Authors: Dal Woo Jung, Il Bum Kwon, 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.
1089
Authors: Seung Bum Kwak, Nak Sam Choi
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.
2864
Authors: Dal Woo Jung, Nak Sam Choi
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.
2860
Authors: Dal Woo Jung, Nak Sam Choi
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.
2681
Authors: Seung Bum Kwak, Nak Sam Choi, Youn Joung Choi, Sei Moon Shin
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.
565
Authors: Sung Hyuk Lee, Cheol Woong Kim, Nak Sam Choi
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.
1825
Authors: Dal Woo Jung, Nak Sam Choi
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.
1757
Authors: Sung Choong Woo, Nak Sam Choi
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.
901
Authors: Sung Hyuk Lee, Hyung Jin Kim, Nak Sam Choi
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.
769
Authors: Hyoung Gu Kim, Hoong Soo Yoon, Nak Sam Choi
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.
763