Saturday, April 30, 2011

Vacuum bagging lessons - the error part

The pump held the vacuum for a good 12 hours and I just unveiled it. 99% good. 1 serious percent bad.

I used paper towels on the surface of the board as breather material on the basis that I was only going to be laminating the bottom and so no need to think about the top. If there was zero excess resin in use underneath then this would have been fine. However, there was a small amount of excess and because the seal to the perspex was so good the excess resin had no choice but to make its way around the rails and onto the top of the board. So, when I unveiled it this morning, I had paper towel and rag laminated to the rails and here and there tacked to the outer edge of the board.

Had I not been using white pigment on the bottom it wouldn't have been any real issue. Just a bit of sanding back. However, the white pigment stuck out like dogs balls and so the amount of time needed to work it back to invisible grew exponentially. Foretunately the resin hadn't penetrated into the balsa on top so was relatively easy, though tedious, to peel back while the resin was still yet to completely harden.

It took about 2.5 hours to scrape off and sand back the overflow but in the end it came up ok with the exception being parts of the rails that look a bit scrappy.

Lessons learned:

i) Never place anything weak material directly on the surface of the board or directly in contact with the resin where it will be in the way for further work. Always use some type of release material and bleeder material to soak up the excess.
ia) Tape off the areas where you can't afford to have excess resin flow.
ii) Cover the board with bleeder material well beyond the edge of the board to soak up excess resin before it has the chance to flow onto the top.
iii) If you can't afford to have pigmented resin on the top surface of the board and you build the board up with layers of core material, it might be better to only prepare the lower part of the core, laminate that then put the top layer on the board once the bottom laminate it done. You could leave it on the rocker table (stuck down) and work on the core and then top laminate in situ.
iv) (After thought) If laminating top and bottom in separate runs, then leave the bottom laminate stuck to the table to that when you laminate the top no resin can flow under the board and mess the finish on the bottom.
v) Random though - I really struggled with getting the vac film off last time because the tacky tape was was damn sticky that it stuck fast to the film and so it stretched in every direction before it let go. This time I took it off early morning when things where cool and ripped it off in one might pull. It came of perfectly!

Tonight the top laminate goes on!!!!

MyVirtualShed reachs 1000 visits!!!!!

A milestone today with the stats pushing through 1000 page visits. Thanks for tuning in everyone. Feel free to comment, correct, concur or add links to your own baord building pages. I'd love to see this blog become integrated into the network of great board building resources out there.


Bagging the bottom - a much smoother ride this time.

The rain has lifted and its just on dusk so time to have a crack at laminating the bottom.

I had planned to try to laminate both sides at the same time for this board but I got cold feet and as it turns out it was probably a good move because of the timing. I am using standard hardener rather than slow this time which gave me workable resin for about an hour ( pot life is about 20-25 minutes i.e when mixing larger batches that sit in a pot which the exothermic reaction heats it up and causes it to go off in 20-25 mins - however when you spread it out into a thin layer the workable time extends, in this case to about an hour). I did some extra work to try to get rid of as many air bubbles as possible and it took me 42 mins before the glass was wet out and the vacuum pump was started.

Here's  a few things I did differently this time and overall it was hell of a lot better job this time.

i) I mixed the right amout of resin. Looking back on my notes from last time, for just a little less glass ( 3 x 6 oz layers) I ended up mixing 750gm of resin. However, I now get what the 'resin %' column in the reinforcement data sheet means. It means the amount that the resin represents in the final laminate. It tends to be around 50% which means there should be as much resin as glass. I weighed the glass and it came out at 350gm. Adding some extra for the glass that extends beyond the edge, the bit that stays in the mixing bowl and you can't get out and the excess you use when hand laying up and will subsequently get squeezed out by the vacuum, I ended up mixing 410gms in total including the 5% epoxy pigment that I used for the bottom. This was perfect. Obviously I wasted a shit load of resin in my last attempt (some of which remains stuck to the rocker table surface).
ii) I didn't use the vacuum through-connector for the vacuum hose but instead ran the vacuum hose half way along the length of the table under the vac film. I had suspected that the through connector was a source of leaking last time. This worked well and also  helped keep an air channel open along the board so that lots of areas were communicating via this channel.
iii) I had one long edge of the plastic stuck down prior to starting to wet out the glass. This reduced the time needed to get the vac film stuck down.
iv) I wet out 2 layers at a time instead of doing each layer seperately. This obviously saved time. I won't know whether this has caused more air bubbles to be trapped until I take the board off.
v) I used paper towels on the top of the board instead of breather material. I'll use proper stuff for the top but this was a cheap way to reduce the materials consumed.

