B. INTRODUCTION TO ASSESSMENT PROCEDURES FOR CRACKED COMPONENTS AT HIGH TEMPERATURES

Transcription

1 B. INTRODUCTION TO ASSESSMENT PROCEDURES FOR CRACKED COMPONENTS AT HIGH TEMPERATURES 374

2 INTRODUCTION The early approaches to high temperature life assessment show methodologies that were based on defect-free assessment codes, i.e. ASME Code Case N-47 and the French RCC-MR, which have many similarities and are based on lifetime assessment of un-cracked structures. More recent methods make life assessment based on the presence of defects in the components. The more advanced codes dealing with defects over the range of creep and creepfatigue interaction in initiation and growth defects are the British Energy R5, the French A16 and BS7910 which have clear similarities in terms of methodology. 375

3 INTRODUCTION The available procedures are implemented in a series of well-defined steps, often shown as flow charts. The individual steps can refer to - a component before it enters service, containing either a postulated defect or one discovered during inspection. - a defect, which has been discovered after a component has been in service for a period of time. The flow charts contain variations and choices available to the user in accordance with their level of expertise and the level of information available on the component under consideration. 376

4 Some typical steps in an assessment are listed here: 1) Establish the cause of cracking 2) Define previous plant history, future operational requirements and relevant stresses 3) Characterise defects 4) Establish material properties 5) Check the fatigue component 6) Perform defect assessment 7) Define Fatigue Crack Propagation Rates 8) Creep Crack Propagation Rate 9) Incubation Period 10) Assessment to Include Creep-Fatigue Loading 11) Others 377

5 6) PERFORM DEFECT ASSESSMENT: 6.1) Determine margin against fast fracture assuming an initial defect or a measured defect dimension using various levels of FAD. 6.2) Evaluate ΔK th and fatigue crack propagation rates. 6.3) Determine the creep rupture life of the component, using initial defect dimensions. 6.4) Evaluate crack propagation rates and estimate the amount of creep crack growth at intervals 6.5) Check the steady creep conditions applied at the crack tip; if not, revise crack growth estimates 6.6) Determine crack dimensions at the end of each interval. 378

6 6) PERFORM DEFECT ASSESSMENT: 6.7) Repeat calculation against fast fracture at the end of each interval. 6.8) If the end of life margin is satisfactory, no remedial action is needed. 6.9) If the end of life margin against fast fracture is unsatisfactory, the intermediate calculations can be used to establish the time at which this margin ceases to be acceptable and to define when a remedial action is necessary. Step 0 Step 1 Step 2 Final Step a 0 a 1 = a 0 + Δa a 2 = a 1 + Δa a f = a f-1 + Δa Margin 0 (M 0 ) M 1 M 2 M final > M min? 379

7 7) DEFINE FATIGUE CRACK PROPAGATION RATES: The fatigue crack propagation rate is generally defined by the Paris equation: da dn f = C ( ΔK ) m C, m: material constants 380

8 8) CREEP CRACK PROPAGATION RATE: Creep crack propagation rate is usually defined in the form: A, q: constants. da = A * dt c ( C ) q A = ε Where the creep ductility of the material is known: for f da dt c in m/h 381

9 8) CREEP CRACK PROPAGATION RATE: Where the creep ductility of the material is not known, crack propagation rates can be obtained from: p K a : da dt c = σ ( K ) p t ( ref ) SIF at maximum depth for a crack of diminsions a and l. t R (σ ref ): time to rupture at the reference stress. ref a R

10 8) CREEP CRACK PROPAGATION RATE: The driving force C * is calculated from: C ε ref * : = σ ε 0 p K ref σ ref 2 creep strain rate from uniaxial deformation data at σ ref The formulation covers primary creep 383

11 9) INCUBATION PERIOD: Where incubation time data are available from test specimens, the incubation time for the component can be correlated with C * provided both specimen and component are in the secondary stage of creep. Then, the incubation time t I can be deduced from: t i, component = t i, specimen C C * specimen * component n n+1 Where data are not available for the material used in the component, procedures provide equations to estimate t I. 384

12 10) ASSESSMENT TO INCLUDE CREEP-FATIGUE LOADING: In most cases, linear summarition of the time dependent creep and the time independent fatigue portions of crack growth adequately describes high temperature failure under cyclic loading: da dn = da dn c + da dn f = da dt 1 * 3600 f da + dn f = A q 1 ( C ) + C ( ΔK ) m 3600 f f: frecuency 385

13 SENSITIVITY ANALYSIS Assuming the final defect size gives an acceptable end-of-life safety margin, a sensitivity analysis is recomended. Different procedures (BS7910, R5, R6...) describe the principles. The sensitivity analysis considers the effects of different assumptions, such as stress levels, material properties, defect sizes, etc. 386

14 BIBLIOGRAPHY / REFERENCES Taylor N., Kocak M., Webster S., Janosch J.J., Ainsworth R.A. and Koers R., Final Report for Work Package 2, State-of-the-Art and Strategy, FITNET/Technical Report/JRC-IE (NSU/NT/ ), September 2003 Dogan B., High temperature defect assessment procedures, Inernational Journal of Pressure Vessels and Piping 80 (2003) Dean D.W., Ainsworth R.A. and Booth S.E., Development and use of the R5 procedures for the assessment of defects in high temperature plant, International Journal of Pressure Vessels and Piping 78 (2001), p British Energy, R5, Assessment Procedure for the High Temperature Response of Structures. Issue 3, Gloucester: British Energy; June