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Structural Fatigue

It is not uncommon to come across the sentiment that structural fatigue is a very complex phenomenon to explain, and an equally complicated issue to solve. A relatively recent high-profile case in Finland, which involved the Finnish Navy, brought structural fatigue into the national spotlight. In September 2015, when explaining that the vibration induced by the propulsion systems of their Rauma-class missile boats was the source of the structural fatigue on their vessels, the Finnish Navy spokesperson also indicated that “structural fatigue is terribly difficult to explain.” But is this the case?

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Structural fatigue is not the easiest phenomena to get one’s head around, but it is also not the hardest. In essence, fatigue sets in all structures that are exposed to dynamic loads and conditions. These loads may include, for example, temperature changes, wind strength changes, corrosion, and varying weight loads, such as experienced on bridges. This holds especially true for structures manufactured from metal. The welded parts of such metal structures are particularly vulnerable to fatigue. 

If the loads are above a certain threshold they can lead to microscopic cracks in the structure, which over time become progressively larger until a tipping point is reached and the structure fractures completely. It is important to note that these loads are individually insufficient to cause a structure to fracture, but their dynamic application over time causes fatigue to set in. 

As a phenomenon fatigue is not new. It has, in fact, been studied quite extensively and nowadays, with the help of modern technologies and tools we can with great confidence dimension structures to avoid the associated risks. 

Load prediction is the most challenging aspect

The most complicated aspect of fatigue dimensioning is load ”prediction”. In essence, this means defining the load in such a way that one can be sure that the load will be incurred during the lifespan of the structure. In the best case scenario, there is already an existing or similar structure that can be used to take load measurements over a period of time. If the structure is unique or a whole new concept, however, load definition becomes more challenging. Fortunately, modern simulation technologies allow the simulation of future structural loads. Historical loads can be simulated virtually with multibody simulations (MBS), which produce information that can be applied in fatigue dimensioning. As such, one is not reliant on any guesswork regarding loads. 

What significance does the magnitude of loads have? If, for example, there is a 10 % difference in the magnitude of loads that affect a part, this translates to a 33 % difference in life span. If a particular part’s lifespan is supposed to be ten years, a larger load can reduce that life span to seven and a half years.

The quality of welded structures a key factor 

If the structures in question are welded structures, particular attention must be paid to the quality of manufacturing, in addition to reliable fatigue dimensioning. It can be said that the life span of an individual weld is in many respects dependent on the “hand” of the welder. It is, as such, not sufficient to only ensure that the structure is designed and dimensioned according to reliable calculations. Manufacturing quality and quality control play a critical role in ensuring that structural integrity is maintained as required. 

Unfortunately, one poor weld can spoil an otherwise perfectly designed and manufactured structure and lead to serious accidents and loss of life. An equipment weld that has been done in the wrong place can also have catastrophic consequences for structural integrity. 

In structures that are exposed to dynamic loads or vibration, the dimensioning of the structure is in most cases a crucial factor with regards fatigue. Structures should be designed and analysed well in order to ensure a sufficient lifespan and at the same time to avoid over-dimensioning, thereby reducing material and production costs. Especially in large welded structures, the location of welds and the type of weld joints used significantly affect the structure’s lifespan and costs. An additional benefit of ensuring structural integrity is that one doesn’t end up in a situation where you have to prohibit the use of structures, machines, or vehicles while they are being repaired, that is if they can be repaired at all. 

Different weld joint types have varying lifespans

What effect does the type of weld joint have? This is most easily illustrated by means of an example where we consider a weld joint that can be welded with either a fillet weld or a full penetration V-weld. There is about a twofold difference between the two weld types’ fatigue classifications, but approximately a threefold difference in their lifespans. One can, therefore, say that a fillet weld would last, for example, two years in a structure, whereas a single-bevel butt weld would last six years. For the owner of a digger, for instance, there is a clear difference between having to repair or replace a component every two years as opposed to running a machine for six years without major repair. 

Structural fatigue is not in the realm of science fiction, it is, in fact, something much closer to home. We solve fatigue related issues every day and as a result, ensure the safety and operational reliability of an extensive range of structures, constructions and machinery. 

Author: Leo Siipola 

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Structural Fatigue
The location of welds and the type of weld joints significantly affect the structure’s life span.

Picture 1. The stress range of a frame in the x-direction.

Picture 2. Calculating the stress range on an adapter’s weld with the Notch Stress method.