Ply Moment
Ply Moment
(OP)
I need help from someone here. Let's say i have a multi-ply laminate subject to plane stress that is loaded in some way (Nx, Ny, Nxy, Mx, My, Mxy). For one particular ply that I'm Interested in, i happen to know the global stresses at the top and bottom of the ply as well as its thickness. I also know the thicknesses of all the other plies. I would like to try and find out how much moment (let's say Mx) is carried by this one ply. I know i need to integrate the stress distibution across the ply, but can anyone provide me some specifics on how to do this? thanks...msn





RE: Ply Moment
Classical Laminate Theory usually considers the moment in the total laminate and then based on strain compatibility, you can deduce in-plane stress/strain of a ply. I have never seen a ply moment used for anything (off the top of my head). The thinking is that for practical applications, it is not relevant.
That said, I think your answer is to just use sigma=P/A + Mc/I of the individual ply. You would subtract the axial component and be left with the stresses that cause bending (i.e. a zero delta top and bottom stress indicates no bending). From there, back calculate out the moment (I=(1/12)bt^3) and c=t/2.
Brian
www.espcomposites.com
RE: Ply Moment
RE: Ply Moment
Consider this, the strain distribution through the thickness of a laminate is linear (for typical laminates). Stress is not, due to varying modulii. At each ply, you also have a linear distribution of strain. A single ply has a single modulus in a given direction (through the thickness). Therefore, if you already have the ply outer fiber stresses you can back calculate the equivalent ply moment. Perhaps you can try a lamination program to verify this, but it should work ( I have not tried myself).
But again, Is there a practical reason or is it just to observe/understand something better?
Brian
www.espcomposites.com
RE: Ply Moment
Perhaps you think you need to integrate as one might when calculating the [D] matrix? However, don't forget that by the time you have the ply stresses, that has already been done. In other words, all the "work" has been done for you already.
The moment that actually remains in the ply should be relatively low, by design, and the overall moment is largely reacted by the plies farthest from the neutral axis. Think of the caps of an I-beam. The cap axial loads react the moment, with very little moment in the cap itself.
Hopefully that makes more sense now. If not, perhaps you can give an actual problem. There are quite a few resources about CLT as well. Have a look at the [D] matrix formulation process and how the integral is used. But remember that is different than the question you are asking. You asking about individual ply moments (as I understand) as opposed to overall moments.
Brian
www.espcomposites.com
RE: Ply Moment