Elastic-Plastic Finite Element Analysis For Piping Elbow With Axial Inside Surface Cracks

It is a trend for the pipes being used to make safety estimation in application of fracture mechanics, and this method has a very important significance. Since the 1970s the fracture mechanics theories and practical applications have been widely developed. However, it is very scarce of the fracture parameters to make safe estimation for some structures with crack in engineering. Based on fracture mechanics, adopting the technique of the finite element method ( FEM ), the studies give the fracture parameters of stress intensity factor K_I and in K_II in elasticity and J-integral in elasticity-plasticity for the piping elbow with axial inside surface, then convert to the shape factor F and full-pasticity coefficients h₁, moreover, resolve the calculation question of the stress intensity factor K_I and K_II, append the database of full-plastic solution. The following contents are included in this paper:1. The using status of the in-service pressure pipelines in china is given briefly and some universal safety assessment methods of pressure pipelines with defects are listed in the paper;and the new method of fracture mechanic in the structure with cracks is studied, which is the foundation of the piping elbow problem starting.2. The method of building the finite element structure model with cracks is studied. Comparisons have been made with the results of the examples with EPRI. The results show that the model method is accuracy. Then the macro programs including physical model, load and solution have been made by APDL for the piping elbow with axial inside surface under different characteristic parameters of geometry, material and load.3. On the basis of the theory of the 1/4 node displacement method, the stress intensity factors of K_I and K_II are calculated under the internal pressure and the moment through the postprocessor of ANSYS, then they are converted to shape factor F.4. The stiffen-exponent n is introduced in the paper and the nonlinear material properties of ANSYS are defined with it. The program of the solution of J-integral is compiled by APDL to analyze the elastic-plastic finite element problem of crack structure model. The full-plastic analysis method is confirmed based on the full-plastic J-intergral method in EPRI.5. The full-plastic J-integral JP of the axial inside surface piping elbows is calculated under the different properties of pressure and moment, then the tables of full-plastic coefficients hi are done.