The designer, the production engineer and the individual welder (or other operator) can influence the fatigue strength by reducing the stress concentrations, the notch effects.
The designer can make all transition areas as smooth as possible and take extra attention of the design of for example weld joints, load attachments and holes.
The production manager is responsible of the quality of the production and the incoming materials condition. How are the surface and the edge condition of the sheets? How is the real quality of the weld joints in the final product? What are the critical parameters during the production process, are they stable enough? And a lot more to think about to ensure the products fatigue performance for the end user.
The welder is influencing the quality of the joint, if the weld geometry and the remaining defects vary, then the probability of fatigue problems increases rapidly.
Note that the classic fatigue design is based on nominal stresses often calculated by conventional engineering methods such as beam theory. Now-a-days FEM-analysis is more commonly used and the stresses are then calculated also locally. Methods as the "Notch Stress" and the "Hot Spot Stress" are available for the very advanced users. These methods are e.g. trying to evaluate the actual stress in the e.g. weld toe, but this is a very deep analysis. Fatigue design of welded structures are using the nominal stress method and all the experience from the different tests that supports this method. What value can you then use? Guidelines for that in the reference list included in the end of this tutorial.
There are generally many references describing the notch effect and giving design recommendations, so we choose not to repeat them here. However, our intention to develop FATcomp is to provide a tool and a database for different solutions and to increase the understanding on the effects of fatigue load in the users own products. Systematically using this software gives a tool for comparing different solutions both during the early design phase and the usage phase.
Picture 1: Stress concentrations, try to avoid them
A weld joint with a bad geometry that is undergoing a constant amplitude loading (full spectrum) has about 16 % of the fatigue strength of parent material under the same load conditions. The life of that bad weld joint is only a small fraction of the life of the parent materials. If you take the possible load variation into account the scatter is enormous, picture 2. The good side of this is that small improvements on design can give large positive effects on the service life. Relative design calculations can then be used if the load parameters are uncertain.
Picture 2: The fatigue strength is very dependent on the local gepmetry.
If you also take the load spectrum variation into account then the scatter is enormous.