Prediction Of Failure Conditions For Full-scale Pipelines Based On Tensile Tesr Of C Wide Plate

With the rapid growth of oil and gas demand,the total mileage of long-distance oil and gas pipelines are increasing year by year.In the long term service,there may be various forms of defects,in which the crack defects have the most serious impact on the safe operation of the pipeline.Therefore,how to get the failure load of the defective pipe and the safety early warning value become very important.The crack mouth opening displacement(CMOD)is a widely accepted fracture parameter in the industry,but CMOD can not be measured for the reinforced pipes and the cracks in the inner surface.In this paper,the strain values around the region of the crack are used as the indirect characterization of the pipe fracture,and the relationship between the strain characteristics of the full size pipe and the tensile specimens of the C shaped wide plate is established,and the failure load is predicted by the tensile specimen of the C shaped wide plate.First,the mechanical properties of L245 steel were tested.The basic mechanical parameters were obtained by uniaxial tensile test,and the fracture parameters were obtained by three point bending test.The results show that the CMOD of three point bending specimen will increase linearly with the increasing of load.After an inflection point,the nonlinear change will appear as the load continues to increase.Then,the tensile specimens of C shaped wide plate were designed with L245 steel,and the axial strain and CMOD on both sides of the crack were obtained by uniaxial tension,and the linear relationship with them was found.The axial strain characteristics of the inner surface area corresponding to the location of the external surface crack are studied,which provides a reference for the safety assessment of the internal surface crack.Finally,a full size pipe with a 120 degree ring surface crack is tested by both internal pressure and bending moment.The test shows that the greater the depth of the ring surface crack,the greater the maximum axial negative strain on both sides of the crack.The CMOD of the full-scale pipeline shows a linear relationship with the absolute value of axial strain on both sides of the crack.Compared with three point bending test,C wide plate tensile test and full-scale pipe test,it can be concluded that: The CMOD of all three showed a linear growth trend with the increase of load,indicating a correlation between the three tests.By comparing the results of full size pipe test and C shaped wide plate tensile test,it is found that the CMOD of the two results has a linear correlation with the absolute value of the axial strain around the crack,which proves the feasibility of using the C shaped wide plate tensile test to predict the full size pipe failure condition.The axial strain value of the crack in the C wide plate tensile specimen can be used as an early warning value for the axial strain of the full-scale pipeline.