Fatigue is the cause of more than nine failures out of ten! Often the reason for the failure is poor knowledge of the fatigue load on the structure or bad design causing high stresses in critical points of the structure. Airplanes, trailers, lorries and cranes are example of products where fatigue is critical. Welded joints, bolted holes and lugs are often critical details.
Picture 1: Fatigue loaded structure.
The stress range, the number of load cycles and the load spectrum are the most important factors for describing the fatigue load. The stress range is simply the difference between the highest stress and the lowest stress during the load cycle. The number of load cycles that the structure undergoes during its lifetime is of course very decisive for how long the design will hold until the cracks grow to an unacceptable length, causing fatigue failure.
The maximum stress levels that cause fatigue failure can be dramatically lower than the yield strength of the material. When a fatigue crack reaches the critical size, the structure will fail suddenly.
Good design of the structures details is the first thing to consider to avoid fatigue failures. The design of welded joints and the weld quality is important, the smoother the joint geometry and the higher the weld quality, the longer the lifetime. Square holes, sharp corners and small bending radii of steel plates are to be avoided. Round holes, smooth fillets and radii gives longer life. Avoid all stress concentrations as far as possible.
Fatigue fracture in a test specimen with fillet welds shown below. The crack starts typically at the weld toe and then propagates further. In some cases, cracks also can start from the root side of the weld.
Picture 2: Fatigue fracture in test specimen
Doubled fatigue strength gives doubled load capacity. However, the good news is that the life can be eight (2x2x2=8) times longer! Picture 2.
Picture 3: Double strength gives eight times longer life
The design criteria summarize the final result, practice to know all the parameters in it!
Picture 4: The design criteria.