract, conserving their magnetic flux. The current density in the thin sheet becomes significantly enhanced, so that microscopic dissipation mechanisms can be expected to be ignited, even if they were absent in the initial state. Finite resistivity enables reconnection, and the growth of magnetic islands as in particle simulations. A comparison of reconnection rates shows that for the chosen initial current sheet thickness, a localized resistivity, corresponding to a Lundquist number (magnetic Reynolds number) of order unity, is necessary to approximate the growth and the reconnection electric field in the particle simulations. Pressure anisotropy, governed by double adiabatic conservation laws (modified by Ohmic heating), leads to reduced growth rates. This reduction is drastic for uniform resistivity. Isotropizing terms, modeling the effects of anisotropy driven microinstabilities again destabilize. HaIl‐MHD simulations can reproduce the fast growth of the particle simulations. The various models show both similarities and dissimilarities in their spatial structure. For similar amounts of reconnected flux, the variation of the normal magnetic field and of the (ion) flow speed along the current sheet are similar, but electric field and current density show significant differences.

Effect of Initial Sheet Thickness on Shear Deformation in Ferritic Rolling of IF-Steel Sheets.

The effect of sheet thickness on shear deformation and texture for the ferrite rolling has been studied. The shear deformation was estimated from the distorted shape of inserted wire before rolling and FEM simulation. The deformed shape of inserted wires and FEM results showed that the shear strain decreased with increasing initial sheet thickness. The measured and simulated textures also showed that the Goss component changed to Dillamore component as the thickness increased. From rolling simulation of 20 % reduction, the minimum shear deformation was found to exist for the given roll bite geometry and friction coefficient.

Prediction of the formability of metastable low nickel austenitic stainless steel sheets

The formability of metastable low nickel austenitic stainless steel sheets is predicted by extending an earlier analysis due to one of the authors and his co-worker. As in the earlier analysis, the size of the imperfection in the thickness direction is associated with surface roughness. Two concurrent modes of flow localisation, viz., geometric instability (thickness strain, surface roughening and void growth) and bulk shear band formation, are assumed to contribute equally and additively to the limiting strain. In this modified method, void growth is described by a refined Avrami relation that takes into account the effect of the strain (stress) state. Also, the β parameter in the analysis that was assumed as unity in the earlier analysis is presented now as a function of the stress (strain) ratio, the material properties and the initial sheet thickness. A reasonably good correlation with experimental results is demonstrated.

Sheet Thickness Optimization for Superplastic Forming of Engineering Structures

This paper explores two optimization strategies; the gradient search and proportional control methods, for determining the initial sheet thickness of superplastic forming to ensure final desired part thickness. A hemispherical dome model was involved in the testing of both optimization methods. Also, a three-dimensional rectangular box model was optimized by the proportional control method. The gradient search technique is shown to be acceptable in terms of the optimized thickness obtained, but displays poor convergence rates. The proportional control approach presented is easy to be implemented, and yields not only more accurate sheet thickness, but much higher convergence speeds that makes such optimization possible on complex geometric models. [S1087-1357(00)01001-7]

Study of thermoplastic foam sheet formation

AbstractThis paper discusses theory and experiments on nonisothermal foam growth during foam sheet formation in an extrusion process. The extruded foam sheet expands and cools simultaneously when exposed to ambient temperature. A viscoelastic cell model in the literature was modified to include heat transfer and gas loss effects during foam sheet formation. Experiments were conducted using a twin‐screw extruder to study the effect of ambient temperature and initial sheet thickness on foam characteristics. The foam was made using low‐density polyethylene with CFC‐12 as the blowing agent. The experimental results are compared with theoretical predictions to check the validity of the model. The results reveal that heat transfer effects become important when sheet thickness decreases to the millimeter range. Agreement between theory and experiment is good when an appropriate boundary condition, to account for the gas loss, is included in the model.

Energy Considerations in Twin-Fluid Atomization

With certain types of prefilming airblast atomizers, the manner in which the atomizing air impinges on the liquid sheet prohibits the wave formation that normally precedes the breakup of a liquid sheet into drops. Instead, the liquid is shattered almost instantaneously into drops of various sizes. This prompt atomization process is characterized by a broad range of drop sizes in the spray and by a lack of sensitivity of mean drop size to variations in liquid viscosity, atomizing air pressure, and initial liquid sheet thickness. Evidence is presented to show that which of these two different modes of atomization will occur in any given flow situation is largely dependent on the angle at which the atomizing air impinges on the liquid sheet. An equation for mean drop size, derived from the assumption that the main factor controlling prompt atomization is the ratio of the energy required for atomization to the kinetic energy of the atomizing air, is shown to provide a good fit to experimental data acquired from atomization studies on water and heating oil, carried out over wide ranges of air velocity, air/liquid ratio, and ambient air pressure.

On the pressure forming of two superplastic alloys

Superplastic forming of the Ti-6Al-4V and Sn-Pb eutectic alloys was attempted using the pressure forming (sheet thermoforming) process. It has been demonstrated that true hemispheres could be formed out of sheets of both the alloys. The thickness strains in both the alloys were less than those predicted theoretically and this could be traced to material flow from the flange and gripped regions. This flow, however, was greater in case of the titanium alloy than the Sn-Pb alloy, on account of the greater strain-rate sensitivity of the former material. Due to the same effect, the “thinning factor” actually increased with deformation in the titanium alloy, but it decreased on increasing deformation in the Sn-Pb alloy. Within the experimental range, the hold-down pressure (titanium alloy) and initial sheet thickness (Sn-Pb alloy) had very small effects, although the deformation became slightly more uniform on decreasing the hold-down pressure or increasing the initial sheet thickness. The thickness and circumferential strains increased with deformation and in particular when the bulge height (h 0) to base diameter (D 0) ratio was greater than 0.35, non-uniformity in deformation along the bulge profile became noticeable. These strains were largest at the pole and its vicinity. On account of its lower strain-rate sensitivity, these effects were more pronounced in the Sn-Pb alloy than in the titanium alloy. Although initially the bulging rate was rapid, later the (h 0/D 0) ratio increased linearly with the forming time and at any instant the bulge profile corresponded to an arc of a circle.

Forming limit curves: Out-of-plane and in-plane stretching

The study of deformation and failure of biaxially stretched sheets is of considerable practical interest in the area of sheet metal press forming. During biaxial stretching of sheet metal, the uniform plastic straining regime is often followed by a localised plastic straining regime which finally leads to fracture. The technique of analysing failures in sheet metal pressing through the use of circle grids and “forming limit curves” has gained considerable popularity. The forming limit curve describes the combination of the two principal surface strains of a biaxially stretched sheet metal at which a localised xone of thinning or necking is bound to occur. Forming limit curves thus provide excellent guidelines for adjusting material, tooling and lubrication conditions in the plant. In spite of their usefulness, however, in an actual forming operation, material parameters interact with the pressing process variables in a complex manner and comparison of different forming limit curves becomes difficult. Forming limit curves are strongly dependent on material parameters, such as the sheet thickness, the straining path and the strain gradients, and also on the geometry of the tooling. The type of stretching process can also influence the forming limit curve. In the present work, forming limit curves have been determined experimentally for annealed aluminium alloy sheets by two methods: out-of-plane stretching, where the sheet is bulged by lateral hydrostatic pressure, and in-plane stretching by tensile forces lying in the plane of the sheet. It is found that the former method produces higher limit strains than the latter under identical degrees of strain biaxiality and initial sheet thickness. A study of the various parameters present in each method which may influence the strain localisation process indicates that the overall strain gradient across the bulged testpiece is responsible for the higher strain limits.