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What are Fatigue Cracks?

Ken Black
Ken Black

Fatigue cracks are cracks caused by repeated stress on a structure over a period of time. Many times, fatigue cracks can be superficial, but at other times, they can cause a catastrophic failure leading to disastrous results. Fatigue cracks are a serious engineering concern that can only be discovered by inspectors, who are able to tell how serious the cracks are.

Perhaps the best known example of how disastrous fatigue cracks can be comes from an incident in Minneapolis on 1 August 2007. On that day, during the afternoon rush hour, a bridge on Interstate 35 over the Mississippi River collapsed, causing 13 people to plunge to their deaths below. It was a national tragedy that caused many states to step up their bridge inspection policies immediately.

Man with a drill
Man with a drill

It was later determined by the National Transportation Safety Board that the bridge collapsed due to the inadequate size of large steel plates used to connect pieces of the bridge together. The inadequate size caused fatigue cracks that eventually led to full-blown fractures. Once this took place, the rest of the plates failed in a chain-reaction event. This is known as a catastrophic fatigue failure.

There are a number of characteristics unique to fatigue cracks. First, they happen in metal structures. Second, they also have predictable stages. These include the initial crack, progressive growth and final fracture leading to fatigue failure.

While fatigue cracks can be a major problem, they can also be very predictable in going through these stages. Only an engineer can tell if fatigue cracks represent a serious problem to a structure. In some cases, redundant pieces may be in place to prevent fatigue failures.

While the thought of fatigue cracks can be terrifying, they can also be predicted and prevented in many cases. Routine inspections often spot fatigue cracks and make note of them. In some cases, repairs can be made quickly and easily by replacing key parts of the structure. The good news is that fatigue cracks happen on the outside, visible sections of the steel the vast majority of the time.

While much of the attention regarding fatigue cracks centers around bridges, it should be noted that they can affect any steel structure and led to catastrophic results anywhere. The steel rails in railroads can cause train derailments if they fail. The steel in buildings, if fatigued enough, can also fail. This is especially dangerous in multistory buildings. However, fatigue failures in buildings are not as common as failures in other applications.

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