• Geomechanics and Engineering
• /
• 제25권2호
• /
• pp.111-121
• /
• 2021

Large deformation and rapid pressure propagation take place inside the rock mass under the dynamic loads caused by the explosives, on quarry faces in order to extract aggregate material. The complexity of the science of rock blasting is due to a number of factors that affect the phenomenon. However, blasting engineering computations could be facilitated by innovative software algorithms in order to determine the results of the violent explosion, since field experiments are particularly difficult to be conducted. The present research focuses on the design of a Finite Element Analysis (FEA) code, for investigating in detail the behavior of limestone under the blasting effect of Ammonium Nitrate & Fuel Oil (ANFO). Specifically, the manuscript presents the FEA models and the relevant transient analysis results, simulating the blasting process for three types of limestone, ranging from poor to very good quality. The Finite Element code was developed by applying the Jones-Wilkins-Lee (JWL) equation of state to describe the thermodynamic state of ANFO and the pressure dependent Drucker-Prager failure criterion to define the limestone plasticity behavior, under blasting induced, high rate stress. A progressive damage model was also used in order to define the stiffness degradation and destruction of the material. This paper performs a comparative analysis and quantifies the phenomena regarding pressure, stress distribution and energy balance, for three types of limestone. The ultimate goal of this research is to provide an answer for a number of scientific questions, considering various phenomena taking place during the explosion event, using advanced computational tools.

• 소성∙가공
• /
• 제30권4호
• /
• pp.195-200
• /
• 2021

In this study, we investigated the dominant processing variables that would influence the microstructural development of AA6061 deformed by differential speed rolling (DSR) at ambient temperature. For this purpose, we carried out a series of DSR on the samples to investigate the effects of roll speed ratio, sample rotation, and number of operation under specific rotation. Among these, the condition with a height reduction of 75% at a speed ratio of 1:4 through rotation along rolling direction (RR) displayed the pronounced results of more homogenous matrix-structure and superior mechanical properties than the others tested in this study. This was mainly due to the cross shearing of macro-shear pattern in segment where dynamic recrystallization took place with ease throughout the sample. Thus, an average microhardness value of 101 Hv was obtained in the present sample deformed by 4-pass DSR with RR where macro cross-shearing was effectively applied.

• 소성∙가공
• /
• 제31권2호
• /
• pp.73-80
• /
• 2022

In this study, a developed Mg-5Bi-2Al-0.4Mn (BAM520, wt%) alloy was successfully extruded at an extremely high speed of 70 m/min. Microstructural evolution during extrusion and the microstructural characteristics and tensile properties of the very-high-speed extruded BAM520 alloy were then investigated. The homogenized BAM520 billet contained only thermally stable Mg₃Bi₂ phase particles without any Mg₁₇Al₁₂ phase with a low melting temperature. Therefore, the BAM520 alloy exhibited excellent extrudability. The very-high-speed extruded BAM520 alloy had a completely recrystallized grain structure and a typical basal fiber texture. Despite the extremely high extrusion speed of 70 m/min, the extruded BAM520 alloy had a high ultimate tensile strength of 280 MPa due to combined strengthening effects of a small grain size, numerous fine Mg₃Bi₂ particles, and strong basal texture.

• Structural Engineering and Mechanics
• /
• 제83권4호
• /
• pp.563-576
• /
• 2022

This study aimed to evaluate the progressive collapse potential of buildings designed using conventional design codes for the merchant occupancy classification and subjected to a sudden column failure. For this purpose, three reinforced concrete buildings having different story numbers were designed according to the seismic design recommendations of TSCB-2019. Later on, the buildings were analyzed using the GSA-2016 and UFC 4-023-03 to observe their progressive collapse responses. Three columns were removed independently in the structures from different locations. Nonlinear dynamic analysis method for the alternate path direct design approach was implemented for the design evaluation. The plasticity of the structural members was simulated by using nonlinear fiber hinges. The moment, axial, and shear force interaction on the hinges was considered by the Modified Compression Field Theory. Moreover, an existing experimental study investigating the progressive collapse behavior of reinforced concrete structures was used to observe the validation of nonlinear fiber hinges and the applied analysis methodology. The study results deduce that a limited local collapse disproportionately more extensive than the initial failure was experienced on the buildings designed according to TSCB-2019. The mercantile structures designed according to current seismic codes require additional direct design considerations to improve their progressive collapse resistance against the risk of a sudden column loss.

