Rapid Assessment Of Fatigue Life For Pressure Equipment With Planar Flaws Based On The Quality Category

The welded flaws are main potential hazards for the safety operation in special equipment,such as pressure vessels,pressure pipelines,etc.Many assessment standards for structures contained flaws have been constituted all over the world.With the development of pressure equipment in large-scale and high-parameter as well as the improvement of inspection requirements,flaws need to be evaluated quickly in some emergency situation and the domestic evaluation methods have not met the requirement at present.The rapid assessment can be accomplished by the quality category approach in BS 7910 standard for structures with flaws based on the principle of fit-for-purpose.However,the theoretical basis and equivalent principle of cracks in the standard for quality category approach are still lacking of systematic investigations.The theoretical basis of the quality category approach is investigated for the fatigue problems of flaws.The equivalent methods and influencing factors are analyzed and discussed,which provides the theoretical basis for the introduction of relevant standards in domestic.Q420 R steel as a new high-strength material in pressure equipment is used widely,especially for moveable pressure vessel equipment.The applicability of rapid evaluation for Q420 R steel based on the quality category approach is worth studying.The fatigue properties of Q420 R steel and the applicability of rapid fatigue assessment for structures with flaws will be analyzed through its joint fatigue tests.First,taken the semielliptic surface crack under axial stress as the study object,the theoretical basis was studied that the semielliptic surface crack was equivalent to extended surface crack and two methods of equivalent fatigue life and strength were proposed.Based on these two equivalent methods,the fatigue crack growth under different parameter sizes was calculated and the results were compared with the standard value in BS 7910.The equivalent flaws size diagram and quality category diagram of the semielliptic surface cracks were drawn.The results showed that the equivalent sizes of flaws were calculated almost in same that based on the two equivalent methods of equivalent fatigue life and equivalent fatigue strength.Compared with the standard values in BS 7910,the relative errors were smaller within 10%.Two equivalent methods can accurately reflect the basic principle of drawing the equivalent flaws size diagrams in BS 7910.Second,the equivalent fatigue crack growth under different crack shapes and loads was calculated based on the crack equivalent methods that had been proposed.The corresponding diagrams of equivalent flaws size and quality category were drawn.The results showed that the calculated results of the semielliptic surface cracks under bending stress and the central embedded cracks under axial stress were better and the errors were within a reasonable range.The eccentric embedded cracks under axial stress were too difficult to determine the theory of the crack growth mode.There was still a gap between calculated results and standard values in BS 7910.In the equivalent calculation of the fatigue cracks growth under different shapes and loads,the equivalent fatigue strength method was better than the equivalent fatigue life method in accuracy and stability.Thus,the equivalent fatigue strength method was more suitable for cracks equivalent.Last,the quality category of Q420 R steel was verified by fatigue tests of its butt joints according to the group method.The P-S-N curve of Q420 R steel was drawn with high survival rate and high confidence level,which compared with quality category curves were drawn in the double logarithmic coordinate system.Fatigue fractures were observed macroscopically and microscopically and the fatigue characteristics in every area were analyzed.The results showed that the P-S-N curve with survival rate P=97.7% and confidence level γ=95% was basically coincided with Q1 category curve which verified the Q1 category corresponding to the butt joints types.The fatigue resistance for the butt joint was the strongest with good stress streamlines.The applicability of rapid assessment and better fatigue performance for Q420 R welded structures could also be reflected from the other view.The typical characteristics of fatigue fractures could be detected from the macroscopic appearance and microscopic structure of the fracture.