What are Fatigue failures in welded structures?

Welding advancements is one of the biggest boons to mankind in the last century. This isn’t an exaggeration. How? From shipbuilding, building bridges to Aerospace, there is no field that is left untouched by the sparks of welding. If it needs more than one piece of metal, welding is there!

Using nuts & bolts to join two pieces of metal can make it sturdy, but it adds a lot of weight to the entire project. Welded joints are light-weight and only add to the aesthetic appeal by minimising any extra metal on the piece or metal structure.

Now, no matter how good the welding material used in the making of the structure, these joints get certain wear and tear over the years and tend to get brittle. A welded joint going weak is an invitation to hazards and accidents.

These problems are often termed as ‘fatigue failures’ in the welding industry and can be reduced to a great extent as well. Let’s get on with ‘fatigue failure’ bit by bit.

Fatigue, in a welded structure, is when a structure or joint develops a small fracture or crack due to continued stress being applied to it. Here, the key is the stress being applied to it. The quality of the welding wire, welding rod and all the welding machinery in general can be great, but over years, could be 50, could be 100, or any other number of years, depending upon the skill of the welder too, fatigue is a common trait. 

There are two phases to this:

1. Formation of the crack or the hairline being formed

2. Propagation of the crack or when the crack continues to get neglected and gets bigger and worse.

Fatigue can and should be taken care of in the first phase itself, otherwise the propagation makes it worse, and one can only calculate how quickly it will become bad, but it’s a gamble nobody wants to bet on.

Other than the stress causing fatigue failure, there are a few more reasons to it, let’s go over them briefly,

1. Inadequate preheating - Preheating  the welding wire with just the perfect temperature of the welding electrode minimises the risk of cracking, reduces metal shrinkage & thus reducing the brittleness.
   
   

2. Porosity
   Gases like nitrogen, oxygen & hydrogen often get trapped in the metal if precautions aren’t taken as these are commonly released during the arc process with welding rods. This gas can cause a bubble or a hole, & the gas exerts pressure on the metal, ultimately aiding the fatigue.
   
   

3. Incomplete joint fusion
   This is caused by the neglection of the welder. No matter what quality of the welding material, if the welder is a novice, they can miss out on one side of the weld. When a side of the weld hasn’t properly fused, it reduces the strength of the weld.
   
   

How fatigue can be controlled

1. Weld geometry improvement
   Holes and sharp corners add to the stress on welded joints. This technique aims on reducing the stress that these joints experience. It includes functionalities like using welding machines like a disc grinding machine which can re-melt a weld to make it furthermore fused and stronger.
   
   

2. Residual Stress Reduction
   This is a result of the expansion that happens due to heat. To reduce it, vibratory stress relief or hammer peening are common techniques used,

All welded joints are susceptible to some amount of wear and tear and can have fatigue due to the aforementioned reasons. But we hope the solutions help you in controlling it!