Dong-Hwan Kang

Densification Behavior of Titanium in Direct Powder Rolling Process

The densification behavior of titanium powder in the direct powder rolling process was investigated. An analytical model for the roll-backlash phenomenon during this process was proposed to conduct a realistic finite element analysis. Furthermore, the roll-backlash was quantitatively analyzed, and the relative densities of the greenstrip along the rolling direction were precisely estimated. Finally, the slip and nip phenomena were identified by calculating the contact pressure and shear stress between the titanium powder and the roller in order to understand the densification behavior of the powder during the rolling process.

Determination of Shock Absorption Performance and Shear Modulus of Rubbers by Drop Impact Test

Abstract Shock absorption performances of various rubbers were investigated by using drop impact test. Several types of rubber such as NR, NBR, EPDM, SR and PUR with three respective levels of shore hardness were used for the test. As in the cases, the absorbed impact energies in rubbers were measured under seven different loads against impact energy between 5-80J. The impact absorption efficiencies of the rubbers then were evaluated by means of both single impact energy condition and summation of all impact energy applied condition. As shown in the results, PUR and EPDM have better shock absorption performances than other rubbers. Further analysis was extended to determine a shear modulus of SR through the finite element implementation with Blatz-Ko model. As can be seen, relatively higher level of absorption energy results in a decreasing shear modulus. 1. 서 론 고무재료는 우수한 충격흡수성능을 지님에 따라 다양한 산업에서 (1)그 활용도가 증가하고 있다. 그러나 고무재료의 충격흡수성능이 고무의 종류, 경도 및 사용환경 등에 의해 다르게 나타남에 따라 실질적인 충격흡수 구조에 적용함에 있어 적합한 고무를 선정하는 데는 상당한 어려움이 따른다. 따라서 충격흡수재의 설계 시 보다 효과적인 고무의 선정을 위해 종류, 경도, 사용환경에 따른 충격흡수성능의 체계적인 평가가 요구되고 있다. 또한 고무재료는 정적거동과 동적거동이 서로 다르게 나타나며 이는 충격거동 해석에서 요구되는 고무의 특성에도 영향을 미친다. 이에 따라 보다 정확한 고무의 충격해석을 위해 요구되는 고무의 동적 특성, 특히 동적 강성(dynamic modulus)의 평가를 위한 다양한 방법들이 제시되고 있다.

Fatigue Crack Propagation and Fatigue Life Evaluation of High-Performance Steel using Modified Forman Model

) Key Words : Fatigue Crack Propagation(피로균열전파), Fatigue Life Evaluation(피로수명평가), Crack Closure Phenomenon(균열 닫힘 현상) 초록: 응력확대계수비와 하중 주파수가 각각 ‘R=0.1, f=0.1 Hz’, ‘R=0.3, f=0.3 Hz’ 및 ‘R=0.5, f=0.5 Hz’인 세 가지 하중 조건에서 피로균열전파 실험을 하였으며 이를 바탕으로 고성능 강재의 피로균열전파거동 분석과 피로수명평가를 수행하였다. 하중 조건에 따른 피로균열전파거동을 모사하기 위해 수정된 Forman 모델을 제안하였으며 제안된 모델은 하한 응력확대계수폭과 균열 닫힘 현상에 의한 유효 응력확대계수폭을 고려함으로써 거동의 전 영역을 모사할 수 있었다. 응력확대계수비와 하중 주파수가 0.1인 경우, 균열이 약 5.0 mm 전파될 때 피로수명을 평가한 결과, Forman 모델은 8,814 cycles, 수정된 모델은 12,292 cycles의 계산결과를 얻었으며, 이를 12,774 cycles의 실제 실험결과와 비교할 때 수정된 Forman 모델이 실제 피로균열전파거동을 보다 효과적으로 모사하고 있음을 확인할 수 있었다. Abstract: Fatigue crack propagation behavior and the fatigue life in-high performance steel were investigated by means of fatigue crack propagation tests under constant loading conditions of ‘R=0.1 and f=0.1 Hz’, ‘R=0.3 and f=0.3 Hz’, and ‘R=0.5 and f=0.5 Hz’ for the load ratio and frequency, respectively. A modified Forman model was developed to describe the fatigue crack propagation behavior for the conditions. The modified Forman model is applicable to all fatigue crack propagation regions I, II, and III by implementing the threshold stress intensity factor range and the effective stress intensity factor range caused by crack closure. The results show that predicted fatigue lives of Forman and modified Forman models were 8,814 and 12,292 cycles, respectively when the crack propagated approximately 5.0 mm and the load ratio and frequency were both 0.1. Comparison of the test results indicates that the modified Forman model showed much more effective fatigue crack propagation behavior in high-performance steel.

DETERMINATION OF MECHANICAL PROPERTIES WITH GEOMETRICAL FACTORS OF CLOSED CELL ALUMINUM FOAMS USING X-RAY TOMOGRAPHY PROCESSING

Factors affecting non-uniform structures of cellular materials include wrinkled morphology, foam shape, and irregular solid structures. These factors result in different mechanical properties. The effect of different cell structure of aluminum foam on mechanical properties has been investigated in terms of several geometrical factors such as foam surface, foam volume, solid surface and solid volume. Uniaxial compression/tension tests have been performed by using a batch of various relative density foams. A 2-D micro-scan data of the foam structure with high spatial resolution was obtained by a Micro-CT through a digital processing technique of X-ray Tomography. The foam structures were then reconstructed as a set of flat cross-sections to interpret the 3-D morphological parameters. As shown by the results, the ratio of foam volume to solid volume has been substituted for the relative density well enough, and thus the higher of the relative density showed the greater of the total cell volume within the solid volume. The ratio of foam surface to solid volume also represented a degree of the total cell surface within a unit solid volume. Energy absorption has been changed with the ratio of foam surface to foam volume. The ratio of foam surface to foam volume in a specific morphology therefore can correctly characterize the complexity of the cell structures for the determination of mechanical properties of closed cell foams.