The photos are interesting, but I am curious about the how structure is made. Is there a block to separate the skins, and the fasteners pass through the block and both sides? Also what is the yellow material on the pictures labelled crack 2006 and crack 3? Is it primer or adhesive?
I also note the "stop" drills on German crack 2007. Someone actually flew this after stop drilling?
My personal thoughts are that stop drilling is a very effective means of determining crack growth rates. If you record the dates and hours when the drill holes were made, you can estimate the growth rate by measuing between the repeated drill holes. (I am being fasetious; I have yet to see an exampe where a stop drill actually worked!
)