Finite element analysis (FEA) is very powerful to investigate a construction for stress concentrations and construction details that are subject to fatigue problems. Knowing the fatigue spectrum of a construction, it is easy to perform a finite element analysis for each loadcase in the spectrum. This way the element stresses of a construction detail are directly related to the spectrum. A survey of the maximum principal stresses occurring in the spectrum is helpful to find the critical details in the construction, which might be sensitive for fatigue.
A crack grows perpendicular to the principal stress. When using FEA to calculate the stresses, the principal stresses are calculated for a particular loadcase in the spectrum. This results in principal stresses pointing in all possible directions. Therefore, the outcome of the finite element analysis as presented by the output file is useless when using FEA to perform fatigue and crack growth calculations.
The program “FEM to spectrum generator” is developed to make to FEA output ready for a fatigue calculation. The program does the following:
- It extracts the stresses for all applicable fatigue loadcases from the FE output files
- It calculates the maximum (most damaging) principal stress for one particular loadcase
- It calculates the stresses for all other loadcases in the same orientation as the most damaging one;
- It creates an input file for the Fatigue and DT program;
The program contains an option to include the temperature stresses. Assuming linear behaviour between temperature and stresses, only 1 loadcase is required to calculate the stresses for all other loadcases. The program does this conversion automatically.
What is the input?
The program “FEM to spectrum generator” needs the following files:
- Loadcase sequence file
- FEMAP report file containing the loadcase stresses covering the complete spectrum
- If required: FEMAP report file containing 1 loadcase containing the temperature stresses
The program comes with a help file included. In this help file it is explained how the above files are generated.
What’s the output?
The program “FEM to spectrum generator” gives the following output files:
- A .csv file containing stresses read from the FEM output files and the calculated spectrum.
- If required a fFatigue input file is generated, which can be used in the fatigue program fFatigue, see www.fFatigue.net
- If reuired a Afgrow input file is generated, which can be used in the crack growth program AFGROW, see www.afgrow.net
The program is written to work with the output generated by FEMAP 9.31 (or higher).
The program works under windows XP or Vista.
An example calculation can be downloaded as well to see how the program “FEM to spectrum generator” is used.