• Transactions of Materials Processing
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• v.1 no.2
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• pp.20-31
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• 1992

대형 강괴의 업셋팅 공정은 주조 조직의 방향성을 없애고, 코깅작업의 효율을 향상시키기 위한 충분한 단조비를 확보하기 위하여 필요한 공정이다. 공정에 영향을 주는 인자로써 상부 금형의 형상을 변화시켜 가면서 해석을 수행하였다. 극단적인 긴 파이프성 기공의 변형거동과 중심부에서 높이에 따른 원형기공의 닫힘거동 및 압하율과 기공폐쇄 정도를 관찰하였다. 충분한 단조비를 얻고 기공의 닫힘 및 압착을 이루기 위한 적절한 압셋팅 다이의 선택 및 업셋팅 공정을 예측하여 공정개선에 기여하고자 한다.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.75-78
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• 2008

The predictive equation of void-closure was developed to evaluate void crush ratio with respect to the process variables in the cogging process of a large round bar. The comprehensive finite element analysis with the process variables such as reduction ratio and die width ratio was carried out. The predictive equation of void-closure for cogging process was established on the basis of the regression analysis with the extensive FE analysis results and verified by comparing the predicted results with FEA results with various forging passes.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.192-197
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• 2004

The anisotropy in coefficient of thermal expansion (CTE) between the in-plane and out-of-plane of 3-dimensional thick composite structures induces residual stresses and the large void content due to insufficient compaction of fabric composites, which results in low interlaminar strengths. In order to reduce the through thickness CTE and the void content, in this work, carbon fabric phenolic laminates were compacted by pressure generated by autoclave and a compressive jig, from which the through-thickness CTEs and the void contents were measured. From the measurement, it was found that the through-thickness CTE and the void content had different characteristics from ordinary composites due to gas produced during the cure reaction of phenolic resin.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1877-1889
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• 1992

Upsetting is performed in open-die press forging to deform metal in all directions in order to enhance soundness of a product and reduce directionality of properties caused by casting. It is necessary to ensure sufficient forging ratio for subsequent cogging operations and consolidate the void along the centerline. To obtain these benefits, the upper die shape (dome and dished shape) is considered as an upsetting parameter. Thermo-viscoplastic finite element analysis has been carried out so as to understand the influence of upper die shape on the effective strain, hydrostatic stress and temperature in the upset-forged ingots without internal defects. The analysis is focused on the investigation into internal void closure in ingots with pipe holes and circular voids. The computational results have shown that the volume fraction of the void is independent of the circular void size and the closure of internal voids is much more influenced by the effective strain than the hydrostatic stress around the void. It is finally suggested that the height reduction must be over 35% for consolidation of internal voids.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.57-60
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• 2008

The studies for internal void closure have been conducted experimentally and numerically for open die forging. The FEM analysis is performed to investigate the deformation behavior of some internal voids in cast ingots during two upsetting stages. The calculated results of void closure behavior are compared with the measured results before and after upsetting. The shapes and sizes of each internal void are scanned by the X-ray scanner. From this result, the criteria for deformation amounts effect on the void closure can be investigated by the types of void. Closed voids could be compressed and eliminated after forging when the applied deformation amounts were larger than the critical effective strains. On the other hand, open voids could not be compressed and removed.

• Transactions of Materials Processing
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• v.19 no.8
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• pp.480-485
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• 2010

In this work, the closing behavior of cylindrical-shaped voids was experimentally investigated according to various parameters such as reduction ratio in height, initial void size and billet rotation during hot open die forging process. The reduction ratio in height, number of path, and billet rotation were chosen as key process parameters which influence the void closing behavior including the change of void shape and size. On the other hand, values of die overlapping and die width ratio were set to be constant. Void closing behavior was estimated by microscopic observation. Based on the observations, it was confirmed that application of billet rotation is more efficient to eliminate the void with less reduction ratio in height. The experimental results obtained from this study could be helpful to establish the optimum path schedule of open die forging process.

