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.

The S-N approach is based on elastic cyclic loading, inferring that the S-N curve should be confined to numbers greater than 1000 cycles. This ensures that no significant plasticity is occurring. This is commonly referred to as high-cycle fatigue.

Figure 1: Low Cycle and High Cycle regions on the S-N curve

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 (Figure 2).

Figure 2: Two segment S-N curve

In OptiStruct various stress combination types are available, with the default being “Absolute maximum principle stress”. In general “Absolute maximum principle stress” is recommended for brittle materials, while “Signed von Mises stress” is recommended for ductile material. The sign on the signed parameters is taken from the sign of the Maximum Absolute Principal value.

In this tutorial, you will be able to evaluate fatigue life with the S-N method through process manager step by step. In brief, the following contents are covered:

 

The following files found in the optistruct.zip file are needed to perform this tutorial. Refer to Accessing the Model Files.

 

Exercise

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A control arm loaded by brake force and vertical force is used, as shown in Figure 3. Two load time histories acquired for 2545 seconds with 1 HZ, shown in Figures 4(a) and 4(b), are applied. The material of the control arm is aluminum, whose S-N curve is shown in Figure 5. Because a crack always initiates from the surface, a skin meshed with shell elements is designed to cover the solid elements, which can improve the accuracy of calculation as well.

Figure 3: Model of control arm for fatigue analysis

(a)

(b)

Figure 4: Load time history (a) for vertical force (b) for braking force

Figure 5: S-N Curve of Aluminum

Step 1: Launch HyperMesh and Process Manager

When finished, the Fatigue Analysis tree will appear.

Figure 6: Glance after launch Fatigue Process Manager (FPM)

Step 2: Import model

This loads the control arm model. It includes a whole definition of two static subcases, elements sets, and material static properties, etc.

Figure 7: Import a Finite Element Model file

Step 3: Create fatigue subcase

fatsub_fpmtut is selected as the active fatigue subcase. Definitions in the following processes (analysis parameters, fatigue elements and properties, loading sequences, etc.) will be for this subcase.

This saves the current definitions and guides you to the next task Analysis Parameters of the Fatigue Analysis tree.

Figure 8: Create and select active fatigue subcase to process

Step 4: Fatigue Analysis Parameters Definition

This saves the current definitions and guides you to the next task Elements and Materials of the Fatigue Analysis tree. For details, consult the HyperWorks 2017 help.

Figure 9: Fatigue analysis parameters definition.

Step 5: Fatigue Elements and Materials Definition

This saves the current definitions and guides you to the next task Load-Time History of the Fatigue Analysis tree.

Figure 10: Material data definition

Figure 11: Elements and material definition

Step 6: Load-Time History Definition

Figure 12: Load-time history definition

Figure 13: Import load-time history

Step 7: Loading Sequences Definition

In this step, one event consisting of two load time history is created; in other words, the linear superposition of the stress caused by the two load time history is requested during analysis. Using this event, one load sequence is constructed.

Figure 14: Associate load-time history with static subcase

Figure 15: Loading sequences definition

Step 8: Submit fatigue analysis

If the job was successful, new results files can be seen in the directory where the OptiStruct model file was written.

The default files written to your directory are:

 

Figure 16: Submit fatigue analysis

Step 9: Post-processing

When fatigue analysis has completed successfully after the previous submit, it will automatically go into this task.

Check the top 0.1%, 1.0%, 5.0% average life, and Top 1, 2, 3 most damage elements lives.

Figure 17: Life results summary

Figure 18: Damage contour in HyperView

See Also:

OptiStruct Tutorials