1 / 21

530.352 Materials Selection

530.352 Materials Selection. Lecture #23 Fatigue Tuesday November 8 th , 2005. Failure even at low Stresses. Failure often occurs even when:  applied <  fracture and  applied <  yielding 90% of all mechanical failures are related to dynamic loading.

kacy
Download Presentation

530.352 Materials Selection

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 530.352 Materials Selection Lecture #23 FatigueTuesday November 8th, 2005

  2. Failure even at low Stresses • Failure often occurs even when:applied < fractureand applied < yielding • 90% of all mechanical failures are related todynamic loading. • Dynamic Loading -> Cyclic Stresses

  3. Examples of Fatigue Failures Plastic Tricycle:

  4. Examples of Fatigue Failures Door Stop:

  5. Versailles 1842 first fatigue problem axial failure Today flaws in 10% of rails. Examples of Fatigue Failures Railway Accidents:

  6. Types of Fatigue • Fatigue of uncracked components • No pre-cracks; initiation controlled fracture • Examples : most small components: pins, gears, axles, ... • High cycle fatigue • fatigue < yield ; Nf > 10,000 • Low cycle fatigue • fatigue > yield ; Nf < 10,000

  7. Types of Fatigue: • Fatigue of cracked structures • Pre-cracks exist: propagation controls fracture • Examples : most large components, particularly those containing welds: bridges, airplanes, ships, pressure vessels, ...

  8. Cyclic Loading  Weight + time -

  9. + max mean  0 time min - Basic Fatigue Terminology: maxmin mean maxmin amplitudemaxmin N = number of fatigue cycles Nf = number of cycles to failure

  10. High Cycle Fatigue • Apply controlled applied < ~ 2/3yield • Stress is elasticon gross scale. • Locally the metal deforms plastically. S-N Curves Mild Steel 50 40 30 10 0 Fatigue limit Stress Al alloys 105 106 107 108 109 Nfailure

  11. Low Cycle Fatigue • Apply controlled amounts of total • total = elastic + plastic • Empirical Observations and Rules • Coffin-Manson Law • Miner’s Rule

  12. Coffin-Manson Law For low cycle fatigue: plastic Nfailure1/2 = Const. log pl y=y/E ~104 log Nfailure

  13. Miner’s Rule Rule of Accumulative damage: Ni Nfailure@ i = 1 N1 N2 N3 Fraction of life time @ i

  14. The Fatigue Process • Crack initiation • early development of damage • Stage I crack growth • deepening of initial crack on shear planes • Stage II crack growth • growth of well defined crack on planes normal to maximum tensile stress • Ultimate Failure

  15. Crack initiation Cracks start at: • Surfaces • Inclusions • Existing cracks Alternate stresses -> slip bands -> surface rumpling

  16. Crack Initiation:

  17. Crack Growth Striation indicating steps in crack advancement.

  18. Propagation in Cracked Structures ao~ adetectible < acritical ao -> acritical = FAILURE !!! K = Kmax - Kmin = (a)1/2 da = A1KmdN Fast fracture log da/dN threshold linear log K

  19. Real world comparisons: Fast fracture log da/dN threshold linear log K

  20. Fracture surfaces:

  21. Fracture Surfaces: Initiation site Fatigue cracking Final fracture

More Related