There are different types of design life analyses in fe-safe. Choosing the FRF analysis may lead to an incorrect factor of strength or incorrect hot-spot identification if the design life is not infinite or if the loading is not constant amplitude. This section will summarize the methods available and their characteristics, in brief.

 There are three types of design life analyses in fe-safe:

1. Factor of Strength (FOS)

2. Fatigue Reserve Factor (FRF)

3. Failure Rate for Target Lives calculation - This analysis calculates a probability of failure for the target lives due to variation in material fatigue strength and the applied loading. Not discussed further in this section.

 Factor of Strength (FOS)

The factor of strength (FOS) is the factor which, when applied to either the loading or to the elastic stresses in the finite element model, will produce the required design life at the node.

This analysis option can be selected by clicking on Factor of Strength (in the Fatigue from FEA dialogue) -> Perform Factor of Strength (FOS) Calculations

Calculating FOS is an iterative process and following procedure is performed at each node to calculate the same.

1. Fatigue life is compared with the design life
2. Elastic stresses at the node are scaled -

• by a factor less than 1.0, if the calculated life is lower than design life
• by a factor greater than 1.0, if the calculated life is greater than design life

Elastic stress history is recalculated using scaled stress. The fatigue algorithm is applied including critical plane search (recalculation of critical plane orientation, if needed) and elastic-plastic correction when applicable Fatigue life is recalculated

The process is repeated with different scale factors until it finds the scale factor to give the required design life. This method is unique to fe-safe.

The FOS values are written as an additional value to the output file and can be plotted as contour plots.

 Advantages:  

1. FOS method recalculates the stresses using scale factor and recomputes the fatigue life based on the scaled stresses. This makes the method more generic and broadens its application to complex loading history and low cycle fatigue analysis.
2. Though the method takes a little longer in computation time but is much more accurate and it applies to all fatigue lives not just infinite lives  

 Fatigue Reserve Factor (FRF)

The Fatigue Reserve Factor (FRF), also referred as the Fatigue Reliability Factor, is a linear scale factor obtained from mean Stress diagrams such as Goodman, Gerber etc. They are used to calculate stress based fatigue safety factors (FRF) for a specified life.

The ratio of the distance to the infinite life line and the distance to the cycle (Sa, Sm) is calculated for each extracted cycle, to produce four FRF types – horizontal, vertical, radial and the worst of the three.

The analysis option is selected from the drop-down menu within the user-defined algorithm in the Group Algorithm Selection dialogue box.

FRF analyses can be applicable when –

• Used for infinite life envelope or the fatigue life based on endurance limit of the material.
• Used for finite life with constant amplitude load.

 This is rarely the case! As in real world problem, the loading is complex and the fatigue life calculation many a times be done for finite life with loading variation.

 Disadvantages:

1. There are problems with FRF calculations when designing for finite life with complex loading amplitude. For example, consider a scenario of an overloaded vehicle hitting a curb. Here, a loading cycle can be defined by a high level load followed by multiple cycles of constant amplitude low level load depicting in-service load. Say the target design life is 10e5 loading cycles. 

The most severe cycle (shown black) i.e. the one that comes close to the Goodman line is plotted on the Goodman diagram and the factor of strength (FRF) is given by the ratio A/B. Currently, the smaller cycles (shown grey) are non-damaging as the stresses are within the endurance limit. However, the low amplitude cycles will be damaging when scaled up by calculated FRF. The FRF analyses DOES NOT re-scale the stresses and DOES NOT re-calculates the fatigue life based on the scaled stresses. It simply calculates the scale factor once based on the most damaging cycle. For this reason, using Factors of Strength (FOS) is strongly recommended instead of FRFs.

2. Using FRF may sometimes lead to a wrong estimation of fatigue hotspot

3. FRF does not consider the elastic stress history into account while calculating the scale factor

In summary,

1.  Factor of Strength (FOS)

• Iterative Process
• Applies to Low Cycle Fatigue and High Cycle Fatigue
• Takes effect of complex loading into account
• Can be performed with any Algorithm
• More accurate than FRF

Fatigue Reserve Factor (FRF) 

• Very quick
• Inferior to FOS calculations
• Can be applicable for infinite life calculation and finite life with constant amplitude loading 
• Available only for Principal Stress/Strain, Brown-Miller, and von Mises algorithms