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Fatigue: Stress-Life

Workshop A12-1. Fatigue: Stress-Life. Goals. Goal:

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Fatigue: Stress-Life

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  1. Workshop A12-1 Fatigue: Stress-Life

  2. Goals • Goal: • In this workshop our goal is to perform a Stress-Life analysis of the connecting rod model (ConRod.x_t) shown here. Specifically, we will analyze two load environments: 1) Constant Amplitude Load of 4500 N, Fully Reversed and 2) Random Load of 4500N. August 26, 2005 Inventory #002266 WSA12.1-2

  3. . . . Start Page • From the launcher start Simulation. • Choose “Geometry > From File . . . “ and browse to the file “ConRod.x_t”. • When DS starts, close the Template menu by clicking the ‘X’ in the corner of the window. August 26, 2005 Inventory #002266 WSA12.1-3

  4. Preprocessing • Change the working unit system to metric (m, kg, Pa …). • “Units > Metric (m, kg, Pa, C, s)” • Verify the material is set to “Structural Steel”. • Highlight the “Part 1” in the geometry branch. • If not, click in the “Material” field and “browse”. 1. 2. 3. August 26, 2005 Inventory #002266 WSA12.1-4

  5. . . . Preprocessing • Select the “Structural Steel” material and then click [OK]. 4. August 26, 2005 Inventory #002266 WSA12.1-5

  6. . . . Preprocessing • Apply the following boundary conditions (see next page): August 26, 2005 Inventory #002266 WSA12.1-6

  7. . . . Preprocessing • Highlight the Environment branch. • Highlight the connecting rod surface shown… • Insert a force load. • “RMB > Insert > Force” • From the detail window change to “Components” and “Z = - 4500 N”. 5. 6. 7. 8. August 26, 2005 Inventory #002266 WSA12.1-7

  8. . . . Preprocessing • Highlight the Environment branch. • Highlight the connecting rod surfaces shown… • Insert a cylindrical support. • “RMB > Insert > Cylindrical Support” • From the Details of “Cylindrical Support” window: • Set Radial=“Fixed”, Axial=“Free”, Tangential=“Free” 9. 10. 11. 12. August 26, 2005 Inventory #002266 WSA12.1-8

  9. . . . Preprocessing • Highlight the Environment branch. • Highlight the connecting rod surface shown… • Insert a fixed support. • “RMB > Insert > Fixed Support” 13. 14. 15. August 26, 2005 Inventory #002266 WSA12.1-9

  10. 17. 18. Solution Setup • Add results to Solution: • Highlight the solution branch. • RMB > Insert > Stress > Equivalent (von Mises). • RMB > Insert > Deformation > Total. 16. August 26, 2005 Inventory #002266 WSA12.1-10

  11. . . . Solution Setup • Insert fatigue tool: • Highlight the solution branch. • RMB > Insert > Fatigue > Fatigue Tool. 19. 20. August 26, 2005 Inventory #002266 WSA12.1-11

  12. 21. 22. 23. 24. . . . Solution Setup • From the Details of “Fatigue Tool” window: • Specify a Fatigue Strength Factor (Kf) of .8 (material data represents a polished specimen and the in-service component is cast). • Specify fully reversed loading to create alternating stress cycles. • Specify a stress-life fatigue analysis (No mean stress theory needs to be specified since no mean stress will exist – fully reversed loading). • Specify that Von Mises stress will be used to compare against fatigue material data. August 26, 2005 Inventory #002266 WSA12.1-12

  13. 25. . . . Solution Setup • Add results to the Fatigue Tool: • Insert “Safety Factor”: • RMB > Insert > Fatigue > Safety Factor. • From the Details of “Safety Factor” window: • Set the Design Life to 1e6 cycles. 26. August 26, 2005 Inventory #002266 WSA12.1-13

  14. 27. . . . Solution Setup • Add results to the Fatigue Tool (cont.): • Insert “Fatigue Sensitivity”: • RMB > Insert > Fatigue > Fatigue Sensitivity • From the Details of “Fatigue Sensitivity” window: • Specify a minimum base load variation of 50% (an alternating stress of 2250N) and a maximum base load variation of 200% (an alternating stress of 9000N). 28. August 26, 2005 Inventory #002266 WSA12.1-14

  15. . . . Solution Setup • Add results to the Fatigue Tool (cont.): • Insert “Biaxiality Indication”: • RMB > Insert > Fatigue > Biaxiality Indication • Solve 29. August 26, 2005 Inventory #002266 WSA12.1-15

  16. Results • View Results • Highlight and plot the “Total Deformation” result. August 26, 2005 Inventory #002266 WSA12.1-16

  17. . . . Results • Highlight and plot the “Equivalent Stress” result. August 26, 2005 Inventory #002266 WSA12.1-17

  18. . . . Results • Highlight and plot the “Safety Factor” result for a design life of 1e6 cycles. August 26, 2005 Inventory #002266 WSA12.1-18

  19. . . . Results • Highlight and plot the “Fatigue Sensitivity” result for a minimum base load variation of 50% and a maximum base load variation of 200%. August 26, 2005 Inventory #002266 WSA12.1-19

