• Composites Research
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• v.26 no.5
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• pp.295-302
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• 2013

In this study, Al5052/CFRP composites were fabricated for an automobile component by compression molding process inside a U-channel mold. Al5052 sheet were treated by sand blasting with two different particle sizes. Accordingly, surface roughness (Ra) values of $4.25{\mu}m$ and $1.85{\mu}m$ were obtained for the treated Al5052 sheets. The effect of surface roughness of Al5052 sheets on the adhesion and mechanical properties of Al5052/CFRP composites have been evaluated. Shear lap test and 3-point bending test were conducted. Results showed that the shear load for the composite fabricated by using the treated Al5052 sheets with Ra value of $1.85{\mu}m$ and $4.25{\mu}m$ were 3 and 5 times higher than Ra value of $0.73{\mu}m$ of the composite fabricated by using the untreated sheet. The bending stress of 200MPa was obtained for the composite fabricated with untreated Al5052 sheets. The bending stress increased to 400MPa when the composite fabricated from treated sheets. However, the bending stress was not influenced by treating condition through sand blasting.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.08a
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• pp.63-71
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• 2004

Recently, a sheet forming process of Mg alloys is highlighted again due to increasing demand for Mg wrought alloys in the applications of casings of mobile electronics and outer-skins of light-weight transportation. Microstructure control is essential for the enhancement of workability and formability of Mg alloy sheets. In this research, AZ31 Mg alloy sheets were prepared by hot rolling process and the rolling condition dependency of the microstructure and texture evolution was studied by employing a conventional rolling mill as well as an asymmetric rolling mill. When rolled through multiple passes with a small reduction per pass, fine-grained and homogeneous microstructure evolved by repetitive dynamic and static recrystallization. With higher rolling temperature, dynamic recrystallization was initiated in lower reduction. However with increasing reduction per pass, deformation was locallized in band-like regions, which provided favorable nucleation sites f3r dynamic recrystallization. Through post annealing process, the microstructures could be transformed to more equiaxed and homogeneous grain structures. Textures of the rolled sheets were characterized by $\{0002\}$ basal plane textures and retained even after post annealing. On the other hand, asymmetrically rolled and subsequently annealed sheets exhibited unique annealing texture, where $\{0002\}$ orientation was rotated to some extent to the rolling direction and its intensity was reduced.

• Journal of Welding and Joining
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• v.20 no.3
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• pp.122-128
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• 2002

The laser welding in the automotive industries has been used widely for the butt joint of blank sheets rather than the lap joint of automotive body panels. But as a substitute far the spot welding of automotive body panels, the so called three dimensional laser welding will be important far the body panel engineers. Specially the laser welding of body panels with a smooth weld line is applied increasingly, for example, to the side panels. So far, some criteria of the laser weld quality was suggested by in-house regulations or national standards from experiences and/or rule of thumbs. In the manufacturing places, a go or no-go criterion is adopted because of the simplicity or a lack of rational criteria. It is true specially for the selection of the process parameters, which gives the basic causes for the good quality of laser welds. In this study, the effects of joint combination, gap and welding speed on the lap joint $CO_2$ laser welding of two mild steel sheets with different thicknesses are obtained through a $2{\times}3{\times}7$ factorial experiment. The results of the weld quality are statistically analysed using analysis of variance (ANOVA) and compared between two characteristic zones, which are separated by the type of sectional shapes and the level of input energy per volume. The thickness combinations are 0.8mm/1.2mm, 1.2mm/0.8mm of mild steel sheets. The welding speed covers from the deep penetration to the partial penetration. The gap size has three levels of no-gap, 0.16m, and 0.26mm. The bead width, penetration depth and input energy per volume are measured and used as the weld quality criteria.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2006.11a
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• pp.5-8
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• 2006

This study investigates the field application, Songdo the # 1st WORLD, on zero-crack construction of the fundamental mass concrete using double bubble sheets and applying low heat mixture. Experimental results of hydration heat analysis showed that crack modulus of concrete incorporating 20% of blast furnace slag cement was 1.0 in 120 hours, representing 50% probability of crack occurrence, thus requiring additional measures. As for a curing method, a specimen insulating two layers of vinyl chloride+double bubble sheets exhibited only $16.5^{\circ}C$ difference between upper and lower sections, and it also showed favorable workability as well as competitive economic side. Therefore it was determined to use it for curing method in this field. For the curing results of practical field, using 2 layers of vinyl chloride+double bubble sheets and applying low heat mixture on the fundamental mass concrete in 3A residential building exhibited less than only $15^{\circ}C$ difference between surface and center section of that in 5 days elapse and less than $20^{\circ}C$ in 9 days. This means that the crack by hydration heat was prevented, and other fields structures also resisted the plastic shrinkage by insulating the vinyl, sinking crack by second temping, even drying shrinkage by the mixture of low unit water. Therefore the crack on the placement has not been found so far, since the construction was started before 6 month.

• Journal of the Korea Institute of Building Construction
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• v.18 no.3
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• pp.251-257
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• 2018

There are various quality deteriorations of concrete such as plastic, drying shrinkage due to abrupt moisture evaporation, slump loss and cold joint under hot weather condition. To protect from above deteriorations, several kinds of modified bubble sheets have been applied to secure heat insulation performance. But, there is not enough application cases of bubble sheets at job site under hot weather condition. The objective of the paper is to investigate the temperature profile and crack occurrence of the concrete covered with five different kinds of surface curing sheets, which is placed under hot weather condition. Single layer transparent bubble sheet, white colored bubble sheet, aluminum metalizing bubble sheet and PE film are adopted for surface curing sheets. Test results indicated that application of aluminum metalizing bubble sheet had most favorable effect on the reduction of on temperature rise and on the crack reduction of concrete. But due to larger reflection of light by aluminum, it brings about visual pollution to the workers. Hence, the application of white colored bubble sheet can be the most desirable alternative to protect the concrete from hot weather in the field.

