XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Investigate Surface Finish/Treatment
MSC Fatigue can compensate for different surface treatments and finishes as you have noticed when setting up analysis jobs. Up to this point we have always set the finish and treatment to either none or a polished finish (which are the same) signifying that we wish to use the material properties “as-is” with no corrections. Do the following to investigate the effect of surface finish and treatment assuming you are still at the Design Optimization main menu of FEFAT:
1. Select Original parameters. This resets the analysis to all original settings.
2. Select Sensitivity analysis | surface Finishes (all).
3. Select Recalculate. This calculates lives based on SWT for all surface finishes.
4. Select Original parameters.
5. Select Change Parameters. Change the Surface Condition to Poor Machined. Leave all other settings as is.
6. Select Sensitivity analysis | surface Treatment (all).
7. Select Recalculate. This calculates lives based on SWT with Poor Machined finish for all surface treatments.
Select new Jobname and redo these steps with the other analysis job if you wish.
To meet the design life of this injection mold we could have left it machined with a poor finish and nitrided it and not imposed a residual stress at all:
Table 6‑4 Surface Condition | mold (no residual stress) | mold (with residual stress) |
Polished | 67,000 Fills | 225,000 Fills |
Ground | 44,000 Fills | 122,000 Fills |
Good Machined | 27,000 Fills | 60,900 Fills |
Average Machined | 20,000 Fills | 41,200 Fills |
Poor Machined | 15,500 Fills | 29,300 Fills |
Nitrided | 3,170,000 Fills | 946,000 Fills |
Cold Rolled | 738,000 Fills | 287,000 Fills |
Shot Peened | 130,000 Fills | 72,400 Fills |
Surface finish and treatment corrections are imposed by changing the material properties. This is accomplished by changing the slope of the S-N curve or for strain-life curves, the slope of the elastic line at the endurance limit. A scale factor for each finish or treatment is stored in the materials database. These factors are based on the UTS of the material and derived from empirical data.
Surface treatments and finishes tend again to only effect HCF jobs. To illustrate, you can perform an exercise similar to that done in the previous section where the load was increased by doing a sensitivity analysis on the scale factor. Except this time do it for different surface finishes or treatments. Note that the answers tend to converge between the various surface finish/treatments at higher load levels. The curve to the right shows two strain-life curves, one with polished and one with some other finish, where only the HCF end is effected.
Note: | Shot Peening is a mechanism used to impose a compressive residual stress into the surface, thus changing the mean stress. However, it is compensated for by surface finish/treatment techniques. |
