I Boundary conditions for a wooden board used for breaking in martial arts

[makamo66]

TL;DR Summary

Boundary conditions for a wooden board used for breaking in the martial arts, clamped or simply supported?

Boundary conditions for a wooden board used for breaking in the martial arts
I wanted to find out if the boundary conditions for supporting the edges of the board are for 2 sides clamped, 2 sides free or 2 sides simply supported and the other 2 sides free.

 

[Dale]

I think that simply supported would be more reasonable than clamped.

 

[makamo66]

But why?

 

[Dale]

Just a feeling. When I have held boards for someone else to break I have focused on keeping my position as stable and stationary as possible. I haven’t tried to press the edges to keep them straight.

You would have to exert a lot of force with the tips of your fingers on the edge to really clamp it. I usually used the heel of my hand mostly

 

[kuruman]

Watch this video. I was the first person to try it.

 

[makamo66]

Thanks for the video. Now I'm wondering what the reason is that the eggs at the fulcrum didn't break.

 

[Ibix]

Just a feeling. When I have held boards for someone else to break I have focused on keeping my position as stable and stationary as possible. I haven’t tried to press the edges to keep them straight.

I think there might be a difference between what you might do to optimise the breaking process and what you might do in a dojo. A quite weak person can brace a board if they understand stance but, as you say, clamping would need the hands of a gorilla. So you'd actually need a strong frame with strong mechanical clamps, and that's more kit to buy and probably makes it slow to change boards. On the other hand you've got a ready supply of reasonably well trained guys who pay the dojo to be there, can do a good enough job by just bracing, and can swap out broken boards in a second.

So clamping might make it easier to break boards (or not - I'm not sure), but it's more expensive and less convenient. And I'm not sure you necessarily want to make it easier - it's meant to be a demonstration of skill and strength after all.

 

[A.T.]

Thanks for the video. Now I'm wondering what the reason is that the eggs at the fulcrum didn't break.

Because the eggs are not at the fulcrum. Once split, each half of the board spins while falling down, such that the instantaneously stationary point is not at the egg support, but somewhere towards the center.

 

[kuruman]

Thanks for the video. Now I'm wondering what the reason is that the eggs at the fulcrum didn't break.

A theoretical calculation (not by me) showed that the middle breaks before the ends receive the information, which travels at the speed of sound, reaches them. Also, there is dispersion and attenuation of the initial pulse so that not all frequencies making up the pulse reach the ends at the same time.

I suspect that the length to cross sectional area ratio is important. What's true for the pieces of wood used in the video may not be true for the planks used in martial arts.

 

[makamo66]

The problem is that I have a book by Ugural that I attached and he only has the options of simply supported, free, sliding, and clamped/built-in for the boundary conditions of the edges of a plate. He discusses these in Chapter 1.7 that has the title Boundary Conditions. I need a boundary condition like fixed which would be less drastic than clamped. I got the error message that the file is too large when I tried to attach Stresses in Plates and Shells by Ugural but I see the attachment link.

 

[makamo66]

I made a PDF of just chapter 1.7 and attached it

 

[kuruman]

The problem is that I have a book by Ugural that I attached and he only has the options of simply supported, free, sliding, and clamped/built-in for the boundary conditions of the edges of a plate. He discusses these in Chapter 1.7 that has the title Boundary Conditions. I need a boundary condition like fixed which would be less drastic than clamped. I got the error message that the file is too large when I tried to attach Stresses in Plates and Shells by Ugural but I see the attachment link.

At least I do not understand what you are trying to do and what you need us to answer. I gather that you are looking for boundary conditions. These are set at the beginning of a problem before you to solve it and depend on, well, what physical situation you are trying to describe mathematically.

It seems that you are trying to describe the breaking of a board as done in martial arts. OK, I suggest that you watch a video where this is done and see how the board is supported before the blow is delivered. The one I saw was selected randomly and shows the plank with its two ends resting on top of cinderblocks. The boundary condition then would be "simply supported at two ends." Is that what you are looking for?

 

[makamo66]

I want to get the boundary conditions for a board held at two edges by participants.

 

[makamo66]

Here's a picture

 

[256bits]

Clamped boards would be more dangerous I believe. When the board breaks, the broken 2 sections become cantilevered sections, and would have springback, much faster than retrieval of the hand. The energy release due to the break could be enough to break the hand.

 

[256bits]

I want to get the boundary conditions for a board held at two edges by participants.

Then you would set the moment at the holding hands to what they are able to sustain, which would be closer to the simply supported case ( 0 ) than the fixed ends case ( -FL/8 ). static loads.

But you do not know F for this dynamic situation.

 

[kuruman]

I want to get the boundary conditions for a board held at two edges by participants.

From the AI overview when I searched "boundary conditions"

So in your case, the boundary conditions would be "the two ends of the board are held." This could mean that the two ends are clamped and prevented from moving in any direction. If you want to allow the ends to move or consider participants' hands moving to some degree, then you need to describe what this means mathematically.

 

[Dale]

he only has the options of simply supported, free, sliding, and clamped

And what is wrong with simply supported?

 

[makamo66]

It doesn't seem to be fixed at the edges, it looks like it can move around at the connection points on both edges of the plank

 

[jbriggs444]

It doesn't seem to be fixed at the edges, it looks like it can move around at the connection points on both edges of the plank

Yes.

The hands are not strong enough and friction is weak enough so that any lateral support (putting the board into tension) is negligible. The ends of the board are free to be drawn together as the board bows away from the kick.

The hands are also not strong enough to resist twisting. The ends of the board are free to rotate as the board bows away from the kick.

The hands are strong enough to resist the linear impulse in the direction of the kick.

Anyone who works with boards knows this intuitively. We do not support a board on two sawhorses like a length of barbed wire by anchoring the ends firmly and applying a tensioning tool. Neither do we support a board on a single sawhorse, screwing one end down firmly so that it can hang cantilevered. Both of those approaches would be hideously inefficient and ineffective. Instead, we simply lay the plank between the two sawhorses. Both horizontal friction and torque at the horse/board interfaces are pretty much negligible.

 

[Dale]

It doesn't seem to be fixed at the edges, it looks like it can move around at the connection points on both edges of the plank

Yes. That seems correct to me.