• Composites Research
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• v.32 no.6
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• pp.395-402
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• 2019

The RFI process is an OoA process that fiber mats and resin films are laminated and cured in a vacuum bag. In case that resin film is insufficient to fill empty space in fibers, it makes void defect in composites and this void decrease mechanical properties of the composites. For this reason, non-destructive testing is usually used to evaluate void of manufactured composites. So, in this study, a manufacturing method of standard void specimens, which are able to be used as references in non-destructive testing, was proposed by controlling resin film thickness in the RFI process. Also, a fiber compaction test was proposed as a method to set the resin film thicknesses depending on target voids of manufacturing panels. The target void panels of 0%, 2%, and 4% were made by the proposed methods, and signal attenuation depending on void was measured by non-destructive testing and image analysis. In addition, voids of specimens for tensile, in-plane, short beam and compressive tests were estimated by signal attenuation, and mechanical properties were evaluated depending on the voids.

• Composites Research
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• v.31 no.1
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• pp.23-29
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• 2018

The RFI process can be applied to very thick structures without limiting the resin viscosity. When the proper thickness of the resin film cannot be set, the resin film creates either the non-impregnated section or the excessive resin contents and this leads to the deterioration of mechanical properties. Therefore, this study proposed a method for setting the resin film thickness in the RFI process. The fiber compaction behavior test was proposed by setting the proper resin film thickness and the properties of composites were evaluated through short beam shear strength test, compression test and porosity measurement to verify the proposed method. The evaluation of physical properties of composites was conducted and an appropriate level of resin film thickness was found based on the results of fiber compaction behavior test.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.1
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• pp.97-107
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• 2015

An analytical model was developed to estimate the viscous and squeeze-film damping ratios of heat exchanger tubes subjected to a two-phase cross-flow. Damping information is required to analyze the flow-induced vibration problem for heat exchange tubes. In heat exchange tubes, the most important energy dissipation mechanisms are related to the dynamic interaction between structures such as the tube and support and the liquid. The present model was formulated considering the added mass coefficient, based on an approximate model by Sim (1997). An approximate analytical method was developed to estimate the hydrodynamic forces acting on an oscillating inner cylinder with a concentric annulus. The forces, including the damping force, were calculated using two models developed for relatively high and low oscillatory Reynolds numbers, respectively. The equivalent diameters for the tube bundles and tube support, and the penetration depth, are important parameters to calculate the viscous damping force acting on tube bundles and the squeeze-film damping forces on the tube support, respectively. To calculate the void fraction of a two-phase flow, a homogeneous model was used. To verify the present model, the analytical results were compared to the results given by existing theories. It was found that the present model was applicable to estimate the viscous damping ratio and squeeze-film damping ratio.

• Transactions of Materials Processing
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• v.22 no.5
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• pp.269-274
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• 2013

Experiments and numerical simulations of the incremental upsetting test were carried out to investigate void closure behavior and mechanical characteristic of a 1.5wt%C ultra-high carbon steel. The experimental results showed that the voids become quickly smaller as the reduction ratio increases. The simulation results confirmed this behavior and indicated that the voids were completely closed at a reduction ratio of about 40~45% during incremental upsetting. After the completion of the incremental upsetting tests, the process of diffusion bonding was employed to heal the closed voids in the deformed specimens. To check the appropriate temperature for diffusion bonding, deformed specimens were kept at 800, 900, 1000 and $1100^{\circ}C$ for an hour. In order to investigate the effect of holding time for diffusion bonding at $1100^{\circ}C$, specimens were kept at 10, 20, 30, 40, 50 and 60minutes in the furnace. A distinction between closed and healed voids was clearly established using microstructural observations. In addition, subsequent tensile tests demonstrated that complete healing of a closed void was achieved for diffusion bonding temperatures in the range $900{\sim}1100^{\circ}C$ with a holding time larger than 1 hour.