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

Documents

Fatigue SN Curve