• 소성∙가공
• /
• 제32권3호
• /
• pp.114-121
• /
• 2023

During rolling, rolling mill rolls endure wear when shaping metal billets into a desired form, such as bars, plates, and shapes. Such wear affects the lifespan of the rolls and product quality. Therefore, in addition to rigidity, wear performance is a key factor influencing the performance of rolling mill rolls. Conventional methods such as casting and forging have been used to manufacture rolling mill rolls. However, powder alloying methods are increasingly being adopted to enhance wear resistance. These powder manufacturing methods include atomization, canning to shape the powder, hot isostatic pressing to combine the powder alloy with conventional metals, and various wear performance tests on rolls prepared with powder alloys. In this study, numerical simulations and experimental tests were used to develop and elucidate the wear analysis mechanism of rolling mill rolls. The wear characteristics of the rolls under various rolling conditions were analyzed. In addition, experimental tests (wear and surface analysis tests) and wear theory (Archard wear model) were used to evaluate wear. These tests were performed on two different materials in various powder states to evaluate the different aspects of wear resistance. In particular, this study identifies the factors influencing the wear behavior of rolling mill rolls and proposes an analytical approach based on the actual production of products. The developed wear analysis mechanism can serve the future development of rolls with high wear resistance using new materials. Moreover, it can be applied in the mechanical and wear performance testing of new products.

• 한국철도학회:학술대회논문집
• /
• 한국철도학회 2007년도 추계학술대회 논문집
• /
• pp.1379-1385
• /
• 2007

철도에서 접속구간은 교량과 토공 또는 터널과 토공 사이의 노반상태가 변화하는 구간이나 궤도구조형식이 변화하는 구간을 말한다. 이들 구간에서는 궤도지지강성이 갑작스럽게 변화함으로써 레일변위가 급하게 변화하여 차량의 이상 진동 및 충격윤중이 발생한다. 열차의 진동은 차량의 주행안정성 및 승차감을 저하시키고 충격윤중은 노반의 침하 및 궤도재료의 열화, 손상을 가속시키는 결과를 초래한다. 자갈도상 궤도구조에서의 접속구간은 노반강성이 차이가 생기는 교량경계부와 터널경계부가 가장 보편적인 천이접속형태이며 특히 교량부와 토공부는 각각 일정한 고유의 탄성을 가지고 있어 서로 큰 강성차를 가진다. 교량부와 토공부의 강성차로 인해 궤도에서 발생하는 탄성변위량이 일치하지 않기 때문에 동적윤중은 하부구조의 과도한 변위를 유발시킬 수 있는 큰 충격하중을 발생시키고 이러한 과하중은 토공부의 궤도구성품의 손상을 유발하게 된다. 따라서 현재 국내에서는 접속구간 설계 시 구간별 노반 및 궤도강성차를 줄여 전체적으로 궤도지지강성 변화를 완화시키고, 토공구간의 소성침하 방지와 접속부에 부정적 영향을 주는 노반구조물의 비이상적 거동발생을 방지할 수 있는 규정을 제정하여 이를 바탕으로 설계하고 있으나 실제 운행선에서 발생하는 접속부 궤도의 동적거동을 감안한 설계지침은 아직 미약한 실정이다. 따라서 본 연구에서는 노반강성별 접속부 궤도의 동적거동 특성을 파악하고 노반구조물과 궤도구조의 상호관계를 입증하기 위해 노반강성별 접속부 궤도의 동적응답을 측정하고 측정 및 이론식을 이용한 접속부 궤도의 탄성력 및 궤도부담력을 산출하여 운행선 접속부 궤도의 동적거동을 감안한 설계지침 개발을 위한 기초자료를 제시하고자 한다.F-1^{(R)}$, $Helioseale^{(R)}$에 brush tip을 적용한 경우보다 많이 발생하였다(p<0.05). 3. 기포의 평균 단면적은 Teethmate $F-1^{(R)}$가 다른 군에 비해 큰 값을 보였다(p<0.05). 4. $Clinpro^{(R)}$와 적용 tip을 달리한 $Helioseal^{(R)}$의 경우 기포의 발생 빈도, 평균 개수, 평균 단면적 모두 유의차를 보이지 않았다(p>0.05).닌, 집이나 학교와 같은 일상의 생활공간이기 때문에 단순한 주의만으로도 외상의 발생을 예방할 수 있을 것으로 사료된다. 보이지 않았다 (p>0.05). 본 실험에서 시도한 두 가지 진정요법이 비교적 높은 임상적 치료 성공률(II군 : 97.14%, III군 : 88.57%)을 보여 만족할 만한 결과를 나타낸 것으로 평가되었다.tosan film보다 큰 수증기 투과도를 보였다.적으로 유의한 차이를 보이지 않았다.y tissue layer thinning은 3 군모두에서 관찰되었고 항암 3 일군이 가장 심하게 나타났다. 이상의 실험결과를 보면 술전 항암제투여가 초기에 시행한 경우에는 조직의 치유에 초기 5 일정도까지는 영향을 미치나 7 일이 지나면 정상범주로 회복함을 알수 있었고 실험결과 항암제 투여후 3 일째 피판 형성한 군에서 피판치유가 늦어진 것으로 관찰되어 인체에서 항암 투여후 수술시기는 인체면역계가 회복하는 시기를 3주이상 경과후 적어도 4주째 수술시기를 정하는 것이 유리하리라 생각되었다.한 복합레진은 개발의 초기단계이며, 물성의 증가를 위한 연구가 필요할 것으로 사료된다.또

