MEASUREMENT OF UNCERTAINTY IN EDDY CURRENT BOLT HOLE CRACK MEASUREMENTS FOR USE IN POD

In order to obtain valid estimates of a90/95 for cracks in bolt holes using eddy current techniques, it is essential that an accurate model for the measurement uncertainty be used in the fit of the a‐hat versus a data. In the absence of explicit error handling schemes, the use of a log axis for the dependent variable implies that the uncertainty is proportional to the variable and a linear axis implies a constant uncertainty. We have analyzed the data from a recent large round robin study of EDM notches and fatigue cracks and demonstrated that, once the calibration errors have been removed, the remaining uncertainty is constant at very low crack sizes and, above a certain threshold (approximately 0.75 mm), increases linearly with the crack size. The proportionality constant appears to be bigger for fatigue cracks than EDM notches.