Why Improve The Fatigue Life of Welds
Welds are where cracks commonly occur from, particularly weld transitions. The strength of the weld is what generally determines the life of dynamically loaded welded steel structures. Welds are crucial parts of steel structures, so it’s vital to create them with longevity in mind. The best way to do this is to improve the fatigue life of welds.
Methods To Improve The Fatigue Life of Welds
The two common ways to improve the fatigue life of welds are: Altering the geometry of the weld and introducing compressive residual stress.
Altering the geometry of the weld
We’ll start with altering the geometry of the weld. Altering the geometry reduces the stress concentration at the weld toes (see image below).
The stress concentration is a result of the change in geometry between the weld and material. This is known as the notch effect. Often a ‘notch’ is created between the weld and material during the welding process, which results in non-uniform distribution of stresses. Reducing the ‘notch’ and smoothening the transition between the weld and material.
Grinding
Grinding the weld toe is a common post-weld process used in industry completed to improve the fatigue life in welds. Grinding the toe removes any cracks that may have initiated during the fusion process of welding. However, it also smoothens the transition from the weld toe to the material/plate welded.
It is optimal to grind at least 0.5 mm below any flaws present in the weld toe (see image below). Along with a more uniform stress distribution at the weld toe, the grinding process also removes flaws in the toe. The grinding process is complete via several methods, for example, burr grinding, disc grinding. These methods all work from the basis discussed above.
Remelting
This method works very similarly to the grinding process discussed above. Remelting melts to weld toe region to shallow and smoothens the transition between the weld and parent material, reducing the stress concentration. This process can be visualised as a dressing process and utilises TIG welding equipment which is known as TIG dressing.
TIG dressing uses TIG welding equipment without filler material. Unfortunately, it is difficult to determine the quality of the process upon inspection. The final result for both remelting and grinding is very similar.
Introducing Compressive Residual Stress
As a result of welding, we see high tensile residual stress. Introducing compressive residual stress will help overcome the tensile stress and increase the fatigue life of the weld. There are numerous methods which can introduce compressive residual stress. We will cover the main ones here.
To obtain the benefits this process should only be used, for structures that undergo a maximum applied stress of less than 80%. As the applied stress increases towards yield, the benefit of the technique decreases.
Shot Peening
Shot peening produces compressive residual stress by plastically deforming the weld toe. It is a form of cold working. The process is very similar to sandblasting, which is an abrasive wear process usually used to remove paint. However, in shot peening, the blast material is small metallic or glass particles.
A great advantage of shot peening is the cost. It is very cost-effective and can easily cover a large area.
Hammer/Needle Peening
Hammer/Needle peening induces compressive residual stress by striking the material. The striking plastically deforms the welds and occurs at a high rate of approximately 5000 strikes/min. Compared to shot peening, we see larger increases in fatigue strength from hammer/needle peening.
This is a result of a greater amount of cold working introduced into the material. The greater amount of plastic deformations, from cold working, results in the compressive residual stress develop at a greater depth of the material.
The difference between hammer and needle peening is the size and material of the tips causing the plastic deformation. Hammer peening uses round steel endings with a diameter between 6-18 mm whereas, needle peening utilises steel wires that are approximately 2 mm in diameter.
The improvement gained from the two methods is very similar however hammer peening gives slightly better increases in weld fatigue strength compared to needle peening.
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