The formulation of an experiment to test the crushing force or crushing pressure on an object is determined by a number of variables. Is the object being crushed hollow or solid? Is the object a regular geometric shape or random shape? Is the object's structure homogeneous or non-homogeneous (steel versus rock, for example)? Is the material of the object ductile or brittle? Solid, ductile objects cannot be crushed by force or by pressure. The definition of the crush point would be impossible to state. Solid ductile objects deform.
Force and pressure are related by the equation p = F / A, where p is pressure in psi, F is force in pounds, and A is the cross-sectional area of the object in square inches. You must be able to measure two of the three variables in order to calculate the third.
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Dead-Weight Test Experiment
Deadweight testing is by far the most straightforward and simplest method of testing crush pressure and crush force. At its most-basic level, it requires a bathroom scale, a vernier caliper for measuring wall thickness and some weights (for a classroom setting, books work fine). Place a can on the scale an write down the weight. Then, start adding weight to the top of the can. If using books, carefully place each book, trying not to side-load it in any way. Watch the scale and continue to stack books until they fall off. Record the weight on the scale when this occurs. Subtract the starting weight and the result is crush force. To calculate crush pressure, you will need to determine the cross-sectional area of the object. If it's a soda can or similar item, measure the wall thickness and the perimeter, and multiply the two. This is the area. Divide the area into the force, and the result is crush pressure in psi.
Atmospheric Pressure Test Experiment
Partial-pressure testing will only work on objects that can be crushed with 14.7 psi of pressure or less. This experiment, again, is useful on soda cans, or other similar cans with a small opening in the top. For this experiment, you will need a small hand-operated vacuum pump with an attached vacuum gauge, duct tape, small diameter vacuum hose and caulk.
Place duct tape over the opening in the can and seal it up tight so that no air can leak into the can. Then, take a Phillips-head screwdriver and punch a hole in the can bottom no bigger than the vacuum hose, maybe even a bit smaller. Insert a length of vacuum hose and run a bead of caulk around the hose to seal it. To the open end of the hose, attach the vacuum pump. Slowly begin pumping the air out of the can until the walls start to crinkle. Record the vacuum level at which this occurs. If the vacuum reading is not in psi, convert it to psi and subtract it from 14.7 psi (atmospheric air pressure). The difference is the pressure required to crush the can. By taking the can's diameter and height, you can calculate its surface area in square inches. Multiply this number by the crushing pressure, and you will determine crush force.
Band Crush Test Experiment
If the object to be crushed has thicker walls, but can still be crushed by bare hands or pliers, an alternative method of testing is called the band crush test. You will need for this experiment a spring scale (also called a fish scale), a 4-foot length of non-stretching strapping material (the plastic banding that comes on copy paper boxes or cardboard boxes will work great), and two pairs of locking pliers.
Measure the width of the strap. Clamp one end of the strap in a pair of locking pliers. Place the object on a flat surface, and wrap the band one complete wrap around the can. Put the closed end of the spring scale on one of the jaws of the locking pliers, then clamp it and the strap tightly using the locking pliers. With a partner holding one end, pull the strap just taut and record the weight. This is the tare weight of the rig. Then, slowly begin to pull on the open end of the spring scale. The strap will tighten, and with continued pulling force will begin to crush the can. Stop when the can starts to crush and record the weight shown on the scale.
The crushing pressure is found by subtracting the rig weight from the crushing weight. With the known width of the strap and the circumference of the can, you can determine the area of the band that was in contact with the can. By dividing the net force by the strap's contact area, you will determine the crushing pressure of the can.
High-Pressure Implosion Experiment
This experiment must be performed with professional, certified and calibrated equipment, but is common in the laboratory and research lab.
If you want to determine the crush pressure of an object that is hollow yet fairly heavy --- a piece of aluminium pipe, for example --- it requires the use of a compression chamber. This is a tank (pressure vessel) filled with water and connected to a pump that can produce thousands of pounds of pressure if needed. In order to effectively crush a sample in this chamber, it must be hermetically sealed. Seal the ends by welding, brazing or crimping, or any necessary means. Before making the last tiny bit of seal weld or braze, let the object cool to room temperature. That way, the air inside the can will be at one atmosphere. Quickly finish sealing the can.
Place the can in the compression chamber and anchor it to the bottom, if necessary. Seal the chamber, and begin increasing the water pressure inside the chamber until you see the object begin to deform through one of the two sight-glasses built into the chamber. Record the pressure and decompress the chamber. You have found the object's crushing pressure, and by determining the surface area, can determine the crush force.
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