Points of The Do’s and Do Not’s for Design Fatigue

These points are summary of design fatigue to give you a short over view about fatigue design consideration.  This study is important because nearly 3 % of a countries income invested in the fatigue failure. The broad study will be posted later to give you better understanding about fatigue.

1. The most common cause of mechanical failure is fatigue.

2. Reliance on safety factors alone cannot overcome poor design procedures.

3. Good fatigue design incorporates synthesis, analysis and testing and is an iterative process.

4. Fatigue durability test verifies rather than develops the design.

5. No one fatigue life model or analytical model applies to all situations.

6. Analytical fatigue life prediction is not the zenith of fatigue design process.

7. Prototyping is done to determine durability and not to develop the product.

8. Accelerated fatigue testing has advantages as well as limitations.

9. Inspection of in-service parts must be done regularly to monitor customer usage.

10. Environmental conditions should be given their due share in analytical and testing aspects.

11. Fatigue is localised, progressive and permanent behaviour involving nucleation and growth of crack and resulting in sudden failure.

12. Fatigue cracks nucleate primarily on planes of maximum shear and grow on plane of maximum tensile stress.

13. Fractured surfaces tell about the cause of failure in post-failure analysis.

14. Putting back fractured surfaces – to see if they fit – will obliterate key fractographic details.

15. Stress-strain behavior at notches may not be same as that under monotonic tensile or compressive loading.

16. Surface cracks are very important as fatigue cracks nucleate at surface.

17. Good resistance to crack nucleation does not necessarily mean good resistance to crack growth.

18. Refer to ASTM, ISO or similar standards for data reduction.

19. Fully reversed fatigue strength can vary from about 1 to 70 percent of ultimate tensile strength (UTS).

20. Cleaner metals and smaller grain sizes at ambient temperature have better fatigue resistance.

21. Frequency effects are small only when corrosion, temperature or other aggressive environmental effects are absent.

22. Surface finish can have substantial influence on fatigue resistance especially for longer life.

23. Compressive mean or compressive residual stress improves fatigue life.

24. Tensile mean or tensile residual stresses decrease fatigue life.

25. If actual data is not available, approximate estimate of median fatigue behaviour can be made.

26. Using monotonic stress-strain curve of cyclic softening material in cyclic loading application can undermine the extent of plastic deformation in it.

27. In strain life fatigue data of smooth uniaxial specimens, failure means formation of 1mm deep cracks.

28. The greatest effect of mean stress is in high cycle fatigue regime.

29. Stress intensity factor, K, describes stress field at the tip of fatigue crack.

30. Low-impurity alloys have better fracture toughness.

31. Most fatigue crack growth usually occurs in mode 1 so opening mode stress intensity factor, ?K1, controls fatigue crack growth.

32. High fracture toughness materials grow longer cracks before fracture so inspection and detection is reliable.

33. Sharp scratches with radii < 0.25mm (0.01 inches) on hard metals are avoided.

34. Theoretical stress concentration factor, Kt, depends on geometry and mode of loading.

35. The stronger the material, the higher the notch sensitivity.

36. Mean stress has more effect on notched parts than on smooth parts.

37. Fatigue crack growth may represent a portion of total fatigue life.

38. greatest influence of residual stresses is at the notches.

39. residual stresses causes more harm in very low cycle (< 103 cycles) applications due to residual stress relaxation.

40. if residual stresses are present, then both compressive and tensile residual stresses exist in pairs.

41. surface tensile residual stresses in high tensile strength parts should be avoided.

42. grinding and welding produces very harmful tensile residual stresses.

43. peening and surface hardening produces very beneficial surface compressive residual stresses.

44. introducing tensile and compressive residual stresses on opposite sides will be detrimental to fatigue resistance.

45. infrequent, one-sided overloads are expected to produce sequence effects.

46. the state of stress at the root of a notch is usually multi-axial even under uniaxial loading conditions.

47. multi-axial stress states can significantly affect the fatigue behaviour.

48. loading is proportional if alternating stresses and strains have fixed principal directions.

49. non-proportional cyclic loading can produce additional cyclic hardening.

50. oxidation is one of the principal causes of fatigue resistance degradation at high temperatures.

51. lowering stress concentration reduces notch sensitivity at high temperatures.

52. thermal stresses become dominant when temperatures are not uniform in a part or component.

53. weldment fatigue resistance depends more on applied stress range and class of weld.

54. stress concentration is reduced by grinding butt welds and dressing fillet welds.

55. butt welds are more preferred than fillet welds.

56. surface compressive residual stresses are induced by shot-peening, surface rolling and local heating.

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