Papers by Author: Do Yeon Hwang

Abstract: Static and dynamic loading fracture experiments were conducted on magnesium alloy under equal and unequal biaxial stress. Cross shape specimens with a crack on their center were used for tests. Fracture behavior was observed by two methods; the caustic and the one point gauge method. From the observation, the stress intensity factors and the fracture toughness values were calculated. It was found that the one point gauge method was not suitable for calculating the stress intensity factor with a strain gauge widely available. The fracture toughness values obtained under equal biaxial stress were 1.2~2.3 times greater than those of magnesium alloy naturally inherited. Moreover, plate thickness was negatively related to the fracture toughness value.

Abstract: This study examined the dynamic penetration phenomena of high-velocity impact of magnesium alloy
 CFRP (Carbon Fiber Reinforced Plastic) composite. Test materials for investigation are magnesium alloy(AZ31B), the heat-treated magnesium alloy (AZ31B-200°C, AZ31B-300°C, AZ31B-430°C), and carbon/epoxy laminated composite materials with fiber direction [0°]8, [0°/45°]4s, [ 0°/90°]4s, [ 0°/45°/90°]3s and [ 0°/45°/-45°/90°]2s. We used a ballistic range (one-stage light gas gun), and the test specimens were set at 0°obliquity at room temperature. A high-speed camera allowed us to capture and analyzed the dynamic penetration phenomena of the test specimen.

Abstract: Recently, high-performance hybrid composites have been used for various applications which require the high strength, high stiffness and low weight. There are growing needs in an automotive, an aircraft, and military applications for composite materials since they have good structural characteristics. They also have good penetration resistance and structural integrity after impact. In order to clarify the mechanism of high-speed destruction for composite materials, this study examined the penetration resistance and the fracture behavior of CFRP (Carbon Fiber Reinforced Plastic) Laminates by using ballistic range (one-stage light gas gun). Test materials for investigation are carbon/epoxy laminated composite materials with fiber direction; [0°]8, [0°/45°]4s, [ 0°/90°]4s, [ 0°/45°/90°]3s and [ 0°/45°/-45°90°]2s. The high speed camera allows us to capture and analyze the dynamic penetration phenomena of the test materials.

Abstract: In this study, dynamic penetration phenomena of high-velocity impact of magnesium alloy were investigated. The surface hardness of magnesium alloy (AZ31B-O) and the heat-treated magnesium alloy (AZ31B-200°C, AZ31B-300°C, and AZ31B-430°C) were examined using Micro Vickers, and the influence of the heat treatment temperature was observed. We analyzed the metal organization using a microscope. We also used a ballistic range (two-stage light gas gun), and the test specimens were set at 0°obliquity at room temperature. A high-speed camera allowed us to capture and analyzed the dynamic penetration phenomena of the test specimen.

Abstract:
Destructive accident sometimes takes place though the equivalent stress is rather low in the viewpoint of strength of materials. The propagation of fatigue cracks under multi-axial stress state and cycling load gives the reason. Fatigue fracture has been considered as one of the most commonly encountered industrial problems that lead to the damage of components in engineering products. In general, the machine structure is always under stress concentration or stress cycles. Moreover, the structure material is usually under two axes or multi-axial stresses instead of uniaxial stress state. It is important, therefore, to clarify the propagation behavior and the fatigue failure problem of the crack under the multi-axial stresses and cycling load from the safety reliability and accident prevention measure. In this study, a biaxial fatigue experimental device was developed which can carry out a wide range of fatigue tests under biaxial stresses. The developed experimental device was identified with a biaxial fatigue experiments including static uniaxial and biaxial tensile test by using the aluminum alloy flat plate as specimens. The propagation behavior of fatigue crack for center notched cruciform specimen in the equal biaxial fatigue test was verified.

Abstract: In this study, the dynamic behaviors of cracks under dynamic biaxial stress are investigated. We conduct dynamic loading fracture experiments on the aluminum (2024-T3) and the magnesium alloy (AZ31B-O) under equitable biaxial stress with a hydraulic high-speed biaxial experimental machine. The processed specimens are cruciform with a crack. Different kinds of cracks are defined by their crack angles. We analyze the results by the caustic method. We obtained the stress intensity factor and the fracture toughness value in the neighborhood of the crack tip under dynamic biaxial stress. We analyzed the obtained data, and then, we compared results.

Abstract: In this study, dynamic behaviors of cracks under dynamic biaxial stress were investigated. We conducted static and dynamic loading fracture experiments on a magnesium alloy (AZ31B-O) under equitable and inequitable biaxial stress by using a hydraulic high-speed biaxial experimental machine. We processed specimens as the cross type with a crack. Different kinds of cracks were defined by their crack angles. We analyzed the results using the caustic method. The experiments revealed some important findings; there is a negative relation between the fracture toughness value and the plate thickness of the magnesium alloy sheet specimen; the fracture toughness value decreases when the crack has an angle different from the loading direction; the fracture toughness value decreases significantly under inequitable biaxial stress.