車體 薄板 構造物의 點鎔接部 耐 久力 補强 및 評價에 關한 硏究 = (A) study on the reinforcement and test method on spot welded area of vehicle structure

다국어 초록 (Multilingual Abstract)

Automakers build hundreds of prototype vehicles in order to test performance and durability of new cars. Some vehicles are used to test structural durability on proving ground. In case of fatigue failure of a prototype vehicle structure during durability test, the vehicle design engineer investigates the failure area in a view of structural durability strength and makes modification to improve its structural strength.
Nearly 70% of failure occurs around the spot welded area, so it is necessary to make an analysis of fatigue life characteristics on spot welded area.
Ch.1 includes overview of dissertation and presents the background and direction of studies.
Ch2. of this thesis includes an analysis of spot weld and plug weld characteristics in a view of durability. Fatigue and tensile tests are performed as well as microstructure investigation for the purpose of estimating the strength of welded joints by using spot welded and CO₂plug welded specimens. Strain history, fatigue failure characteristics due to the load range and direction, and phase transformation due to the proceeding of fatigue test was presented. In CO₂plug welded tensile shear specimens, the main factor effecting on tensile strength and fatigue life is the nugget diameter of welding. Interface gap of plates, however, gives influence in fatigue strength when the gap is wider than 0.3mm. In L-type tension specimens, the tensile strength of spot welded specimens is superior to that of CO₂Ф6 plug welded specimens, while the fatigue strength in high cycle range shows an opposite trend. The heat affected zone of CO₂plug welded specimen is much larger than that of spot welded specimen, which increase fatigue strength of high cycle range in L-type tension specimens.
Partial reinforcement method to the fatigue failed spot welded area without building a new prototype car by applying an appropriate gas metal arc plug welding is suggested in fabricating automobile body at Ch.6
It is most common to weld two layer sheet plates together, but the fabrication limitation requires multi-layer spot welding of three or four layers over lapping. In welding the multi-layer sheets, it is more difficult to achieve adequate welding conditions. The appropriate conditions of multi-layer spot welding are studied by analyzing static strength and fatigue life. A methodology has been developed based on static and fatigue test results for converting multi-layer spot welding to that of twofold layer with equal strength. Also requirements on minimum strength and fatigue life are suggested according to the sheet conditions.
The tensile and fatigue characteristics of aluminum spot weld were also included in a view of comparing with steel spot weld. And the hardness distribution was contrary to that of steel spot weld.
There are several loading factors of spot or plug weld according to driving conditions and characteristic of each vehicle driver. Automaker produces vehicle strong enough to satisfy the several driving conditions in a view of vehicle durability around spot weld. In order to develop a car in a short period, automaker engineer tests vehicle at several accelerated durability test roads. Before testing the vehicle durabling of spot weld area, test engineer must know how much the test road is severe comparing with general field road which is composed of high way, city road, paved road, and unpaved road. Two test methods on road severity that is using relative and absolute fatigue life predictions are suggested, and the merits and demerits of two methods are also presented. By the results, appropriate durability test mode of passenger car around spot weld can be set.

목차 (Table of Contents)

• 목차
• List of Tables = ⅴ
• List of Figures = ⅶ
• 기호설명 = xiii
• 제1장 서론 = 1
• 1.1 연구배경 = 1
• 1.2 연구동향 = 5
• 1.2.1 점용접 피로거동 = 5
• 1.2.2 CO₂플러그 용접의 피로거동 = 9
• 1.2.3 다겹용접부 강도특성 = 11
• 1.2.4 내구가혹도 평가의 연구동향 = 11
• 1.3 본 논문의 구성 및 각 연구내용 = 12
• 1.4 참고문헌 = 18
• 제2장 점용접과 CO_(2)플러그 용접의 피로거동 연구 = 21
• 2.1 서론 = 21
• 2.2 실험재료 및 방법 = 23
• 2.2.1 실험재료 = 23
• 2.2.2 용접 시험편 제작 = 24
• 2.2.3 실험방법 = 26
• 2.3 실험결과 및 고찰 = 27
• 2.3.1 용접부위의 조직과 경도 측정 결과 = 27
• 2.3.2 점용접부 특성 = 30
• 2.3.3 점용접부 균열시 플러그 용접으로의 보강효과 = 36
• 2.3.4 점용접과 CO_(2)플러그 용저브이 인장강도 = 38
• 2.3.5 인장전단 시험편에 대한 피로 시험 결과 = 41
• 2.3.6 L형 인장 시험편에 대한 피로시험 결과 = 48
• 2.3.7 판재의 틈새(Interface Gap)가 Co_(2) 플러그 용접부의 피로수명에 미치는 영향 = 49
• 2.4 결론 = 52
• 2.5 참고문헌 = 53
• 제3장 다겹 점용접부의 강도 특성 = 55
• 3.1 서론 = 55
• 3.2 실험장치 및 검토 = 57
• 3.3.1 인장강도 = 59
• 3.3.2 피로강도 = 63
• 3.3.3 인장강도와 피로수명의 관계 = 65
• 3.3.4 다겹강판을 상당 이중겹판으로 환산 = 66
• 3.3.5 용접방법 표준 설정 = 68
• 3.4 결론 = 69
• 3.5 참고문헌 = 69
• 제4장 경량화 재료의 점용접 피로거동 = 71
• 4.1 서론 = 71
• 4.2 실험재료 및 방법 = 74
• 4.2.1 실험재료 = 74
• 4.2.2 용접시편 제작 = 75
• 4.2.3 실험방법 = 77
• 4.3 실험결과 및 고찰 = 78
• 4.3.1 점용접부 단면조직 관찰 = 78
• 4.3.2 인장전단강도 비교 = 84
• 4.3.3 하중 및 피로수명별 용접부의 피로균열 변화 = 88
• 4.3.4 용접조건에 따른 피로수명 및 경도 변화 특성 = 93
• 4.4 결론 = 100
• 4.5 참고문헌 = 101
• 제5장 점용접부 내구력 평가 = 102
• 5.1 서론 = 102
• 5.2 변형율 측정 방법 = 104
• 5.3 피로수명 예측법 = 104
• 5.3.1 절대수명 평가법 = 104
• 5.3.2 상대수명 평가법 = 109
• 5.4 절대수명 예측법을 이용한 각 도로의 노면 가혹도 평가 = 111
• 5.5 내구시험 도로에서의 내구목표거리 설정 방법 = 111
• 5.6 내구시험로에서의 내구목표 설정예 = 113
• 5.7 결론 = 118
• 5.8 참고문헌 = 118
• 제6장 CO_(2) Arc 점용접기에 의한 보강용접 = 120
• 6.1 서론 = 120
• 6.2 실험 재료 및 방법 = 120
• 6.2.1 실험 재료 = 120
• 6.2.2 시험편 제작 = 122
• 6.2.3 실험 방법 = 130
• 6.3 실험 결과 및 고찰 = 132
• 6.3.1 용접 부위의 조직 관찰과 경도 측정 결과 = 132
• 6.3.2 균열 선단에 행한 아아크 점용접의 균열저지능력 평가 = 142
• 6.3.3 저항 점용접 시편과 균열 저지 시편의 피로 수명 비교 = 146
• 6.3.4 저항 점용접 시편과 보강 용접 시편의 피로 수명 비교 = 155
• 6.3.5 각 시편에 대한 정적 인장 시험의 결과 비교 = 161
• 6.4 결론 = 165
• 6.5 참고문헌 = 166
• 제7장 결론 = 168
• Abstract = 171