Modification Of CTOD Allowed Values Based On Probabilistic Fracture Mechanics And Study Of Conservativeness

“Made in China 2025” will further promote the release of marine structures will be large-scale,deep-sea development,resulting in large thickness,high strength structural steel welded joints of the fracture toughness of a higher demand.CTOD as a toughness index is widely used in the assessment of toughness at domestic and abroad specifications,of which CTOD allowable value as CTOD test toughness of the key acceptance criteria,how to set reasonable allowable value has been the concern of many scholars.Based on the CTOD experiment of high strength steel welded joints with large thickness EQ70,the allowable values of CTOD are studied by the theory of probability fracture mechanics and integrity assessment.Provides a new method to allow the correction and setting of values,and extends a new idea.The main contents and conclusions are as follows:(1)According to the probabilistic fracture mechanics theory,the CTOD fracture toughness value obeys the specific probability distribution model.The relevant research only develops the pipeline steel and the shell steel with small thickness and low strength,and few studies on structural steel with large thickness and high strength.Based on the CTOD experimental data of high strength steel welded joints of large thickness EQ70,the non-parametric test and the distribution fitting optimization of the CTOD test probability distribution model of weld fusion line and weld center are made respectively,The results show that the optimal probability distribution of CTOD fracture toughness value is normal distribution in weld fusion line,and the optimal probability distribution of CTOD fracture toughness value is Weibull distribution in weld center,And the expression of probability distribution is obtained,which provides the design basis for probability integrity assessment of high strength steel with large thickness.(2)The relevant toughness assessment norms set allowable values conservative,likely to cause a lot of waste of resources,do not meet the “Fitness for Purpose “principle.To solve this problem,Based on the optimal probability distribution model of CTOD fracture toughness for welded joints of EQ70 steel,a probabilistic iterative correction method of permissible value probabilities is proposed,which provides a complete set of allowable value correction methods and ideas,And The safety evaluation of themodified permissible values was verified by the SINTAP specification,demonstrating the safety of the modified method,Which makes up the subjectiveness of the proposed amendment in the past,does not provide a complete correction process,and does not carry out the safety assessment of the modified allowable value,and extends the idea for the allowable value correction.Into the iterative correction found,weld fusion line the allows value can be set to 0.069 mm,weld center the allows value can be set to 0.064 mm.(3)Based on the principle of " Fitness for Purpose " and referring to the concept of safety margin in American Highway Bridge Design Code,the conservative degree of permissible value is studied numerically for the first time,and the conservative formula of permissible value is obtained.A reasonable allowable value interval is proposed based on the numerical value of conservative degree of permissible value,which provides a new design basis for the setting and modification of allowable values.At the same time,the process of toughness evaluation of CTOD is improved,which makes the process of toughness more consistent with the principle of “Fitness for Purpose".