변형률 수명 방법과 파괴역학 접근 에 의한 적층 제조 된 AlSi10Mg 의 피로수명 평가

학위논문 (석사) -- 서울대학교 대학원 : 공과대학 항공우주공학과, 2021. 2. 윤군진.The alloy manufactured by the selective laser melting method, which is widely known in 3D printing technology / Additive manufacturing (AM), has advantages in design flexibility and production efficiency compared to the conventional process. Titanium and aluminum (Al) alloys processed by AM are commonly used in aviation and medical fields. However, in this process, heat treatment is performed to remove internal defects such as pores in the alloy, although complete removal is inevitable. For this reason, the fatigue life of AM alloy should be evaluated in consideration of the effect of pores. In this thesis, the fatigue life was analyzed by generating a representative volume element (RVE) with pores as computed tomography (CT) image of an Al alloy processed by AM. For fatigue life analysis, the strain-life method was applied to the crack initiation cycle and Fracture Mechanical approach was applied to the crack propagation cycle. RVE was generated by increasing the image's quality and separating the pores of the CT image through an image processing algorithm. Stress and strain were obtained through 3D finite element (FE) analysis by applying periodic boundary conditions to multiple RVEs for statistical analysis. The value required for fatigue life analysis and FE analysis was calculated using commercial software ABAQUS, and FE-SAFE and the total fatigue life was calculated through post-processing and numerical integration by Python script and MATLAB code. As a result of analyzing the relationship between the pore information of the RVE and the fatigue life, the volume of pores was a critical factor, and compared with the experimental results of reference. It was good agreement that the fatigue life values of multiple RVEs had a somewhat similar range3D 프린팅 기술 / 적층제조 에서 널리 알려져 있는 선택적 레이저 용융방식으로 제조된 합금은 기존의 공정방식에 비해 설계의 유연성과 생산 효율성의 장점을 가진다. 특히 적층제조로 공정된 티타늄과 알루미늄 합금은 항공 및 의료분야에서 많이 이용된다. 하지만 이 공정과정에서 합금에 생기는 기공과 같은 내부적 결함을 제거하기 위해 열처리 과정을 진행하나 완벽한 제거는 불가피하다. 따라서 적층제조의 합금의 피로수명은 기공의 영향을 고려해서 평가되어야 한다. 본 논문에서 적층제조 방식으로 공정된 알루미늄 합금의 단층촬영(CT) 이미지로 기공이 포함된 대표체적요소(RVE)를 만들어 이에 대해 피로수명을 해석하였다. 피로수명 해석을 위해 균열 시작주기는 변형률-수명 방법을, 균열 성장주기는 파괴역학 접근을 적용하였다. RVE는 CT 이미지를 이미지 처리 알고리즘을 통해 이미지의 품질을 증가시키고, 기공의 입자를 분리시켜 만들었다. 통계적 해석을 위해 다수의 RVE에 주기적 경계조건을 적용하여 3차원 유한요소 해석을 통해 응력과 변형률을 구했다. 유한요소 해석과 피로수명 해석에 필요한 값들을 상용 소프트웨어인 ABAQUS 와 FE-SAFE를 이용하였고, Python 과 MATLAB을 통해 후처리 및 수치 적분으로 총 피로수명을 계산하였다. RVE의 기공 정보와 피로수명의 관계를 분석한 결과 기공의 부피에 가장 큰 영향을 받았으며, 문헌의 실험 결과값과 비교하여 다수의 RVE의 피로수명 값과 어느 정도 비슷한 범위를 가지는 것을 확인할 수 있었다.Abstract i Table of contents iv List of figures v List of tables vi 1. Introduction 1 1.1. Motivation 1 1.2. Objectives and Thesis Overview 6 2. Theoretical background 8 2.1. Representative volume element approach 8 2.2. Fatigue life prediction 11 2.2.1. Strain-life method 12 2.2.2. Fracture Mechanical approach 16 3. Microstructure modeling including pores 21 3.1. Material properties 21 3.2. Image processing 23 4. Fatigue Life Prediction Methods 26 4.1. Fatigue crack initiation model considering K_t 26 4.2. Fatigue crack growth model 29 5. Result and Discussion 34 5.1. Simulation result of crack initiation model 34 5.2. Simulation result of crack propagation model 43 5.2.1. Fatigue crack growth rate 43 5.2.2. Computational result of N_p 46 5.3. Estimate of Total fatigue life 47 5.3.1. Total fatigue life (N_f) 47 5.3.2. Validation of N_f with simulation results 49 5.3.3. The effect of pores on fatigue life 51 6. Conclusion and future works 52 6.1. Conclusion 52 6.2. Future works 53 국문초록 61Maste

