Dongyoung Lee

Investigation into manufacturing of very long cups by hydromechanical deep drawing and ironing with controlled radial pressure

Abstract High-quality cups of deep drawing ratio of more than four cannot be simply drawn by conventional drawing and redrawing process. In the present study, after the first deep drawing process, subsequent hydromechanical reverse redrawing with controlled radial pressure and final ironing to control the thickness and outer surface appearance are employed. In order to increase the deep drawing ratio much more than four, the radial pressure is controlled independently of the chamber pressure and thus an optimum forming condition can be found by varying the radial pressure. The final ironing process ensures the fine surface quality as well as improved deep drawing ratio without inducing any forming defect such as slight puckering and surface waviness. The process has been subject to finite element analysis by using the rigid-plastic material modeling considering all the frictional conditions induced by the hydrostatic pressure. The comparison of the computation with the experiment has shown that the finite element modeling can be conveniently employed for the design of the process with reliability from the viewpoint of formability.

Interlocking Membrane/Catalyst Layer Interface for High Mechanical Robustness of Hydrocarbon‐Membrane‐Based Polymer Electrolyte Membrane Fuel Cells

A physical interlocking interface that can tightly bind a sulfonated poly(arylene ether sulfone) (SPAES) membrane and a Nafion catalyst layer in polymer electrolyte fuel cells is demonstrated. Owing to higher expansion with hydration for SPAES than for Nafion, a strong normal force is generated at the interface of a SPAES pillar and a Nafion hole, resulting in an 8-fold increase of the interfacial bonding strength at RH 50% and a 4.7-times increase of the wet/dry cycling durability.

Consideration of geometric nonlinearity in rigid-plastic finite element formulation of continuum elements for large deformation

Abstract A rigid—plastic finite element formulation for the continuum elements employing the geometric nonlinearity during an incremental time step is presented. In sheet metal deformation, the displacement for each step is considerably large even though the effective strain increment is very small. For such large displacement problems, geometric nonlinearity must be considered. In the elastic—plastic finite element using continuum elements, general incremental formulations to include the geometric nonlinearity are available. However, in the conventional rigid—plastic finite element analysis using continuum, elements, the geometric nonlinearity has not been considered properly during an incremental time step. In this paper, in order to incorporate geometric nonlinearity to rigid—plastic continuum elements during a step, the convected coordinate system is introduced. To show the stability of strain distributions by the effect of geometric nonlinearity according to incremental step size, two sheet metal forming processes, stretching and deep drawing process, are analysed with various step sizes. Then the computed results using the derived equation are compared with those obtained without considering geometric nonlinearity.

Three‐Dimensional Interlocking Interface: Mechanical Nanofastener for High Interfacial Robustness of Polymer Electrolyte Membrane Fuel Cells

A scalable nanofastener featuring a 3D interlocked interfacial structure between the hydrocarbon membrane and perfluorinated sulfonic acid based catalyst layer is presented to overcome the interfacial issue of hydrocarbon membrane based polymer electrolyte membrane fuel cells. The nanofastener-introduced membrane electrode assembly (MEA) withstands more than 3000 humidity cycles, which is 20 times higher durability than that of MEA without nanofastener.

Investigation into hydromechanical reverse redrawing assisted by separate radial pressure : process development and theoretical verification

Abstract High-quality cups with a deep drawing ratio of more than four cannot be simply drawn by conventional drawing and redrawing. A special technology is required to form cups of a high deep drawing ratio. In the present study, after the conventional mechanical deep drawing process, subsequent hydromechanical reverse redrawing with controlled radial pressure has been developed. In order to increase the deep drawing ratio by more than four, the radial pressure is controlled independently of the chamber pressure and thus an optimum forming condition can be determined by varying the radial pressure. The process has been verified by a rigid-plastic finite element (FE) analysis considering all the external force boundary conditions induced by the hydrostatic pressure. The pressure distribution on the sheet is calculated numerically from the simplified Navier-Stokes equation. Through the experiment and the FE analysis, it has been shown that hydromechanical reverse redrawing assisted by separate radial pressure, developed in the present study, helps to increase the drawability of cylindrical cups, and thus it is useful when forming long cups.

Silver nanowire networks embedded in a cure-controlled optical adhesive film for a transparent and highly conductive electrode

A transparent conductive electrode (TCE) based on silver nanowire (AgNW) networks has been intensively studied as an alternative to indium tin oxide (ITO) for optoelectronic devices. However, for practical use, it has several critical issues, including high surface roughness, low adhesion between AgNWs and the substrate, poor contact junction between AgNWs, and severe oxygen corrosion. To resolve these issues, we present a novel AgNW-based TCE which is prepared by embedding AgNWs in the surface region of a photo-curable NOA 85 film. The key enabler of our work is the use of cure-controlled NOA 85 as a AgNW embedding matrix, the hardness of which can be tuned to allow the intrusion of the deposited AgNWs just below the surface under a low pressure of 2 MPa. The resulting AgNW/NOA 85 composite film exhibits an ultra-smooth surface, firm adhesion, high conductivity due to monolithically fused AgNW junctions, and outstanding oxidation stability, thereby leading to considerable improvements over as-deposited AgNW films or commercially available indium-tin-oxide (ITO) plastic films. In addition, we demonstrate a transparent touch panel by using the AgNW/NOA 85 composite film which outperforms a device with the ITO film.

Development of multifunctional carbon composite bipolar plate for vanadium redox flow batteries

The carbon/epoxy composite bipolar plate is a promising substitute for the conventional graphite bipolar plate for the vanadium redox flow battery due to its high mechanical property and productivity. The carbon/epoxy composite bipolar plate, a multifunctional structure, requires expanded graphite coating or additional surface treatments to decrease the interfacial contact resistance. However, expanded graphite coating has low durability under vanadium redox flow battery operating condition and surface treatments are costly to implement. In this work, excess resin-absorbing method is developed with polyester fabric to uniformly remove the resin-rich layer and expose carbon fibers on the surface of the carbon/epoxy composite bipolar plate. This method not only decreases the interfacial contact resistance by exposing carbon fibers but also forms a unique ditch pattern which is desirable to fix carbon felt electrode in place. Durability against acidic environment, mechanical property, and gas permeability of the composite bipolar plate manufactured by excess resin-absorbing method is investigated.