plate dimension 100*100mm, material graphite/epoxy, stacking sequence (0/90/0/90/0/90/0) ply thickness=1/7, total thickness of laminate=1mm , applied stress=1MPa,
but after solving the problem stresses in 0 deg. ply are 1.7MPa. how this is possible?
It is possible as only the force is equivalent. But because of heterogeneity, 0 deg ply will take more load while 90 ply takes less load. You might want to try SwiftComp, a general-purpose composite modeling software freely accessible in the cloud at https://cdmhub.org/resources/scstandard.
https://cdmhub.org/resources/scstandard
Is it a in plane stress applied? If so, since the material is composite, it is quite possible to get more stress than applied.
It comes down to how you've applied the traction at the boundary:
1. If you've applied it as a load per unit length (1 MPa *1mm):
Since the laminate is symmetric, (there is no bend-stretch coupling) you would see this load redistribute itself amongst the individual laminae based on their stiffnesses (as Prof. Yu describes). The total internal stress resultant = 1.65MPa*4/7 + 0.133MPa*3/7 =~ 1 will equal the total applied load per unit length = 1MPa*1. This is of course assuming that the stress is uniform within each lamina depth (as I presume it would be). I get these stress values from the graph you've attached.
A quick check for this is to plot the strain profile through the thickness: it should be constant through the depth. This is similar to the case of having 7 springs in parallel attached to a rigid bar onto which is applied a load of 1MPa*1mm*100mm. This load would simply redistribute amongst the 7 springs such that each spring extends the same amount (because of symmetry of the structure)
The 1 MPa that you talk about at the edge would be an "average" representation of the stress through the laminate. It's similar to replacing the laminate with a single layer and asking what the stress would be in that layer (for a statically equivalent case).
The crux is this: For equilibrium, the net internal FORCE must equal the external force. (NOT stresses).
Another check is the Q11 values of the 0 and 90 deg layers. Since, the strains are constant through the depth, the stresses in each layer would simply be Q11*strain. So the ratio of stresses in the 0 and 90 deg layers would be equal to the ratio of Q11 in each layer.
2. You've applied the traction as an "outward pressure" (Force/area) :
In this case, you've applied a traction of 1MPa on each lamina, so the internal stresses must equal the applied loads at the interface.
I doubt this is the case. I have a feeling you've modeled the laminate as a single shell (probably the mid-plane) and provided the material properties of each layer as a "section assignment". So you might have specified the traction as a load/length on the edge, corresponding to case 1 above.
Hope this helps!
- Badges
- Science topic