Research On Life Assessment Method Considering Constraint Effect For P92Pipe With Defects At Elevated Temperature

The constraint level could affect the incubation and propagation of crack in thecomponents containing defects at high temperature. The variation of crack depth andstructure size would induce the variation of crack tip constraint. As a result, theresidual creep life of the components would be changed. Hence, understanding theeffect of constraint on creep crack behavior and establishing the correspondingresidual life assessment approach played an important role in assuring the reliabilityof the component with the presence of defects at elevated temperature. In this paper,the effect of constraint on the creep crack growth property was qualitatively studiedand quantitatively characterized, which were carried out on ASME P92steel mainlyapplied in Ultra Supercritical Power Plants Components by experimental investigationand numerical simulation.The creep properties of distinct zones of P92steel welded joint, i.e. base metal(BM), welded metal (WM), coarse grain heat affected zone (CGHAZ) and fine grainheat affected zone (FGHAZ), were obtained using small punch creep test method.This was achieved by establishing the relationship of load in small punch creep testand stress in uniaxial creep test and by determining the correlation of deflection andstrain on the basis of the comparison of uniaxial creep results and small punch creepresults.The creep crack growth tests were carried out on the specimens with initial crackslocated in the welded joint distinct zones to determine the creep crack growthproperties of micro zones of P92steel welded joint. The relationships between creepcrack growth rate a and high temperature parameter C*for micro zones of weldedjoint were obtained. The results revealed that at the same value of C*, the highestcreep crack growth rate occurred in the FGHAZ specimen while the lowest presentedin the BM specimen. This was due to the fact that the creep properties of FGHAZspecimen was low and the constraint in it was high compared with those of othermicro zones.Based on creep continuum damage mechanics and coupled with finite elementmethod analysis, the virtual creep crack growth technique was achieved. The creepcrack growth behaviors calculated by numerical simulation exhibited agreement withthose of experimental data. Furthermore, the evolution of the creep damage and the variation of stress and strain in the crack tip zone during creep crack propagationprocess were also obtained.Creep crack growth tests with specimens of various crack depths and specimensizes were carried out to investigate the effect of constraint on the creep crack growthbehavior. The tested results revealed that the constraint induced by crack depth andspecimen size could affect the relationship between creep crack growth rate a andC*. Then, the variations of the crack tip stress distribution and the constraintparameter Q during creep crack growth process were obtained using creep crackgrowth numerical simulation. Furthermore, a modified creep crack growth predictionapproach based on the C*-Q theory was proposed. Compared with conventionalpredictions provided by single C*parameter, the creep crack growth rate determinedby the modified C*-Q approach exhibited agreement with experimental results andnumerical results as the value of C*was the same. These phenomena meant that thisapproach could be employed to give a more accurate residual creep life and to assurethe reliability for the in-service components with defects at elevated temperature.