In the example shown here, data was collected on specimens that were specially manufactured, and had much lower noise levels than signals taken from bolt holes on aircraft in service. Empirical measurements were used to estimate the effect of hole quality, and to add this back into the POD estimate from manufactured specimens. In this particular example, the a90/95 value changed from and estimate of 0.011'' to 0.067'', a very significant difference. This may be overly conservative, due to effects counted both in the original manufactured coupons and the uncracked aircraft holes. For example, probe tilt, instrument noise, and material variability are all present in both. Models could be used as shown in reference to estimate the magnitude of these effects and subtract them out of this answer to be less conservative. There has not been to date a study of the POD on naturally occurring fatigue cracks in aircraft components corresponding to the example data shown here, so the accuracy of the original versus the corrected POD estimates is unknown. Models can also be used for this purpose, for example, reference [2] shows the use of models to estimate the variability in a surface ET inspection due to varying probe tilt as might be expected from manual scanning. The geometry due to hole quality could also be simulated, and effects estimated from the model results. Using the statistical model for POD from the USAF MIL-HDBK-1823 allows the decomposition of NDT system performance into individual components. Thus these components can be modelled or measured empirically, and combined into a POD estimate.
Build your own POD
Konstruiere deine eigene Fehlernachweiswahrscheinlichkeit
2009
8 Seiten, 3 Bilder, 11 Quellen
Conference paper
Storage medium
English
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