|عنوان فارسی مقاله||منحنی S-N برای سازه های دریایی جوشکاری شده، غیر جوشکاری شده یا جزئیات جوشکاری بهبود یافته|
|عنوان انگلیسی مقاله||S-N Curves for Welded, Non-Welded or Improved Welded Details of Marine Structures|
|رشته های مرتبط||مهندسی عمران، جوشکاری، سازه های دریایی و بازرسی جوش اسکلت فلزی|
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|نشریه||الزویر – Elsevier|
|مجله||پنجمین کنفرانس طراحی خستگی – 5th Fatigue Design Conference|
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بخشی از ترجمه فارسی مقاله:
5- نتیجه گیری
بخشی از مقاله انگلیسی:
The development at the end of the sixties, in shipbuilding and offshore, of design tools using computers allowed the development of large and very large units with structural strength and so steel weight optimization [1,2,3,4]. The reduction of the steel weight obtained by the adjustment of the safety margins thanks to the new design tools, associated with the use of high strength steels, lead to make cumulative fatigue the major in-service failure mode. To handle this challenge, a large European cooperative research program was launched within the CECA (European Community for Coal and Steel) the results of which were published in 1987 . During the same period classification societies developed researches on cyclic fatigue of ship structures  with publication of guidance notes  and rules for very large tankers . The research results and the return experience have shown clearly that welded joints are the weak points with respect to cumulative fatigue, so the design rules were only focussed on welded joints. Different approaches were considered, nominal stress with tables of details  and , hot spot stress [11,12,13], notch stress  and local stress [24,26]. But more recent fatigue failure return experiences on FPSOs  and the development of ULCS/Ultra Large Container Ships  with thick non welded hatch corners raised the question of the verification of non welded ship structure details. FPSO repairs considering welded joint post weld treatments also raised the question of the S-N curve to be used. Since the seventies, numerous information and data have been accumulated and published. The presented work aims to provide a synthesis of these published data with proposal of S-N curves for linear welded joints adapted to the various ship and offshore building problems.
2. Background The existing rules for fatigue verification of welded steel ship and offshore structure provide S-N curves defined in terms of stress range and applied for all used steel strength [11,17].
2.1. Available data For the performed work, available data have been collected and analysed. The oldest test data for linear weld joints are given for as-welded joints in terms of nominal stress without consideration of the mean stress assuming that the welding residual stresses are high enough to delete the mean stress influence on the fatigue life. Two main sources have been considered [9,18]. The documents also give data for non-welded details: as-rolled plates (Table 1).
Based on publication of S-N curves for welded linear joints and the mechanical component fatigue limit practice, formula for S-N curves applicable to notch stress for welded details, and hot spot stress plus roughness effect for non welded details, also applicable to welded, stress released and post weld treated steel details have been developed. The S-N curves are two slopes curves, different for low cycles and high cycles domains. The low cycles domain curves have a constant m parameter and a constant K value function of the steel yield strength. The point of slope change is a function of the steel yield strength. The high cycles domain curves have a parameter m function of the R ratio, except for as welded details for which m = 3, and a constant K function of the R ratio and yield strength excepting for maximum value of R (as-welded details) The proposed formula have been applied to two container ship deck details showing that the found Miner sum are in good agreement with the actual practice and return experience. The proposed formula represents a first approximation but fills a lack in ship and offshore design as the existing standards only provide S-N curves for as welded details. New publication of data, in particular concerning the low cycles domain and the non welded details, will allow to improve the proposed formula.