Failure Analysis & Durability Improvement

Fatigue Basics

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Fatigue is the failure of a component due to repeated loading & unloading. Each loading & unloading is called a cycle. The simplest demonstration of fatigue is take a paper clip, straighten it out, and then bend it back & forth. You'll probaly cause failure fairly quickly. It has been estimated that 90% of engineering failures are due to fatigue.

R Ratio

Factors influencing fatigue life include the stress range and the R ratio. The R ratio is defined as the minimum stress divided by the maximum stress. For exmaple, if the stress is cycled between -50 and 50 ksi, the stress range is 100 ksi and the R ratio is -1 (-50/50). Half of the stress range is called the alternating stress, which in this case would be 50 ksi. Two common R ratios are -1, which is often referred to as "Fully reversed" and 0, often called "Zero to Max". Not surprisingly, the greater the stress range, the shorter the fatigue life.

S-N Curve

If fatigue life is plotted against stress range, we get something like this:

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This is called an S-N curve, since it relates stress range (S) to cycles (N). Typically, the Y axis is linear while the X-axis is logrithmic. Note that the slope of the curve decreases as life increases. Some materials have what is called an "Endurance limit" or "Fatigue limit". In these cases, the curve goes flat, and there is a stress range below which the material will never fail, no matter how many load cycles are applied. S-N curves are also influence by R ratio. The higher the mean stress, the lower the fatigue life. If we plot S-N curves with different R ratios, we get something like this:

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Causes of Fatigue

Even when designed by top-flight engineers, fatigue failures may come about for a number of reasons:

Unrecognized Stress Concentration Factor - Most fatigue cracks begin at stress concentrators. These can be fillets, where two right angle surfaces come together, holes drilled for attachment purposess, or any other discontinuity in the component. A good example of this type of failure is the de Havilland Comet. It was the world's first commercial passenger jet, and a commercial success until a couple were lost due to fatigue that started at the window corners. With a pressurized cabin, the fuselage was stressed every time the plane reached cruising altitude, where the exterior pressure was much lower. The stress concentration at the window corners was greater than anticpated, and fatigue cracks initiated there.
Unexpected Loads - Loads that engineers think a component will experience in the field are not always what it actually experiences. If this is the case, no amount of testing will reveal the problem. This could be due to the way the operator uses the component, or it may be due to unforseen forces. An example of this is the failure of the Tacoma Narrows Bridge in 1940. Unexpected aerodynamic loads caused it to sway, earning it the nickname "Galloping Gertie", and eventually causing it to fail.
Microscopic defects: Typically, be voids or contaminants. These defects act as stress concentrators and cause small cracks to initiate very quickly, diminishing the overall fatigue life.
Incorrect microstructure - Microstructure is very important and often overlooked. Two components with exactly the same alloy can have vastly different mechanical propoerties. This is because of different microstructure, which can be influenced by heat treatment. If a part is cast and cooled very slowly, large grains can form. If it is cooled quickly, there is not enough time to form large grains, so the grains will be small.

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Failure Analysis & Durability Improvement

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Ed Pope
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Ed@Failure-Analysis-Durability.com

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