Fatigue Safety Factor

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.

Fatigue Wizard allows the definition of two different FSF type diagrams known as the:

• Goodman diagram.
• Haigh diagram (more generic).

By default the FSF is calculated using the standard ‘Goodman’ diagram as shown here.

Goodman Diagram

Generic ‘Haigh’ Diagram

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.

NoteValues of ultimate strength and yield strength do not have to be the same in compression as they are in tension. This note can apply to materials such as cast irons, which show greater values of compressive strength.

Calculation of FSF

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.

Fatigue Wizard calculates two standard methods of forming the intersect with the infinite life line; the method of constant stress ratio (constR), and the method of constant mean (constM).

Method of Constant Stress Ratio (constR)

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.

Method of Constant Mean (constM)

The basis for the method of constant mean calculation of the FSF is the assumption that when scaling stresses in the cycle to the ‘infinite life’ line, only the alternating stresses are scaled.

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.