  20. . . . Results • Find the sensitivity of available life with respect to loading for a maximum base load variation of 400%. Note, must resolve to obtain the new Fatigue Sensitivity results. August 26, 2005 Inventory #002266 WSA12.1-20

  21. . . . Results • Highlight and plot the “Biaxiality Indication” result. Note, the stress state near the critical location is not far from uniaxial (.1~.2), which gives an added measure of confidence since the material properties are uniaxial. Recall, a biaxiality of zero corresponds to uniaxial stress, a value of –1 corresponds to pure shear, and a value of 1 corresponds to a pure biaxial state. August 26, 2005 Inventory #002266 WSA12.1-21

  22. Solution Setup • Insert a second fatigue tool to analyze a random load of 4500N. Assume that we have strain gauge results that were collected experimentally from the component and that we know that a strain gauge reading of 200 corresponds to an applied load of 4500N: • Highlight the solution branch. • RMB > Insert > Fatigue > Fatigue Tool. 30. 31. August 26, 2005 Inventory #002266 WSA12.1-22

  23. 32. 33. 34. . . . Solution Setup • From the Details of “Fatigue Tool 2” window: • Specify a Fatigue Strength Factor (Kf) of .8 (material data represents a polished specimen and the in-service component is cast). • Specify fatigue loading as coming from a scale history and select scale history file containing strain gauge results over time (browse and open the “SAEBracketHistory.dat” file). • Define the scale factor to be .005 (we must normalize the load history so that the FEM load matches the scale factors in the load history file): August 26, 2005 Inventory #002266 WSA12.1-23

  24. 35. 36. 37. . . . Solution Setup • From the Details of “Fatigue Tool” window (cont.): • Specify Goodman theory to account for mean-stress effects. • Specify that a signed Von Mises stress will be used to compare against fatigue material data (use signed since Goodman theory treats negative and positive mean stresses differently). • Specify a bin size of 32 (Rainflow and Damage matrices will be of dimension 32x32). August 26, 2005 Inventory #002266 WSA12.1-24

  25. . . . Solution Setup • Add results to the Fatigue Tool 2: • Insert “Life”: • RMB > Insert > Fatigue > Life • Insert “Safety Factor”: • RMB > Insert > Fatigue > Safety Factor • From the Details of “Safety Factor” window: • Set the Design Life to 1000 cycles. 38. 39. 40. August 26, 2005 Inventory #002266 WSA12.1-25

  26. . . . Solution Setup • Add results to the Fatigue Tool (cont.): • Insert “Fatigue Sensitivity”: • RMB > Insert > Fatigue > Fatigue Sensitivity • From the Details of “Fatigue Sensitivity” window: • Specify a minimum base load variation of 50% (an alternating stress of 2250N) and a maximum base load variation of 200% (an alternating stress of 9000N). 41. 42. August 26, 2005 Inventory #002266 WSA12.1-26

  27. . . . Solution Setup • Add results to the Fatigue Tool (cont.): • Insert “Biaxiality Indication”: • RMB > Insert > Fatigue > Biaxiality Indication • Insert “Rainflow Matrix”: • RMB > Insert > Fatigue > Rainflow Matrix 43. 44. August 26, 2005 Inventory #002266 WSA12.1-27

  28. . . . Solution Setup • Add results to the Fatigue Tool (cont.): • Insert “Damage Matrix”: • RMB > Insert > Fatigue > Damage Matrix • From the Details of “Damage Matrix” window: • Set the Design Life to 1000 cycles. • Solve 45. 46. August 26, 2005 Inventory #002266 WSA12.1-28

  29. Results • View Results • Highlight and plot the “Life” result. August 26, 2005 Inventory #002266 WSA12.1-29

  30. . . . Results • Highlight and plot the “Safety Factor” result for a design life of 1000 cycles. If the loading history corresponded to the loading experienced by the part over a months time, the damage and FS will be at a design life of 1000 months. Note that although a life of only 77 loading blocks is calculated, the needed scale factor (since FS @ 1000=.60) is only .60 to reach a life of 1000 blocks. Note, the “scale factor” is the scale factor for the loading to make it meet the life of 1000 months. August 26, 2005 Inventory #002266 WSA12.1-30

  31. . . . Results • Highlight and plot the “Fatigue Sensitivity” result for a minimum base load variation of 50% and a maximum base load variation of 200%. August 26, 2005 Inventory #002266 WSA12.1-31

  32. . . . Results • Highlight and plot the “Biaxiality Indication” result. August 26, 2005 Inventory #002266 WSA12.1-32

  33. . . . Results • Highlight and plot the “Rainflow Matrix” result. Here, one can see from the rainflow matrix that the majority of the cycle counts are for low mean stress and low stress amplitude (range). August 26, 2005 Inventory #002266 WSA12.1-33

  34. . . . Results • Highlight and plot the “Damage Matrix” result. Although, from the previous slide, one saw that most of the counts were for the low mean and range bins, these do not cause the most damage at the critical location, as shown in this damage matrix.  Instead, the 'medium' stress amplitude cycles cause the most damage at the critical location. August 26, 2005 Inventory #002266 WSA12.1-34

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