(A) STUDY ON THE FORMATION OF CERAMICS THROUGH THE CUTTING OF 3 DIMENSIONAL FORMS

예술은 언제나 그 시대의 정신적 상황을 대변하여 왔으며, 시공을 초월한 산 증인이다. 따라서, 예술은 시대에 따라 변모되어 왔고, 현대에 살고 있는 우리들은 현대의 모습을 현대의 조형 언어로써 표현해야 할 사명이 있는 것이다. 현대의 예술은 자신의 강렬한 표현 욕구 충동을 충족시키기 위해서 내면 세계를 표출한 것이다. 또한 일체의 기존체제를 부정하여 기존의 미학을 일변시키고 미술의 존재 방식조차 재고하게 하였다. 따라서 추상과 구상의 대립을 비롯한 수 많은 이념과 양식이 공존하게 되었다. 본 연구에서는 현대의 대립되고 복잡한 구조 속에서 좀 더 단순한 형태인 기하학적 형태의 입체를 표현 소재로 하여 적재·반복 등의 다양한 변형을 통해 조형화하고자 하였다. 본론에서는 기하학적 입체의 일반적인 개념과 입체를 절단한 형태에서의 변형에 대해 알아보고 아울러 입체를 절단했을 때 나타나게 되는 조형성에 대하여 고찰하였다. 직선이나 곡선을 이용한 입체의 절단이 단지 절단하는 행위에 그치는 것이 아니라 절단된 형태에 조형미를 부여하기 위해서는 변형의 단계가 필요하며, 절단을 통한 입체의 단위형태들이 이상적인 질서에 의해 변형되는 경우에 진정한 미적 가치가 생겨나게 되는 것이다. 조형의 요소가 총합되어 형성되기 까지는 공통의 원리가 있게 마련이다. 따라서, 절단면의 표현을 반복·대비·조화·통일의 조형원리로 설명하였다. 작품제작 방법에 있어서는 절단선의 움직임을 보다 간결하고 매끈하게, 선과 면을 통해 보여주기 위해서 석고형을 이용한 이장주입기법(泥裝注入技法)을 사용하였다. 그리고 태토로는 백색도가 높은 고려도토의 백점토와 요업개발의 자기토를 사용하였으며, 작품 6과 작품 10을 제외한 모든 작품에 투명유를 분무법으로 시유하였다. 1차소성은 석유를 연로로 하는 0.7㎥가마에서 9시간에 걸쳐 900℃까지 소성 온도를 올렸으며, 2차소성은 가스를 연료로 하는 1.5㎥가마에서 1시간동안 산화소성하여 1,240℃에서 마무리 하였다. 기하학적 대상인 입체를 통해 현대의 예술가들이 얼마나 다양한 표현욕구를 시도할 수 있는지 알게되었고, 하나의 입체에서 직선·곡선의 절단 과정으로 또 다른 입체를 표현하는데 있어 무한히 많은 형태의 변화가 존재한다는 것을 깨닫게 되었다. 또한, 반복, 대비, 조화, 통일의 조형원리를 기본으로 작업에 임하게 될 때 더 큰 효과를 거두게 되는 것이다. 이러한 과정을 통해 본 연구자는 절단의 과정이 처음 입체의 형태를 보다 조형성 있게 변형시키게 되며 기하학적 형태의 변형이 객관적인 단순화가 아니고 조형성의 재창조임을 이 연구를 대하는 사람들에게 인식시키고자 하였다.;The arts have constantly and historically represented the emotional state of the times, and to this day are transcending time and space in witnessing history. Therefore, the arts experience change adapting to the era, and thus, contemporary man must be dedicated to expressing the present time with the modern formative language. The modern arts are results of an exposed inner-world, satisfying the motivational and passionate need to express oneself. Also, modern art aims to defy in absolute terms the existing system and bring about change in the existing field of esthetics, re-evaluating even the very mode of existence of art itself. Accordingly, innumerable theories of art and modes of art, including the contrast between abstract and construct, have come to exist in unity. In a conflicting and complex modern world structure, this study purports to introduce 3 dimeusional geometric forms, which are more rational and simplified, as expressional tools and to format them by applying vaciations such as repeating through loading. In order that cutting 3 dimensional form with strait and curved lines does not only mean the cutting itself but also put the beauty of formation to the cut 3 dimensional form, the stage of modifying format is required and the ture value of beauty comes in case that 3 dimensional unit form through cutting being modified by ideal system. Common principles are always being existed in the formation element's coming together and being shaped. Thecefore, I explained expression of cut face as formation principle of repetition, comparison, harmony and reunion. In the main discussion, the study examines the general coucept of 3 dimensional forms by cutting with straight and curved lines, and the organized formation resulting from such divisions. As for the method applied in manufacturing the pieces of work, the slip casting method using gypsum was chosen in order to display in clarified and smooth lines and surfaces the impact of the cut lines. As for the clay body, the white clay of Koryo Doto ceramics company and the ceramic clay of Yo'op Development company was chosen for the high degree of white coloring and except for No. 10 work, all were glazed by the speclding method. The primary firing process was carried out in a 0.7㎥ kiln using gas fuel, the temperature was raised to 900℃. The secondary firing was done in a 1.5㎥ lciln using gas fuel, the temperature reached 1,240℃ in an oxidated firing and finishing process. I have been known that modern artists can try various expressional desire through 3 dimensional of geometric structure and realized that there are innumerable modification of forms in t he process of cutting strait and curved lines from a unit of 3 dimensional form and to express other forms. Also, I can have bether results when I have worked in the basis of repetition comparison, harmony and reunion. This researcher applying the mentioned methodological process, found that by implementing the cutting process, the original 3 dimensional form underwent a more formative change and the variations of geometive forms presumed a renewed creativity as opposed to an objective simplification, and the study aims to re-evaluate the values of such findings.논문개요 = ⅶ Ⅰ. 서론 = 1 A. 연구의 목적 = 1 B. 연구의 내용과 방법 = 2 Ⅱ. 본론 = 4 A. 기하학적 입체의 개념 = 4 B. 절단을 통한 변형 = 5 C. 입체의 절단면에 의한 조형성 = 12 Ⅲ. 작품 제작과 해석 = 18 A. 작품제작 과정 = 18 B. 디자인 과정 = 20 C. 작품해석 = 26 Ⅳ. 결론 = 46 참고문헌 = 47 ABSTRACT = 4