So the laminating schedule this time was (from the bottom up):

6 oz eglass 0/90 degrees
4 oz s-glass 0/90 degress
6 oz eglass 45/45 degress ( used 2 overlapping pieces).
3 layers paper towel
rag drapped across the baord and the head of the vac hose to keep everything communicating.

mixed 410gm resing/hardner/pigment and used about 400gm. With excess squeezed off its likely to be close to the 50% resin percent.

Used standard hardener with R180 resin from FGI which gave about an hour working time. Clamped it under 23 inHg because I thought the stiffer board would need more pressure to bend the extra concave. Not so. 23inHG ( 11.5 psi) was more than enough. In fact I think this was a bit too much as it squeezed out a bit more resin then I hoped. So I backed it off to 20 inHg.

Rocker table showing the vac hose ( 12mm garden hose) run half way down the table.

Board drapped with paper towel used as breather/ bleeder material

The board clamped under 23 inHg. Within about 5 mins a lot of resin had wicked away to the edge of the excess glass.

The board is curing away as I type with the pump engaging about every 5 mins and runs for just a few seconds.

Close inspection of the step  from the balsa to the rail shows that the vac film has no problems conforming tho the circa 4mm radius on the filler between the sidewall and the rail.

Thursday, April 28, 2011

BoardOff Design Tool - Shameless Outline Rip-off

I discovered a neat way to trace the outline of the boards and get the parameters into Board-Off (the MS Excel Kiteboard design tool I've been working on) using pictures of the baords from the manufacturers web-sites.

 It turns out that the excel lets you put an image behind the graph and by making the chart area transparent the Board-Off cubic spline drawing functions lets you trace the outline by tweaking the control points for the splines used to make the different section of the board. Here' a couple of examples.

Brokite - Pop Series 130 x 40 board outline
Cabrihna Calibre without the concave tips 133x41cm
Board-Off then produces the templates that can be printed off, stuck together and used to cut out the board without having to take to your mates' boards with butchers paper and pen:)

Half template produced by Board-Off

Monday, April 25, 2011

Rocker Table #2

I had been tossing up whether to make a fully adjustable rocker table. A recent seabreeze post showed the details of one that let you dial in any rocker and concave profile you liked. I ended remaking the existing one purely because I could be bothered putting in all the effort to make the adjustable version. In the end it may not have saved much time at all because with the additional rocker and concave in this board I have ended up needing to massively reinforce the rocker table to stop the compound curves I'm trying to bend into the mould surface from completely distorting the frames I used. If ever there was confirmation of the impact of concave (coupled with rocker) than this provided it in truckloads. With 45mm rocker and 7 mm concave (measured at the board edge so at the jig edge the concave was more like 30mm) my full weight of 75kg could no get the jig to bend the shape into the mold and the mold surface is on 4mm (low quality) plywood.

So adjustable rocker table has gone onto the list of time saving projects!

In the end the rocker table has turned out ok. Here are some snaps of the finished product.

Side shot showing the wire ties needed to clamp the jigs down

End shot showing the concave (7mm at board edge exactly)

Underside of the rocker table showing the center and edge jigs

Sunday, April 24, 2011

Calibrating Board-Off Flex Profiling - Not too bad

I stumbled across the bag weighing scales that I had been looking for to calibrate the setting in the Board-Off Design Excel Workbook.

Here is a picture of the 'lab' and the sophisticated test equipment. A foam block with cm internals marked on it. The board clamped across the middle with a plank of wood and the bag weighing scales used to lift the tip of the board.

The scales are marked in kg but to get Newtons you just multiply by 9.8.

Testing the point load flex in the carefully controlled test environment in our car port!

Testing a 132x41cm full carbon Crazy Fly Raptor
I also put my Raptor on the test bed so that I can start building up some points of reference.

Here's the rest results for my first board and the predicted values. Drum roll please.........

Not too bad I have to say. I think the 20kg prediction /measured gap may be a confluence of experimental errors but at least the figure is in the ball park and if its within 10% then I think that is great given that there would be a large number of in situ effects that aren't taken into account in the model.

