Research On Safety Monitoring Technology Of The Pressure Pipe With Inner Surface Oblique Crack

Pressure pipes are widely applied in the fields of chemical and nuclear energy.Defects are unavoidable in the process of manufacture and application.There is a hiden danger in the operation of pressure pipeline containing defects.When the defect is extended to a certain state,the pipe will leak or even explode.But in practical production process,if the pipe is replaced as soon as cracks are found on the pressure pipe,it`s neither economical nor satisfies the requirements for continuous production.Moreover,according to the principle of Fitness for Service in fracture mechanics,components with defects can also operate safely under certain conditions.In this paper,the safety of pipe with crack-like defects and the monitoring of the growth state of the crack are studied.The method of monitoring the propagation condition of inner crack by measuring the strain of the outer surface on the pressure pipe was improved and perfected.The finite element method was applied to model the pressure pipeline with oblique cracks on the inner surface and the relationship between the crack size and the maximum equivalent strain on the surface of the pipe was calculated within a certain range which was verified by the strain test experiment.The calculation of the stress intensity factor in pressure pipelines with cracks is an essential step in its safety assessment.However,for oblique cracks,the analytical formula of stress intensity factor is not provided in evaluation criteria,which brings inconvenience to safety assessment in engineering practice.The finite element method was applied to simulate the singularity of the oblique crack tip in this paper.The stress intensity factor at the crack front was calculated and converted into a crack shape factor under different crack sizes.The formula between the crack shape factor and the crack size was obtained within a certain range,and it is convenient to calculate in practical application.When calculating with the finite element method,firstly,the method of modeling the inner surface crack in the pipeline was explored and the appropriate mesh size of the model,considering the calculation time and accuracy comprehensively,was determined.The correctness of the crack model was varified by compareing with the existing examples in standards and results of the strain test,which provides the guarantee for the following calculation.Then,a more complex finite element model of the pipeline with inner elliptical oblique cracks was established.The crack deflection angle(6 and three dimensionless parameters,crack shape ratio(6?(8,crack depth ratio(6? and radius thickness ratio ? were defined to characterize the size of the crack.After a large number of finite element calculation,the relationship between the above-mentioned parameters and the maximum equivalent strain of the pipe surface and crack shape factors was fitted under certain conditions.The connection between the strain on the pipe surface and the dangerous condition of the crack is established,which provides a solid theoretical basis for the safety monitoring of the pipe with crackes.