Fatigue Analysis Overview

Fatigue analysis establishes whether your model is susceptible to fatigue damage when subjected to a varying load. You can use constant amplitude loading for situations in which the stress cycles are regular, such as a rotating shaft operating at a constant speed. For situations where the stress cycles are random, you can define a variable amplitude loading pattern for your model.

You must first define a static analysis before you can define a fatigue analysis. The stress results from the static analysis are multiplied by the load factors you specify for the fatigue analysis to find the loading variation for one life cycle.

• Log Life—the estimated number of cycles until your model breaks. Because of the exponential nature of fatigue, it is useful to express life as a logarithm.

• Log Damage—the ratio between accumulated fatigue cycles and the total number of cycles to failure. A value greater than unity indicates failure. A value of 0.5, for example, represents a loss of 50% in the useful life of the model. Because of the exponential nature of fatigue, it is useful to express the damage ratio as a logarithm.

Accumulated fatigue cycles are the number of cycles the model experiences before the fatigue analysis is run. You cannot specify this value, and Creo Simulate assumes it to be 1. Therefore the relation between Log Life and Log Damage can be expressed as

• Factor of Safety—the permissible factor of safety on the input load. When the fatigue life calculated for your model is greater than the target design life, the software carries out a back calculation to determine a permissible factor of safety on the input load. This represents the extent to which the amplitude of the load can be increased without compromising the target design life.

If you want the software to calculate the factor of safety, select the check box in the Output area at the bottom of the Fatigue Analysis Definition dialog box.

• Confidence of Life—the ratio between the calculated life and the target design life. Because of the statistical nature of fatigue, the greater the confidence the better. Values below unity indicate failure. Values greater than 3.0 usually reflect an adequate confidence of achieving the desired target life.

You can display Confidence Of Life results in a tricolored fringe display to give an overall view of where the model will break first and where the model will last for a greater number of cycles. Red signifies a confidence of life from 0 cycles to the number of cycles entered for desired endurance on the analysis dialog box. Yellow signifies a confidence of life that ranges from the number of cycles for desired endurance to 3 times that number. The difference between these numbers is considered the marginal life. Green signifies any number of cycles over the marginal life.

For background information on fatigue and details about the methodology used in fatigue analysis, see the online document Understanding Fatigue Analysis.

Fatigue Advisor is optimized so you can obtain a rapid indication of whether a design is sensitive to fatigue without having to provide the full range of input normally required to solve this problem. The software accomplishes this by asking for input that is relatively straightforward to obtain and by internally setting very conservative defaults for input that you do not directly provide. Advanced fatigue users may want to alter these defaults to examine less conservative scenarios.