Fatigue Failure Slides

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Fatigue Failure Slides

  1. 1. Lecture 17 introducing FATIGUE FAILURE Atta ul Haq GIK Institute-Fall 2013 1
  2. 2. Just to have an overview where we are now Atta ul Haq GIK Institute-Fall 2013 2
  3. 3. Major Issues to be addressed; What is FATIGUE STRESS CYCLES S-N CURVES Atta ul Haq GIK Institute-Fall 2013 Fatigue Testing 3
  4. 4. What IS a FATIGUE ? Atta ul Haq GIK Institute-Fall 2013 4
  5. 5. Getting Started- Fatigue basics Object --> Repeated Motion--> Weakness/tiredness --> Fatigue Example: Your legs/ muscles decrease in output due to constant running Aircraft Automobile Fatigue in materials Metals --> Weak/fracture due to repeated loading 90% service failure  Fatigue Turbine blades, Examples: materials in continuous loading-unloading applications like bridges… bolts/ shafts (Brittle type fracture—why?) “The PREMATURE failure/lowering of SRENGTH of a material due to repetitive stresses which may be below or above its yield strength ” Atta ul Haq GIK Institute-Fall 2013 5
  6. 6. Requirements for Fatigue Failure Plastic strain Tensile stresses Cyclic loading • This type of failure occurs in Metals and Polymers, While Ceramics do not undergo this type of failure!! Atta ul Haq GIK Institute-Fall 2013 6
  7. 7. Fracture Surface in a Fatigue failure: STAGES OF FATIGUE FAILURE FATIGUE FAILURE Two Main fracture ZONES  Smooth Surface  Crystalline Surface 1) CRACK initiation 2) Crack Propagation 3) Catastrophic rupture Atta ul Haq GIK Institute-Fall 2013 7
  8. 8. Fracture Surface in a Fatigue failure: Fatigue in Engine Crankshaft Striation High magnification SEM Image Atta ul Haq GIK Institute-Fall 2013 8
  9. 9. Fracture Surface in a Fatigue failure: Macroscopic Examination Microscopic Examination start End progress SEM image Clamshell/beach markings Finer markings-Striations SEM image @ high magnification Intrusions and Extrusions Dislocation motion leads to PERSISTENT SLIP BANDS (PSBs) These SLIP BANDS involve INTRUSIONS and EXTRUSIONS (slip on different set of planes depending on loading cycle)  Tiny Steps  Stress Raisers  Microcrack Initiation (along planes of high shear stress ) Atta ul Haq GIK Institute-Fall 2013 9
  10. 10. CRACK Initiation and Propagation !  Crack formation and propagation is a Micromechanism in fatigue  Fatigue crack initiation is a surface phenomenon  SLIP BANDS Formation (Persistent Slip Bands)  Crack initiates • Atta ul Haq GIK Institute-Fall 2013 10
  11. 11. CRACK Propagation Mechanism ! • Cyclic Load • Crack Initiation (microcrack) • Crack Propagation Stage I • Crack Propagation Stage II • Fracture We will study Crack propagation Stages again once we get familiarize with the loading cycles Atta ul Haq GIK Institute-Fall 2013 11
  12. 12. STRESS CYCLES Atta ul Haq GIK Institute-Fall 2013 12
  13. 13. Types Of Fluctuating Stresses Reversed Stress cycle Repeated Stress cycle Random/irregular stress cycles Atta ul Haq GIK Institute-Fall 2013 13
  14. 14. Stress Cycles in Fatigue Mean Stress: Amplitude Stress: Range: Radius Ratio: Amplitude Ratio: Atta ul Haq GIK Institute-Fall 2013 14
  15. 15. Atta ul Haq GIK Institute-Fall 2013 15
  16. 16. CRACK Propagation Mechanism ! • Cyclic Load • Crack Initiation (microcrack) • Crack Propagation Stage I • Crack Propagation Stage II • Fracture Atta ul Haq GIK Institute-Fall 2013 16
  17. 17. Let us consider three samples with same initial crack length subjected to different repeated cycles. Please note that material is also same; Higher Crack Growth rate at higher magnitude of applied stresses and vice versa Materials life to Fracture is now highly dependent on magnitude of applied stress and final crack length (which is related to materials structural property (resistant to deformation etc..) Atta ul Haq GIK Institute-Fall 2013 17
  18. 18. Fracture Mechanics for Fatigue Fatigue Life Atta ul Haq GIK Institute-Fall 2013 18
  19. 19. S-N Curves Wöhler curve/SN curves - A graph showing stress as a function of number of cycles in fatigue. Atta ul Haq GIK Institute-Fall 2013 19
  20. 20. Presenting Fatigue Data Atta ul Haq GIK Institute-Fall 2013 20
  21. 21. Fatigue Limit/Endurance Limit: “The stress below which a material will not fail by fatigue” ferrous Non-ferrous Atta ul Haq GIK Institute-Fall 2013 21
  22. 22. Fatigue Life: The number of cycles permitted at a particular stress before a material fails by fatigue. Fatigue Strength: The stress required to cause failure by fatigue in a given number of cycles. Atta ul Haq GIK Institute-Fall 2013 22
  23. 23. Fatigue Testing Atta ul Haq GIK Institute-Fall 2013 23
  24. 24. Cantilever loading Four Point Bending Set up Atta ul Haq GIK Institute-Fall 2013 24
  25. 25. Failure Criteria can also be established by plotting fatigue test data Modified Goodman Relation a = e (1 – m/ Predicts safer regions for fatigue u) Gerber Relation a = e (1 – m/ 2 u) Soderberg Relation a = e (1 – m/ y) Atta ul Haq GIK Institute-Fall 2013 25
  26. 26. Miner’s Law and Cumulative Damage Fatigue life consumed at S1 Remaining cycles permissible at S2 Fatigue Failure will occur when N1 = lifetime at S1 n1 = Number of cycles applied till S1 Atta ul Haq GIK Institute-Fall 2013 26
  27. 27. Factors Effecting Fatigue Loading and Loading Cycles Surface Conditions Design Consideration Nature of Environment The nature of Surface strongly influence fatigue strength: -Surface Hardness -Surface Roughness -Residual Stresses Atta ul Haq GIK Institute-Fall 2013 27
  28. 28. How to Improve Fatigue Performance 1. Surface Treatments, induce compressive stressess. --Method 1: shot peening --Method 2: carburizing shot put surface into compression 2. Remove stress concentrators. C-rich gas bad better bad better Atta ul Haq GIK Institute-Fall 2013 28
  29. 29. Fatigue in Polymers Fatigue behaviour and SN curves similar to that of metals except the crack initiation: There is no Concept of Persistent Slip Bands (PSBs) in Polymers , rather crack is initiated with CRAZING phenomenon. Atta ul Haq GIK Institute-Fall 2013 29
  30. 30. QUIZ-5/6 ON THURSDAY, 8TH WEEK @5:30PM MLH Atta ul Haq GIK Institute-Fall 2013 30

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