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With proper training, board breaking is an excellent way to demonstrate the power of Taekwondo. A scientific understanding of the mechanics behind board breaking is helpful to all board breakers.

Board breaking is a relatively simple task, but, as with all applications of force, it can be empirically analyzed. The following is an analysis of the speed and force required to break a board.

Board breaking consists of applying a large force to a piece of wood for a short time. Since both force and instantaneous striking velocity are important, this breaking analysis was undertaken relative to breaking energy. The boards were assumed to be standard 12 in. x 12 in. x ¾ in. White Pine (P. Strobus) held tightly at both ends (so all striking energy goes into breaking the board). Boards were assumed to be free of defects (no high local stress concentrations) and modeled according to column loading. This means that the maximum energy that the board can store before fractures begin to propagate is given by:

Umax=V*sb/2E

Where sb is the breaking stress, V is the board volume, and E is Young’s Modulus. Since the boards break mostly in tension, sb can be replaced with Mr, the modulus of rupture. The modulus of rupture is the highest tensile stress a material can undergo in bending before fracturing.

Figure 1 shows a 3D representation of the breaking energy applied to a board as a function of apparent mass (ma) and striking speed. Apparent mass is a function of the striking technique. In the case of a punch, someone could punch with only the mass their arm, or they could put their entire body mass behind the punch by snapping the shoulder and hip into the strike. Striking speed is defined as the instantaneous velocity of the fist at the point of contact with the board. 

The standard case is a fresh pine board with a volume of 0.0017 m3. With Mr=0.061 GN/m2 and E=8.81 GN/m2, the breaking energy is 359 J.  The energy required to break the board is shown as a horizontal plane with z=359 J.  Any combination of striking speed and apparent mass above this plane will result in fractured board, any combination below the plane will result in a fractured hand.  

Several recognized limitations exist with this analysis. First, the breaking stress energy is defined as the average energy per unit volume. During a strike, the stress energy is highly localized, and therefore neither shape nor geometry variations are adequately addressed. Secondly, at least some energy from the strike is used to displace the board, and some is dissipated as heat and sound.

Overall, the predictions of the model are consistent with experiential evidence and provide a meaningful estimate of parameters that can determine whether a given board will break or not break.

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