Quasi-isotropic test directions 1

[TheBigM]

If using a standard (+45,0,-45,90)3$ lay up for a QI test specimen (made from UD prepreg), what difference in compressive strength could I expect to see (if any) if I cut some of my test specimens so that the centre plies are at 0° and the others so that the centre plies are at 90°? If there is a difference, why?

 

[SWComposites]

Yes, in general there will be a difference. There are three general reasons:
- edge effects: some stacking sequences are more prone to edge delaminations, which can reduce compression strength
- the support to the load carrying fibers (0° plies relative to the load direction) is provided by the plies outside of the outer 0° ply; compression strength is lower for layups with 0° plies on the outside of the laminate relative to layups with the 0° plies covered up by plies with other fiber angles.
- there can be test induced differences, as layups with lower bending stiffness (0° plies towards the laminate center) can bend/buckle at lower loads, thereby resulting is lower apparent compression strength.

Also, as multiple 0° plies are grouped together, compression strength tends to decrease.

 

[TheBigM]

Thanks SW,

further to this, which configuration will give the highest compressive strength?

(+45,0,-45,90,+45,0,-45,90,90,-45,0,+45,90,-45,0,+45)
OR
(-45,90,+45,0,-45,90,+45,0,0,+45,90,-45,0,+45,90,-45)


Thanks.

 

[SWComposites]

If I had to bet a quarter, probably:

(-45,90,+45,0,-45,90,+45,0,0,+45,90,-45,0,+45,90,-45)

assuming that this is for a typical carbon fiber/epoxy unidirectional tape material tested at room temp, and assuming that there are no testing issues (grips, support, alignment, etc).

Then again, intuition with CFRP is often wrong.

Which test method/standard are you using?

 

[TheBigM]

It's actually an open hole compression test. I suspected that the orientation would not have a dramatic effect at either 0 or 90 but a colleague disagrees.

I had thought that the isotropy was the same and so any edge effects etc would be too small to be detectable. After all you have the same number of plies in each direction, my only concern is that having the 90° plies so close to the outer surface might accentuate the edge effect.

 

[SWComposites]

This is a common misconception: "isotropy" related to a laminate only applies to stiffness, not strength.

There is typically less difference with OHC than unnotched compression strength relative to the loading direction.