1. Lecture 17 introducing FATIGUE FAILURE Atta ul Haq GIK Institute-Fall 20131

2. Just to have an overview where we are now Atta ul Haq GIK Institute-Fall 20132

3. Major Issues to be addressed; 1 What is FATIGUE 2 STRESS CYCLES 4 Fatigue Testing 3S-NCURVES Atta ul Haq GIK Institute-Fall 20133

4. What IS a FATIGUE ? 1 Atta ul Haq GIK Institute-Fall 20134

5. Object --> Repeated Motion--> Weakness/tiredness --> Fatigue Example: Your legs/ muscles  decrease in output due to constant running Example: Your legs/ muscles  decrease in output due to constant running Fatigue in materials Metals --> Weak/fracture due to repeated loading 90% service failure  Fatigue Examples: materials in continuous loading-unloading applications like 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 ” Getting Started- Fatigue basics Atta ul Haq GIK Institute-Fall 20135 Automobile Turbine blades, bridges… Aircraft

6. Requirements for Fatigue Failure This type of failure occurs in Metals and Polymers, While Ceramics do not undergo this type of failure!! Atta ul Haq GIK Institute-Fall 20136 Cyclic loading Tensile stresses Plastic strain

7. FATIGUE FAILURE Fracture Surface in a Fatigue failure: STAGES OF FATIGUE FAILURE STAGES OF 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 20137

8. Striation High magnification SEM Image Fatigue in Engine Crankshaft Atta ul Haq GIK Institute-Fall 20138 Fracture Surface in a Fatigue failure:

9. Atta ul Haq GIK Institute-Fall 20139 Fracture Surface in a Fatigue failure: Macroscopic Examination Microscopic Examination Clamshell/beach markings Finer markings-Striations Intrusions and Extrusions SEM image @ high magnification SEM image start progress End 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 ) Schematic diagram showing Intrusions and Extrusions

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 201310

11. CRACK Propagation Mechanism ! Atta ul Haq GIK Institute-Fall 201311 Cyclic LoadCrack Initiation (microcrack) Crack Propagation Stage ICrack Propagation Stage IIFracture We will study Crack propagation Stages again once we get familiarize with the loading cycles

12. 2 STRESS CYCLES Atta ul Haq GIK Institute-Fall 201312

13. Atta ul Haq GIK Institute-Fall 201313 Types Of Fluctuating Stresses Reversed Stress cycleRepeated Stress cycle Random/irregular stress cycles

14. Stress Cycles in Fatigue Atta ul Haq GIK Institute-Fall 201314 Mean Stress: Amplitude Stress: Range: Radius Ratio: Amplitude Ratio:

15. Atta ul Haq GIK Institute-Fall 201315

16. CRACK Propagation Mechanism ! Atta ul Haq GIK Institute-Fall 201316 Cyclic LoadCrack Initiation (microcrack) Crack Propagation Stage ICrack Propagation Stage IIFracture

17. Atta ul Haq GIK Institute-Fall 201317 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..)

18. Fracture Mechanics for Fatigue Atta ul Haq GIK Institute-Fall 201318 Fatigue Life

19. 3 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 201319

20. Presenting Fatigue Data Atta ul Haq GIK Institute-Fall 201320

21. Fatigue Limit/Endurance Limit: “The stress below which a material will not fail by fatigue” Non-ferrous ferrous Atta ul Haq GIK Institute-Fall 201321

22. Fatigue Life: The number of cycles permitted at a particular stress before a material fails by fatigue. The stress required to cause failure by fatigue in a given number of cycles. Fatigue Strength: Atta ul Haq GIK Institute-Fall 201322

23. 4 Fatigue Testing Atta ul Haq GIK Institute-Fall 201323

24. Atta ul Haq GIK Institute-Fall 201324 Four Point Bending Set up Cantilever loading

25. Modified Goodman Relation a = e (1 – m / u ) Soderberg Relation Gerber Relation Failure Criteria can also be established by plotting fatigue test data Predicts safer regions for fatigue a = e (1 – m / u ) 2 a = e (1 – m / y ) Atta ul Haq GIK Institute-Fall 201325

26. Miner’s Law and Cumulative Damage Remaining cycles permissible at S 2 N 1 = lifetime at S 1 n 1 = Number of cycles applied till S 1 Fatigue life consumed at S 1 Atta ul Haq GIK Institute-Fall 201326 Fatigue Failure will occur when

27. Factors Effecting Fatigue  Loading and Loading Cycles  Surface Conditions  Design Consideration  Nature of Environment Atta ul Haq GIK Institute-Fall 201327 The nature of Surface strongly influence fatigue strength: -Surface Hardness -Surface Roughness -Residual Stresses

28. --Method 1: shot peening put surface into compression shot --Method 2: carburizing C-rich gas 2. Remove stress concentrators. bad better 1. Surface Treatments, induce compressive stressess. How to Improve Fatigue Performance Atta ul Haq GIK Institute-Fall 201328

29. Atta ul Haq GIK Institute-Fall 201329 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.

30. QUIZ-5/6 ON THURSDAY, 8 TH WEEK @5:30PM MLH Atta ul Haq GIK Institute-Fall 201330