DBPIA-NURIMEDIA

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1 37 기술논문 압연및용접방향이같은맞대기용접강판의하중방향에따른피로균열진전특성 이용복 * 김성엽 ** 오병덕 ** * 홍익대학교기계 시스템디자인공학과 ** 홍익대학교대학원기계공학과 Characteristics of Fatigue Crack Propagations with Respect to Loading Directions in Butt-Welded Steel Plates with the Same Direction of Rolling and Welding Bead Yong-Bok Lee*, Sung-Yeup Kim** and Byung-Duck Oh** *Dept. of Mechanical and System Design Engineering, Hongik University, Seoul , Korea **Dept. of Mechanical Engineering, Hongik University Graduate School, Seoul , Korea Abstract Most of the welding steel plate structures have complicated mechanical problems such as rolling directional characteristics and residual stresses caused by manufacturing process. For the enhancement of reliability and safety in those structures, therefore, a systematic investigation is required. SS400 steel plate used for common structures was selected and welded by FCAW butt-welding process for this study, and then it was studied experimently about characteristics of fatigue crack propagations with respect to rolling direction and welding residual stress of welded steel plates. When the angles between rolling direction and tensile loading direction in base material are increased, their ultimate strength not show a significant difference, but yielding strength are increased and elongations are decreased uniformly. It is also shown that fatigue crack growth rate can be increased from those results. When the angles between welding bead direction and loading direction in welded material are increase, fatigue crack growth rate of them are decreased and influenced uniformly according to the conditions of residual stress distribution. In these results, it is shown that the welded steel plate structures are needed to harmonize distributed welding residual stress, rolling direction and loading direction for the improvement of safety and endurance in manufacture of their structures. * Corresponding author : yblee@wow.hongik.ac.kr (Received September 14, 2005) Key Words : Rolling direction, Loading direction, Welding residual stress, Fatigue crack propagation, Base material, Welded material, Ultimate strength, Yielding strength, Elongation, Fatigue crack growth rate 1. 서론 산업의발전과더불어모든기계나산업구조물들의설계상더안전하고, 경제적인설계를위하여정밀성, 기능성, 효율성및내구성향상에대한필요성이증대되고있다. 대부분의강구조물은제조과정에서압연, 인발및압출등의공정을거치게되고, 이들과정에의하여제조된강판재와형강재를사용하여나사결합, 리벳결합및용접의가공과정을통하여제작되고있다. 이러한과정들중에서압연된강판재를용접하여제작하는방법이많은비중을차지하고있다 1-2). 이들구조물재료의제조과정및가공과정에서발생하는결함의존재여부는물론재료의압연방향과용접잔류응력을정확하게파악하고, 이들을고려하여구조물을설계하는것은안전상매우필요하다 3-5). 따라서이러한제조과정과가공에대한개선방법이꾸준히진행되고있으나, 사용조건이더욱가혹해지고, 산업재해문제가크 大韓熔接學會誌第 23 卷第 6 號, 2005 年 12 月 533

