Rivet strength tests

24 May 2005 Rivet strength tests

From Bill Marvel:

Two days ago I got around to doing something that I had planned last
year — actual pull tests on riveted aluminum coupons to see how
critical it is to drive rivets to the correct height. All of us
building or with completed RVs (as will those those planning on it in
the future) have had to wonder which imperfect rivets to drill out and
which are OK. The answer is obvious when there is a severe cosmetic
problem, but when strength is at issue, how much does a slightly under
or overdriven rivet affect strength? How much does a grossly under or
overdriven rivet affect it? Frankly, I had made the decision that the
risk of damage from drilling out a flush rivet is greater than the
benefit of doing so, unless an obvious cosmetic defect or really bad
rivet is at issue. Now I have some hard data to go by.

What I did was to make up 10 test coupons. Each of these consisted of
two pieces of .032 2024-T3 sheet 1.5 inches wide and 4 inches long.
These two pieces were overlapped by 1.5 inches and riveted together with
two parallel rows of 3 rivets each. Of the 10 total coupons, five
involved the use of universal head AN 470 AD3 rivets and the other 5
used AN 426 AD3 flush rivets. In the latter case, both pieces of
aluminum were dimpled at each rivet location, as is routinely done in
Van’s airplanes. In fact, the coupon construction is similar to the
double rivet line where the lower outboard wing skin overlaps the lower
inboard wing skin. This joint is loaded in tension normally for
positive G flight and gave me the idea to mimic it for the pull tests.

Before getting into the results, let me ask you a question. Please
think about the answer before proceeding. Just how many pounds of force
do you think it would take to destroy one of the sheets used in making
up the coupons? Remember this is .032, 2024-T3 sheet 4 inches long and
1.5 inches wide with no holes or rivets in it. Think about grabbing and
suspending it at one end with some sort of clamp across the entire 1.5
inch width and then hanging weights on the other end from another clamp.
How much weight would it take to break this .032 inch thick sheet?
Would a 100 pound set of barbells do it? A 500 pound set? A 1200 pound
small car? A gross weight RV8 at 1800 pounds? A gross weight Grumman
Tiger at 2400 pounds? More than that? Come up with some sort of gut
feel before proceeding. I was surprised by the answer. You may or may
not be, depending on your knowledge in this area.

Since some of you will cheat and read on, I’ll hold the answer for a
moment! Each of the 5 test coupons, both with the universal head rivets
and the flush head rivets, was riveted to a different degree. One was
grossly under driven, one was slightly under driven, one was correct per
the rivet gauge, one was slightly over driven and the last was grossly
over driven. The slightly under driven and slightly over driven rivets
were such that you would probably need a rivet gauge to detect them — I
did this because I suspect that most of the rivets in our planes fall
into this category. The grossly over and under driven rivets were
really gross. The over driven were squashed nearly flat and the under
driven were barely set at all. I did this to see just how poorly a
joint make of this sort of gross error would hold up. You would easily
see these and know there was a problem immediately. You’ll find the
results interesting………

The idea was to put each coupon in a pull test machine and expose the
riveted joint to a slowly increasing force until it yielded. This was
done at a structural test lab in Paramount (Southern CA city) that works
mostly with civil engineering construction materials. A stress/strain
graph was running and we monitored it to see the first indication of
joint failure as indicated by a decrease in force required as the coupon
stretched, cracked, broke in two, sheared or tipped rivets, etc. I was
interested in the force required to cause the initial failure, as well
as the nature and appearance of that initial failure; ie, what actually
happened first. We agreed to stop the machine at the incipient
indication of failure, thus preserving the coupon in its early failure
state without destroying the joint completely. I was very curious as to
how things would fail and really had no idea other than the thought that
the dimpled, flush riveted joint would probably be stronger than the
undimpled one with the 470 universal head rivets. In contrast, one of
the owners of the lab came in to watch and thought the opposite would be
true. In his 50 years in the business, he had never seen this test
done. What do you think would hold best?

That said, here is the answer to my prior question. A force of 2300
pounds was required to break the test material with no rivets or holes
in it. It failed catastrophically shortly after some initial stretching
was noted. I had no idea that a cross section of this 2024 T3 sheet,
.032 inches thick and 1.5 inches wide, would sustain anywhere near that
load. Frankly, I was surprised when it passed 1000 pounds and still
going strong.

Before showing you the numbers, I will give a brief summary of them:

1. The dimpled, flush riveted construction was stronger, but not by as
much as I had thought. However, and this is really important, initial
failure of the dimpled construction was generally not catastrophic and
occurred as rivet tipping and rivet head distortion. In contrast,
initial failure of the AN 470 undimpled construction was generally
catastrophic by rivet shear. I am really happy Van uses the flush
riveted, double dimpled joints throughout most of the airplane!

2. Slightly under driving or slightly over driving a rivet makes an
observable and thus measurable difference in the joint strength.

3. Slightly over driving is stronger than slightly under driving and
results (in my opinion) in an insignificant difference in strength as
compared to properly driven rivets.

4. In the one test of slightly over driven AN 470 rivets, the joint was
actually stronger than with properly driven rivets. This may have just
been the luck of the draw for this single sample, so I wouldn’t put any
real faith in it.

5. A joint made of grossly over driven rivets is stronger joint than a
joint make of grossly under driven ones.

6. A grossly under driven AN 470 joint is much weaker than a grossly
under driven AN 426 joint.

7. No joint was as strong as the parent material itself.

To summarize the summary, try for properly driven rivets but realize
that minor over driving is preferable to minor under driving and results
in nearly the same strength as does the condition of properly driven
rivets.

AN 426 AD 3 Table

Condition Force at failure Nature of failure

Gross under 1650 Rivet tipping, head distortion

Slight under 1775 Same

Correct 2025 Same

Slight over 1975 Same

Gross over 1825 Sheet tear at rivet line

AN 470 AD 3 Table

Gross under 1100 Rivet tip plus one sheared rivet

Slight under 1600 5 sheared rivets!

Correct 1625 6 sheared rivets!

Slight over 1750 6 sheared rivets!

Gross over 1500 Rivet tip plus sheet tear at

rivet line

Anyway, those are some real numbers for an area we have undoubtedly
thought about at one time or another. My opinions, FWIW: I think an
occasional rivet that is slightly under driven or slightly over driven
is utterly no big deal and can safely be ignored. We all have some of
these flying in formation in our airplanes. A line of them would be
another matter. Even an occasional grossly over driven rivet is
probably OK, especially if getting rid of it could cause damage. And if
underdriven too much, just whack it again. Hope you learned something
from this. I certainly did.

Bill Marvel