Most importantly the model can be used to determine the relative affects of different parameters and now I have a benchmark from my first board to have a real sense of what that stiffness feels like.

Centre and tip flex profiles (blue and pink respectively)
 One interesting thing in the analysis was the affect of the he direction of the plies. In board #1, one of the plies was laid off axis at about 30 degrees (rotated anticlockwise about 30 degrees). I was trying to lay it at 45 degrees but the cloth wasn't wide enough and I didn't want to join it cause I wasn't sure how to make sure it was strong enough. The 30degree offset has a disproportionate affect on the elastic modulus in the direction of the length of the board and with the standard weave it reduces the elastic modulus of that ply to about 60% of the axial value. Because the glass fibre's elastic modulus is about 6 times that of the resin the overall affect with all 6 layers of reinforcement is to reduce the 'average' elastic modulus by about 10-15%.

A valuable addition to Board Off will be some more granularity around the laminate properties given that there is so many permutations and combinations of lay-ups that possible.

Saturday, April 23, 2011

Core ready for laminating

The core for board # 2 is now ready to start laminating. The wood veneer looks great and generally the insert setting went well.

Total weight now is 850gm. Of this about 160gm is resin used to stick the to layers of Klegecell, balsa veneer, rails and inserts. This means that the board should come out at 2.1kg again as last time I used 1.2kg of glass and resin for 6 layers of 6oz (200gm) glass. This time I'm subing in 2 layers of 4 oz (140gm)  s-glass for 2 of e-glass layers. The slightly lower weight (60gm of cloth for the 2 layers) and slightly less resin about 30 gm ( has baout 50% resin content when vac bagging) means that the board should come out around the same weight as the last board despite the core being 170gm heavier. With the s-glass, increased concave and the (higher modulus) balsa the board should be stronger despite being the same weight.

Core #2 - The first board designed using the Board-Off Design tool in MS Excel.

One thing that I'll need to work on next time is getting the q-cell right. It seems that I didn't get rid off all the solid bits in it. The q-cell I've got is a bit stuck together and so I needed to crush it up to get it back to a fine powder. Obviously I didn't manage to do this because the small pieces floated to the top of the resin as it cured and the top layer of the poured resin had a number of them set in the resin. Fortunately I had over filled each of them ( I knew it would need to be sanded anyhow) and so when I sanded it back it took off most of the solids layer. I thought about using a flour sifter last time but didn't have anything on hand that I could use. Sifter is the go next time.

Otherwise, I'm very happy with the extent to which this core is has improved over the last one. Next step is to tweak the rocker table.

Balsa Veneer and new inserts

With the Easter long weekend I've had a chance to do some solid work on board #2 and as I type its outside with all the epoxy for the inserts and the fins curing away.

First thing was to decide on the orientation of the balsa planks. Although I am trying to minimise the stiffness of the core as its thicker and the tapered tips will also be stiffer running the grain on the tips at 90 degrees to the center section just looked wrong so I decided to stagger them like below. I'm going to thin the balsa down to maybe 3/4 mm to reduce the stiffness of it so hopefully it shouldn't matter.

To avoid having the foam core showing through under the balsa when looking at it from the side I trimmed about 5 mm of foam away from the edge and glued balsa spars along the edge. Sewingpins are ideal for holding basla in place.

In this picture you can also see the footstrap inserts that I've made instead of trying to track down the stainless t-nuts. I used the smallest hole cutter (25mm) and cut out 4 washers out of ABS plastic offcuts. I opened the hole in the middle so that the nut could be pressed into it and then I used 5 min epoxy to glue it in place. I also drill  2 x 1/32" holes on each washer. These holes were to make it easier to set the inserts in place. I'll provide some more detail below.
Then I mixed up 60 gm of epoxy and q-cell and wet out the surface of the foam. Placing the pre-cut balsa pieces on the surface I laid the 4mm ply wood that I used for the rocker table surface over the top of the balsa and weighted it........ and waited for it to cure. I was a bit worried that the epoxy would be enough as I had sanded the surface with 60 grit paper pretty hard before in order to clean it up and rough the surface for a better bond and I thought that this may have opened up some of the otherwise closed cells in the foam which would suck up the epoxy as might the balsa. But in the end it seemed all ok. 60m was just enough to wet out the surface and the balsa stuck nicely.

