Papers by Author: Sung Ho Yoon

Abstract: This paper focuses on the modeling of the cyclic super-elastic behaviors of SMA. Cyclic tensile experiments are performed on the NiTi SMA to test its cyclic super-elastic behaviors. A material parameter evolution equation is supposed to express the cyclic effects on the material parameters of the SMA. A cyclic constitutive equation is developed to describe the stress-strain relationship of the SMA under cyclic load. Numerical results show that the cyclic constitutive model, which includes the material parameter evolution equation and cyclic constitutive equation, well predicts the super-elastic behaviors of the SMA subjected to cyclic load.

Abstract: A shape memory alloy (SMA) torsion actuator is designed by using SMA wires and a thin-walled tube. A mechanical model, which predicts the thermo-mechanical behaviors of the SMA torsion actuator, is developed based on the knowledge of solid mechanics and constitutive relation of SMA. The relationship of the torsion-angle and temperature of the SMA torsion actuator is numerically described by using the mechanical model coupled with Tanaka’s, Liang’s and Zhou’s phase transformation models of SMA respectively. Results show the mechanical model well predict the thermo-mechanical behaviors of the SMA torsion actuator.

Abstract: In this study, the effect of combined environmental factors such as ultraviolet ray, high temperature and high moisture on mechanical and thermal analysis properties of glass fabric and phenolic composites are evaluated through a 2.5KW accelerated environmental aging tester. The environmental factors such as temperature, moisture and ultraviolet ray applied of specimens. A xenon-arc lamp is utilized for ultraviolet light and exposure time of up to 3000 hours are applied. Several types of specimens - tensile, bending, and shear specimens that are warp direction and fill direction are used to investigate the effects of environmental factors on mechanical properties of the composites. Mechanical degradations for tensile, bending and shear properties are evaluated through a Universal Testing Machine (UTM). Also, storage shear modulus, loss shear modulus and tan δ are measured as a function of exposure time through a Dynamic Mechanical Analyzer (DMA). From the experimental results, changes in material properties of glass fabric and phenolic composites are shown to be slightly degraded due to combined environmental effects.

Abstract: Interlaminar fracture behavior of carbon fabric/epoxy composite, which is one of the candidate composites for the Korean tilting train carbody, was investigated. Specimens were made of a CF3227 plain fabric with an epoxy resin. An initial starter crack was formed by inserting a 12.5
thick Teflon film at the one end of the specimen. Interlaminar fracture toughness was evaluated using the mixed mode flexural fixture, which provides a wide range of mixed mode deformation by varying the length of lever arm. According to the results, the crack growth was progressive and stable under mode I dominantly mixed mode ratio and was relatively rapid and unstable under mode II dominantly mixed mode ratio. The mixed mode interlaminar fracture behavior can be predicted by a mixed mode fracture criterion depending on the point at which the crack growth was associated.

Abstract: This paper explains the structural safety evaluation of a hybrid composite train carbody. The composite carbody with length of 23m was manufactured as a sandwich structure composed of a 40mm-thick aluminum honeycomb core and 5mm-thick woven fabric CF1263 carbon/epoxy face. In order to evaluate the structural safety of it, the dynamic force of ±0.2g was applied to the full weight carbody by two 50-ton capacity hydraulic actuators. The excitation frequency was determined by the first bending natural frequency evaluation test under full weight condition. The test was conducted for 2x106cycles. During the test, the nondestructive tests using X-ray for the composite body structure and liquid penetrant test for the welding region of the steel underframe were performed.

Abstract: Phase transformation temperatures of the SMA annealed at different annealing temperature were investigated through a series of DSC test. A new phase transformation model was proposed to predict the phase transformation and a numerical analysis was conducted to demonstrate the applicability of the new model. According to the result, phase transformation temperatures would be affected on annealing temperatures and the new phase transformation model would be applicable to predict the phase transformation under the stress-free state or the external stress state.