fatigue life estimation crack initiation & damage

1. Presentation on A ,Fatigue life estimation, fatigue crack
initiation Mechanism, avoidance of fatigue Damage
Presented by- Guided by-
Aqib Jawed Dr Afzal Khan

2. overview
I. Fatigue
II. How fatigue occurs
III. How start fatigue
IV. Fatigue life estimation
V. What is S-N curve
VI. Failure of a material due to fatigue
VII. Mechanism of Fatigue Crack Initiation
VIII.Avoidance of fatigue damage

3. fatigue
 in materials science, when materials fails at stresses below the yield point,
fatigue is structural damage that occurs when a material is subjected to cyclic
loading.
Fatigue failure of a crankshaft

4. How fatigue occurs
 Fatigue occurs when a material is subjected to repeated loading and
unloading. If the loads are above a certain threshold, microscopic cracks
will begin to form at the surface. when a crack will reach a critical size,
and the structure will suddenly fracture. The shape of the structure will
significantly affect the fatigue life; square holes or sharp corners will lead
to elevated local stresses where fatigue cracks can initiate.

5. How start fatigue
 In metals and alloys, the process starts with dislocation movements,
eventually continuing slip bands that nucleate short cracks.
 Fatigue is a often showing considerable scatter even in controlled
environments.
 Damage is cumulative Materials do not recover when rested.

6. Characteristics of fatigue
 Fatigue life is influenced by a variety of factors, such as temperature,
surface finish, microstructure, presence of oxidizing or inert chemicals,
residual stresses, etc.

7. Fatigue life estimation
 The fatigue life is estimated by S-N curve
What is S-N curve?
The S-N curve is very useful way to visualize time to failure for a specific
material . The "S-N" means stress verse cycles to failure,

8. Main Steps in the S-N Fatigue Life Estimation Procedure
 Analysis of external forces acting on the structure and the component.
 Analysis of internal loads in chosen cross section of a component,
 Selection of individual notched component in the structure,
 Identification of the stress parameter used for the determination of the S-N
curve (nominal/reference stress),
 Determination of analogous stress parameter for the actual element in the
structure.
 Identification of appropriate stress history.
 Calculation of fatigue damage,
 Determination of fatigue life in terms of number of stress history
repetitions, the number of cycles to failure, N.
 The procedure has to be repeated several times if multiple stress
concentrations or critical locations are found in a component or structure.

9. Failure of a material due to fatigue
failure may be viewed on a microscopic level in three steps:
 Crack Initiation: The initial crack occurs in this stage. The crack may be
caused by surface scratches caused by handling, or tooling of the material
threads ( as in a screw or bolt)
 Crack Propagation: The crack continues to grow during this stage as a
result of continuously applied stresses
 Failure: Failure occurs when the material that has not been affected by the
crack cannot withstand the applied stress. This stage happens very quickly.

10. Mechanism of Fatigue Crack Initiation
 The fatigue life of a material is defined as the number of cycles necessary
to form an “engineering” crack, During cyclic loading.
Fatigue crack initiation has been divided into two stages:
1. initiation stage that involves the growth of micro structurally cracks
2. propagation stage that involves the growth of mechanically small cracks.

11. Initiation of fatigue crack (stage I)
 In stage I, the fatigue crack tends to propagate initially along slip planes
(extrusion and intrusion of persistent slip bands) and later take the
direction normal to the maximum tensile stress
 The crack propagation rate in stage I is generally very low on the order of
nm/cycles giving featureless surface

12. Initiation of fatigue crack (stage II)
 The fracture surface of stage II crack propagation frequently shows a
pattern of fatigue band.
 Each scratch is produced by a single stress cycle and represents the
successive position of an advancing crack front normal to the greatest
tensile stress.

13. Factors causing fatigue failure
Basic factors
 A maximum tensile stress of sufficiently high value
 A large amount of variation or fluctuation in the applied stress.
 A sufficiently large number of cycles of the applied stress.

14. Factors causing fatigue failure
Additional factors
 Stress concentration
 Residual stress
 Combined stress
 Corrosion
 Temperature
 Overload
 Metallurgical structure

15. Avoidance of fatigue damage
1. Cyclic stress state
2. Geometry
3. Material Type
4. Residual stresses
5. Size and distribution of internal defects
6. Environment
7. Temperature

16. Cyclic stress state
 Depending on the complexity of the geometry and the loading, one or
more properties of the stress state need to be considered, such as stress
amplitude, mean stress,
in-phase or out-of-phase shear stress and load sequence,

17. Geometry
 Notches and variation in cross section throughout a part lead to stress
concentrations where fatigue cracks initiate

18. Material Type
 Fatigue life, as well as the behavior during cyclic loading, varies widely for
different materials, e.g. composites and polymers differ markedly from
metals.

19. Residual stresses
 Welding, cutting, casting, and other manufacturing processes involving
heat or deformation can produce high levels of tensile residual stress,
which decreases the fatigue strength.

20. Size and distribution of internal defects
 Casting defects such as gas porosity, non-metallic inclusions and
shrinkage voids can significantly reduce fatigue strength.

21. Grain size
 For most metals, smaller grains yield longer fatigue lives, however, the
presence of surface defects or scratches will have a greater influence than
in a coarse grained alloy.

22. Temperature
 Extreme high or low temperatures can decrease fatigue strength.

23. 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