Research On Safety Margin Model Of Attenuation Path Velocity Integration Considering Dynamic Time-Variation Characteristics Of Fatigue Cracks

As pressure-bearing special equipment,large-scale pressure vessels play a pivotal role in the engineering field.Congenital defects such as slag inclusions and loose holes are inevitable during the manufacturing process.In the course of use,due to factors such as random load,alternating stress,etc.,the expansion of congenital defects leads to a significant decrease in the overall safety and reliability of the pressure vessel.Among them,the most dominant macroscopic manifestation of the growth of birth defects is crack growth Including embedded cracks,surface cracks and penetrating cracks.For the calculation of safety margin with cracked defects,traditional methods usually judge the remaining safety margin of pressure vessels with cracked defects from a static perspective,but they cannot accurately characterize the range of dynamic safety margins.For this reason,this paper studies the attenuation path of crack propagation based on the law of crack propagation change.Based on the concept of attention path velocity integration proposed by Professor Long Wei,we try to put forward a more accurate path velocity integration calculation formula,and develop a more effective safety The margin characterization method provides an effective analysis method for the reliability of pressure vessels with cracks,ensures the safety of equipment operation,and reduces the incidence of major safety accidents and property losses.Firstly,the influence of different types of cracks with different initial values on the safety attenuation path and path velocity attenuation safety margin model was first studied,and the correlation function of different types of crack growth was obtained.Through the analysis of the crack propagation mechanism,the correlation change function of the depth and the length of the buried crack and the surface crack during the propagation process was established.Based on the Paris formula,obtained the crack growth law formula with iterative method,which applied the theory of Newman et al.to calculate K,and considered the changes in the growth law of embedded cracks and surface cracks under the crack closure effect.The material coefficient m in the Paris formula needed to be changed to prove that the crack growth change was related to the material,and used Python to program the model to obtain the crack change pattern under different initial values,and then obtained a function which could present the relationship between depth and length of two kinds of cracks at different initial crack points.For penetrating cracks that have no relationship between depth and length,the single correlation function of the length of the farthest propagation point of the crack front was solved based on the crack length of the surface observation under the condition of considering the safety evaluation and the closure effect.The attenuation path of different crack types on the R6 failure assessment chart was analyzed by the correlation function model.Then,based on the attenuation path model and the concept of velocity integration,a residual safety margin model for large pressure vessels was established.According to the method of calculating the assessment points in the newly revised national standard GB/T 19624-2019,an attenuation path model was obtained and the breakpoints of surface cracks in the simulation of the attenuation path were discussed.From the attenuation path,considering the time-varying behavior of crack propagation,a path velocity integration model was proposed,which was used to characterize the dynamic safety margin of a pressure vessel with defects.The surface crack was taken as an instance to carry out an example analysis.Through a series of transformations of the correlation function model,attenuation path model and path velocity product model,the integral was used to calculate the area and the value of the safety margin and performed it with the ray method and the fuzzy evaluation method.In comparison,the path velocity integration safety margin model was evaluated from the perspective of life safety margin,and the results were in good agreement.Finally,the remaining life model under crack propagation was derived.Finally,a visual engineering application platform for the dynamic safety margin model of pressure vessels with defects was built.Use Visual Basic(VB)language to program the proposed correlation function model,attenuation path model and path velocity integration model for visualization platform programming,which could facilitate the import and export of data,and finally achieved the engineering application of each model,and quickly determined residual safety margin of the large pressure vessels with defects.