Some advice along the way is more than welcome since this is my first board building attempt
. I am not a regularly kiteboarder so I decided to build a wakeboard (for a cable park) instead since I do that more often these days. I want it to be strong and durable so I can have lots of fun with it for a decent amount of time. This means that I will have to add a p-tex base and use a proper amount of glass in the layup. I intend to use ABS sidewalls as well. I am somewhat familiar with fiber reinforced plastics so I am up for the challenge.
I have some wooden poplar strips laying around which I like to use as the core material. Already sorted out some nice ones without to many knots. I aligned the strips such that the inserts are not placed exactly on the glue bond between 2 strips.
Sooo... I started to do some calculations regarding the bending stiffness (flex) of the board. I found the following (relevant) properties of poplar wood on http://www.wood-database.com
. I also added properties of Paulownia and Balsa.
I verified the young’s modulus of my poplar strips using a simple three-point-bending test. The values were close to 10000 MPa. This means that the wood core will become approximately twice as stiff compared to a more frequently used Paulownia core. I see some troubles at the horizon. Wakeboards generally have a rocker of about 60-80 mm which will become a challenge to achieve with such a stiff core (large spring back). I hope that pre-bending the wood core will help.
I used my current wakeboard as a reference for the bending stiffness. A little side note; I noticed that most board builders use the term 'flex' which is, in my opinion, a bad unit. A better unit is the bending stiffness per unit width of the board (since boards have different widths). I used a three point bending test and the corresponding flexural formula (d=FL^3/48D) to determine the bending stiffness of my current wakeboard. F is the force per unit width, L is the span length and D is the bending stiffness per unit width. I applied my own weight on the board and measured the deflection using a span length at which the width has a fairly constant value of 445 mm. The board deflected 17 mm over a span length of 0,9 meter under a force of 2245 N/m. So the bending stiffness per unit width of the board is D = FL^3/48d = 2245*0,9^3/48*0.017 = 1826 Nm.
Now I have some reference for the poplar wood board build. I will be using a tri-axial 830 g/m2 E-glass fabric 0/+45 °/-45 ° (425 g/201 g/201 g) and epoxy resin. I am aiming for a fiber volume fraction (Vf) of 40% using a vacuum bagging technique. Thickness of the cured layup is approximately 0.8 mm and weights 1250 gr/m^2 when assuming a Vf of 40%.
Now I can determine the desired thickness of my core which gives me a bending stiffness (per unit width) in the desired range of 1826 Nm. I use the classical lamination theory for this matter using the following layup as an input [0/45/-45/core/-45/45/0]. For people who are into this, the D11 term from the ABD matrix is equivalent to the bending stiffness per unit width. I will spare further details.
Core thickness (Bending stiffness)
7 mm (852 Nm)
8 mm (1147 Nm)
9 mm (1501 Nm)
10 mm (1919 Nm)
11 mm (2408 Nm)
I go for a core thickness of 10 mm as the initial assumption of 0.8 mm for the faces may be slightly less in practice when vacuum bagging. I started to make the following design:
The weight of the board is determined using the volumes and surface areas from the CAD model together with the density’s and areal weights of the core, rails, faces and base material. The total weight of the board will be 4050 gram (without the bindings). Which I think is acceptable considering my current wakeboard weights 3510 gram. Not sure yet if I will add a top sheet which will add 150 gram or so. Another thing that may add some weight is the absorption of resin by the poplar wood. I have no clue how much this will be. A test sample hopefully gives some insight into this. The alert reader may already noticed in the calculations above that I am not that lightweight (93 kg), so the wakeboard will experience relative high loads. That is why I chose 830 gr/m^2 glass fabric for the faces.
I ordered some base material (ptex), topsheet, sidewalls and inserts from junksupply. I kinda know that I will like building boards so I also ordered a paulownia core for a future project. Besides, it was financially more interesting to buy material for 3 boards since everything is sold per meter. I bought the fiber glass and epoxy at http://www.carbonwinkel.nl
amongst some other stuff for the vacuum bagging stage.