Core with balsa veneer in place

Next I marked out the insert holes and the areas to reinforce for the fins. Both of these I cut right through the board and cleaned the holes up. I placed masking tape on both sides of the holes. The bottom layers will hold the resin in when its poured in and the top ones will be cut open with a super sharp Stanley knife and be used to keep mess of the board when the resin gets poured in.

Footstrap and fin insert holes taped and ready for pouring the epoxy in.
Amount of resin for inserts and filler for rails.

I mixed 72gm of resin ( 60+12gm resin/hardner) and this was about 15gm too much for not only filling all the insert and fin holes but also for using as filler where the rail and the balsa meet ( to get rid of the right angle corder that the fibreglass won't be able to conform to).

I only used a small amount of c-qcell in the mixture for the inserts because it was mostly ofr coloring that I used it. However, for the filler I heaped the q-cell in until it was like peanut paste. It barely flowed at all. this is the consistency that you want for the filler because any thinner and then you put it in place it needs to stay bridging the right angle and flow away.

Setting the inserts

As mentioned earlier the 1/32" holes help make it easier to set the inserts in place. The idea when setting the inserts in the board is to drill the 25mm holes (same size as the ABS washers that the nuts are pushed into) for the footstrap inserts and open the holes a bit so that the ABS/flange nuts are a snug fit. I drilled these holes right through the board, placed marking tape on the underside of the holes and filled the footstrap insert holes with about 2mm of 5 min epoxy to assist in preventing an over tightened screw from pushing into the lower laminate and delaminating it.

I used some 6 x 25 mm stainless bolts and screwed them through the flange nut and out the other side so that the end of the bolt touched the 5 min epoxy barrier and the washer sat about 2-3mm below the top surface of the board.

When the 5 minute epoxy was cured I 2/3 filled the each hole q-cell/ epoxy (not 5 min). Q-cell is only for lightening the mix and colour so I didn't use much at this stage. With the holes 2/3 filled I pushed the inserts into the resin. Because they are a snug fit, the 1/32" holes let the resin run through and over the top of the ABS washer. I then filled the rest of the insert hole with resin so that it sat just above surface of the board.

Its important to make sure that there are as few air bubbles as possible here so its work getting a toothpick or the like and stirring and poking around to get any out.

Friday, April 22, 2011

Impact of concave on flex - the theory (TBC)

I've been doing a bit more upgrading of the design workbook that I've used for board #2. Previously I modelled the flex in the board by ignoring the concave and just modeling it as a flat rectangular cross section with the laminate and the core modelled separately because the elastic modulus of the two are orders of magnitude different.

The latest version of the worksheet incorporates the concave profile along the length of the board.

The key quantity that you need to calculate to determine the flex profile is the Second Moment of Area which, because the cross-section of the board is symmetrical, is equal to the average value of y^2 over area of the cross section at a point along the board. The Flexural Rigidity is defined as the Elastic modulus of the material times the second moment of area. To work out the radius of curvature at a point on the board due to a force applies at (say) the tip, you divide the value of the moment ( force x distance from the point of interest to the force) divided by the Flexural Rigidity. the formula is

1/ R = M/EI

where R - radius of curvature at the point x
M - is the moment at x
EI - flexural rigidity ( E- elastic modulus, I - SMOA)

You can use this to plot out the curve that the board makes when it bends under various forces.

The SMOA for a rectangular section is

I  =  t^3*w/12                 -      (1)

where t-is the thickness, w is the width. The equations in the DIAB Sandwich Handbook for a closed beam cross section comes straight from this.

After a lot of algebra and googling around beam bending theory sites, the correction term that I came up with for the SMOA when the rectangular section has concave of 'c' is

4/9*t*c^2*w                      -          (2)

To get an idea about the size of this term, when concave is about 50% of the core thickness, this term equals the value of (1). That is, it doubles the flexural rigidity. Bearing in mind that the concave is not constant throughout the length of the board and typically tappers of at the tips you need to do the full calcs to see just what it does to the total flex of the tip.

When I loaded this term into Board-Off (the design worksheet) for the current board, with 7mm concave tapering to zero at the tip then it reduce the amount the tip flexed when the board is bent from the middle was about 22% compared to no concave.

The flex of the tip (from the footpad to the tip) was negligible which corresponds with concave being just 1-2mm at that point.

