Joon-Tae Yoo

Postbuckling Failure Behaviours of Hat-Stiffened Composite Panels with Different Bonding Methods

Postbuckling failure behaviors and strength were investigated in compression tests of the hat stiffened composite panels manufactured with different bonding methods and different stiffener section shapes. The skin-stiffener separation loads, separation failure behaviors and final collapse loads were different with respect to bonding methods and stiffener shapes. As the separation failure was initiated early and propagated larger area, collapse loads and structural performance of the panels decreased. The test results showed that the co-curing with or without adhesive film and open type stiffeners were most beneficial for the postbuckling strength.

A study on tensile shear strength of IN 718 welded joint

Abstract It is well known that nickel-based superalloys are suitable for the high-temperature applications, like turbine blades and discs in aircraft engines and power generation plants. Due to the high cost of manufacturing and repairing with difficulty using conventional mechanical processing, the joining of superalloys has been of a major concern to the aerospace and nuclear industries. Fusion welding is one of the desirable methods and has been investigated with a major problem of the formation of hot cracks, while as other solid state welding technologies, like diffusion bonding or brazing, normally require long bonding times with extra heat treatments. Present work studies the tensile shear strength of welded specimens of Inconel® alloy 718 with electron beam welding, laser beam welding, diffusion bonding and hot isostatic pressing. The purpose was to determine the optimum welding method for overlap bonding of Inconel® alloy 718 for superplastic blow forming.

Study of Blank Thickness Optimization in Free Bulging for Maximizing Bulged Height

Thickness profiled blank is designed using optimization techniques for maximizing the bulged heights during the free bulging of Inconel 718. The thickness of the blank was described by the Bezier curve and the locations of the control points were used as the design variables for optimization. The maximization of the bulged heights within the limited strain range served as the objective function and constraints for optimization. The equivalent static loads method for non-linear static response structural optimization (ESLSO) was used and the result of the optimization revealed 22 increased bulged heights. A free bulging test using a blank with an optimized profile was conducted to verify the optimization process. The results were compared with those of numerical analysis in terms of bulged height and deformed shape.

A study on blank optimization for blow forming of Inconel 718 to secure formability at high temperature

In this study, thickness profiled blank is designed using optimization technique to get high formability in blow forming of Inconel 718 by equivalent static loads method for non-linear static response structural optimization (ESLSO). When ESLSO is applied, optimization with wide design range is difficult because the effectiveness of equivalent static loads is not valid if there are wide changes in design values. To maximize the design range in ESLSO, move limit scheme is applied during non-linear iteration considering the effectiveness of equivalent static loads. Optimization results show increased formability within limited strain range and possibility of gas forming using non-SPF grade Inconel 718. Free bulging test using blank which has optimized profile is conducted to verify the optimization process. Result is compared with that of numerical analysis in bulged height, deformed shape and thickness and they show good agreement. Verified optimization process is applied to obtain profile of blank thickne...

Superplastic Forming of Duplex Stainless Steel for Aerospace Part

In this study, the high temperature forming behavior of duplex stainless steel has been characterized and the outer shell of a combustion chamber was fabricated with pressure difference of hot gas. It consists of two parts which are the outer skin made of stainless steel to sustain the internal pressure and the inner shell made of copper alloy for regenerative cooling channels. Two outer skins partitioned to half with respect to the symmetric axis was prepared by hot gas forming process with a maximum pressure of TMPa following to FEM analysis. For inner layer, copper alloy was machined for cooling channels and then placed in the gas pressure welding fixture. It is shown that the optimum condition of gas pressure welding is 7㎫ at 890℃, for one hour. EDX analysis and scanning electron microscope micrograph confirm the atomic diffusion process is observed at the interface and copper atoms diffuse into steel, while iron and chrome atoms diffuse into copper. The result shows that the manufacturing method with superplastic forming and gas pressure welding of steel and copper alloy has been successful for near net shape manufacturing of scaled combustion chamber of launch vehicle.