2 38 이용복 김성엽 오병덕 게대두되면서설계상안전성및신뢰성확보에어려움을겪고있다 6-7). 재료의제조과정으로부터유발되는압연에의한방향성이기계적성질과피로특성에미치는영향, 용접으로인한국부적인잔류응력의존재, 외부작용하중의크기및방향과관련한피로균열특성등복잡한역학적관계로특히용접구조물상의안전설계를위하여는종합적이고, 체계적인연구가필요하다 8). 이와관련하여본연구에서는먼저압연강판재의압연방향에따른기계적특성과피로특성을파악하고, 이를기초로하여용접재료에서의하중방향에대한용접비드방향및압연방향을같이하여용접잔류응력이피로균열진전에끼치는영향을실험적으로규명하고, 효율적이며안전한용접구조물설계방법에대하여고찰해보고자하였다. 2. 실험 2.1 시험편의준비 본연구를위하여 Table 1의화학성분을갖는일반강구조물재료로많이사용하고있는 3mm 두께의열간압연강재 SS400을선택하였고, 인장시험을위하여하중방향에대하여압연방향을 0, 30, 45, 60 및 90 가되도록채취하였다. 그리고 Fig. 1에나타낸바와같이 KS 5호규격에따라 0.01mm 이내의오차범위로와이어절단 (wire cutting) 으로가공하였다. 모 Table 2 FCAW condition for fatigue specimen Voltage (V) Electric Current (A) Electrode (Dia.mm) Gas Flux (L/min) Velocity (cm/min) 재피로균열시험편은 Fig. 2의규격으로인장시험편과같이하중방향에대하여압연방향각도를주어와이어절단가공하였다. 또한, 용접재의피로균열시험편은 Fig. 3에나타낸바와같으며, 강판의압연방향과같은방향으로용접비드방향을잡고, 용접으로인한재료의과대변형을막기위하여용접재료를지그로고정시킨후앞뒤양쪽을 FCAW(flux cored arc welding) 법으로맞대기용접하였다. 용접조건은 Table 2와같으며, 230A의전류와 26V의전압으로 12L/min 의송출유량의혼합가스분위기에서 40cm/min 의속도로용접을수행하였다. 2.2 시험방법 모재시험편의압연방향에따른기계적특성을알아보기위하여 Fig. 4에나타낸 10톤용량의재료시험기 (UTM, MODEL T48-0) 를사용하였고, Fig. 1의시 Table 1 Chemical compositions of specimen Material Chemical compositions (wt.%) C Si Mn P S SS Fig. 1 Configuration of tensile specimen Fig. 2 Configuration of base metal specimen for fatigue test 534 Journal of KWS, Vol. 23, No. 6, December, 2005

3 압연및용접방향이같은맞대기용접강판의하중방향에따른피로균열진전특성 39 Fig. 3 Configuration of welded steel plate specimen for fatigue test Fig. 5 Apparatus for fatigue test(instron 8511) Table 3 Conditions of fatigue test Frequency of loading (Hz) Maximum load (kn) Minimum load (kn) Stress ratio R = σ min σ max Tempera ture ( ) 당성있는잔류응력분포도를 Terada 식과결부시켜구하고, 모재의피로균열진전시험편과동일조건하에서실험을수행하였다. 3. 실험결과및고찰 Fig. 4 Apparatus for tensile test(utm: MODEL T48-0) 험편규격에따라하중방향에대한압연방향각도가 0, 30, 45, 60 및 90 로구분하여가공한 4편씩의시험편을표점길이 50mm로표시하여준정적하에인장시험을하였고, 공칭응력변형률선도를구하였다. 피로균열진전시험을위하여 Fig. 5에나타낸 5톤용량의피로시험기 (Instron 8511) 를사용하였고, 앞에서설명한바와같이준비한시험편을하중방향에대한압연방향별로 3편씩사용하여 Table 3에나타낸바와같이응력비 R=0, 최대하중 24.5kN, 반복수 10Hz 로피로균열진전시험을하였다. 이때일정하중싸이클마다광학현미경으로 1/100mm의정확도로균열진전길이를측정하였다. 용접재에대한시험은먼저자기식응력측정기로용접부의잔류응력을여러번측정하여타 3.1 모재의압연방향에따른기계적성질및피로특성 모재시험편재료의압연방향각도에따른인장시험결과를종합적으로나타내면 Fig. 6과같다. 인장강도는하중방향에대한재료의압연방향이 45 및 90 의경우가 0, 30, 60 의경우보다 6~7% 의높은값을나타나고있으나전반적으로큰차이는없고, 항복강도는전체적으로 18~25% 의높이차이로인장강도의경우보다큰차이를보이고있으며, 45 의경우약간의차이를제외하고, 하중방향과압연방향각도가클수록높게나타나는경향을보이고있다. 그리고연신률은압연방향각도가클수록낮게나오는경향을보이고있는데, 이결과는항복강도가높을수록연신률은낮게나타나고있음을알수있다. 피로균열진전특성은 Fig. 7과같으며, Paris law 9) 로부터구한재료상수값을나타내면 Table 4와같다. 재료의압연방향각도가 0, 30, 45, 60 및 90 로증가함에따라피로균 大韓熔接學會誌第 23 卷第 6 號, 2005 年 12 月 535

