Structural Integrity Assessment of Pressure Vessels with Defect in Welded Joints
Vencislav Grabulov,
Stojan Sedmak,
Aleksandar Sedmak,
Zijah Burziæ
The examples of practical application of fracture mechanics parameters in cracked pressure vessel structural integrity are presented.
The application of fracture mechanics depends on available data, material behaviour, environmental effect and loading. In predominantly static loading material behaviour can be described as linear elastic, based on the stress intensity factor, KI, when its ductility is negligible, and as elastic-plastic, with crack opening displacement or the J integral as parameters.
Linear elastic fracture mechanics was applied in a typical service problem when non-allowable defects according to ISO 5817 had been detected by non-destructive testing in regular in-service inspection in welded joints on the cylindrical storage tanks for pressurized air. The defects have been replaced by corresponding cracks for conservative estimate. Simplified application of the stress intensity factor as a parameter and a two parameter fracture assessment diagram from PD6493 Procedure have shown that structural integrity of the analyzed storage tanks is not endangered.
For the detailed elastic-plastic analysis the J integral is used. The experimental pressure vessel was manufactured by welding high strength steel (yield stress 700 MPa) and then precracked artificially for J integral direct measurement.
The theoretical and experimental analyses can not fully describe the behaviour of the cracked welded joint and its heterogenious microstructure. For that purpose, the numerical analysis can be helpful, if applied in a proper way. An example of the finite elements numerical analysis of welded joints is presented.
Key words: structural integrity, pressure vessels, welded joint, welded defects, crack, fracture mechanics, stress intensity, j integral, method of finite elements.