applied fracture mechanics

Structural Reliability Improvement Using In-Service Inspectionfor Intergranular Stress Corrosion of Large Stainless Steel Piping 353Figure 12. Improvement Factors versus Dsigma for 4-Year ISI Interval with various POD Curves6. ConclusionsStress corrosion crack growth can occur under constant loading conditions, and istherefore very different from crack growth driven by the cyclic loading. Our originalprobabilistic fracture mechanics model incorporated a simple model of stress corrosioncracking, based on the assumption that crack growth velocity in either the radial orcircumferential direction is controlled by the value of stress intensity factor (for a givenmaterial and environment) at the crack tip. This model described crack kinetics by asimple functional relationship between crack growth rate and stress intensity factor.This probabilistic stress-corrosion cracking model was applied to assess the effect ofvarious inspection scenarios on leak probabilities. This chapter has also discussedprobability of detection curves and the benefits of in-service inspection in theframework of reductions in the leak probabilities for nuclear piping systems subjectedto IGSCC. The results for typical NDE performance levels indicate that low inspectionfrequencies (e.g., one inspection every 10 years) can provide only modest reductions infailure probabilities. More frequent inspections appear to be even more effective. Thegreatest benefits are predicted for the “Very Good” NDE technology and procedures,for which an order-of-magnitude improvement on the leak probability can be achieved