A couple of queries on Board-Off

classic Classic list List threaded Threaded
3 messages Options
Reply | Threaded
Open this post in threaded view
|

A couple of queries on Board-Off

Chris
Hi Matt,
I've been trudging through board off and it's very impressive, definately helps with everything I've been toying with over the last few months.

When it comes to the stress and deflection stuff (appologies if I'm barking up the wrong tree and it's already like this) I have a couple of queries/comments:

1) Does it take into account the binding positions when calculating the bending and stress diagrams? This will change the BM diagram considerably by adding a point load onto the model in compression and in tension on the other side.
2) The inter lamina stresses could be plotted so you can see if the applied stress is higher than the allowable at the laminate/core interface to help with delamination issues.(or this could be displayed like a classic stress diagram through the section)
3) The shear stress at the 1st binding fixing (or other maximum) would be useful when trying to avoid core crush in thin laminates
4) I note in the laminate tab there are no properties in the E transverse column, is this referenced anywhere or is it just there fore future development?

my 2c

Chris

Reply | Threaded
Open this post in threaded view
|

Re: A couple of queries on Board-Off

Matt Ma
Hi Chris

Mate, thanks for getting down and dirty with BoardOff and thanks for the feedback and the ideas - much appreciated. I know its not the most user-friendly spreadsheet so 10/10 for persisting with it. It sounds like you may have a mechanical/civil engineering background?

The flex/stress model was a really interesting exercise. It started out primarily to compensate for my lack of experience in building boards and the idea of the model was primarily to be able estimate the flex and compare that to the flex in production boards I had ridden. One of the big challenges though was there is a lot of variation in the physical properties of the materials from one batch to the next and the conditions that the resin cures and core properties (in particular in wood) plus other process variations. The best way to make the flex model useful, I found, was to put the average theoretical values in from the data sheets and then when I did my first couple of boards I calibrated it with 'engineering factors' to bring the measured and predicted values closer. I use the same suppliers and mostly the same technique and so now the model does an acceptable job of predicting the flex when I change my designs around. So now I have a feel for the flex in the boards I like to ride I've now got some good rules of thumb for making design changes.

There are a few challenges with the stress/strain model in terms of trying to make it useful in predicting failures. At a big picture level the flex model uses linear bending theory and one of the conditions needed is that there are no big shear stresses present. In the case of heal damage to a board my feeling is that the shear forces right under the heal would make the linearity conditions fail and so not be of much use. Also, and again its just speculation, but I suspect that failure is more often due to local buckling of the laminate under the heal. So to make the stress model useful I think it would need to bring in all more factors such as core-laminate bond strength +++  to calibrate it would possibly require destructive testing which for a carport operation is a bit too painful. Also buckling isn't something I know much about.

What are your thoughts about the modes of failure under the bindings?

E transverse is a place holder for when I thought I include some torsional stiffness calculations. I didn't really take this any further because to be honest I can't really distinguish the performance impact of it. Not that there isn't an effect, I think I am just not a good enough rider to know what the impact is.

An extension that I am a bit interested in is modelling the dynamic behaviour of it. The rates of energy storage and release I though might be interesting in quantifying behaviour in chop and, although I don't understand it in detail, I have started to see some marketing material ( Slingshot in particular) talking a 'reflex' which seems to be referring the rate of recovery of the board to the original position.

Now that you've used BoardOff quite a bit I would really welcome and ideas you have to make it a more user friendly tool. I have really enjoyed building it but it is sorely in need to user feedback as I would like to get more people making use of it and sharing their designs. Any ideas/things you like/don't like etc would be really welcomed.

What sort of boards have you been working on? Would love to see some pic's on the forum if you have some. Even just the finished products would be great to see.

Cheers
Matt








Reply | Threaded
Open this post in threaded view
|

Re: A couple of queries on Board-Off

Chris
You're close, I'm a Structural Engineer here in the UK, so I like to think of myself as somewhere between a Mechanical and Civil Engineer

I've always played with boats and cars, this is my first board build so looking forward to getting stuck in. I've just got to get my oven rebuilt since moving it to a different workshop (read garage to hoosive shed). It's 2400x1200x800 high with a process controller for temp ramp rate and holding. It'll hold 80deg +/-1degC so well chuffed for a home brew. My testing results last year were pretty consistant so I think I can control the cure process pretty well.

Here's a couple of thoughts I've had on various things...

img041

Loads more where that came from knocking around between the ears, just need to write it down!

If you've got some pix of the failure at the heal that may be beneficial to working out the failure mode but I think it's more like core crush due to thin laminate under the heal or local buckling in the compression face where it delams from the core (due to the bending moment). You have loads of complex forces going on in that area in multiple directions so you have to really simplify the model down.

I'm looking at building a wakeboard so the thing has to be a load stronger than a kiteboard, my tip loading is going to be in the region of 150-200kg to simulate hitting a slider (probably should be more in reality)

When I was looking through the books earlier for that buckling equation, I found a subnote about limiting the total deflection to somewhere in the region of span/400  so the relative shear deflection of each lamina isn't too high or doesn't exceed the strain failure of the matrix.