According to the IIW Recommendations for Fatigue Design of Welded Joints and Components (2nd Edition, 2016), fatigue is the leading cause of failure in welded structures under cyclic loading.
The key principle is that welded joints have an inherent fatigue class based on geometry and loading direction. For example, a transverse butt weld ground flush has a FAT class of 125 MPa, while a non-load-carrying fillet weld drops to FAT 80 MPa.
Common pitfalls include:
• Ignoring the weld toe as a stress raiser.
• Assuming post-weld treatment (TIG dressing, hammer peening) always upgrades the FAT class.
• Overlooking the effect of plate thickness: thicker sections reduce fatigue strength.
The IIW recommendations provide a systematic method: determine the detail category, apply thickness and stress ratio corrections, then compare the design stress range to the allowable fatigue strength.
For welding coordinators, the takeaway is that fatigue design is not a material property but a detail property. A well-designed joint geometry often matters more than the base metal grade.
What is the most common fatigue-related non-conformance you encounter on welded fabrications?
Source: IIW Recommendations for Fatigue Design, 2016