4 40 이용복 김성엽 오병덕 Fig. 6 σ y, σ u and elongation vs. angles between ng and tensile loading direction rolli Fig. 8 Distribution of residual stress in weldments Fig. 7 Crack growth rate vs. stress intensity factor r ange Table 4 Material constants by Paris law C m 열진전속도는높게나타나고있음을알수있고, 이와같은결과는재료의항복강도가높을수록, 그리고연신률이낮을수록피로균열진전속도는높은경향을보임을알수있다. 또한, 인장강도는압연방향에별로영향을받지않으므로, 동일재료에서의피로균열진전속도는인장강도와관련지을수없음을알수있다. 따라서동일재료에서의압연방향에따른피로균열진전속도는인장강도와는거의관계가없고, 항복강도의증가및연신률의감소에따라증가하는경향이있음을알수있다. 3.2 용접재의피로균열진전특성 용접시험편의잔류응력분포는 Fig. 8 과같고, 용접 비드방향이하중방향 ( 시험편의길이방향 ) 과일치 ( θ =0 ) 하는경우높은잔류응력분포로부터두방향사이의각도가커질수록하중방향에대한잔류응력분포는낮게나타나는데, 이분포도는 θ=0 인경우를기준으로 Terada 의용접잔류응력분포식과측정값을결부시켜나타낸것이다 10). 용접재의피로균열진전은 Fig. 3에나타낸바와같이 CCT(center cracked test specimen) 시험편이므로재료중앙으로부터양쪽으로균열이진전하도록되어있고, 용접잔류응력의영향을받게되므로응력확대계수는 K res =2 a π 로부터구할수있다. 여기에서 0 a σ r (x) a 2 -x 2 dx σ r (x)=σ o [1-( x b ) 2 ]e ( x b ) 2 + σ a 이며, σ o 는용접선중앙의최대인장잔류응력값이고, b는용접선중앙에서인장잔류응력으로부터압축잔류응력으로변하는변곡점까지의거리, σ a 는작용응력이다. 또한 a는균열길이를나타낸다. 유효응력확대계수는 ΔK =(K max +K res )-(K min +K res )=K max -K min 이며, 유효응력비는 R eff = K min +K res K max +K res 따라서, 이들값을 Forman's equation (11) da dn = C(ΔK) m (1-R eff )K cf -ΔK 536 Journal of KWS, Vol. 23, No. 6, December, 2005

5 압연및용접방향이같은맞대기용접강판의하중방향에따른피로균열진전특성 41 에대입함으로써, 용접재의피로균열진전특성을나타낼수있다. 위식에서 c와 m은모재시험의피로시험결과로부터얻은재료상수값으로, Table 4의값을취하였고, K cf 값은피로시험시급속파손때의 K I 의임계값을대입하였다. 용접시험편에대한피로균열진전시험결과는모재시험편의피로균열진전시험결과를바탕으로작용응력과용접잔류응력의중첩에의한피로균열예측선과함께하중방향에대한용접및압연방향각도에따라균열진전속도를 Fig. 9~Fig. 13에나 Fig. 12 Crack growth rate, da/dn) vs. S.I.F.R. ΔK [ θ =60 ] Fig. 9 Crack growth rate, da/dn vs. S.I.F.R. ΔK [ θ =0] Fig. 13 Crack growth rate, da/dn vs. S.I.F.R. ΔK [ θ =90 ] Fig. 10 Crack growth rate, da/dn vs. S.I.F.R. ΔK [ θ =30 ] Fig. 11 Crack growth rate, da/dn vs. S.I.F.R. ΔK [ θ =45 ] 타내었다. 용접재시험편에서의피로균열진전속도는초기인장잔류응력지역에서모재시험편의경우보다높게나타나고균열이진전됨에따라모재의균열진전속도와접근하여나중에는같은균열진전속도로나타난다. 이와같은경향은초기에시험편중앙부의높은인장잔류응력영향을받아용접시험편의피로균열진전속도가빠르게진전하여점점균열이진전되면서인장잔류응력이줄어들어그영향이감소하고, 인장잔류응력이끝나는지역으로부터는용접잔류응력영향을받지않기때문인것으로생각된다 12). 이러한경향은하중방향에대한용접선방향의각도가다른모든시험편에서동일하게나타나며, 하중방향에대한용접선각도가커짐에따라균열진전방향에분포하는잔류응력은감소하여균열진전속도에대한그영향은감소하는것으로나타나고있다. 이와같은결과로부터용접재에서의피로균열진전은용접잔류응력분포에예민하게영향을받고있음을알수있으며, 따라서용접에의한구조물들의안전사용및안전설계를위하여잔류응력의분포를정확하게파악하고하중방향과용접방향그리고재료의압연방향을잘조화시켜고려하는것이중요하다고사료된다. 大韓熔接學會誌第 23 卷第 6 號, 2005 年 12 月 537

