FatiguePro Fatigue Crack Growth Module
SIB-95-126

A fatigue crack growth analysis module is available for the FatiguePro fatigue monitoring system which has, as an objective, the on-line prediction of growth of flaws in reactor components. Calculation of the fatigue crack growth is a major feature which has been added to the FatiguePro system's capabilities, since a basic input assumption for most ultrasonic examinations is that cracks of some defined size may have escaped detection. In addition to hypothetical flaws, actual as-found cracks may also be tracked with the fatigue crack growth module. The crack growth module of FatiguePro allows in-situ evaluation of flaws (either actual or postulated) to ensure that they remain within allowable structural margins, thereby justifying longer inspection intervals or continued operation without repair.

Methodology
Once a crack has been initiated in a component, or has been assumed in a component, its propagation is driven by the magnitude and frequency of not only the peak surface stress, but also of the stress distribution through the wall of the component. Fatigue crack growth is a function of changes in the stress intensity factor, K, which is itself a function of the time dependent through-wall stress distribution.

Fatigue crack growth calculations for flaws under a specified set of loads are commonly performed using linear elastic fracture mechanics (LEFM) techniques. The component geometry and stress state through the component wall are used to determine the stress intensity at the flaw location, and the flaw growth resulting from a stress cycle is predicted as a function of K. This is usually accomplished by means of the ASME Code, Section XI crack growth relationships of the form:

The stress intensity is a function of the through-wall stress distribution and the flaw depth, and the stress intensity range between two extremes (K_max-K_min = K), and R-ratio are the controlling variables in the crack growth relationships.

The stress intensity factor is calculated as a function of time based on actual plant operation. The FatiguePro system uses measured values of plant temperature, pressure and flow to calculate through-wall stress distribution transients in real time as the plant operates. These stresses are in turn used to calculate stress intensity as a function of time at the locations of interest. The resulting "K transient" is then sorted and filtered to eliminate cycles with ranges below the K threshold, the level below which no crack growth will occur. The filtered history is then used to calculate the incremental crack growth, using the appropriate bilinear fatigue crack growth law.

The user specifies the initial flaw size a_o, to be used by the FatiguePro system for each monitored component. The value of a_o is the crack depth of an as-found flaw or, in the case of a hypothetical flaw, is a conservative estimate of the depth of flaw which may have been missed by external ultrasonic examination of the component. FatiguePro calculates the incremental crack growth which would be predicted from the actual plant transients and updates the flaw depth to reflect any predicted growth.

Since stress intensity is a function of crack depth, the calculated stress intensities are based upon the updated crack depth from previous calculations. The process is performed continually during plant operation to provide a conservative estimate of flaw depth.

For the case of a hypothetical flaw, if subsequent inspections reveal that no flaws greater than a_o are present, the flaw depth can be "reset". The crack growth analysis may be repeated at any time, using the filtered stress intensity history for the entire monitoring period. This is useful if an alternate crack growth law becomes available, or if it becomes necessary to change the stress intensity relationships used by FatiguePro.

Application Tracking fatigue crack with FatiguePro has equal application to both BWR and PWR units. This capability has been used to track actual flaws in PWR steam generator feedwater nozzles, and has been used to evaluate hypothetical flaws in BWR feedwater and CRD return nozzles. In particular, the fatigue crack growth module can be used to perform the fracture mechanics assessment recommended by the alternative rules to NUREG-0619.

If you would like more information on any aspect of FatiguePro, please contact SI.