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7/28/2019 Fatigue SN Curve
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Fatigue S.N curves
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Fatigue
NNii = ?= ?
NNpp = ?= ?
NNTT = ?= ?
Metal FatigueMetal Fatigue is a process which causes premature irreversible damage oris a process which causes premature irreversible damage or
failure of a component subjected to repeated loading.failure of a component subjected to repeated loading.
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FatigueFatigue A sequence of several, very complex phenomena encompassing several disciplines: motion of dislocations
surface phenomena
fracture mechanics
stress analysis
probability and statistics
Begins as an consequence of reversed plastic deformation within a single crystallitebut ultimately may cause the destruction of the entire component
Influenced by a components environment Takes many forms:
fatigue at notches
rolling contact fatigue
fretting fatigue
corrosion fatigue
creep-fatigue
Fatigue is not cause of failure per se but leads to the final fracture event.
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The Broad Field of FractureThe Broad Field of Fracture
MechanicsMechanics
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Intrusions andIntrusions and ExtrusionsExtrusions
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c
Schematic of Fatigue Crack Initiation Subsequent GrowthSchematic of Fatigue Crack Initiation Subsequent Growth
Corresponding and Transition From Mode II to Mode ICorresponding and Transition From Mode II to Mode I
Locally, the crack grows in shear;
macroscopically it grows in tension.
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The Process of FatigueThe Process of FatigueThe Materials Science Perspective:The Materials Science Perspective:
Cyclic slip,Cyclic slip,
Fatigue crack initiation,Fatigue crack initiation, Stage I fatigue crack growth,Stage I fatigue crack growth, Stage II fatigue crack growth,Stage II fatigue crack growth, Brittle fracture or ductile ruptureBrittle fracture or ductile rupture
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Fatigue in composite materials
Composite materials exhibit very complex failure mechanisms
under static and fatigue loading because of anisotropiccharacteristics in their strength and stiffness.
There are four basic failure mechanisms in composite
1. Matrix cracking
2. Delamination
3. Fibre breakage
4. Interfacial debonding
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The different failure modes combined with the inherentanisotropies, complex stress fields, and overall non-linear
behavior of composites severely limit our ability to
understand the true nature of fatigue.
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Typical Comparison of Metal and
Composite Fatigue Damage
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Comparison of Metal and Composite Stiffness
Reduction
Conclusion: composite materials have better
fatigue strength than metals
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Fatigue models for composite
materials
Existing fatigue models for composite
materials can be classified into threecategories:
.
2. residual strength or residual stiffness model
3. progressive damage model
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The S-N curve
The basic method of
presentingengineering fatigue
- .
It is a plot of stress(S)against number of
cycles to failure(N)
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Numerical Model of Fatigue Life
Prediction
It is basically an S-N curve-based fatigue progressive damage
model and ignores the detailed analysis of the local failure. It uses fatigue data from the family of S-N curves and uses a
special damage variable to account for the multiaxial fatigue
.
where max S and min S are the maximum and minimum
cyclic stresses, R is the stress ratio,
N is the fatigue life, and R A and R B are the strength
coefficients
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S-N curve example The fatigue test data for balanced laminates using material
D155 are chosen. The unidirectional S-N curve functionsunder stress ratio 0.1 are listed in table
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Results
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References
Fatigue Damage Modeling of Composite
Laminates by Y.M. Liu and S. Mahadevan,9th ASCE Conference on Probabilistic
(PMC2004)
Mechanical metallurgy by George E.Dieter