Strength of single vs double layer

[Dorfie]

I've been wondering if there is much of a difference in strength between, lets say single 0.060 alum and two layers of 0.030 pieces riveted together.
Does a doubler make a structure as strong (or stronger) than single piece of same thickness (combined thickness of doubler and base material).
Thanks.
Johan

[Aiki_Aviator]

It definitely makes the outcome much stronger/stiffer in terms of ability to bend. It will of course depend on the amount of rivets as to how much that will be and what its cycle time is etc...

The riveted pieces complement each other with compression and tension. If the top layer is under tension, the bottom layer is under compression. A single piece of material will do this at its top and bottom layers but it will have effectively a place that the material can "work" its way from one area to the other.

With two single sheets this does not happen in the same way.

Of course, this question has another question behind it and you need to look at the application of this approach as to whether it is correct or not. Some parts need the ability to move, others less so.

[bret]

The bending moment would be much stronger in two ply to where you would have one in tension and one in compression with four walls.

[vfrazier]

Now to start a really interesting discussion....

Anyone care to touch the 3rd rail and compare the laminated RV-4 or 6 spar to the machined bars on the 7, 8, etc. ?

Not that it matters. They are all well proven at this point.

[krwalsh]

Originally Posted by Dorfie

Does a doubler make a structure as strong (or stronger) than single piece of same thickness (combined thickness of doubler and base material).

Beam theory would conclude that unless a riveted doubler resists shear exactly the same as if the two pieces were completely mechanically joined, then the doubled piece is not as stiff (or strong) as the single piece. Think of it this way, the stiffness of a cantilevered beam is:

d=(F*L^3)/(3*E*bh^3/12)

d=deflection
F=Force
E=modulus of the material
b=width of the beam
h=height of the beam

So, you can see that for a given force and length of a beam, the stiffness goes with the width of the beam, and the depth cubed. So absent the rivets between the two layers, the single thickness is much stiffer. Two layers that can slide are basically only twice as stiff as one layer. The thicker layer is 8 times as stiff as the thinner section, 4 times stiffer than the two layers stacked with no rivets. The rivets add the ability to carry shear between the two, but at a maximum you can approach the stiffness of the thicker section in the first place.

Now, there are a LOT of reasons to laminate the layers separate from this argument (lighter structure for a spar when the loads at the tips diminish, ability to withstand damage, easier to build, costs less to manufacture, etc.)

[David Paule]

To replicate the bending strength you'd need enough rivets to replace most of the shear stiffness of a solid piece, not easy. That also applies to using this riveted assembly in an area which could buckle - in all probability, the resulting buckling strength will be lower.

And you'd be introducing an additional failure mode, interrivet buckling, that didn't exist before.

If buckling isn't an issue you'd have a part that would still be weaker for in-plane loads, due to the additional rivet holes.

Bottom line, don't do it unless it's for original design and you've done the analysis.

Dave

[Dorfie]

Thanks for all the replies. Gets too complex for me!
The question was prompted by wanting to replace the bottom cowl hinge on RV 10 with 0.063 alum, about 6 x 2". See post 10 onwards: http://www.vansairforce.com/communit...ad.php?t=76004
Problem was I don't have 0.063. So I was thinking to make a double layered piece from what I have to at least the 0.063 thickness.
I think it will work.
Thanks to all.
Johan