엘리베이터 시스템의 응용

학위논문(석사) - 한국과학기술원 : 전산학과, 1992.2, [ [ii], [46] p. ]한국과학기술원 : 전산학과

Injection of contrast media using a large-bore angiography catheter with a guidewire in place: Physical factors influencing injection pressure in cerebral angiography

Background: We have used a contrast injection scheme termed as "guided catheterization method (guided method)." By using a large-bore 5-Fr catheter and 0.032-in guidewire, a contrast medium could be injected without removing the guidewire. Using a neurovascular phantom, we studied the influence of leaving the guidewire on the contrast injection pressure. Image quality was compared with that obtained using a 4-Fr regular angiography catheter (conventional method). Methods: Actual contrast injection pressure, flow rate, flow volume, and several variables from the time-density curve (TDC) were obtained using the guided method and the conventional method. Catheterization parameters included flow rate, the viscosity of a contrast medium (CM), and catheter length. The pressure limit of a contrast injector was set as 1200 psi. Digital subtraction angiography (DSA) images on the neurovascular phantom were acquired. The DSA images were processed, and TDC on a specific region of interest was obtained. Variables from TDC were calculated and compared between the different catheters. Results: The ranges of actual contrast injection pressure with the conventional and the guided method were 138-299 psi and 184-451 psi, respectively. A minimal reduction of the actual flow rate was found under some conditions with the guided method. Comparable opacifications in DSA images were achieved in all conditions. Although peak intensity was different by flow rate or CM, all TDC variables did not differ based on the catheter. There were no significant harmful events during the 90 experiments. Conclusions: With adjustment of the pressure limit, cerebral angiography using the 5-Fr, large-bore catheter without removal of the guidewire is feasible, safe, and expected to provide image quality comparable to that of the 4-Fr regular catheter

Staged Approach for Stent-Assisted Coiling of Cerebral Aneurysms after Failure of Initial Intra-Saccular Catheterization

Purpose: Microcatheter navigation into an aneurysm sac can present difficulties through negative interactions between the deployed stent mesh and microcatheter. We hypothesized that endothelialization of the stent mesh would minimize these interactions. We aimed to assess the feasibility of staged coiling after stenting by reviewing our experiences with unavoidably staged embolization cases.Materials and Methods: Between 2011 and 2019, 7 patients (mean age 57.2 years, range 49?76 years) including 5 females, experienced 9 unruptured aneurysms treated with staged stenting and coiling due to unstable microcatheter navigation into the aneurysm after stent placement. The aneurysms were in the paraclinoid internal carotid artery (ICA) (n=3), ophthalmic origin ICA (n=1), superior cerebellar artery origin (n=2), basilar tip (n=2), and the middle cerebral artery bifurcation (n=1). The stents used were the Neuroform Atlas (n=4), Neuroform EZ (n=2), and Low-profile Visualized Intraluminal Support Blue (n=1).Results: The mean interval between stenting and coiling was 15 weeks (range, 12?21 weeks). The average navigation time between the first roadmap imaging and microcatheter insertion in the sac was 14 minutes (range, 8?20 minutes). One aneurysm was occluded without further coiling on follow-up. Staged coiling successfully treated the remaining aneurysms (n=8). No complications were identified.Conclusion: In cases of difficult intra-saccular catheterization, intentional staged coiling may be a feasible option for stent-assisted coiling of the cerebral aneurysms

Rare Neurovascular Diseases in Korea: Classification and Related Genetic Variants

Rare neurovascular diseases (RNVDs) have not been well-recognized in Korea. They involve the central nervous system and greatly affect the patients' lives. However, these diseases are difficult to diagnose and treat due to their rarity and incurability. We established a list of RNVDs by referring to the previous literature and databases worldwide to better understand the diseases and their current management status. We categorized 68 RNVDs based on their pathophysiology and clinical manifestations and estimated the prevalence of each disease in Korea. Recent advances in genetic, molecular, and developmental research have enabled further understanding of these RNVDs. Herein, we review each disease, while considering its classification based on updated pathologic mechanisms, and discuss the management status of RNVD in Korea