• Geomechanics and Engineering
• /
• 제23권3호
• /
• pp.245-259
• /
• 2020

Tunnels have become an indispensable part of metro cities. Blast resistance design of tunnel has attracted the attention of researchers due to numerous implosion event. Present paper deals with the non-linear finite element analysis of rock tunnel having shear zone subjected to internal blast loading. Abaqus Explicit schemes in finite element has been used for the simulation of internal blast event. Structural discontinuity i.e., shear zone has been assumed passing the tunnel cross-section in the vertical direction and consist of Highly Weathered Granite medium surrounding the tunnel. Mohr-Coulomb constitutive material model has been considered for modelling the Highly Weathered Granite and the shear zone material. Concrete Damage Plasticity (CDP), Johnson-Cook (J-C), Jones-Wilkins-Lee (JWL) equation of state models are used for concrete, steel reinforcement and Trinitrotoluene (TNT) simulation respectively. The Coupled-Eulerian-Lagrangian (CEL) method of modelling for TNT explosive and air inside the tunnel has been adopted in this study. The CEL method incorporates the large deformations for which the traditional finite element analysis cannot be used. Shear zone orientations of 0°, 15°, 30°, 45°, 60°, 75° and 90°, with respect to the tunnel axis are considered to see their effect. It has been concluded that 60° orientation of shear zone presents the most critical situation.

• 소성∙가공
• /
• 제14권4호
• /
• pp.384-391
• /
• 2005

The nickel-based alloy Nimonic 80A possesses the excellent strength, and the resistance against corrosion, creep and oxidation at high temperature. Its products are used in aerospace engineering, marine engineering and power generation, etc. Control of forging parameters such as strain, strain rate, temperature and holding time is important because change of the microstructure in hot working affects the mechanical properties. Change of the microstructure evolves by recovery, recrystallization and grain growth phenomena. The dynamic recrystallization evolution has been studied in the temperature range of $950\~1250^{\circ}C$ and strain rate range of $0.05\~5s^{-1}$ using hot compression tests. The metadynamic recrystallization and grain growth evolution has been studied in the temperature range of $950\~1250^{\circ}C$ and strain rate range $0.05,\;5s^{-1}$, holding time range of 5, 10, 100, 600 sec using hot compression tests. Modeling equations are proposed to represent the flow curve, recrystallized grain size, recrystallized fraction and grain growth phenomena by various tests. Parameters in modeling equations are expressed as a function of the Zener-Hollomon parameter. The modeling equation for grain growth is expressed as a function of the initial grain size and holding time. The modeling equations developed were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The grain size predicted from FE simulation results is compared with results obtained in field product.

• 소성∙가공
• /
• 제29권1호
• /
• pp.11-19
• /
• 2020

For microstructural analysis of a friction stir welded (FSWed) joint of advanced high-strength steels, dual phase (DP) and complex phase (CP) steels, are studied. FSWed joints are successfully fabricated in the following four cases: (i) DP/DP; (ii) CP/CP; (iii) DP/CP, where the advancing side is DP and the retreating side is CP; (iv) CP/DP, where the advancing side is CP and the retreating side is DP. The stir zone (SZ) of (i) the DP/DP joint mainly consists of lath martensite, while the stir zone of (ii) the CP/CP joint consists not only of lath martensite but also of bainite. In the case of (iii) DP/CP and (iv) CP/DP, they exhibit a similar microstructure including acicular-shaped phases in the joints; however, cross-sections of the joints show differences in material mixing in each case. In (iv) the CP/DP joint, temperature towards the CP steel is sufficient to cause softening, thus leading to better mixing than that in (iii) DP/CP. The phases of the SZ in each of the four cases are formed by phase transformation during the FSWed process; however, the transformed phase volume fraction of CP steel is lower than that of DP steel, indicating that dynamic recrystallization occurs mainly in CP steel. The hardness values of the SZ are significantly higher than those of the base materials, especially, the SZ of (iii) the DP/CP joint has the highest value due to highest fraction of lath martensite.

• 소성∙가공
• /
• 제22권1호
• /
• pp.23-29
• /
• 2013

In this study, a micro/nano-random-pattern-structure surface was machined by electric discharge machining (EDM) followed by replicating the EDM surface with a silicone elastomer having low energy and greater hydrophobicity. The variation of hydrophobicity was of prime interest and was examined as a function of the surface roughness of the replicated silicone elastomer. The hydrophobicity was evaluated by the water contact angle (WCA) measured on the relevant surface. For the experiments, the original surfaces were machined by die sinking electric discharge machining (DS-EDM) and wire cutting electric discharge machining (WC-EDM). The ranges of surface roughness were Ra $0.8{\sim}19{\mu}m$ for the DS-EDM and Ra $0.5{\sim}4.7{\mu}m$ for the WC-EDM. In order to fabricate a hydrophobic surface, the EDM surfaces were directly replicated using a liquid-state silicone elastomer, which was thermally cured. The measured WCA on the replicated surfaces for DS-EDM was in the range of $115{\sim}130^{\circ}$ and for WC-EDM the WCA was in the range of $123{\sim}150^{\circ}$. Additionally, the dynamic hydrophobicity was evaluated by measuring an advancing and a receding WCA on the replicated silicone elastomer surfaces.