Breaking Materials

Breaking2

Materials that are brittle and whose strength comes mostly while in they are in compression are used in breaking, such things as cinder blocks, bricks, and ice. Other things, such as soft rocks and re-breakable plastic boards, may also be used. However, wood is the most widely used material for breaking.

Breaking materials vary as to consistency. Wood provides the most inconsistent qualities, even when from the same tree. Board sizes may vary but the usual size is 12x12x1, which is always broken with the grain. Long boards, such as 2x4s, are always broken against the grain. Tiles provide the most consistent qualities. Bricks also provide consistent qualities, but only when they come from the same batch at the manufacturer. Ice is extremely consistent as long as is the same temperature throughout, which is hard to maintain. Plastic (as in re-breakable practice boards) provides consistent qualities, but repeated use will quickly degrade in performance, and may give the practitioner a false sense of achievement when moving on to more difficult breaking mediums. Also, the breaking resistance of plastic boards decreases with an increase in temperature.

Also, there is a concept called "spring co-efficient." This co-efficient is different between softer materials like wood and plastic, than in harder materials like brick and tile. If you were to look at a super slow motion video of a material being broken, you will notice that the material does not break at the point of impact, it "gives." The more the "give", the more "bounce" the material has. Softer materials, such as wood, have more give than harder materials, such as tiles. If your technique does not break these softer materials, this bounce is reflected back into the arm or leg and you will feel it. If the amount of force required to break a piece of wood is the same for a tile, you will probably feel more pain from a bad technique into wood than from the tile.

Wood Boards

Usually 12x12x1 white pine boards (actual dimensions are a little smaller). Wood boards can break at some point other than the impact point. Wood does not have to be hit in the exact center. When choosing board to break:

Choose ones with least number of knots (wood will break through a knot not through it). Avoid wood with knots that run along the length of the board. Small round knots are not a problem but avoid large knots in the center of the board.
Choose ones lightest in weight (less moisture)
Choose ones lightest in color (less sap)
Choose ones with rings closest together. Since you are splitting the board along the grain, a board with grain running in narrow bands along the length will be easier to break. The more grain that runs in a vertical direction, the harder the wood will be to break.

Only break in one place, so they must be hit dead center. Plastic interlocking "fingers" that create the board's resistance to breaking are susceptible to temperature. The warmer they are the easier the board is to break. Breaking resistance decreases the more times a plastic board is broken. When re-chambering the hand or foot after a break, the board fingers may dig into the arm or leg and cause injury.

Wood Sticks, Planks, or Bats

This includes any type of wood that is broken across the grain. In this case, you want grain that runs wide and flat.

Patio Blocks

Though actually slightly smaller, their dimensions are listed as 8x16x2. They are made of cement with a small amount of aggregate (additional rock chips and pebbles mixed into the block to strengthen it). Beware of reinforced blocks. They may look similar to regular blocks but they have a stronger aggregate and are much harder to break. These blocks most often come in gray and red and may be flat on both sides or the bottom may have small ridges. The color makes no difference but ridges may make it very slightly stronger. These blocks have a consistent hardness. Wet blocks are harder to break so insure blocks that have been stored outside are dry. One block equals about five strong boards and is cheaper than one board.

Rocks

Rock breaking requires a very solid and reliable base, such as an anvil or head of a sledgehammer. Because of molecular structure and complex irregular shapes of different rocks, rock breaking is unpredictable. Diamonds are split along fault lines. Choose rocks with fault lines that run in the direction of your break.

When striking a rock, do not lay it on the base and start hitting it. The rock must be lifted slightly from the base, so there are two support points. Since rocks do not flex, the lift only needs to be a small amount, never less than 1/4" or more than 1/2". Then strike the rock between the two support points, usually using a knife hand strike. You cannot hold back on the strike, so you should have a lot of experience with breaking before tackling a rock break.

