Yer doc, this is an interesting thread for sure.
I’m feeling stoked, because i’m studying engineering at the moment, and i’ve been chained to the desk reading all about materials science for the last few weeks for upcoming exams. It’s exciting to have a link between surfing and engineering to talk about, well more exciting than other engineering babble.
I’ve got some points, facts and figures to add from my books that can help out. What i say may not be right, but hopefully the examiners dont know that
the board flexes a good couple inches – those inches are all attributable to the core being compressed rather than the skins actually being shortened or elongated?
Flex is very complex is composites.
Percentage elongation (%EL) is probably a good thing to start with when talking about flex. Basically its about how much the material stretches before it fractures. The elongation (and also the force sustained) is greater along the longitidinal direction (length wise) compared to the transverse (width wise) direction.
The %EL for a composite of E-GLASS AND EPOXY is around 2 - 3 % (longitudinal) . and the %EL for the same composite is 0.4% (transverse)
So lets say we had a 6ft epoxy and e-glass board, this is about 1800 mm board lengthwise and 20" wide so 508mm wide. The one thing that is very difficult is assume how the load is applied. There is the loading from the board on the wave face (millions of combinations) and the point impact loading of the riders feet etc. Stretch or elongation of the board would be a combination of transverse and longitudinal depending on the forces.
Anyways, we can just imagine some constant uniform force along the entire board, but the board will stretch in both directions.
So with 2-3% elongation longitudinally, this relates to the length changing from 1800 to 1845. Thats 45mm increase lengthwise.
And with 0.4% elongation transversly, this gives a change from 508mm to 510mm. Only a change of 2mm
So from this you can see that the flex will come from extending the board lengthwise, but this will create a compressive force on the core inside. Using the belt as an example again, if you fold the belt in half, and start pulling at either end (extending the belt lengthwise) then the two halves pull together and meet in the middle, place someones arm in there and pull, and they will feel a compressive force. Its interesting to note, that this compressive force they feel will be significantly less than the force you are pulling the belt with.
The fatter the arm, the more compressive force they will feel, but if you change it to just having a few fingers in there, the height of arm(fingers) is really small, and so most of the tensile forces stretching the belt, or surfboard, stay in tension, as they are acting in a pretty straight line.
I guess its easier explained in a diagram.
The green arrows will be providing more force as compared to the yellow arrows with the red arrows being the tension forces.
So the thicker the board, the more the core will be compressed, but the core compression occurs due to stretch of the skin, so they are related in the fact that the stiffer or stronger the core, the less compression, and consequently the less stretch or elongation of the board.
So if the core could collapse (compress 100%) then all the flex would be taken by the skins… so i would say that the thickness, and compressive ability (or hardness) of the core is a big factor.
Its hard in topics like these because the real scenarios are so so much more complex, but hopefully the babble i put out there can be made sense of and some usefull ideas made.
Happy brain strainining