Papers by Author: Nak Sam Choi

Abstract: A temperature-compensating fiber Bragg grating(FBG) sensor having two different FBGs in one fiber line was applied to the real time measurement of mechanical normal strain in structures. Measurement of mechanical strains of the aluminum beam surface by the double FBG sensor was performed under various thermal conditions, which was compared with results of electrical resistance strain gage. The FBG sensor fabricated in this study could detect accurately values of mechanical strains without containing any thermal strain component.

Abstract: Coolant rubber hoses for automobile radiators under thermal and mechanical loadings can be degraded and thus failed due to the influences of contacting stresses of air, coolant liquid and to the locally formed electricity. In this study, degradation behavior of the radiator hose made of EPDM rubber was evaluated. The thermo-oxidative aging test showed that the surface hardness IRHD of the rubber increased together with a reduction of failure strain. By the electro-chemical test it was shown that the penetration of coolant liquid into the skin of the rubber hose arose inducing an increase in weight of specimens as well as a decrease in failure strain and IRHD hardness. The penetration of coolant liquid altered considerably the micro-structure and the micro-hardness distribution along the depth in the rubber hose. On the basis of the above results failure mechanisms of degraded EPDM rubbers were suggested according to the kinds of contacting stresses.

Abstract: Fatigue fracture behavior of a hybrid joint part for the tilting car body was evaluated in comparison to the case of static fracture. The specimens of hybrid joint part applied in the real tilting car body were fabricated for the bending test. Characteristic fracture behaviors of hybrid joint part specimens under cyclic loads were obviously different from the case under static loads. Static bending load caused the shear deformation and fracture in the honeycomb core region, while cyclic bending load did the delamination along the interface between composite skin and honeycomb core layers as well as the fracture of welded joint part. Experimental results obtained by static and fatigue tests were reflected in modifications of design parameters of the hybrid joint structure in the real tilting car body.

Abstract: Fatigue fracture behavior of a hybrid joint beam for the tilting car body was evaluated in comparison to the case of static fracture. Specimen beams of the hybrid joint part attached in the real tilting car body were fabricated for the bending test. Characteristic fracture behaviors of hybrid joint beam specimens under cyclic transverse loads were very different from the case under static loads. Static transverse load caused shear deformation and fracture in the honeycomb core region, while cyclic transverse load brought about delamination along the interface between composite skin and honeycomb core layers as well as fracture of the welded joint. Fracture characteristics obtained by the transverse fatigue tests were reflected for improving the hybrid joint structure in the real tilting car body.

Abstract: In this study, for EPDM rubber conventionally used as a radiator hose material the aging behaviors of the skin part under the electro-chemical stresses were nondestructively evaluated. On account of the penetration of coolant liquid into the skin part the weight of rubber specimens degraded by ECD test increased, whereas their failure strain and IRHD hardness decreased largely. The penetration of coolant liquid seemed to induce some changes in inner structure and micro hardness distribution of the rubbers. Consequently, EPDM rubbers degraded by ECD could be characterized nondestructively by micro-hardness and chemical structure analysis methods.

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: Fatigue fracture behavior of a hybrid composite joint with riveting was evaluated in comparison to the case of static fracture. Hybrid composite joint specimens for shear test were made with layers of carbon fiber/epoxy composite and stainless steel. Characteristic fracture behaviors of those specimens were obviously different under static and cyclic loads. Static shear loading showed the fracture of a pure shear mode, whereas cyclic fatigue-shear loading caused the local stress concentration of a tensile mode and thus brought about the tensile fracture at that site. Experimental results obtained by static and fatigue tests were considered in modifications of design parameters of the hybrid joint.

Abstract: Dominant frequency characteristics of acoustic emission (AE) from single-edge-notched (SEN) glass fiber aluminum laminates (GFAL) under tensile loads were analyzed in relation to fracture mechanisms. The first and second peak frequencies expressed the characteristic changes of fracture processes in SEN-GFAL specimens such as macro-crack propagation and/or delamination between aluminum and fiber layers. On the basis of the above frequency analysis and of the fracture observation with ultrasonic through-scan and various microscopes, characteristic models for crack propagation of SEN-GFAL was proposed according to various orientations of fiber layer.

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: Theoretical formulas for effective elastic modulus and Poisson's ratio of honeycomb core materials were proposed considering the bending, axial and shear deformations of cell walls. Theoretical results obtained by the formulas showed orthotropic elasticity and large Poisson’s ratio, which were comparable to results by finite element analysis(FEA). Tensile test of honeycomb sandwich composite(HSC) plates was performed for analysis of their deformation behaviors and interlaminar stresses. Equivalent plate model using the theoretical results of honeycomb core layer show that interlaminar shear stress occurring due to large difference of Poisson’s ratio between skin and honeycomb core layers led to the delamination in HSC plate under tensile loading. Load-displacement behavior of HSC specimen simulated by equivalent plate model coincided fairly with that of detailed FEA model similar to experimental results.