So, if these calcs are correct then the mid section stiffness is quite sensitive to concave. I've been told that recently the trend is to no concave in the boards and I wonder if it might be that the increasing use of carbon which incredibly stiff and strong negates the need to have concave for strength reasons. ...

To really make any use of the flex calcs the model still needs to be calibrates with real figures for the elastic modulus of the laminate. I'm using a figure that I have no real point of reference so once the board is done I'll be able to put the theory to work in designing number 3.

Interesting test

Below is the flex profile for 20mm concave. The thin black line shows the radius of curvature from the tip to the middle of the board.

The bulge in the curvature line towards the left hand side means that board is getting flatter ( big radius = flatter section). One possible reason for this is that with 20mm concave and a board thickness of just 12mm the neutral line ( the centroid) which is where the stress in the board changes from compression to tension. When the board has no rocker this line is about in the middle of the board cross section. In this situation you have one side of the board in tension and the other in compression and the characteristics of the laminate are different in these two situations. However, when the centroid lies outside the board it means that board sides will be tension at the same time when you try to bend it in the middle.This means a lot more stiffness and strength. As you progress to the tip and the concave reduces to nothing the centroid as some point falls inside the cross-section and you sides start working together in a complementary way rather than the co-operative way when board are in tension simultaneously.

Wednesday, April 20, 2011

Whats it going to cost??

Today I bought all the materials to finish the baord off over the up and coming easter long weekend.

A couple of small changes

i)  I've decided to replace 1 layer of 6 oz eglass with 4 oz s-glass. I'm hoping to improve the flex and reduce the weight a bit. While it s-glass on paper is 30% strong and about 10% stiffer than e-glass its also about twice the costs at almost $10/m.
ii) Its proving too hard to get hold of stainless steel t-nuts for the foot strap inserts. They seem to be pretty uncommon and the one source I found for them (Brookvale Stainless) would only sell the box of 100 and it was going to cost $130. So I've decided to use stainless steel flange nuts instead. The 6mm nut is also about 6mm from the bottom of the flange to the top and so my idea is to make a small ABS plastic washer to that the nut will fit through and then suspend the whole set up in qcell'd resin. About 20c per nut it beats the 2 dollars a t-nut and you can get them anywhere. I also go 6x10mm bolts.

So here's a round up of the costs on a per board basis.

Potential cost savings on the board - no balsa veneer, use builders plastic instead of vac film, replace peel ply ripstop nylon from an old kite.

Because you end up having to over purchase some things because of the minimum quantities you can buy the per board price is the price you'll end up paying if you make 3 or more boards. I guess if you were buying production quantities there would be some volume discounts......

Tuesday, April 19, 2011

Core ready to add veneer

So after 48hr of curing !! the rails are finally set in place and the core is ready to add the balsa veneer.
Already the board is feeling more like a board than this time last board. Things actually fit nicely!

There is still overflow epoxy/qcell on the foam which I am going to thin back and rough up to make sure when the veneer goes on it will have a grippy surface to attach to. After having to wait 48 hours for this slow curing epoxy to harden I'm definitely going to be investing in some quick curing resin to put the veneer on.

With the tapered tips on the board my estimate is that the tips are going to be around 20% stiffer than the last board which had a step profile. So ideally I'd like to do what I can avoid the veneer contributing to any further stiffness of the tips. While the elastic modulus of balsa is much greater than Klegecell ( 1.5-2 GPa vs 30 MPa) it is still an order of magnitude smaller than that of the laminate which as near as I can find out is likely to be around 15-20GPa)  So this means that the veneers contribution to the board strength and stiffness still primarily derives from the core thickness it adds (stiffness being proportional to core thickness cubed: 10% increase in thickness - > 30% increase in flexural rigidity).

Why am I bollocking on about this. Well I'm trying to figure out whether I need to be concerned about where the joins are in the veneer. If the balsa was adding significantly to the strength through its intrinsic properties then having all all the balsa planks aligned at the short end would introduce a 'fault' line because the grain will be discontinuous. However, this is not the case and so I'm thinking that it doesn't matter and that what I should do is just do whatever looks good.

Sunday, April 17, 2011

Board #2 Core and Rails

Board 2 is underway. I'm a bit more excited about this one than the first one because there are a lot fewer unknowns in getting this one underway. Also, the outline, rocker concave and flex profile have all been designed using the Excel workbook I pulled together so its a plagiarism free board too!!!!