I glued the strips together with some water resistant wood glue (water resistant for the pre bending stage). I also clamped to top of the strips (not in picture) to prevent them from buckling outwards. I then used the cad model to make a true scale print of the outline for the sidewall channel. I transferred the print to a mdf board and used it as a guide for the router. I shaped the ABS strips in the desired shape using a heat gun. As you can see in the picture I cutted the strips under an angle and placed the bond off centre; it just felt like a smart thing to do. After proper sanding I cleaned the ABS sidewall with alcohol and gave it a flame treatment right before gluing it onto the core with the same epoxy as I will be using for the vacuum bagging stage. After letting the board+sidewall cure I called in a favour at a local woodworker who has a machine to trim down wide wood panels to a desired thickness, which is 10 mm in my case.
It is stressed everywhere on the web that the flame treatment of ABS is very important, so I decided to do a small test with 2 small left over pieces ABS. Both samples were sanded and cleaned with alcohol. But 1 sample was glued to the poplar wood without flame treatment and one with the flame treatment. After curing I pulled the sample without the flame treatment off pretty easily while the other has a very strong bond with the poplar. I found it nice to actually witness this in practice.
At this stage I checked the bending stiffness of my core using the 3 point bending setup below. I used a 15 kg dumbbell for the load. The deflection over a span length of 1,0 meter was 10,0 mm. The bending stiffness (per unit width) is D=748 Nm. Note that D is defined as: D=(1/12)*h^3*E. In which E is the young modulus and equals 8973 MPa. The bending stifness of 738 Nm for the core seems pretty good. Because, adding 2 fiberglass+epoxy faces with a fiber volume fraction of 40% to this bending stiffness gives a stiffness for the board of 1853 Nm... Right on target
. (Side note; the correct term for E in this case would be the flexural modulus instead of the young’s modulus. The young’s modulus has different values for tension and compression. In the case of bending both tension and compression occur so the E hidden in the bending stiffness is sort of a combination of the bending and compression modulus. When the material behaves very different in tension and compression the flexural modulus may be a better thing to use or say. )
The final shape will be achieved using a belt sander and some elbow grease. The desired tips of 5mm are simply achieved by checking it with a caliper during sanding. I did not find it worthwhile to build a router table for this purpose. An even slope is achieved using some colouring wax and a straight wooden strips to indicate the high spots.
The result is the picture below. btw, the inserts are epoxied in from the bottom since I felt this would be a little bit stronger.
I let water soak into my core overnight and fixed it onto a rocker table having 8 cm rocker. This wasn't a success. Only 1 cm rocker was left and the board now has a slight concave in the wrong direction :/
I am currently working on my vacuum bagging setup. I used an old refrigerator compressor. I know it doesn't look that fancy with all the rust but it is the inside that matters
. The setup right now can pull a gauge pressure of -96 kPa. I am using an automatic switch to control the on/off switching of the compressor. Otherwise the pump gets way to hot. Furthermore, I use an old fire extinguisher as a vacuum reservoir. The pressure drops with 2 kPa every 15 minutes when the pump is switched off (I did however used a small vacuum bag ). I would like to install a one-way valve between the pump and bag to rule out leaks from the pump. The outlet of the pump is connected to a jar with some foam to catch the oil vapours.
I bought a vacuum bag for storing clothes. But the bag had a poor connector so I build my own bag using the sealing of the commercial vacuum bag together with a sheet of vinyl. I sealed the edges with tacky tape.
I made a test sample and cut it open. I am not too happy about it since it seems that there is some air trapped under the epoxy. Also, I could pull out some dry fiber yarns.
I did not use a breather fabric for the test sample, so maybe I should get some peelply+breather fabric in order to pull an even vacuum throughout the product.
I am trying to figure out what the best plan of approach is… But first I definitely want to make more test samples.