Abstract

Fatigue crack growth laws are typically dependent on the ratio between minimum and maximum Stress Intensity Factor (SIF), referred to as the load ratio (R). When part of the SIF range is compressive (and hence R is negative) the amount of growth for a given SIF range is reduced due to crack closure effects.

Methods for capturing the effect of crack closure were presented in a previous PVP paper [1]. These methods are based around defining a scaling factor (q0) which is dependent on R and applied to the SIF range before calculated growth. Equations were provided for both best fit and bounding q0 factors.

This paper presents a comparison between these methods and results of testing. The specimens used were square cross-section bars and were made from Type 304L stainless steel with an initial corner crack. A range of load ranges and R ratios (including some positive R values) were used and the testing was undertaken at 250°C in both air and a simulated PWR environment.

The growth rate observed in the tests was used to derive the effective q0 factor observed in each stage of the testing. These values were then compared with the q0 methods that are used in actual defect tolerance calculations. The results agreed very closely with the derived best estimate q0 curves, with no discernible difference between the air and water results.

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