2. FATIGUE
Progressive, localized and permanent damage that
occurs when a material is subjected to cyclic or
repeated or fluctuating strains at nominal stresses
that have maximum values less than the static yield
strength of the material
Failure occurring under conditions of dynamic loading
3. Characterization of Fatigue
• σ fatigue failure = σ staticfailure
• Apparently brittle even in ductile materials
• Fracture surface is normal to the principle tensile stress
• Sudden and catastrophic – No prior warning
• Result of initiation and propagation of a crack
• Fatigue is failure due to time-varying stresses
• Accounts for about 90% of industrial failure
2
1
4. • Initiates from the surface irregularities, stress
raisers like
– Key holes
– Notches
– Slag inclusions
– Just under a surface coating
• Fracture appearance is mix of smooth (Brittle) and
Rough (Ductile)
Characterization of Fatigue
10. Factors required to cause fatigue
Three basic factors
1. Max tensile stress (S) of sufficiently high value
2. A large enough variations or fluctuations in
the applied stress
3. A sufficiently large number of cycles (N) of
applied stress
13. Stages
Occurs in Five stages
1. Cyclic plastic deformation prior to fatigue crack initiation
2. Initiation of one (or) more micro cracks
3. Propagation (or) Coalescence of micro cracks to form one (or)
more macro cracks
4. Propagation of macro cracks
5. Final Fracture – Ductile manner
15. S-N Curve
• Method of presenting the fatigue data
• S = Stress σa, σmax, σmin
• N = No of Cycles, Life
• Strength (S) vs Log of No of Cycles (N)
• Log of strength (S) vs Log of life (N)
• Each curve is for a constant probability (p) of failure
21. Fatigue Design Approaches
1. Infinite life design
– Very old design type
– Based of fatigue limit
– Stress are some fractions (or) well below the
endurance limit
2. Safe life design
– Finite life design – Life is fixed
– Implies life can be predicted
– Need safety margin to account for
uncertainties
22. 3. Fail-safe design
– Recognizes some structural damage is inevitable --
Relies on detection and repair of cracks before
failure
4. Damage-tolerant design
– Is and extension of fail safe design incorporating
materials that have slow crack growth - high
fracture toughness
– Fracture mechanics approach
– Example pressure vessel “leak before burst”
26. PROCEDURE
• Most methods for completely reversed cycles
σmean = 0
• S = Stress σa, σmax, σmin
• Determined for a specific value of σmean,R or A
• Take 8 – 10 specimens
• Test the first specimen at higher stress (2/3 of Su)
• Find out the N value at failure
• Then reduce the stress gradually for succeeding samples
• Again find out the corresponding N values
27. • Further reduce the stress until one or two samples do not fail in the
specified no of cycles
• Usually 107
cycles
• The highest stress at which a non failure is obtained taken as
fatigue or endurance limit
• For materials without a clear fatigue limit, test is usually terminated
at N = 108
OR 5 x 108
cycles
• Finally plot a graph between Strength (S) vs Log of No of Cycles (N)
29. Factors affecting S-N diagrams
• Material composition
• Grain size
– Fine-grained are superior at room temp
– At high temperature coarse grained superior
• Grain direction
• Residual stresses
• Environment
