Fatigue and creep rapture

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Fatigue and creep rapture

  1. 1. Fatigue rapture Failure under Fluctuating Stress Creep rapture
  2. 2. Muhammad Umair Bukhari Engr.umair.bukhari@gmail.com www.bzuiam.webs.com 03136050151
  3. 3. The failure of metal under alternating stresses is known as Fatigue. Under fluctuating / cyclic stresses, failure can occur at lower loads than under a static load. 90% of all failures of metallic structures (bridges, aircraft, machine components, etc.) Fatigue failure is brittle-like – even in normally ductile materials. Thus sudden and catastrophic! Fatigue Failure under fluctuating stress
  4. 4. Fatigue: Cyclic Stresses Characterized by maximum, minimum and mean Range of stress, stress amplitude, and stress ratio Mean stress m = (max + min) / 2 Range of stress r = (max - min) Stress amplitude a = r/2 = (max - min) / 2 Stress ratio R = min / max Convention: tensile stresses  positive compressive stresses  negative
  5. 5. Fatigue: S—N curves (I) Rotating-bending test  S-N curves S (stress) vs. N (number of cycles to failure) Low cycle fatigue: small # of cycles high loads, plastic and elastic deformation High cycle fatigue: large # of cycles low loads, elastic deformation (N > 105)
  6. 6. Fatigue: S—N curves (II) Fatigue limit (some Fe and Ti alloys) S—N curve becomes horizontal at large N Stress amplitude below which the material never fails, no matter how large the number of cycles is
  7. 7. Fatigue: S—N curves (III) Most alloys: S decreases with N. Fatigue strength: Stress at which fracture occurs after specified number of cycles (e.g. 107) Fatigue life: Number of cycles to fail at specified stress level
  8. 8. Fatigue: Crack initiation+ propagation (I) Three stages: 1. crack initiation in the areas of stress concentration (near stress raisers) 2. incremental crack propagation 3. rapid crack propagation after crack reaches critical size The total number of cycles to failure is the sum of cycles at the first and the second stages: Nf = Ni + Np Nf : Number of cycles to failure Ni : Number of cycles for crack initiation Np : Number of cycles for crack propagation High cycle fatigue (low loads): Ni is relatively high. With increasing stress level, Ni decreases and Np dominates
  9. 9. Fatigue: Crack initiation and propagation (II)  Crack initiation: Quality of surface and sites of stress concentration (microcracks, scratches, indents, interior corners, dislocation slip steps, etc.).  Crack propagation  I: Slow propagation along crystal planes with high resolved shear stress. Involves a few grains. Flat fracture surface  II: Fast propagation perpendicular to applied stress. Crack grows by repetitive blunting and sharpening process at crack tip. Rough fracture surface.  Crack eventually reaches critical dimension and propagates very rapidly
  10. 10. Factors that affect fatigue life  Magnitude of stress  Quality of the surface Solutions:  Polish surface  Introduce compressive stresses (compensate for applied tensile stresses) into surface layer. Shot Peening -- fire small shot into surface High-tech - ion implantation, laser peening.  Case Hardening: Steel - create C- or N- rich outer layer by atomic diffusion from surface Harder outer layer introduces compressive stresses  Optimize geometry Avoid internal corners, notches etc.
  11. 11. Factors affecting fatigue life Environmental effects  Thermal Fatigue. Thermal cycling causes expansion and contraction, hence thermal stress. Solutions:  change design!  use materials with low thermal expansion coefficients  Corrosion fatigue. Chemical reactions induce pits which act as stress raisers. Corrosion also enhances crack propagation. Solutions:  decrease corrosiveness of medium  add protective surface coating  add residual compressive stresses
  12. 12. The Macroscopic Character of Fatigue Failure  Because of the manner in which the fracture develops, the surfaces of a fatigue fracture are divided into two areas with distinctly different appearances.  In most cases, the surface will have a polished or burnished appearance in the region where the crack grew slowly.  In the last stage, the surfaces developed are rough and irregular.
  13. 13. Fractograph of fatigue failure in SAE 1050 pin, induction hardened to a depth of 5 mm ( 3/16 in.) and surface hardness of 55 HRC. Core hardness: 21 HRC. Fatigue initiated inside the grease hole at the metallurgical notch created by the very sharp case-core hardness gradient.
  14. 14. Schematic representation of fatigue fracture surface marks produced on smooth and notched components with circular cross sections under various loading conditions.
  15. 15. Creep Creep testing Furnace Time-dependent deformation due to constant load at high temperature (> 0.4 Tm) Examples: turbine blades, steam generators. Creep test:
  16. 16. Stages of creep
  17. 17. 1. Instantaneous deformation, mainly elastic. 2. Primary/transient creep. Slope of strain vs. time decreases with time: work-hardening 3. Secondary/steady-state creep. Rate of straining constant: work-hardening and recovery. 4. Tertiary. Rapidly accelerating strain rate up to failure: formation of internal cracks, voids, grain boundary separation, necking, etc. Stages of creep
  18. 18. Parameters of creep behavior Secondary/steady-state creep: Longest duration Long-life applications (creep rate) Time to rupture ( rupture lifetime, tr): Important for short-life creep t/s  tr /t
  19. 19. Creep: stress and temperature effects With increasing stress or temperature: The instantaneous strain increases The steady-state creep rate increases The time to rupture decreases
  20. 20. Creep: stress and temperature effects Stress/temperature dependence of the steady-state creep rate can be illustrated by
  21. 21. Mechanisms of Creep Different mechanisms act in different materials and under different loading and temperature conditions:  Dislocation Glide  Dislocation Creep  Diffusion Creep  Grain boundary sliding Different mechanisms  different n, Qc. Grain boundary diffusion Dislocation glide and climb
  22. 22. Dislocation glide- Involves dislocations moving along slip planes and overcoming barriers by thermal activation. This mechanism occurs at high stress levels. Dislocation creep- Involves the movement of dislocations which overcome barriers by thermally assisted mechanisms involving the diffusion of vacancies or interstitials. Mechanisms of Creep
  23. 23. Diffusion creep- Involves the flow of vacancies and interstitials through a crystal under the influence of applied stress. This mechanism occurs at high temperatures and low stress levels. Grain boundary sliding- Involves the sliding of grains past each other. Mechanisms of Creep

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