6 42 이용복 김성엽 오병덕 4. 결론 본연구에서는일반적으로강구조물로많이사용하는 SS400 강재를선택하여구조재료의압연방향과용접으로인한잔류응력의영향을고려하여피로균열진전특성을파악하고, 강용접구조물의안전설계를위한방법에대하여실험적으로고찰해보고자하였다. 연구결과를요약하면다음과같다. 1) 하중방향에대한재료의압연방향이커짐에따라항복점은대체적으로증가하며, 연신률은일률적으로감소하는경향이있고, 피로균열진전속도는높게나타나는경향을보인다. 2) 용접재에서의피로균열진전은전반적으로용접잔류응력분포상태에따라일률적이고, 예민하게영향을받으며, 하중방향에대한용접선방향이클수록잔류응력의영향은감소한다. 3) 종합적으로용접구조물의안전설계를위하여는용접에의한잔류응력분포를정확하게파악하고, 하중의크기와방향그리고재료의압연방향을잘조화시켜고려하는것이중요하다고사료된다. 후 기 이논문은 2004년도홍익대학교교내연구비에의하여지원되었음. 참고문헌 1. O.V. Green : Trans. of American Society for Steel Treating, 18 (1930), G. R. Irwin : Analysis of Stress and Strain near the End of a Crack Traversing a Plates, Trans. ASME, 24 (1957), Yong Bok Lee and Byung Duck Oh : Characteristics of Fatigue Crack Propagations with respect to the Angles between Rolling and Tensile Loading Directions of Steel Plates, J. of KSMTE, 14-3 (2005), (in Korean) 4. Yong Bok Lee, Chin Sung Chung and Nam Il Cho : A Study on Fatigue Behavior Considering Effects of Redistributing Tensile Residual Stress and Crack Closure in SS330 Weldment, J. of KSME(A), 20-7 (1996), (in Korean) 5. Yong Bok Lee, Chin Sung Chung, Ho Kyung Kim, Nam Il Cho and Sang Heup Park : A Study on Fatigue Behavior concidering Effects of Redistributing Compressive Residual Stress and Crack Closure in SS330 Weldment, J. of KSMTE, 6-2 (1997), (in Korean) 6. R.C. Juvinall : Engineering Considerations of Stress Strain and Strength, McGRAW HILL BOOK Co., M. Isida : On the Determination of Stress Intensity Factors for Some Common Structural Problems, Eng. Fracture Mechanics, 2-1 (1930), Yong Bok Lee, Nam Il Cho and Kang Eun Park : A Study on Surface Fatigue Crack Behavior of SS400 Weldment, J. of KWS, 14-2 (1996), (in Korean) 9. P. Paris and F. Erdogan : A Critical Analysis of Crack Propagation Law, J. of Basic Eng., Trans. of ASME, 8 (1963), Sang Chul Kim, Yong Bok Lee : The Application of Forman Equation for Fatigue Crack Propagation in Welding Residual Stress Region, J. of the Korean Welding Society, 5-1 (1987), (in Korean) 11. R. G. Forman and V.E. Kearney : Numerical Analysis of Crack Propagation in Cyclic Loaded Structures, J. of Basic Eng., Trans. of ASME, 9 (1967), Yong Bok Lee, Chin Sung Chung, Young Keun Park and Ho Kyung Kim : Effect of Redistributing Residual Stress on the Fatigue Behavior of SS330 Weldment, Int. J. Fatigue, 20-8 (1998), Journal of KWS, Vol. 23, No. 6, December, 2005

DBPIA-NURIMEDIA

DBPIA-NURIMEDIA 22 연구논문 SPC 용접부의강도특성에미치는 CO 2 용접조건의영향 송준희 * 최준용 ** 임재규 *** * 전북대학교공학연구원자동차산학협력원, 성연산업 ( 주 ) ** 전북대학원기계설계학과 *** 공학연구원공업기술연구센터, 전북대학교기계항공시스템공학부 Effect of CO 2 Welding Conditions on Property of Strength in