I've been pondering this for a while and figured I'll ask. Does anyone have any idea how the crack would have developed without the lightening holes?
It seems like the crack would have occurred anyway. If so, wouldn't it eventually have the same result, even without the holes?
Or did the holes increase the likelihood of the crack starting?
Having an education in this as well as failure analysis back ground.
Short answer, Yes the drilled screw holes where they were are a problem.
Personally I never liked the screws in the spar caps. At least these spar caps. Wrong thing to do to a high stress region, but these do not rear their head often. To me the fastener should be into the web of the spar, or the cap increased to accept the disrupted load path. But that is generally just not truly necessary.
This area of the rear spar is one of the highest loaded sections of a Cub and has required reinforcement when utilized at higher gross weights. This has been known for a long time and is accepted practice.
Now to me, someone comes along and wants to build a light Cub, and with little to no experience in material properties or stress analysis decides to lighten one of the highest stress regions of the structure.
He lightened a region the factory adds reinforcement too.
In this case, having a screw hole lined up with the greatly reduced cross section of the spar is a guarantee of failure. Had the screws been in the region of full web, between the added holes, maybe it would not have failed at the loading it did. Or as soon as it did in the count of stress cycles.
In engineering logic, if you are going to remove material from the spar web, do so in reduced load regions, particularly bending loads. These would be outer span of the structure and with smaller holes on the inner span, but just not in the region of the highest load, the strut attachments.
And the case in hand here, do not add stress risers in the weakest spots.
And yes, I expect these spars would have failed in time.