Is there any reason not to run the grain of a wood core parallel to the CF? For example a simple 2 inch square beam? I know balsa core normally has the grain perpendicular to the CF faces that are bonded to it.
I would be looking for maximum stiffness not maximum strength.
Thank you.
Scott
Theoretically, it doesn’t matter. There is compression and tension through the balsa shear web at approximately 45* to the longitudinal axis. Vertical grained shear webs are often a little stronger but only after there has been an initial failure within the balsa shear web. I’ve done several rounds of testing this and found that that vertical grain shear webs are 3 to 4% stronger. Stiffness is nearly identical.
Vertical grain balsa shear webs typically ends up heavier since more adhesive is needed to bond the end grain to the carbon spar cap.
The best thing to do is wrap the whole thing with some light bias-cut fiberglass so the fibers are at 45* to the longitudinal axis. This puts fibers in line with forces within the shear web. The wrap also prevents the carbon caps/flanges from buckling away from balsa shear web. Very little adhesive is needed between the balsa and the carbon.
If the beam is made in a press you have to be careful to not crush any horizontal grain shear webs. This will drastically reduce its strength.
Look at Banova balsa core:
Thank you for your thorough response Wyowindworks. It is greatly appreciated!.
I have to disagree that the grain direction does not matter especially for balsa core, http://www.gurit.com/files/documents/balsaflexv3pdf.pdf
The shear/compression strength and modulus values are much higher in the grain direction vs the transverse direction and thus the reason for everyone making end grain balsa core.
If your ultimate goal is to have a sandwich structure needing a core with a high resistance to crush i.e. high compressive strength, don’t use balsa. Much lighter and stronger core materials out there.
That is true if the forces are direct shear. When they are between two flanges the forces in the web are roughly at 45* to the length of the beam. You have tension in one direction and compression in the other. When you look at the failure of glass wrapped balsa cored spars the fracture lines are at 45* to the length of the beam. I’ll post pictures later.
Vertical grain cores are often used because the compression strength is better when impacted, walked on, etc.
Thanks WW for the thorough responses. With that in mind, if I’m making something like a front end splitter on a car with a fiberglass/balsa/fiberglass structure, I could get away with using a standard sheet of balsa and not end grain? It seems to me that having the grains go fore to aft (or front to back on the car) would be beneficial to the bending.
The core, being in the middle of the laminate, is subjected to almost pure shear forces particularly at the neutral axis. I completely agree that if you have pure shear loading in the end grain direction this leads to tension/compression at 45 degrees to the grain (stress transformation).
However the original question was, Is there a reason not to run the grain parallel to the CF? The answer is yes. The higher compression strength is needed in the directions the loads will be applied which is typically to the top or bottom of the beam.
For a splitter you could probably get away with long grain balsa if you are not going to bolt through the core. However, I don’t think you will see much benefit in bending since the modulus values for the carbon or glass fibers in the skins are much, much higher.
Please elaborate?
I am surprised there is not more reaction on the Banova bidirectional balsa picture in this thread.
The are a lot of variables in a structure. Any structure is only as strong as it weakest component. Increasing the strength of the shear web only helps if that is the weakest part. If the carbon flanges fail/buckle before the shear web reaches failure then the grain alignment is mout. How large do the flanges have to force be to force a balsa web of either orientation to fail? How do you prevent the caps from buckling away or towards the balsa web? How do you economically fabricate a beam that meets the objectives? Creating a Web that exceeds the strength requirements is an waste of time.
In my experience, when it comes to beams, wrapping the entire beam with light glass will do signicantly more for you than fussing with the grain orientation. This prevents thinner flanges from buckling. Flange buckling is a common failure mode. As the flanges become thicker they are less prone to buckle. They also can carry a greater load and require a stronger shear web.