Elastic-plastic fracture analysis of pressure pipelines with axial cracks based on the interaction integral method

Abstract

The proposed work aims to demonstrate the significance of the plastic zone at the tip of an axial crack in a pipeline for managing Stress IntensityFactors(SIF). The three-dimensional finite element model of pressure pipeline with axial cracks was built by utilizing the Ramberg-Osgood X80 material model of pipeline. according to Von Mises yield criterion, the size of plastic zone at crack tip was determined, and the fracture parameters were calculated based on interaction integral method, the plastic stress deformation law, determination of elastic-plastic limit load and plastic correction of SIF at crack tip of pressure pipeline with axial crack were discussed. Consequently, it is observed that the elastic-plastic limit load diminishes as the initial crack length increases under specified pipe geometry and material conditions. the plastic zone dimensions at the crack tip of the pipeline expand proportionally with the relative crack length (δ). Moreover, the relative error between the Stress Intensity Factors (SIF) before and after plastic correction exhibits nonlinear growth in response to increasing internal pressure within the pipeline. Notably, when assessing coefficients prior to plasticity corrections, it becomes evident that the maximum error may exceed 20% as the internal pressure rises. Importantly, the empirical verification data substantially aligns with the previously mentioned theoretical analysis results in a noteworthy concurrence.

Fig 1. Crack tip arbitrary integration loop Γ at crack tip.

Fig 2. Crack tip plasticity correction: (a) Crack tip plastic zone; (b) Corrected plastic zone.

Fig 3. Node number corresponding location map.

Fig 4. Axial penetration cracks in pressure pipelines.

Fig 5. (a) Finite element model of pipeline with axial cracks and (b) Pipeline restraint method.

Fig 6. Pipeline test points.

Fig 7

Test devices (a) XL2118C strain gauge. (b) S-SY-33 pressurized pump.

Fig 8. Von Mises stress at crack tip under different pressures P.

Fig 9. Variations in the plastic deformation of the split tip with time at different internal pressures.

Fig 10. Relationship between the crack tip plastic strain and load step length of the pipeline.

Fig 11

Variation law of stress intensity factor K_I after plasticity correction: (a) Variations in K_I with a at different R. (b) Variations in K_I with a at different t.