Creep and wear

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Creep and wear

  1. 1. Creep & Wear High Temperature Irreversible Deformation
  2. 2. Objectives <ul><li>Identify the primary mechanisms of creep deformation </li></ul><ul><li>Determine the threshold temperature for creep for an alloy of known melting temperature </li></ul><ul><li>Use creep data to determine creep model parameters </li></ul><ul><li>Read and apply a creep rupture curve </li></ul>
  3. 3. Creep  Time dependant permanent deformation  At constant stress, strain continues to increase
  4. 4. Creep Time Stress Strain
  5. 5. Mechanisms of Creep <ul><li>High rates of diffusion permit reshaping of crystals to relieve stress </li></ul><ul><li>Diffusion significant at both grain boundaries and in the bulk </li></ul><ul><li>High energy and weak bonds allow dislocations to “climb” around structures that pin them at lower temperature </li></ul>
  6. 6. Mechanisms of Creep
  7. 7. Creep Test <ul><li>Usually a tensile bar </li></ul><ul><li>Dead load applied </li></ul><ul><li>Strain is plotted with time </li></ul><ul><li>Test usually ends with rupture (failure) </li></ul>
  8. 8. Classical creep curve
  9. 9. Application of Classical Curve <ul><li>Steady state creep rate determined from straight section (secondary creep) </li></ul><ul><li>Ahrennius model captures temperature dependence (diffusion!) </li></ul><ul><li>Several tests at different temperatures & stress levels can be used to determine material model parameters </li></ul>
  10. 10. Creep Constitutive Models d  = K 2  n exp (-Q C / RT) dt For T = constant d  = K 1  n dt
  11. 11. Creep Rupture Method <ul><li>When rupture (catastrophic failure) is the only concern (eg boiler tubes) a simpler method will do </li></ul><ul><li>Maps of time to rupture as a function of stress and temperature are used to determine the expected life </li></ul>
  12. 12. Stress Rupture Curve (Applied stress vs. time to rupture)
  13. 13. Threshold for Creep The Critical Temperature for Creep is 40% of the Melting Temperature. If T > 0.40 T M  Creep Is Likely
  14. 14. Example Will Lead Creep at Room Temperature? T M = 327  C = 600  K T ROOM = 23  C = 296  K
  15. 15. Creep Example Will Lead Products Creep at Room Temperature? T M = 327  C = 600  K T ROOM = 23  C = 296  K 100 x 296 / 600 =49.3 %  Will Creep
  16. 16. Creep in Ice <ul><li>How cold does your freezer need to be to avoid creep in the ice cubes? </li></ul>
  17. 17. Creep in Ice <ul><li>How cold does your freezer need to be to avoid creep in the ice cubes? </li></ul><ul><li>0C = 273 K </li></ul><ul><li>0.4 * 273 = 109.2K = -163.8 C! Brrrr! </li></ul>
  18. 18. Creep of Ice <ul><li>Ice creeps at most terrestrial temperatures </li></ul><ul><li>People have studied this for use in weather models & ship design </li></ul>Budd and Jacka, 1989).
  19. 19. How Do We Deal With Creep ? <ul><li>Reduce the effect of grain </li></ul><ul><li>boundaries </li></ul><ul><ul><li>Use Single Crystals </li></ul></ul>
  20. 20. Stress Rupture in Single Crystals CMSX-4 & CM 186 (yellow) are Single Crystals Advanced Materials & Processes April 1995
  21. 21. Single Crystal Turbine Blade <ul><li>Allison AE 2100 Turboprop engine </li></ul><ul><li>First Stage vane doublet from CMSX-4 alloy </li></ul>Advanced Materials & Processes April 1995
  22. 22. How Do We Deal With Creep ? <ul><li>Reduce the effect of grain </li></ul><ul><li>boundaries </li></ul><ul><ul><li>Use Single Crystals </li></ul></ul><ul><li>Change Materials </li></ul>
  23. 23. How Do We Deal With Creep ? <ul><li>Reduce the effect of grain </li></ul><ul><li>boundaries </li></ul><ul><ul><li>Use Single Crystals </li></ul></ul><ul><li>Change Materials </li></ul><ul><li>Change Operating Conditions </li></ul>
  24. 24. Wear <ul><li>Three Mechanisms </li></ul><ul><ul><li>Adhesive </li></ul></ul><ul><ul><li>Abrasive </li></ul></ul><ul><ul><li>Fretting </li></ul></ul>
  25. 25. Adhesive Wear <ul><li>Compatible Materials </li></ul><ul><li>Special coatings </li></ul><ul><li>Welding of Asperities </li></ul><ul><li>Welds break during translation of surfaces </li></ul>
  26. 26. How Do you Prevent Adhesive Wear? <ul><li>Incompatible materials </li></ul><ul><li>Lubricate </li></ul><ul><li>Polish </li></ul>Fundamentals of Machine Component Design, Juvinall & Marshak, Wiley
  27. 27. Abrasive Wear <ul><li>Three body wear </li></ul><ul><li>Contamination or </li></ul><ul><li>wear debris does </li></ul><ul><li>much of the damage </li></ul><ul><li>Why you use an oil filter. </li></ul>
  28. 28. Fretting Wear <ul><li>Parts in Contact </li></ul><ul><li>Small relative motion </li></ul><ul><li>Microwelding causes spot damage </li></ul>

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