Elastic-Plastic Analysis Of Constraint And Failure Of 2D And 3D Crack For X100 Pipelines

With the rapid development of the industrialization , the request and capacity of energy transport such as oil and gas much more than before . Accompanied by the progress west-east natural gas transmission and south-to-north water project, large diameter high-pressure pipeline was used more and more widely. Micro crack exist in the form of all manufacturing defect and welding defect and would propagated to be macroscopic later in service, therefore, it is rather important to have strength analysis for the structural that contains crack.Fracture toughness is a key parameter when have the safety evaluation, failure probability and life assessment of X100 pipelines which contains defect. The geometric, loading and crack dimensions of the specimen result in different constraint effect, further more, fracture toughness and J_R resistance curve were relevance with it also. Therefore, the constraint effect must be considered carefully so that failure predict can be more accurately.First, constraint effect and fracture behavior for X100 pipeline steel with axial crack have been quantitative researched by elastic-plastic fracture theory combined FEA. Considering the plastic effects, a parameterβ_Q was introduced on the basis of J-Q theory and the fracture criterion of J-β_Q double parameters have been established. The failure curve of X100 was fitted from the dates of experiment. And Failure pressures for pipelines with internal and external axial crack were predicted, separately. The results show that constraint effect of external crack is little stronger than internal crack, but opposites when a/t≤0.2. For shallow cracks (a/t≤0.40), crack tip fields are not very constrained and yield failure is most likely to occur as plastic deformation is larger. Then critical pressure of PCORRC is safe. For deep cracks, strengthened constraint effect result in small plastic deformation and fracture failure would to take place.Second, in connection with surface crack that usually exist in engineering structures, stress triaxiality theory and strain energy density theory were used to analysis half semi-elliptical surface crack in infinite plate. The results show that the equivalent plastical strain, stress triaxiality, strain energy density and opening stress are rapidly changed near the free surface point B where equivalent plastical strain has achieved its maximum and stress triaxiality obtained its minimum value, but changed slightly along crack line and almost the same with deepest point A. Conclusion can be used to predict the direction of elliptical crack steady propagation .These works have certain directive significance to design safety assessment of engineering structure.