rattlesnake wrote:
....
roughly the setup was like this:
- plane upright
- tail and elevator loaded by about 430 lbs of bricks
- ailerons and flaps loaded by about 290 lbs of bricks
- both wings supported by 1.5 ton car jacks
- engine pushed down by about 1.100 lbs at position of the four attachment
points

I got a bad feeling before the test, but I have only two choices in my
country:
1: do this silly static test
2: reject it and never receive the permit to fly

I think the lower attach points can be repaired. Probably solid 3/4" rond
bars will be welded into the remaining tubes. However I lost some of my
confidence in this flying machine because I don't know what (invisible)
secondary damage may have occurred.


Let me ask you this: how do feel about a tube that was loaded with 275
lb (if the loading was equal and local) that failed?
Pulling numbers out of the air, lets say the failing tube was 3/4 inch
diameter and the material was 30 ton steel - how thick would it be?
There's something evidently very wrong with my numbers, or your loading
conditions: the tube wall thickness would have been
(using 30 ton sq in = 60000 psi steel)
275 lb force = 60000 X pi X 0.75 X wall thickness
So wall thickness = 275 / (60000 X pi X 0.75 ) = about 2 thousandth
inch??
Certainly not! Perhaps it was light alloy tube rated at 20000 psi?
That leads to a wall thickness of 6 thousandth inch? Certainly not!

So maybe they loaded the engine itself though its centroid.
The tube did not fail in crushing, it looked like it failed in shear??
That's the weakest modulus - but not THAT weak - so I am missing
something about the geometry: a long long engine mount over a narrow
area bulkhead?? I just don't get it!

It should not be possible to weld up an engine mount WEAK enough to fail
at the load you mentioned.....
But inserting a solid rod into a thin tube is an unfavorable fix - the
stress concentration is inviting another failure just past the end of
the rod insert.....

Perhaps you might let someone look at the engine mount drawing. There's
something strange about it. At the very least, there was no post weld
heat treat ?? Critical structure should ALWAYS yield (if its metal)
not crack destructively.

Good luck

Brian W
p.s. If the designer was an absolute genius, and the materials all
produced exactly on specification, at the load test, if the engine mount
was just 2% stronger, then one or several other parts would have yielded
(but NEVER cracked) before the engine mount YIELDED.
You can take it for granted that nobody is that good!