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What is a Fatigue Test?
Mary McMahon
Mary McMahon
A fatigue test is a type of materials test in which the goal is to determine the point at which the material will fail. Fatigue is a problem for all materials, caused by wear and tear as they are used. In a fatigue test, fatigue is deliberately induced to find the stress limits of the material. Materials for certain applications may need to meet specific requirements in terms of fatigue. For example, metals used in a bridge need to be able to endure very high stress loads.
Fatigue is caused by things like motion, oscillations, vibration, pressure, wear, and loading. To borrow the example of a bridge again, bridges are subjected to stress as they move in the wind, stress during seismic events, and the daily vibration and load of thousands of cars. Over time, fatigue leads to cracking in the material, and will eventually cause failure; either the material will shear off, or the cracks will cause a fracture of structural components, leading to collapse.
Whether it's a bedframe or a bridge, fatigue is an issue. In a fatigue test, the materials used are subjected to vibration and other kinds of stress and carefully monitored. As soon as signs of fatigue start to appear, they are noted and tracked, charting how long it takes for fatigue and eventual failure to occur. The point at which fatigue starts to happen is the lower stress limit. Fatigue testing is sometimes referred to dynamic testing, because it involves a variety of techniques to pummel and essentially assault the material to find its stress limits.
With something like a piece of furniture, the furniture itself may be put through a fatigue test. With a chair, for example, a machine might simulate someone flopping into the chair over and over, or might create the twisting stresses which happen when the chair is dragged across a floor, with or without an occupant. In the case of large equipment, individual machine components are put through fatigue testing, since the entire piece of equipment may not fit in the testing area.
The results of fatigue testing are kept on file, and may need to be filed with a government agency, in some cases. For example, people working on a high rise building need to show proof that a fatigue test was done on components and that the components passed. This information can be useful if a failure occurs, creating a paper trail which can be used to figure out when things started to go wrong.
Mary McMahon
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AboutMechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
Learn more...
Mary McMahon
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AboutMechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
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Man with a drill
Discussion Comments
The results of fatigue tests are used to set warranty periods for furniture and appliances. The company estimates how long it would take the average person to use the product the number of times it proved faithful in the test, and then it offers a warranty for that amount of time.
My uncle worked for a furniture factory, and he had to write reports involving fatigue tests. He had to help calculate how many years a warranty should last on each product.
If the products fail to last throughout the warranty period, then the customer can return them for a refund. If the warranty has expired, then they are stuck with the product.
@seag47 - It makes sense to have a robot doing that mundane, tiring job. I had often wondered exactly how companies do fatigue tests on their products.
One product I really wondered about was a mattress. I thought maybe they actually hired people to come and jump on the bed until it broke! This would be a fun treat for children, and their parents would appreciate the outlet it provided for their extreme energy levels.
Logically, they probably use a machine instead. I guess they have some sort of robot that presses down on the mattress repeatedly until something happens to it.
I am always nervous when driving over bridges. I know I probably shouldn’t be this anxious, because the materials have undergone a fatigue test.
However, what makes me a little uneasy is that they can only test a small scale model of the bridge. I fear that they don’t know what could actually occur in the big version.
Outside of the unpredictability of natural disasters, these tests probably predict necessary maintenance times with accuracy. I don’t even live in an area where something like a big earthquake is a possibility, so I should probably just relax. I’m no mechanical engineer, after all.
My friend works for a company that designs window blinds. They are always coming up with new models and features, but they have to test every new product that they invent.
He told me that they have a large computer that resembles a robot. It has a mechanical arm that is programmed to pull the cord on the blinds over and over again.
The robot continues to pull it until the product breaks or no longer functions normally. They also have some blinds that you raise and lower by lifting them up with your hand, so the machine can do this test, too.
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