XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Concluding Remarks
This was a very simple analysis, the results of which should be obvious. The lowest life was naturally predicted at the highest stressed location. Because the loading was simple, perhaps a detailed fatigue analysis as performed here, was not necessary. In fact you could have simply extracted the highest principal stress (333 MPa) and gone directly to the S-N curve using PFMAT to assess the life. This, of course, starts to become very impractical with anything much more complicated as you will see in subsequent examples.
As an exercise, go back to the Material Info... form and invoke the materials database manager, PFMAT, again and plot the S-N curve as done before. With the S-N curve plotted you can use the left mouse button positioned on the curve to read off coordinate values (reported in the lower left corner). You can also use the right mouse button to zoom in on the curve (click once on one side of the curve and again on the other side to zoom). To restore the curve, select the View | Full Plot option. You can read the life value right from the curve.
Hint: | To read the correct life value from the curve for this exercise, you must multiply the maximum principal stress at Node 1 by two (666 MPa) since the total range of the signal is twice the stress determined by the FE analysis since it is experiencing full reversal. |
Note: | Note about plasticity: as mentioned in Introduction, fatigue cannot occur without some local plasticity. The S-N method makes no effort to define the amount of plasticity or compensate for it in any specific manner. All plasticity is built into the S-N curve itself. The S-N curve used in this exercise is known as a material S-N curve. This is significant because you must know beforehand what the S-N curve you use actually represents. In this case the S-N curve is representative of the actual material and relates local stress (σ) to life. That is, the monitored stress used to create the S-N curve is the stress at the actual failure location. This will become more clear when we discuss another type of S-N curve (component S-N) in a later exercise. |
Exit from Pre & Post when finished with this exercise. Keep the files and directory for use in the next exercise.