Papers by Author: Sung Hyuk Lee

Abstract: Ankle Foot Orthosis (A.F.O) should endure the uncountable repeating impact and fatigue loadings due to the gait characteristics. This study investigated the impact deflection and relationship between the absorbed energy and the residual strength rate using the cross ply GFRP (glass/epoxy) and the woven AFRP (aramid/epoxy) for the leaf spring in A.F.O. In conclusion, the equation was suggested to evaluate the absorbed energy and the residual strength rate by the different impact velocities. When the cross ply GFRP and the woven AFRP was selected for the leaf spring in A.F.O, it was reasonable to use the cross ply GFRP for the parts subject to the large impact and the woven AFRP for the parts to require the high elastic energy such as the large deformation.

Abstract: The behavior of a circular piezoelectric actuator for volumetric micropump has been investigated by using theoretical and finite element analyses. A modified theoretical model was developed to predict the behavior of a piezoelectric actuator induced by the applied voltage. The theoretical results for the diaphragm deflection were in good agreement with the results from numerical simulation. Based on the theoretical analysis, the effects of several important parameters on actuation performance have been investigated. These parameters include the dimensions and mechanical properties of the piezoelectric disk, bonding layer and elastic diaphragm materials. Consequently, it is thought that above theoretical model might be employed as a tool for design and optimization of the piezoelectric actuator for micropump application.

Abstract: Bending collapse behaviors and energy absorption characteristics of aluminum-GFRP hybrid tube beams were evaluated by using experimental tests combined with theoretical analysis. Hybrid tube beams composed of glass fiber-epoxy layer wrapped around on aluminum tube were made in autoclave with the recommend curing cycle. The hybrid tube beams showed a considerable improvement in their bending performance. The maximum bending moment and specific energy absorption of the hybrid tubes were higher than those of the aluminum tubes. They were also evaluated as a function of ply orientation and thickness of GFRP layer. A modified theoretical model was developed to predict the resistance to the collapse of hybrid tube beams subjected to a bending load. Theoretical ultimate bending moments and moment-rotation angle curves of hybrid tube beams were in good agreement with experimental ones. Hybrid tube beams strengthened by GFPR layer with 90°/0° and 45°/-45° ply orientation showed an excellent bending strength and energy absorption capability, respectively. Therefore, on the basis of above results, it was concluded that aluminum-GFRP hybrid tube beams might be employed as reinforcing and/or energy absorbable light weight space frame.

Abstract: To analyze the stress distribution and the crack initiation due to the location of the defects, artificial defects were made on the different locations of the high strength monolithic Al and GLAss fiber REinforced laminate (GLARE). Experimental study shows that the defect location in the vicinity of the circular hole was changed from ° = 90 4 θ to ° = 0 1 θ , the stress concentration was increased. The stress concentration of GLARE was about 15% higher than that of the monolithic Al. When the defect was at ° = 30 2 θ , Multi Site Damage (MSD) crack was found in the monolithic Al but not in GLARE. Instead of MSD crack, a delamination was made in GLARE and that it resulted in the prevention against the second crack initiation.

Abstract: Bending deformation and energy absorption characteristics of aluminum-composite hybrid tube beams have been analyzed for improvement in the bending performance of aluminum space frame by using experimental tests combined with theoretical and finite element analyses. Hybrid tube beams composed of glass fabric/epoxy layer wrapped around on aluminum tube were made in autoclave with the recommended curing cycle. Basic properties of aluminum material used for initial input data of the finite element simulation and theoretical analysis were obtained from the true stress-true strain curve of specimen which had bean extracted from the Al tube beam. A modified theoretical model was developed to predict the resistance to the collapse of hybrid tube beams subjected to a bending load. Theoretical moment-rotation angle curves of hybrid tube beams were in good agreement with experimental ones, which was comparable to the results obtained from finite element simulation. Hybrid tube beams strengthened by composite layer on the whole web and flange showed an excellent bending strength and energy absorption capability.

Abstract: Bending performances of aluminum square tube beams reinforced by aluminum plates under three point bending loads have been evaluated using experimental tests combined with theoretical and finite element analyses. Basic properties of aluminum materials used for initial input data of the finite element simulation were obtained from the true stress-true strain curves of specimens which had bean extracted from the Al tube beams. True stresses were determined from applied loads and cross-sectional area records of a tensile specimen with a rectangular cross-section by real-time photographing. True strains were obtained from in-situ local elongation measurements of the specimen gage portion by the multi-point scanning laser extensometer. Four kinds of aluminum tube beam specimens adhered by aluminum plates were employed. The bending deformation behaviors up to the maximum load described by the numerical simulation were in good agreement with experimental ones. An aluminum tube beam strengthened by aluminum plate on the upper web showed an excellent bending capability.

Abstract: To analyze the bending collapse behavior of an aluminum square tube under the bending moment load, a finite element simulation for the four-point bending test has been performed. Using an aluminum tube beam specimen partly inserted with two steel bars, local buckling deformation near the center of the tube beam was induced. Simulated moment-rotation angle curve obtained during the post-collapse period of the aluminum tube with steel bars were in good agreement with experimental result, which was comparable to the result obtained from Kecman's theory. Using a combination of the four-point bending test and its finite-element simulation, analysis of the local buckling and the bending collapse behavior of an aluminum tube beam could be quantitatively accomplished.