The problems here are described in Further Reading (App. 16). They consist of ten simple notched geometries which were tested at various constant amplitude load levels. Four different material types were used, whose properties are given later. In some cases, the measured life was to failure while for others it was to crack initiation. Figure 14‑9 shows the various notched geometries.

For this series of benchmark problems, the geometries were created in MSC.Patran. Due to symmetry, only half of each specimen was actually modeled. MSC.Patran FEA was used as the analysis code using plane stress elements. The MSC.Patran neutral files and MSC.Patran FEA results files can be found in the examples directory delivered with your MSC.Fatigue system. The neutral files containing the geometries are in files notchxx.out where xx = 01 - 10. The stress-strain results are in files notchxx.txt which need to be converted to binary format using the MSC.Patran utility program RESTXT. See previous examples. Files can be found in

<install_dir>/mscfatigue_files/examples

Table 14‑11 shows the four materials used in the study of these 10 specimens. They must be input to the MSC.Fatigue materials database manager PFMAT. Follow the method used in Problem 3: Comparison to Another Code, 1237, Step 2: Material Characterization, 1238 to input the four material properties and name them MATA, MATB, MATC, and MATD.

Hint: You can set the units preference to KSI so that you do not have to convert the values in Table 14‑11 to SI units. Use the Preferences option from the PFMAT main menu.

Loading was performed with constant amplitude load levels. The first seven specimens used a stress ratio of R = -1 (fully reversed) while the last three specimens used a R ratio of 0.1. The magnitudes of the loads are given in Table 14‑12. They were created in PTIME by making just one triangular cycle with a max. and min. value of ±20kips/in for R=-1 and entering X-Y points with max=20000, and min=2000 for R=0.1. These time histories were then scaled in FEFAT for the rest of the load levels and specimens. All MSC.Patran FEA loads are in pounds/inch and the scale factors used properly convert the time history loads from ±20kips/in. to the appropriate kip/in depending on the specimen geometry and load level. (Specimen 4 used kip loading as opposed to kip/in.)

The following keystrokes create these two simple constant amplitude load histories. This assumes you are starting in a clean, empty directory:

Enter MSC.Patran and use the following operations to setup the MSC.Fatigue job for the first specimen. Accept all defaults unless otherwise specified:

At this point, the job has been submitted and can be monitored as to its progress if desired. Once the message Preprocessing completed successfully appears as the status message, the results can be determined by FEFAT. Note that we did not run the analysis to completion and only went through the preprocessor stage.

Table 14‑12 and Figure 14‑10 show results for measured vs. MSC.Fatigue predicted lives. Node 1 for all notch specimens has the most damage. To duplicate the results presented in Table 14‑12 for the first specimen using FEFAT, use the following keystrokes.

You may repeat this entire exercise for all other notch configurations if you wish to reproduce all the results in Table 14‑12. If you do, do not forget to use the correct material (A,B,C, or D) and the correct load time history (R=1, or R=0.1).

It might be pointed out that the lives at high load levels do not appear to give good results compared to those at lower loading levels. This is due to the fact that at these higher loading levels the specimens are being literally torn apart where wide areas are experiencing stresses exceeding the yield stress. When this happens the fatigue theory begins to break down since it is based on a local strain approach. There are other correction methods in MSC.Fatigue, namely the Mertens‑Dittmann and the Seeger‑Beste, that can better compensate for high plasticity.