What Is The Smallest Crack You Can Find
The proof of the pudding that this newsletter is driven by its readers is much more obvious to us than perhaps to the readers. But a sterling
example has just occurred, as a reaction to a recent article in the May 2014 PENETRANT PROFESSOR.The reader wrote as follows:
“There are many like me who have been long time readers and benefit from the wisdom of the professor so I thought I would pass a bit of
info regarding POD and NASA to you in the event your readers need to deal with how small a crack you can find. NASA STD 5009 contains a table of flaw sizes that can be found with the “standard” penetrant process as defined in the standard. I believe the standard is in the public domain and could be passed along as reference material for the inquisitive test lab.”
This note prompted us to immediately GoogleTM NASA STD 5009, which we found extremely interesting. It is basically a tutorial, but it
lists the sizes of the smallest defects that can be found using various NDT methods. However, there are cautionary notes, as an experienced inspector might expect. The following quotes are from the document:
“Nondestructive inspections of fracture critical hardware are required to detect the initial crack sizes used in the damage tolerance fracture analyses with a reliability of 90/95 (90% probability of detection at a 95% confidence level). The minimum detectable crack sizes for the Standard NDE methods shown in Table 1 (U. S. Customary Units) or Table 2 (Syteme International (SI) Units) meet the 90/95 capability
requirement. The crack size data in Table 1 or 2 are based principally on an NDE capability study that was conducted on flat, fatigue cracked 2219-T87 aluminum panels early in the Space Shuttle program. Although many other similar capability studies and tests have been conducted since, none have universal application, neither individually nor in combination. Conducting an ideal NDE capability demonstration where all of the variables are tested is obviously unmanageable and impractical.
The components geometry features, such as sharp radii, fillets, recesses, surface finishes, and cleanliness, material selection, and other conditions, can influence the capability of the applied Standard NDE method. When this occurs, the method shall be evaluated to ensure that the Standard NDE detection capability is not reduced (see section 4.2.3). The conditions and their evaluation shall be documented in the NDE Summary Report.”
As one might expect, there is no clear cut answer to the question of how small a defect can be found. There are data that describe what
has been found on flat aluminum panels, but relating those findings to other situations and materials is not a simple task. The NASA standard recognizes this and is clear that different situations must be evaluated to ensure the capability of locating defects.