Sunday, June 12, 2011

Obituary for Board #2

I gave board #2 a punishing workout at both the cable park in Cairns and on the choppy water at Port Douglas and I am very sad to report that it was not up to the challenge and cracked under the footpad after an out of control landing though not from any great height.

Although this very clearly sucks a great deal, it does give a great opportunity that I would not have otherwise been happy to do - destructive testing on a board to see where the faults are so that I can continue to improve it.

Analysis of the failure

The board failed catastrophically right under my trailing foot heal after a crash landing. The crash was not so big that it could have been solely responsible for the fracture and so my guess is that the failure started earlier and perhaps at the cable park where I really loaded heavily for a couple of hours.

A few things are noticeable:

A) It was a compression failure - the crack appeared on the top of the board on landing when the top surface was subject to strong compression. There was a small crack on the underside of the board ( about 1cm starting at the rail) but I think this was probably a secondary effect of the failure of the top surface leading to the laminate being bent beyond some critical radius and breaking. The crack on the top extends half way across the board.

This is the most expected failure as laminate has very little strength in compression. The glass reinforcement adds nothing to the compressive strength, unlike tension where its very large E (c. 70 GPa) drastically improves the properties. In compression the resin bears all the compressive load and the mode of failure tends to be rupture in the laminate due to buckling and delamination. The strength then depends on the compressive strength of the resin and the strength of the adhesion to the core material. I suspect that the rupture location would be at some irregularity in the surface and difficult to predict.

B) The top laminate delaminated from rail - I think that this may be the most significant issue. When one surface is free to move relative to the other ( this is the case when the laminate is not bonded to the rails) a huge amount of strength is lost.

When the laminate is bonded to the rails the cross section of the board form a closed rectangular section. The vertical walls of laminate create 'I' beams on either side of the cross-section which add enormously to the strength because of the thickness squared contribution of material to the cores strength. When these sides are removed the top surface and the bottom surface can move much more freely relative to each other the strength of the board depends largely on the elastic modulus of the material which for foam cores is very small and its likely the the shear forces ( the force parallel to the surface of the board) are no longer negligible as its possible for the radius of curvature can get quite small.

Close inspection of the rail where it delaminated suggests a few things:

i) The rail material is quite smooth where its delaminated. This suggests that maybe I did not rough the surface up enough to get a good mechanical bond ( surface roughness creates 'grab handles' for the epoxy).

ii) The chemical bond was not good. I have read in many places that the bond between resin and ABS plastic is not good and hence the need for flaming the plastic first. Flaming it ( literally running it through flames and stopping short of it burning) oxidises the surface and so that the chemical composition of the plastic bonds better to the resin. I did heat treat the plastic in the course of bending it into shape. There was lots of heat applied to the plastic pieces that protect the tips but less so on along the sides where it probably needed it the most. Perhaps these side rail pieces need more flaming that i gave it.

In Board #1 I fully encapsulated the rails in resin by leaving a1-2mm layer of resin on the outside vertical part of the rail. I did this because i was concerned about delamination. Board #1 has taken a lot of punishment including hitting a rock at full speed on a 20 knot day. If the resin on its own is tough enough to do the job it raises a question mark over the value of using ABS in DIY boards. (in production boards I understand it speeds the process up).

iii) The rail material cracked about 10cm away from the heal point. This has got me a bit puzzled. I've tried to snap ABS plastic by bending it repeatedly to fatigue it and even after bending it in 2 and working it still did not break. Interesting the crack is on the top half of the rail which means that it happened after my heal went through the deck as this crack could only happen by the top half being under tension. To it to crack like this suggests that the bond with the laminate to the right of the crack is very good and so after the laminate to the left separated, the section to the right stayed fixed and so concentrated the stresses in the rail sufficiently to crack. However, this doesn't seem complete. To get this very ductile material to fail like that seems very strange. Either the material was also more brittle at that point for some reason or the dynamic behaviour of the material under impulse loads rather than slow bending is very different than expected.

The faulty bond with the rail seems to be where all the evidence points.

D) The top laminate cracked half the width of the board. I was surprised to see the laminate crack. I was under the impression that the resin may have failed under compression but that the strength of the glass would have meant that it would have remained intact. Obviously as the board began to fail the radius of curvature at the point where it buckled and delaminated it reach a critical radius that caused the glass to fail. In e-glass the strain to failure is 1.5% so it must have been that this limit was reached. 

E) I did not provide any extra reinforcement under the  footpad as I was thinking that the plastic plate under the foam footpad would have spread the load. Obviously not the case. Footpad reinforcement goes back on the must have list.

Having a broken board means that I can not run some tests on the board to see what the actual strength is as maybe use this to calibrate the BoardOff's be continued.


  1. Sorry to hear that! Q: is your board shaped on the top like on "More thoughts on channels" picture?
    If so, this would mean the impact is not on the flat surface but more on curved one and being under foot straps it's releasing energy differently.
    I think the board has got lower thickness in the middle and more on rails side. Exactly where it needs it!
    What do you recon?

  2. Something like this, with 3 layers and 4mm concave, this would give measurements every 1cm from the rail:

    Only middle layer is sanded up to 0.29mm if possible. The top layer is 4mm thick sanded near tips with middle layer as well. This would give 12mm thickness near rails, where we need most, and 8mm in the middle.

  3. Hey Dean . that's a neat idea. having a flat top would also make it easier to budthe board upsidedown if you wanted to put channels on the bottom as the inverted rocker table would not need to have any concave bent into it.

    I wonder if ot would be easier to start with a 12mm thick block and then carve the concave out of it rather that build it up in 3 pieces as the wedge seems like it needs a fair bit if precision and especially if the concave reduces as to move towards the tip.

    my dead board wasn't actually shapped like the more thoughts on channels. it was . it was 12mm thick right to the start of the rails. I've convinced myself that it was the poor resin bind with the rails at that point that bought it all undone. once that's delaminated there's nothing short of a really stiff core material that could have saved it.

    how's your design coming along?????


  4. Today I'm pouring epoxy to routed rails. It's quite cold in WA so need to build that warm box first of many used fish boxes from the local market (smelly but free :)

    12mm tick block will be way to much work I think. In my case, 1st layer is marine ply, 2nd is bamboo ply 90 degrees on 1st (just to save some expensive bamboo and give more strength) and 3rd is bamboo again.

    Hopefully, won't be so heavy...


  5. cool. can't wait to see the pix!!!!

    finally got some wind today but had to drive 50kms. board #1 ( now nicknamed the pig) is going like a champion!!!!!!

  6. Done!

    On my native tongue:)