Hi Higgler,
In my experience at an aircraft engineering organisation, the smallest crack that we would typically ask inspectors to find would be a 1mm x 1 mm corner flaw in aluminium alloy that was typically 4 mm to 20 mm thick. These would usually be cracks emanating from bolt holes and an often-encountered problem was that the hypothetical crack would be situated underneath the bolt head, making detection difficult. We would usually assume that the crack shape for a 1 mm x 1 mm crack was quarter-circular.
We had no minimum crack size below which cracks were of no concern. In addition to structural design, my group was also responsible for analysis of in-service findings, and all detected cracks were a concern, regardless of their size. Under the right conditions, inspectors were able to find cracks significantly smaller than 1 mm x 1 mm, and we sweated over them just as much as larger findings.
As to what size was “very dangerous”, we calculated the critical crack size under limit-load conditions, ie - if the structure experienced a limit load condition, the crack would fast-fracture, leading to structural failure. I don’t know if you have an aviation background, so limit load is, in a nutshell, the maximum expected load that the airframe would ever see in its life, usually due to some abnormal operating condition, such as a 2.5 g manoeuvre, large gust, hard landing, or whatever. We were dealing with commercial aircraft - military aircraft have a much different loading environment.
In some cases we were operating in plane strain conditions, in other cases we were in the plane stress regime, so critical crack sizes ranged from, typically, 10 mm from the bore of the hole to 150 mm tip-to-tip. I emphasise that these are just typical critical crack sizes, not an absolute envelope. If the critical crack size was not a through-crack, our analyses would predict the crack growth rates along the bore of the hole, and growth rates along the surface of the material. These crack rates would be different, so you could end up with, say a 15 mm (bore) x 10 mm (surface) crack from an assumed 1.27 mm x 1.27 mm initial flaw, and we would assume that the profile of the crack tip was elliptical between the bore crack and the surface crack. Hope that makes sense. Too difficult to draw a diagram in ASCII!
I can’t comment on non-metallic structure. While we did analyse plastics from time to time, our core competence was analysis of aluminium.
Your proposal for detecting cracks of 0.005” size, and maybe even and order of magnitude smaller is interesting. These detectable crack sizes are surprisingly small for aircraft inspection and I am sure that bringing this capability to in-service aircraft would provoke some discussion in the industry. Some people would love it. Others would hate it.
I am aware of cases in other organisations where, to keep the aircraft in operation rather than stuck in a hangar for repairs, they would not want to do anything about a crack less than, say, 7mm long, so they would deliberately calibrate their inspection methods such that cracks less than 7 mm long would not be found, despite the fact that the inspection technique could easily find much smaller cracks with appropriate calibration.
Without going into the details, there are situations where this is a sound engineering philosphy, but at other times, it is analogous to simply “looking the other way” and those employing such an approach will not want a technique that can find 0.005” cracks or smaller.
There is also the issue of discriminating between true cracks and simple scratching of the material as a result of bolt installation, etc, but I am sure that your discussions with others have highlighted this and more. I can see an immediate application to better understanding of the benefits of manufacturing techniques such as cold expansion of holes.
I hope that this is useful.
Good luck! I would love to be able to ask our technical publications people to write up a technique that can find a 0.005” crack.