I got a bit ahead of myself with the last post and forgot to record the cores and rails before actually starting to glue things up. So for posterity here's some snaps of the pieces so far.

This is the bottom piece of the core - 6mm Klegecell. The paper template was printed from the design workbook which I've christened 'Board-Off Kiteboard Designer'. Its printed on A4 sheets and then tapped together. Once the designed stabilise a bit more I'll commit them to wood templates but while the experimenting is continuing paper works fine.

Unlike the last board which had a step profile on the core this board is going to have tapered tips. The modelling suggests that this will mean that the tips will flex about 20 less because the mean thickness of the tips is greater. To help minimise this I've cut a couple of recesses in the top layer to reduce the average thickness and also to give the area where the fin reinforcement goes a flat place for the screws to connect to. 

The picture also shows my new 2011 Crazyfly footpads. Awesome pads ( its all about the toe holds) but also work out to be the same price as all the rest of the materials for the board. Working out how to make these is high on the priorities for keep the overall board coast down.

I used a plank of wood wrapped in sand paper to put the tapper on the top layer. I did this before the top and bottom where glued together. I placed a piece of 4mm ply over the top of the sheet and moved it around until I length of wood I was using as a sanding block was on the angle of the tapper that I needed. Then just spent 20 mins slowly sanding it with 60 grit sand paper. I thought this would be too rough but any finer paper and it just wasn't removing the foam quickly enough.

Also, I'm going to lay a 1.5mm balsa wood veneer over the top so in order to keep the maximum core thickness down to 12mm I need to plan about 1.5mm off the thickness of the foam. It was way too big a job to do by hand and so I resorted to an orbital sander with 60 grit paper again and planned off the needed amount from the lower core piece. This also made the lower core piece the same height as rail material and so removed one step that would need to be either sanded off of filled with q-cell.

The picture below shows the tip's tapper

 Here's the rails being glued in place.  Used the same technique as last time but used thicker mixture of q-cell and resin. I think that next time using 5 min epoxy and doing one bit at a time is the way to go. This step is unnecessarily long and because it is vertical faces being glued and the faces don't match perfectly, the epoxy does run away and pools underneath.

I started with the tip pieces first and clamped them between 2 pieces of wood. I used offcuts of klegecell to put end pressure on both end rail pieces to make sure they fitted snugly to the end of the board.

Its turned quite cold here in Sydney and the curing time for the epoxy has consequently gone way up. I really think that 5minute epoxy is the way to go with this. Less mess , less fuss.......

Friday, April 15, 2011

Rails - revised technique

I've cut out the templates for the new board and just finished making the rails up. This time the process and the outcome was far superior to the first attempt. In the first attempt I traced the core out on the ABS plastic sheet and cut the plastic exactly to shape. There were 2 things wrong with this i) It wasted expensive plastic because there are now pieces of plastic with curved edges that can't easily be used for anything and ii) I hand cut the plastic because the dremel like tool I've got spun at a rate that melted the plastic so I needed to cut it out with a hacksaw and this took about hours where I only took out time to swear at the fucker.

This time a very different story and a great outcome. I used a jigsaw with a metal cutting blade to cut long straight strips 10mm wide. To help get the thickness constant I put a small G-clamp on the base plate 10mm in from the blade so that I had a good guide. I did the same at the front and back of the base plate so that the blade would run parallel to the edge. I cut of 3 long strips - 2 x long edges and another to be cut in half for wrapping around the tips.

Then , thanks again to Brokites video, I took the half length and heated them slowly over a gas BBQ to get them to soften them. There is a critical temperature at which the plastic looses its rigidity and becomes like Plasticine. I took the softened ABS and shaped it around the tips and bingo a perfectly shaped rails to wrap around then end of the board and down the length of it for about 10cm.

It wasn't necessary to heat the side rails cause there is so little curvature in them that it will be very easy to stick them in place. The result is perfectly formed rails. Before I stick them on the board I will rough them up with 80 grit sand paper and flame them again as I believe that this changes the surface chemistry of the plastic and forms a better bond with the epoxy.

 Tomorrows work is to prepare the core properly and apply the balsa wood veneer......

Tuesday, April 12, 2011

The Birth of a Brand!!!

This is turning out to be a really fun project(s) so thought it would be good to get a brand onto the boards. Here it is.....

