Calculate the strength of several polycarbonate sheets laminated together

[Plat]

Based on dimensions and the materials' Young's modulus, I know how to calculate the expected deflection of a rectangular sheet of material under a given distributed or point load and given various edge support conditions.

I have discovered however that multiple thinner sheets when bonded together are stronger than a single, thicker sheet of the same total thickness.

I am curious as to:

1. Why and how this is possible
2. How the strength of such multiple-sheet laminations may be calculated in terms of deflection under load assuming no bonding failure?

 

[Mech_Engineer]

What kind of adhesive are you using, and how does its strength compare to that of the polycarbonate?

 

[Plat]

I have been using simple lacquer thinner to wet the surface of one sheet and then press another against it.

My understanding is that this solvent causes the polycarbonate from each piece to fuse together, leaving no foreign material or adhesive.

I have done deflection testing with a 20cm beam simply supported on each end, and applying three different masses to the center and measuring the downward deflection each produces with a micrometer. This deflection decreases exponentially (somewhere between power of 2.5 and 4.5) with each additional layer added.

With 4 layers of 2mm each, the 20cm inch beam can easily support at least 110kg in the center without permanent deformation.