Mapping reliably detectable dye penetrant crack size at external corners with fillet radii

Typically, reliably detectable crack size for dye penetrant testing is determined by using test specimens with known size fatigue cracks. Certain surface crack size was qualified in original Standard dye penetrant Probability of Detection (POD) studies. The flaw size was qualified to a minimum 90% POD with 95% confidence (conf.). No corner crack size was qualified by direct POD testing. Typically, the corner and other crack sizes were established based on crack face area equivalency analysis. However, there is no supporting empirical data for crack face area equivalency. The volume of reservoir between crack faces available for dye penetrant to seep in and then bleed back after removal of excess surface dye penetrant is an important factor is forming a dye penetrant indication. The volume available for dye penetrant to seep into a crack would be proportional to the crack face area for cracks. Therefore, the approach has some merit, but other factors such as aspect ratio of crack length-to-depth must be considered. The paper provides an approach to interpret POD demonstrated surface crack size to a corner crack on a corner with any radius based on consideration for crack face area, crack chord equivalency and minimum depth. The reliably detectable flaw sizes for dye penetrant testing are used for fracture mechanics damage tolerance analysis. The paper provides an analysis approach to qualify the reliably detectable dye penetrant corner crack size based on POD demonstrated surface crack size.