Monday, April 11, 2011

Flex Profiles - Modelling

I've been spending a bit of time improving the board design worksheet that I'm using to produce the board outline and the rocker jigs. I've added in some very basic modelling of the flex profile based on the beam bending theory in the FGI DIAB Sandwich handbook.

As you would expect modelling it properly is extremely complex and, like all good engineering problems, by approximating the hell out of it you can get pretty charts with numbers on them but really need experiments to calibrate the many constants and error terms that appear in the analysis.

The modelling I did was very basic. It modelled the board as a beam witha thin rectangular cross-section (i.e no concave) with uniform thickness laminate. The elastic modulus of the core is negligible compared to that of glass/resin and the rails have been assumed to be wrapped (although the shear strength of ABS probably makes this ok'ish). The core elastic modulus condition is probably ok for foam and lighter balsa but not for any more dense wood. The calcs do take into account the actual width of the board and thickness profile that can be either a step or flat in the middle with tapered ends, the number of layers and strength of the laminate. Also the applied bending forces can be set at either a point force at the tip or a uniformly distributed load.

I googled around to try to get some of the material properties for the different materials used and found some properties that seem about average. To do this well you'd ideally determine them by experiment. Of course the real divergence between the theoretical values and the actual values is in a big part driven by the influence of the actual quality of the construction. There's no way to model a crap job of removing air bubbles or resin that doesn't stick to the core because of greasy finger prints.

All this does make the exercise of limited value in calculating the absolute values of the flex profile but where it does come in very useful is to determine the relative values for different core profiles, board shapes, layers and distribution of glass to look at the effect on the flex. Even better will the case when I've made a few boards using the tools and both calibrate the model and potentially generate some really values to compare with.

So here is the output for the parameters for the last board I made. 3layers of 6 oz glass each side, step profile core ( 2 layers of 6mm klegecell) , 24 " stance , no tip thinning.

The top chart shows the flex profile under a 40kg load applied only to the tip of the board. The solid lines show the profile of the board when the board it clamped at the center and then at center of the footpad. The deflection (does not include rocker) that results. The dotted line shows that radius of curvature at each point along the board - bigger equals flatter.

The second chart shows the calculated stresses and strains in the top layer of the board under the same test conditions. The calculated stress can be read off the left hand axis and the strain ( % elongation experienced at the point along the board) can be read off the right. The two curves coincide because I have used the simple linear stress strain relationship determined by the elastic modulus of the laminate (glass+resin)

By way of comparison, I changed the profile to have the thickness of the core tapper from the 12mm midsection down to tips at 6mm and you can see from the first plot below that the stiffness of the tips flex about 20% less. This seems intuitively right. The strain is mislabelled and should be teh strain of the underside of the baord.

Saturday, April 9, 2011

Board Project 2 - The outline and rocker.

Kicking off Board #2

Board Number 2 is underway after some serious time on Google working out what to tweak to improve the flex without compromising strength. A couple of other goals for this project are

  • Make the board better in chop and better pop for freestyling.

  • Use wood in the core construction to have a natural wood grain finish

  • Improve the construction techniques based on what I learned from board 1.
Chop and Pop

More rocker seems to be the key to both of these. More rocker means less of digging in the nose and board diving like a shag. More rocker also, apparently, improves pop because it allows you to hold the edge in longer while loading the board. My theory here is that with more rocker, when you edge hard the water is able to flow around the board rather then with a flatter board where it has no choice but to go under the board and hence lift the board up and out of the water (so you loose your edge). Also a continuous rocker means more drag as you have a thicker cross section your pulling through the water compare to a flat section. More drag means more load on the lines means more pop.

Flex is also an important feature here. A flexible board will bend in the middle as you edge and the mid section will flatten and you are effectively trying to pop with a flat(ter) board. Not what you want. My feeling about the last board with 12mm PVC foam and 3x6 oz glass both sides was that it was just a bit too stiff. This was mostly fine except in chop the board didn't really seem to absorb much and so it was rough on the knees. However, my reason not to change the flex is that the flexible boards I ridden just feel really 'mushy' when you're trying to pop and you get less pop out of it. So I've decided to keep the save glass schedule but to thin the core a touch. From the previous google discovery, flex is proportional to core thickness cubed(!!) so a 10% reduction in core thickness (1.2mm) is almost 30% reduction in the flexural rigidity-very sensitive!! Part of this will be clawed back by the use of balsa laminate on the deck. The higher modulus of elasticity of balsa, I am hoping, will reduce this loss of rigidity a bit.