• Steel and Composite Structures
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• v.10 no.2
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• pp.187-205
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• 2010

The existing CFT columns present the deterioration in confining effect after the yield of steel tube, local buckling and the deterioration in load capacity. If lateral load such as earthquake load is applied to CFT columns, strong shearing force and moment are generated at the lower part of the columns and local buckling appears at the column. In this study, axial compression test and beam-column test were conducted for existing CFT square column specimens and those reinforced with carbon fiber sheets (CFS). The variables for axial compression test were width-thickness ratio and the number of CFS layers and those for beamcolumn test were concrete strength and the number of CFS layers. The results of the compression test showed that local buckling was delayed and maximum load capacity improved slightly as the number of layers increased. The specimens' ductility capacity improved due to the additional confinement by carbon fiber sheets which delayed local buckling. In the beam-column test, maximum load capacity improved slightly as the number of CFS layers increased. However, ductility capacity improved greatly as the increased number of CFS layers delayed the local buckling at the lower part of the columns. It was observed that the CFT structure reinforced with carbon fiber sheets controlled the local buckling at columns and thus improved seismic performance. Consequently, it was deduced that the confinement of CFT columns by carbon fiber sheets suggested in this study would be widely used for reinforcing CFT columns.

• Earthquakes and Structures
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• v.15 no.6
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• pp.617-627
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• 2018

After earthquakes FRP sheets are often used for the rehabilitation of damaged Reinforced Concrete (RC) beamcolumn connections. Connections with minor to moderate damage are often dealt with by applying FRP sheets after a superficial repair of the cracks using resin paste or high strength mortar but without infusion of thin resin solution under pressure into the cracking system. This technique is usually adopted in these cases due to the fast and easy-to-apply procedure. The experimental investigation reported herein aims at evaluating the effectiveness of repairing the damaged beam-column connections using FRP sheets after a meticulous but superficial repair of their cracking system using resin paste. The investigation comprises experimental results of 10 full scale beam-column joint specimens; five original joints and the corresponding retrofitted ones. The repair technique has been applied to RC joints with different joint reinforcement arrangements with minor to severe damage brought about by cyclic loading for the purposes of this work. Aiming at quantitative concluding remarks about the effectiveness of the repair technique, data concerning response loads, loading stiffness and energy absorption values have been acquired and commented upon. Furthermore, comparisons of damage index values and values of equivalent viscous damping, as obtained during the test of the original specimens, with the corresponding ones observed in the loading of the repaired ones have also been evaluated and commented. Based on these comparisons, it is deduced that the technique under investigation can be considered to be a rather satisfactory repair technique for joints with minor to moderate damage taking into account the rapid, convenient and easy-to-apply character of its application.

• Structural Engineering and Mechanics
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• v.78 no.5
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• pp.611-622
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• 2021

Composite material-due to low density-causes weight savings, which results in lower fuel consumption of transport vehicles. The aim of the research was to change the existing base-plate of the aluminum airplane container with the composite sandwich plate in order to reduce the weight of the containers of cargo aircrafts. The newly constructed sandwich plate consists of aluminum honeycomb core and composite face-sheets. The face-sheets consist of glass or carbon or hybrid fiber layers. The orientations of the fibers in the face-sheets were 0°, 90° and ±45°. Multi-objective optimization method was elaborated for the newly constructed sandwich plates. Based on the design aim, the importance of the objective functions (weight and cost of sandwich plates) was the same (50%). During the optimization nine design constraints were considered: stiffness, deflection, facing stress, core shear stress, skin stress, plate buckling, shear crimping, skin wrinkling, intracell buckling. The design variables were core thickness and number of layers of the face-sheets. During the optimization both the Weighted Normalized Method of the Excel Solver and the Genetic Algorithm Solver of Matlab software were applied. The mechanical properties of composite face-sheets were calculated by Laminator software according to the Classical Lamination Plate Theory and Tsai-Hill failure criteria. The main added-value of the study is that the multi-objective optimization method was elaborated for the newly constructed sandwich structures. It was confirmed that the optimal new composite sandwich construction-due to weight savings and lower fuel consumption of cargo aircrafts - is more advantageous than conventional all-aluminum container.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.469-474
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• 2002

Recently fiber sheets are used for strengthening the damaged concrete structures due to its many advantages such as its durability, non-corrosive nature, low weight, ease of application, cost saving, control of crack propagation, strength to thickness ratio, high tensile strength, serviceability and aesthetic. However, the lack of analytical procedures for predicting the nominal moment capacity by the fiber sheet reinforcement leads to difficulties in the effective process of decisions of the factors in the strengthening procedure. In this work, flexural strengthening effects by fiber sheets bonded on soffit and web of the member are theoretically studied for the reinforced concrete T beam. The analytical solutions are compared with experimental results of several references to verify the proposed approach.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.75-80
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• 2001

The purpose of this study is to investigate the characteristics of springback for various process conditions of the 2D draw-bending operation. The process variables are the forming temperature, the geometry of tools such as punch profile radius(Rp) and die profile radius(Rd). Especially, in order to control the springback, the use of the warm forming method is applied. For the warm draw-bending, five steps of temperature ranges, from room temperature to $200^{\circ}C$, were adopted. And two kinds of steel sheets, namely SCP1 and TRIP(transformation-induced plasticity), the newly developed high strength Steel, were adopted. As a result, the springback was affected by the elevated temperature and the geometry of tools in two kinds of steel sheets.