Other Materials

Other materials include ice, stones, cinder blocks, tiles, clay bricks, coconuts, etc. Be careful when experimenting with other materials. Red bricks a will weaken if baked in the sun. Bricks may vary greatly in hardness due to the manufacturing process and the amount of sand (aggregate) in them. Generally, lighter colored bricks are easier to break.

How do wood boards break?

If trees were rigid, unmoving objects, they would snap in high winds. However, the composition of wood allows trees to flex with the wind, absorbing the force of the wind without breaking, at least until their flex limit is exceeded, and then the trees break. When a single board is struck, it bends like a bow, until the flex limit is exceeded and the internal fibers in the board begin to tear apart. When a board is flexed, the bottom surface must stretch more than the upper because it moves in a larger arc of a circle. This means that the bottom will start to crack first. Once cracked, the wood becomes very weak along the break along the inside of the crack and the crack immediately starts to deepen into the wood. The board actually tears open form the bottom to the top.

When more than one board is used, the bottom board feels more stress than the other boards since there is no board beneath it to resist its flexing. When the bottom board breaks, the break propagates through each board from the bottom up, breaking each board in turn. Since the stress needed to break two boards is greater than the stress to break one board, when the bottom board breaks there is suddenly a much larger stress on the top board than is necessary to break it, so it breaks quickly.

If the strike is too weak to break the boards, they will only bend, storing all the energy in their fibers. Then the boards will transfer that energy back into your hand or foot, and pass it along through the joints, causing pain or injury. Make sure to use enough arm or leg motion to force the boards past their maximum flexing point or all your punching energy must be absorbed by your body.

When the board is struck, it begins to vibrate sinusoidally with a certain frequency and amplitude. The frequency of vibration is represented by f=1/2Π√k/m where k is the spring constant for the board (in this case 274 lbs/in ) and m is the apparent mass of the part of the board that is moving (0.5 lb mass or 0.22 kg). This gives a frequency of vibration of about 54 Hz a note that is at the lowest level of our hearing to hear, the sound you hear is that of the board breaking apart. The amplitude of the vibration is related to frequency times the velocity of the striking. This is represented by A=v √m/k where v is the velocity imparted to the board at contact. The breaking speed v in our example would be 4 m/s or 14.8 km/hr. Although this seems low, for an inelastic collision, the speed of your fist must be twice this speed or 29.6 km/hr. To attain this speed, the arm (0.6 m in length) must snap forward in less than 0.3 s.

An elastic collision is almost twice as efficient, which means that hitting the board with a fore fist punch takes less power than with hammer fist. However, because a board hits back with the same force with which it is hit, you must be careful when hitting objects harder than your fist. For these objects, hitting with a softer part of the hand, such as with a hammer fist or knife hand strike, will be less painful.

As you double or triple the number of boards, you must similarly double or triple the striking speed. To break three boards, it takes a breaking speed of about 90 km/hr.

Wood Grain Alignment

There are two basic wood grain alignments of a board used for board breaks.

Figure 1 is the top edge view a board cut from the center of a tree, where the grains are such that one face is the mirror image of the other. The amount of force needed to break the board will be the same no matter which side is struck.

Figure 1

Figure 2 is the top edge view a board cut slightly off-center from the center of a tree. The left side was closer to the center of the tree than the right side. Since the grain alignment is not symmetrical, on which side should the board be struck?

Figure 2

Striking the left face of the board as it is shown Figure 2 will cause the board to break more easily than striking the right face of the board. When a board with a grain alignment similar to that shown in Figure 2 is held on the edges by a board holder and the board is struck, three forces act on the board: two forces from the board holder pushing back against the board edges and one from the board breaker who is pushing against the board center. When struck, the board bends and the backside begins to stretch and split while the front side is compressed. The grain on the left side of the board is closer together and is more easily compressed. The grain on right side is straighter than the grain on the front side and thus easier to stretch. If the board were struck from the right side, the opposite would be true making the board harder to break.