Balsa in core

I really like the look of the wood grain finish on kiteboards and so I want to include wood in this board. A full wood core or paulownia seems like a pretty common thing in current boards but its proven a bit hard to track down the small quantity I need. I have seen full balsa boards (12mm , 3x6oz e-glass top and bottom) that look great but my reservations about balsa are that its not water proof and I'm not confident that I won't rupture the glass at some point (already have on board #1 when on a 20 knot day I hit a large rock and came to a griding holt... interesting insights in to the nature of the failure of the material... subject for another post) and it is also very expensive to get high quality stuff with reasonably similar density and elastic modulus. Also, I have heard that the moisture content in balsa can lead to delamination of the glass if the board is left in the sun for long periods of time. This appears to be why some all balsa surfboards have vent screws on them.

So the trade off is to use Klegecell left over from my first board and put a 1.5mm ( 1/16 inch) balsa veneer on the top deck. It should look good and will need to be accompanied by a thinning of the core to ensure that the 10% increase does lead to a board 20% stiffer that board #1.

Board #2 Parameters

Here's what I've decided to go with:

  • length 133cm (bit longer than the last one to assist with increasing flex and to compensate for the narrower board)

  • width - 38cm

  • continuous rocker 45mm measured along the centre

  • single concave 7mm

  • 10mm klegecell core with 1.5mm balsa veneer

  • 3x6oz e-glass ( although need to price up s-glass and potential use 3x4oz s-glass and leave the klegecell core at 12mm) top and bottom. 90 degrees, 45 degree, 90 degrees.

  • thin the core from the foot straps to the tip instead of using leaving a step. Thin the tips to about 3mm so tips are a bit more flexible.

  • use either glecoat for bottom surface or lay cotton material or possible ripstop nylon between layer 1 and 2 on bottom. Pure for decorative purposes.

The outline and rocker.

I've revamped the excel spreadsheet I put together for creating the board outline to include the creation of rocker templates for the center line as well as for the jigs that will clamp the edge of the rocker table surface. This time I have calculated the correction needed to account for the rocker table being almost double the width of the board (using the beam bending equation). In board 1 my estimate was out by about 50-100% and so I ended up with 1/2 the concave I wanted. This time it works very nicely.

Here are the parameters and the plots for the rocker jigs. The light blue line shows the concave profile along the length of the board. The outline of the board was chosen to be something like a Cabrinha Calibre. The drawing tool I created in excel uses cubic splines to make smooth curves on each of 4 section of the outline. The short edge, the tip (corner), the mezzanine section and finally the mid-section. On the drawing below the red dots show each of the points being joined by the cubic splines for these section. These points being joined are referred to as 'knots' or control points in the parameters below. In parameters, knot location and width for each section relate to the section in isolation and referred to the distance between the outer two points in the section and the height different between the lowest and highest point respectively and not to the total length or width of the board. This the upper left quadrant of the board that the above parameters produces. In the spreadsheet I just reflected this chart to make a full half of the board, stretched the chart out to be 7 pages long and calibrated it so that it printed out to the correct length. The paper template was then transferred to directly to the core.

Thursday, April 7, 2011

Material Properties

Found some more great info comparing the properties of different reinforcement material.

Checkout the back section that compares high performance reinforcements such as s-glass, carbon and Kevlar. Here's the killer table

A few interesting things that come out of this.

Carbons strain to failure is just 1.2% ( the % its fibres can stretch before they break) which is why you often hear carbon layups referred to as quite brittle. The don't' flex much but when they do they break catastrophically.

From this table it looks like S-glass has some real benefits for strength and flex over the rest except where weight is really critical. While carbon is strong and light its also very stiff so so it seems that carbon might best be used to stiffen sections of a board rather than make a board completely out of it.

Interesting that Kevlar compressive strength and modulus a so low. Other info in the above link explains this. it seems that Kevlars mode of compressive failure is different to the other reinforcements that are typically fai'l catastrophically ( all of a sudden crack and rip apart) Kevlar fails via fibre elongation ( distort and bend) rather than catastrophically and so when it fails the laminate and structure stay intact and so keep water out. This is handy for boats I guess but if you've got a bent board then you might as well have a cracked board because the bin does care how its stuffed up.

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.