Fatigue using S-N (Stress - Life) Method
OptiStruct uses the S-N approach for calculating the fatigue life. The S-N approach is suitable for high cycle fatigue, where the material is subject to cyclical stresses that are predominantly within the elastic range. Structures under such stress ranges should typically survive more than 1000 cycles.
Since S-N theory deals with uniaxial stress, the stress components need to be resolved into one combined value for each calculation point, at each time step, and then used as equivalent nominal stress applied on the S-N curve.
- Fatigue Material Properties (S-N Curve)
- FATDEF
- Defines the elements and associated fatigue properties that will be used for the fatigue analysis.
- PFAT
- Defines the finish, treatment, layer and the fatigue strength reduction factors for the elements.
- MATFAT
- Defines the material properties for the fatigue analysis. These properties should be obtained from the material's S-N curve (Figure 3). The S-N curve, typically, is obtained from completely reversed bending on mirror polished specimen. S-N curves can be one segment or two segments.
- Fatigue Parameters
- FATPARM
- Defines the parameters for the fatigue analysis. These include stress combination method, mean stress correction method (Figure 4), Rainflow parameters, Stress Units.
- Fatigue Sequence and Event Definition
- FATSEQ
- Defines the loading sequence for the fatigue analysis. This card can refer to another FATSEQ card or a FATEVNT card.
- FATEVNT
- Defines loading events for the fatigue analysis.
- FATLOAD
- Defines fatigue loading parameters.
The following files found in the optistruct.zip file are needed to perform this tutorial. Refer to Access the Model Files.
ctrlarm.fem, load1.csv and load2.csv
The model being used for this exercise is that of a control arm, as shown in Figure 1. Loads and boundary conditions and two static loadcases have already been defined on this model.
Launch HyperMesh and Set the OptiStruct User Profile
Import the Model
Set Up the Model
Define TABFAT Load Collector
Define TABLOAD Load Collector
- In the Model Browser, right-click and select .
- For Name, enter FATLOAD1.
- Click Color and select a color from the color palette.
- For Card Image, select FATLOAD from the drop-down menu.
- For TID (table ID), select table1 from the list of load collectors.
- For LCID (load case ID), select SUBCASE1 from the list of load steps.
- Set LDM (load magnitude) to 1.
- Set Scale to 3.0.
- Repeat the process to create another load collector named FATLOAD2 with FATLOAD as card image and pointing to table2 and SUBCASE2.
- Set LDM to 1 and Scale to 3.0.
Define TABEVNT Load Collector
- In the Model Browser, right-click and select .
- For Name, enter FATEVENT.
- For Card Image, select FATEVNT.
- Set FATEVNT_NUM_FLOAD to 2.
- Click on the Table icon next to the Data field and select FATLOAD1 for FLOAD(1) and FATLOAD2 for FLOAD(2) in the pop-out window.
Define TABSEQ Load Collector
Define Fatigue Parameters
- In the Model Browser, right-click and select .
- For Name, enter fatparam.
- For Card Image, select FATPARM.
- Verify TYPE is set to SN.
- Set STRESS COMBINE to SGVON (Signed von Mises).
- Set STRESS CORRECTION to GERBERGOODMAN.
- Set STRESSU to MPA (Stress Units).
- Set RAINFLOW RTYPE to LOAD.
- Set SURVCERT in CERTNTY to 0.5.
Define Fatigue Material Properties
The material curve for the fatigue analysis can be defined on the MAT1 card.
Define PFAT Load Collector
- In the Model Browser, right-click and select .
- For Name, enter pfat.
- For Card Image, select PFAT.
- Set LAYER to TOP.
- Set FINISH to NONE.
- Set TRTMENT to NONE.
Define FATDEF Load Collector
- In the Model Browser, right-click and select .
- For Name, enter fatdef.
- For Card Image, select FATDEF.
- Check the box next to PSHELL.
- Click next to the Data field and select shell for PID(1), and pfat for PFATID(1) in the pop-up window.
- Click Close.
Define the Fatigue Load Step
- In the Model Browser, right-click and select .
- For Name, enter Fatigue.
- Set the Analysis type to fatigue.
- For FATDEF, select fatdef.
- For FATPARM, select fatparam.
- For FATSEQ, select fatseq.