Fatigue Life: What it is and Why it Matters

Fatigue life is a mechanical and scientific term that relates to how long an object or material will last before completely failing because of concentrated stresses. There are a number of different factors that can influence fatigue life including the type of material being used, its structure, its shape and temperature changes.

Calculated as the Number of Stress Cycles

In most cases, fatigue life is calculated as the number of stress cycles that an object or material can handle before the failure. There are several different types of stress values that are considered when computing fatigue life, including the nominal maximum stress value, which is usually less than ultimate tensile stress limits. There is another value, the yield stress limit, which may be higher than ultimate tensile stress limits.

When a material is put into use, the design can increase the stress that is put on the object. For instance, certain sharper angles, such as the corners in a square object, can be a significantly higher stress area than a rounded area, which disperses the weight and stress of a load more evenly over a larger area.

Fatigue Begins with Microscopic Cracks

The beginnings of fatigue may not be immediately noticeable, with microscopic cracks and pinpoint holes forming along the site of the highest level of stress. Eventually, though, the crack will widen or elongate to a point where it is not only noticeable but is obvious. Once one of these imperfections starts, it is usually not much longer before the object totally fails, and there are certain materials and items that are pulled out of use at the first, notable sign of fatigue related damage because they can be dangerous once they are under stress.

An example of this damage can be seen in metal parts that look bent or warped but may not be noticeable until the part snaps in half. Helicopter blades, which were previously created from pure metal, are subject to a number of stress factors including wind shear and temperature changes, which can increase the number of small cracks and fissures along their length. Because altitude changes can also increase the amount of stress, it is important that these be inspected, especially once they get to one half the normal fatigue life calculated for that material. Helicopter blades are currently being made out of other materials that are meant to accept the stress load with more give to increase its fatigue life.

Some Materials are More Prone to Stress Damage

Some materials, especially very thin or very rigid materials, are more prone to stress damage than others. On the other hand, there are materials that are meant to absorb and displace the stress in the object better than others. Sorbothane® material is used in certain types of objects because it absorbs the stress and displaces a load better than other types of materials. Because it has a blend of viscous and elastic properties, it employs the best and makes the object last longer. Sorbothane can also be used to lighten the overall weight of the object it is being built into.

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