Wednesday, April 6, 2011

Where to remove glass from ?

I've started designing board number 2.

One of the issues with the previous board was that it was very stiff and so I've been exploring the best approach to improving the flex. The most promising things that I've been considering are

i) reducing the amount of glass on one or both sides

ii) changing the core thickness.

ii) using different reinforcing material (s-glass, biaxial.... carbons still to expensive) so that less can be used

iv) changing the amount of concave as more concave stiffens the board.

v) changing the core material

Each of these has pros and cons and the issue of flexural strength is obviously the big issue to consider when changing things around. There is a detailed treatment of sandwich structures in the DIAB Sandwich handbook that you download from the literature section at

Ignoring and assuming a lot, flexural rigidity of a laminated core material is proportional to the core thickness cubed(!) but only linearly dependant on the elastic modulus of the material and the thickness of the laminate.

where Ef and Ec are the elastic moduli of the glass/resin (faces) and the core respectively.

This suggests that thinning the core is likely to have a bigger impact on the flex of the board than reducing the amount of glass. However, if you are trying to thin the board by hand this also means that uneven thinning of the core, even very small amounts, may have the affect of concentrating the strain at these points an introducing weaknesses.

To put some numbers around it reducing a 10mm core by 10% or 1mm reduces the flexural rigidity by 28% (i.e. 1-0.9^3) and similarly increasing the thickness by 10 % results in 28% stiffer board.

Other reasons (perhaps) against reducing the amount of glass is that glass/resin have very different strength under compression compared to tension. It is much weaker in compression and often fails by buckling around local imperfections such as wrinkles in the fibreglass, misaligned fibers. The chart below gives a comparison of the strength glass/resin laminates. No ideas what the units are but the more important thing is the relative strength under compress vs tension.

One implication is that areas that are under compression need more not less laminate. Given that the section between you feet on the underside of the board will be in compression when you land a big jump but the underside of the tips will be in tension as the tips flex up, using this idea of reinforcing compressed section would lead to a real patchwork of glass. My current thinking is that keeping it simple by using the same amount of glass which seems to be handling the loads well but thinning the core by say 10% might be the simplest and safest approach.

Using other reinforcement materials also offers some interesting possibilities. S-glass instead of e-glass. S-glass is stronger due to different compounds in the glass and you can use less of it and resin (about 10% less resin seems to be the going estimates). There are lots of different opinions on the net about just how much strong and stiffer it is but the estimates range from 10% to 30% but these figures are apparently not often reached because the layup technique and issues introduce weakness that mean the theoretical maximum is not achieved. People are unanimous in say that it is a waist of time using s-glass with polyester resin as polyester does not stretch enough to take advantage of the better characteristics of s-glass. S-glass is denser than e-glass so you loose some of the weight advantage of being able to use less.

One thing to bear in mind with s-glass is that its strain to failure ( how much it stretches before it breaks) is about 4.5%. Apparently most polyester resins strain to failure is around 2%. So using s-glass with polyester resin is a waist as the resin will fail before the more extra flex capability of the s-glass is reached.

Biaxial glass is not woven like e-glass and s-glass where there strands of glass go under and over each other. This under and over means that only the under or the over part of the fibre is pt under tension when you bend the laminate not both at the same time. This means that the woven mat is more flexible (less fibres resisting it bending) but not as strong as biaxial glass which does not have a weave. The fibres are just layed down (stitched) in the same plane and then a second layer placed of the top at 90 degrees. then used in laminates the entire fibre laying along the axis being bent are placed under tension and so resists the flex and also making the material a lot stronger in tension and compression as well.

S-glass and biaxial are more expensive - that is the trade off. To get the amount of right seems like it will take a bit of trial error so I am going to keep to the e-glass/ epoxy for the next board and try to control the thickness to get the desire flex.

Concave, stiffens a board. To see this try bend a playing card and then try bending when it folded over into and semi-circle. Just how much 5-10mm of concave will stiffen a board ? Dunno. But it should be possible to get some feel for it theoretically.

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