All materials, whether they are plastic or metal, can be subject to failure and eventually will succumb to fatigue failure. Thinner, more brittle materials may break more quickly, while thicker more flexible materials can take more weight or movement before breakage occurs. In the case of all materials, failure typically refers to damage that prevents the material and components from working correctly or to complete breakage.
The more movement between working parts, the more heat and friction can be built up. Metal tends to absorb energy from a load it is bearing and then expands under the pressure. When the load is removed from its surface, the metal will return to its normal position. Eventually though, the load will expand the metal beyond its normal capacity, which means that it will not be as able to return to its original condition any longer. This might result in small pits along the surface of the metal, which can weaken it, or in larger fractures that will eventually lead to a complete break.
In humans, fatigue failure happens during certain types of motion- whether it is caused by distance, speed or a combination. Pounding feet on the pavement causes an impact of solid (foot or shoe) to solid (ground), which is typically felt as vibration from the sole of the feet and up the body. This impact will eventually cause the legs and feet to become exhausted and the runner/walker will have to stop. In other words, they have reached their point of fatigue failure. Like other materials, the human body can take only so much of these impacts without damage. Eventually, small tears or other injuries will build up and may eventually lead to a complete failure for the body in question. Reducing fatigue caused by impact and vibration is key to preventing some of these injuries.
Designers for both machines and athletic shoes as well as for other items have long looked for ways to absorb some of the generated energy and to safely dissipate it before it causes fatigue failure. Absorbing some of the most serious impact and vibrations might be the key to accomplishing this and may be the way to increase the useful life of certain machines as well as to allow a human body to exercise for a longer period of time, to cover greater distances or to strive for faster speeds without worry of increasing the risk of serious injuries.
Sorbothane is frequently used for a number of these applications; it is used in machines to reduce the vibrations that can cause components to break and the machine to fail. It is also frequently used in athletic or other types of footwear so that humans can move as they are meant to without the accumulating stress causing them to tire as quickly as they normally would. Because it is light and flexible, it can be used to create a comfortable insole for shoes of all types that does not change their fit at all but can allow for greater performance.
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