substitute aluminum for 4130?

Earlier, Robert Loer wrote:
Plans call for wing spar to fuselage carry through attach fiitings to be
7/8x1/8 4130 steel. Advisability of using 9/16x 3/16 6061-T6 aluminum as I
have it on hand.

Your thoughts please. I have plans for another ship which uses the 6061-T6.

Thanks

**** Disclaimer: I'm not an engineer. This is not engineering advice.
This is stuff anyone with high-school physics oughta grasp. /disclaimer
****

Executive summary: normalized 4130 steel is good for 90000 psi in
tension. 6061-T6 aluminum is good for 42000 psi in tension. Do the
math.

Just for grins, let's take a swag at the proposed substitution.

For the sake of this swag, let's start by guessing that the fitting is
a simple tang loaded longitudinally through a 1/4" bolt in a hold
drilled through the middle of the metal strap.

The cross-sectional area of the steel strap is (7/8)/8=0.109in^2. The
hole subtracts (1/4)/8=0.031in^2, so the area at the hole station is
0.078in^2. If we say that the ultimate tensile strength of the steel is
90000 psi (90 ksi), that drilled strap might be good for a longitudinal
tension of 90000*.078=7030 pounds force (lbf).

For the proposed aluminum substitution, the cross-section area of the
strap is 9*3/(16^2)=0.105". The hole subtracts (3/16)/4=0.047in^2,
leaving a cross-sectional area at the hole of 0.058in^2.

If we wanted to, we could stop right there. Since the aluminum strap
has less cross-sectional area at the hole than the steel strap, and
since aluminum is generally softer, weaker, and more elastic than
steel, and has less forgiving fatigue properties, we know that the
proposed substitution cannot be as strong in simple tension as the
steel original. That doesn't mean that it isn't strong enough, but we
have way too little information to figure out exactly what "strong
enough" is. All we have to go on is "as strong as the original design,"
so that has to be our guide in this.

However, just for grins let's look at how much weaker:

The proposed substitution has a cross-section at the hole of 0.058in^2.
If we say that the ultimate tensile strength of 6061-T6 is 42 ksi, that
means that our drilled strap might support 42000*.058=2440 lbf. That's
only about a third of what the original will do in simple tension.

Oh, and it gets worse, much worse. Since we're talking about a major
structural attach fitting, there is no guarantee it is only loaded in
simple tension or compression. In fact, it is almost certain that it is
loaded in bending as well. Bending strength and stiffness are related
to the cube of the depth times the width.

I could go on and on about this, but I think we've already made as many
guesses and suppositions as we ought. Without specific details of the
design and how it is loaded we can do no more meaningful analysis. In
fact, the analysis I suggest above is highly suspect without knowing
how the strap is installed and secured.

To wrap this up, I'll not say that you should never consider such
substitutions. After all, homebuilt aircraft are all about making what
you need out of what you can get.

However, it oughta stand to reason that when you substitute weak stuff
for strong stuff, you're going to need a lot more of the weak stuff.
Trying to trade straight across isn't ususally a good idea, and trying
to get away with even less of the weak stuff than that is probably a
bad idea.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24
http://www.hpaircraft.com/glidair