Prediction And Analysis Of Creep Crack Initiation Times Considering The Constraint Effect And Residual Stress For The High Temperature Structure

For in-service components with defects(such as cracks)under high temperature and pressure,the creep crack initiation(CCI)time occupied a large proportion of service life.To ensure safety of in-service components under high temperature environment,CCI time was significant to be accurately estimated.In this work,the CCI times for high temperature structures was investigated and relevant predicting models of CCI times were proposed.Influence of constraint effect,residual stress and creep properties on CCI times was analyzed.Suitability and accuracy of the analytical approaches were also verified.Detailed research contents were summarized as follows:Firstly,based on the creep ductility exhaustion model,theoretical prediction models of CCI times,denoted as C*-Q* two-parameter approaches,were established by considering constraint effect.The CCI times of different specimen geometries with different thicknesses and crack depths,and pipelines with various semi-elliptical surface cracks and long surface cracks were studied by using finite element method,which indicated that the order of constraint level and creep damage accumulation rate was: CT > CST > SENB > SENT > DENT > MT.With increase of specimen thicknesses or crack depths,constraint level increased,which resulted in acceleration of creep damage accumulation and decrease of CCI times.Variations of CCI times and the distributions of constraint levels along the crack fronts were discussed.It was found that axial surface crack and internal surface crack were more dangerous than circumferential surface crack and external surface crack,and CCI were more prone to occur in formers.Besides,long surface crack was the most dangerous types of cracks.Suitability and accuracy of C*-Q* two-parameter approaches were verified in above specimens and pipeline structures.Specially,K-RR and HRR-RR models that denoted CCI were occurred under transient creep stress state could obtain better predictions,because these prediction models took into account of the entire stress variation and redistribution of creep process.Secondly,based on the ductility exhaustion model and by considering influence of residual stress,a prediction model of CCI times under combined loading condition was proposed,which incorporated elastic follow-up factor Z and reference stress method.The calculation methods for different fracture parameters under combined loading condition were established.Then by considering constraint effect,analytical models of CCI times coupled residual stress and constraint effect were developed under combined loading condition.Using CT specimen,residual stress was introduced by pre-compression loads and combined with primary tension loads in FE analysis.The effect of residual stress on variation of creep stress,creep damage and CCI times were studied.Results showed that the CCI times decreased when residual stress was considered,and initial residual stress was relaxed within a very short time,so the influence of residual stress mainly focused on initial creep stage.The comparison verified that coupled models can more accurately predict creep damage and CCI times than the models only considering residual stress,especially for K-RR and HRR-RR models.Thirdly,by considering stress-regime creep ductility,enhanced C*-Q* twoparameter approaches for predicting CCI times were proposed.Compared with experimental data(P92 steel and 316 H steel),it could be indicated that conservatism due to using constant creep properties was optimized and reduced,and enhanced C*-Q* two-parameter approaches were more accurate.Finally,enhanced C*-Q* two-parameter approaches were developed into engineering application where 0.2 mm was denoted as critical distance of creep crack initiation.Effectiveness of predicting approaches under engineering condition was studied by comparison with experimental data.