NASA uses special dye penetrant nondestructive evaluation (NDE) process to provide reliable detection of very small cracks. Typically, the surface crack length sizes are 0.030” and 0.050” for special dye penetrant process. Qualification requires demonstration of crack detection on a set of cracks with average crack size smaller than or equal to the qualification crack size. The demonstration is called point estimate demonstration. A set of corner cracks can be used to determine reliably detectable corner crack size using the point estimate demonstration method. However, dye penetrant demonstration on surface cracks can be used to assess reliably detectable corner crack sizes by using similarity in the penetrant processes between the two cases. The paper provides similarity analysis engineering approaches for determining the reliably detectable corner crack sizes for given a point estimate demonstrated surface crack size. Three approaches are given. These are: A. surface length equivalency, B. crack face area equivalency, and C. projected length equivalency. Corner fillet radius and accessibility to perform dye penetrant process on both sides of crack are two important considerations in choosing between the approaches. These approaches are recommended to be used only when corner crack demonstration data is not available and responsible engineering board accepts such assessment.
Ajay M. Koshti, "Assessment of dye penetrant crack detectability in external corners using similarity analysis," Proc. SPIE 10599, Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, Civil Infrastructure, and Transportation XII, 105991L (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 08, 2018; Published: 27 March 2018); https://doi.org/10.1117/12.2286788.
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