13th International Conference on Fracture June 16–21, 2013, Beijing, China 1 CTOD fracture toughness assessment method of High-strength steel based on BS7910 Ziyu Xia1, a，Zhangmu Miao1, b ，Tao Ma2, a ，Gang Chen2, b，Sheng Peng1, a 1.School of Transportation, Wuhan University of Technology，Wuhan 430000 2 .Shanghai Waigaoqiao Shipbuilding Company Limited，Shanghai 200137 awhut_xzy@126.com, bzmmiao1957@yahoo.com.cn Abstract As one of the essential criteria of assessment to welding toughness, CTOD(Crack tip opening displacement) is widely applied in engineering. The research on allowable values of the CTOD of high strength and thickness steel has been a hot issue, therefore the theoretical research in China lags behind its overseas. The present paper takes the single edge fatigue precrack of CTOD specimen as a flaw with the technical route of the British Standard 7910-2005, that is "Guide to methods for assessing the acceptability of flaws in metallic structures". In the meanwhile, make the grading assessments of the welding seam center of the EQ70/56 joints, combining the results of the CTOD tests. The assessments on welded joints of EQ70/56 show that the CTOD values are in the acceptable range. This method provides some useful guidance in researches of allowable CTOD values. Key Word Welded joint of High strength steel，CTOD，BS7910，FAD ,EQ70/56 1 Introduction With the development of the ocean engineering, the high-strength steel and ultra-high-strength steel are widely applied in the Deep-water Semi-Submersible Drilling Platform. As the complicated welding joints, high-strength and big thickness, the welding procedure of the marine structure becomes the key to the construction of the platform. The specification and classification societies around the world, including the CCS (China Classification Societies), are beginning to take the CTOD tests as a means to assess the toughness of the welded joints and the high-strength and heavy steel plates. However, there is much difference on the allowance value of CTOD with different societies, especially for the new high-strength and heavy steel plates. As the development of the methods for assessing the acceptability of flaws of the welded metallic structures, the British Standard 7910:1999, called "Guide to methods for assessing the acceptability of flaws in metallic structures", replaced the old standard PD 6493:1991[1] and was being widely applied in the engineering[2]. Basing on the fracture mechanics, the stand BS7910 fulfills the assessments with the FAD (failure assessment diagram) and takes the compare between the crack resistance of the structure and the actual ability to crack loading as its main idea. When majority standards of flaw assessment of metal structure were applied on the issue of ‘Fitness for Purpose’, the BS7910:1999 was amended and extended to an update version BS7910:2005[3], which has three levels of fracture assessment for the flaws in metallic structures. The choice of level depends on the materials involved, the input data available and the conservatism required. In the standard BS7910, flaws include Planar flaws, Non-planar flaws and Shape imperfections. In the assessment of high-strength and heavy steel plates, the specimen of the CTOD test was pre-carved with crack throughout the thickness. The welding seam center of the specimen was determined by the Level 2B. In the FAD of Level 2B, the axis is assessment index of fracture toughness, appears as Kr which is the ratio of the stress intensity factor, or r δ , the square root of CTOD fracture ratio. In the CTOD tests, the single edge running through crack [4] could be taken as the flaws assessed by FAD with the fracture ratio as ordinates against load ratio (Sr) as abscissa. The applied Level 2 assessment has an assessment curve given by the equation of a curve and a cut-off. If the assessment point lies within the area bounded by the axes and the assessment curve, the flaw is acceptable; if it lies on or outside the curve, the flaw is unacceptable. The CTOD toughness assessment of the heat affected zone and welding seam center of
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