# Fatigue tool for the last step of a multiple-step analysis

Stat
Member

in Structures

Hi everyone,

I am performing an analysis with multiple steps and I want to calculate the fatigue life of the model when only the load of the last step is applied as cyclic loading.

I have been told that I can do that by using tabular data. However I was not able to find where do I define the tabular data.

Can anyone help me with that?

## Comments

In addition, will the program calculate the fatigue life of the model due to cyclic loading implemented on the last step by default?

For Constant Amplitude, Proportional loading (r-ratio) or Non-Constant Amplitude Proportional loading (e.g., history data), the Mechanical Fatigue Tool will use whatever result set that you specify (in Details Window). By default, the last result set is specified.

I appreciate the answer!

At the "Display Time" tabs in "Definition" I understand that I can set the ending time of the cyclic loading, but always the starting time is at 0 sec. How can I define the starting time to be at the 4th sec, since my cyclic loading is implemented at the 4th till the 5th sec.

The Fatigue Tool (FT) just uses the FE results from the specified result set. In a static analysis, time does not have any physical meaning. It is just a counter. When you set the Display Time to be equal to 2 seconds, you are just telling the FT to use the FE results that are associated with a solution time of 2 seconds.

If you want to examine the fatigue damage caused by cycling between result set 4 and result set 5, you need to do a Constant Amplitude, Proportional Loading analysis. You need to set up a Solution Combination that contains result set 4 and result set 5; and then place a FT under that Solution Combination. The FT will use the alternating stress between result set 4 and result 5 to determine the fatigue damage.

Thank you for the answer once again!

Does this has to be on a discrete analysis?

Do I have to export the remaining stresses from the previous steps and import them in a new analysis in order to perform the last step and calculate the fatigue damage?

I am not sure what you mean by a "discrete" analysis. You need to result sets from the same model. They can be 2 results from the same multi-step solution or they can be 2 result sets from separate solutions that share the same mesh.

I meant to say "two" results sets from the same model.

I understood what you mean and I appreciate your interest!

I am afraid that I do not know how to extract the 2 result sets that I am interested in, from the multi-step solution and how to prepare the simulation that will consider cycling loading between these two results sets.

You need to use a Solution Combination. The image shown below uses 2 results sets from the same analysis. You could also use 2 results sets from different analysis.