The basis for a Fatigue Safety Factor (FSF) calculation is a perceived ‘infinite life’ of the component. To achieve the perceived ‘infinite life’ beyond N number of cycles, construct a diagram that shows values of mean stress and alternating stress, which achieve N cycles (or infinite life). You can generate these diagrams experimentally. However, for fatigue analysis, it you can approximate these diagrams from monotonic material properties, such as ultimate strength, yield strength and the fatigue strength.
The generic ‘Haigh’ diagram is constructed with six points. The points are calculated from input of the material UTS (tensile and compressive), material yield (tensile and compressive), and the material fatigue endurance limit. The key points in the ‘Haigh’ diagram are then defined as shown in the previous diagram.
The basis of the Fatigue Safety Factor (FSF) calculation is the ratio of a point representing the stress cycle in question (mean and alternating), and an intersection point with the ‘infinite life’ line.
The basis of the method of constant stress ratio calculation is the assumption that when scaling stresses in the cycle to the ‘infinite life’ line, both the mean and alternating stresses are scaled with the same ratio.
The FSF calculation gives a safety factor with respect to ‘infinite life’. In calculating this safety factor, the assumption is that there is no damage summation of stress history constituent cycles (as is the case with a ‘life’ calculation). An FSF is calculated for all cycles individually, and the worst safety factor is reported. This report assumes that if the material/component can give infinite life when subjected to the worst cycle, then any subsequent cycles within the stress history do not have any further effect. This assumption is reasonable for ferrous materials that show a distinct endurance limit. However, exercise care if you apply the same assumption to a material that does not show this phenomenon.