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지민기,박준건,김도형,정요한,박진규,문일철,Ji, Min-Gi,Park, Jun-Keon,Kim, Do-Hyeong,Jung, Yo-Han,Park, Jin-Kyoo,Moon, Il-Chul 한국시뮬레이션학회 2018 한국시뮬레이션학회 논문지 Vol.27 No.1
토목 공정 계획은 건설 공정 관리에서 중요한 과제 중 하나이다. 수학적 방법론에 기반을 둔 최적화 기법, 휴리스틱에 기반을 둔 최적화 기법 그리고 행위자 기반의 시뮬레이션 등의 방법론이 건설 공정 관리를 위해 적용되어왔다. 본 연구에서는 가상의 토목 공정 환경을 개발하고, 가상의 토목 공정 환경에서 강화학습을 이용한 시뮬레이션을 통해 토목 공정의 최적 경로를 찾는 방법을 제안하였다. 강화학습에 있어 본 연구에서는 상호작용 하며 서로 다른 행동을 하는 굴삭기와 트럭 에이전트들 에 대해 순차적 학습과 독립적 학습에 기반을 둔 두 가지의 Markov decision process (MDP)를 사용하였다. 가상의 토목 공정 환경에서 두 가지 방법 모두 최적에 가까운 토목 공정 계획을 만들어 낼 수 있음을 시뮬레이션 결과에 따라 알 수 있었으며, 이 계획은 건설 자동화의 기초가 될 수 있을 것이다. Earthwork planning is one of the critical issues in a construction process management. For the construction process management, there are some different approaches such as optimizing construction with either mathematical methodologies or heuristics with simulations. This paper propose a simulated earthwork scenario and an optimal path for the simulation using a reinforcement learning. For reinforcement learning, we use two different Markov decision process, or MDP, formulations with interacting excavator agent and truck agent, sequenced learning, and independent learning. The simulation result shows that two different formulations can reach the optimal planning for a simulated earthwork scenario. This planning could be a basis for an automatic construction management.
극저온이 22MnB5강의 냉간 스탬핑 마모에 미치는 영향
지민기,노연주,강현학,전태성 한국트라이볼로지학회 2022 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.38 No.6
This paper presents the effects of cryogenic temperature on the wear behavior of 22MnB5 blank under cold stamping. After immersing the blank in liquid nitrogen (LN2) for 10 min, a strip drawing test was performed within 10 s. The hardness was measured using the Rockwell hardness test, which increased from 165 HV at 20oC to 192 HV at cryogenic temperature. The strip drawing test with 22MnB5 blank and SKD61 tool steel shows that for the different wear mechanisms on the tool surface with respect to temperature; adhesive wear is dominant at 20oC, but abrasive wear is the main mechanism at cryogenic temperature. As the friction test is repeated, sticking gradually increases on the tool surface at 20oC, whereas the scratch increases at cryogenic temperature. For the friction behavior, the friction coefficient rapidly increases when adhesive wear occurs, and it occurs more frequently at 20oC. The results for nanoindentation near the worn blank surface indicate a difference of 1.3 GPa at 20oC and 0.8 GPa at cryogenic temperature compared to the existing hardness, indicating increased deformation by friction at 20oC. This occurs because thermally activated energy available to move the dislocation decreases with decreasing temperature.
Al-Si 도금층 두께가 22MnB5강의 핫스탬핑 공정 마모에 미치는 영향
지민기,손현성,오진근,김성우,김경목,전태성 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.8
Al-Si coated ultra-high strength steels (UHSS) are widely used in automotive applications because of their high strength-to-weight ratio, corrosion protection and good weldability. The hot stamping (also called hot press forming) process is an effective and suitable technique for producing automotive parts from Al-Si coated UHSS. However, critical issues, such as the transfer of the coating materials and the build-up of these materials on the tool surface, have been encountered. These defects affect tool life and product quality. The wear behaviour of Al-Si coated 22MnB5 and Cr coated SKD11 tool was investigated using a high temperature friction test which mimicked the actual hot stamping environment. Two kinds of Al-Si coated 22MnB5 with ~19.6 (AlSi-A) and ~29.3 (AlSi-B) μm coating thicknesses were used in this study. After the friction test the coated layer of AlSi-A was found to be worn up to the diffusion layer, while for AlSi-B, the coating layer mostly remained after the friction test. Adhesive wear predominantly occurred on the tool surface in both cases, but the wear increased significantly in case of AlSi-B. This suggests that the coating thickness and the associated surface roughness are critical factors affecting wear behaviour.