Fatigue Failure in Structures

Fatigue is a type of failure that occurs when a material fails at loads below the specified yield strength. It usually occurs from stress from cyclical loading over a period of time. Fatigue failure also results in brittle failure of an element from loads which would normally be resisted if it were static in nature and not cyclic. The first sign of fatigue manifests in the form of cracks in concentrated stress areas which is immediately followed by crack propagation and thereafter fracture failure. Examples of cyclical loading include high fluctuating wind actions, traffic on bridges, the opening of doors in buildings est.

Although fatigue occurs in buildings and bridges, bridge fatigue appears to be more severe than building fatigue. For example, in bridges, the flow of traffic is repetitive and heavy trucks are usually disrupted from time to time. Also, since fatigue loss occurs relatively quickly. It is crucial to inspect the areas of concentrated stress in bridges to avoid fatigue failures

Crack propagation in a typical composite bridge
Figure 1: Crack Propagation in a Composite Bridge due to Fatigue

In buildings, fatigue occurs in only small parts and its almost always impossible to cause a total collapse of the entire structure. Only the moving parts will be damaged and need to be replaced. Both concrete and steel structures are both susceptible to fatigue failure, although reinforced concrete structures are less prone to it.

Fatigue in Steel Structures

Steel members have intrinsic microscopic defects. These defects can be revealed when the steel part is exposed to fatigue and so their existence has an effect on the resistance of the member. Steel ‘s inherent fatigue resistance can be assumed to be approximately half of its ultimate strength. This applies as long as the element is small, has a polished surface and there is no significant concentration of stress in the element. When an element deviates from these conditions, its fatigue resistance is reduced. For example, if the surface is not polished, the flaws would remain in the steel. There would also be no areas of high-stress concentration relative to the size of the element, as would be the case.

Corrosion of Steel Members

Corrosion must be considered for fatigue analysis of the steel structures. It has been shown that, due to the particular increased brittle condition of steel caused by corrosion, its fatigue resistance is greatly reduced. Conversely, the deposits generated due to corrosion can fill the previously described flaws, partially counteracting the corrosion’s brittle effect. It does this by reducing the spread of crack growth within the material. This is of great importance when designing elements that are naturally corrosive to marine environments.

Fatigue and Connection Type

The type of connection used in a steel structure has a great influence on the resistance of the structure to fatigue. A welded connection will react differently as wilt a bolted connection. Therefore a thorough understanding of how these types of connection resist fatigue is necessary when designing steel structures that have tendencies of being subjected to this form of loading.

Structures made up of a significant amount of welded connections are weak against fatigue. This is due to the difficulty in creating connections, which leads to stress concentrations due to poorly applied welding. The flow of stress passes directly through the butt welds and, as a result, is the most effective type of weld in terms of fatigue resistance. Fillet welds, on the other hand, do not offer the same level of resistance due to the change in the direction of stress within the connection.

Bolted connections resist fatigue through the friction they create between the elements fixed through them. The clamping mechanism produced by the bolts prevents the element from moving and thus the friction between the components resists the effects of fatigue. This applies only to simple connections only. 

BS-EN 1993-1-9 is the head document for assessing the resistance of steel elements and joints for fatigue loading.

Fatigue in Concrete Structures

As mentioned earlier, reinforced concrete elements are less prone to fatigue failure due to relatively low-stress levels than their steel counterparts. Concrete structures that are subject to high stress are more likely to subjected to fatigue effects than those that are exposed to cyclical impact loading. Road bridges are a good example of such structures.

Usually, the failure mode consists of premature load cracking, which is about half of what the structural element was designed to withstand in a static condition. Reinforced concrete components withstand corrosion effects through the steel reinforcement bars within them. According to tests and research carried out on the subject, straight bars have a much greater resistance than those that are bent.

Pre-pressed concrete elements are almost resistant to the effects of fatigue if the pre-compression level is such that no cracks are created inside the concrete. This is due to the fact that the entire concrete structure resists stress changes inside it, and not just steel reinforcement, making it more effective.

When pre-stressed concrete elements have cracks, resistance to fatigue effects becomes significant. This is particularly the case in exposed environments, which may lead to water ingress. This would cause corrosion in the pre-stressing strands within the concrete and thus reduce their effectiveness.

Impacts of Fatigue on Design Life

To conclude this post, the fatigue effect on the design life of the structures can be pronounced. Elements intended to withstand the effects of fatigue can not be expected to do so indefinitely. At some point, the element will succumb to a fragile failure, but it is the responsibility of the designer to ensure that the point at which this occurs is beyond the design life of the structure.

References

Young W.C., Budynas R.G. and Sadegh A.M. (2012) Roarkโ€™s Formulas for Stress and Strain
(8th ed.) New York: McGraw Hill

The Institution of Structural Engineers (2013)- Introduction to Fatigue – Technical Guidance Note 25 (level one)

Thankyou!

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