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•  S-N (Stress-Life) Relates nominal or local elastic stress to fatigue life

•  ε-N (Strain-Life)

Relates local strain to fatigue life

•  LEFM (Crack propagation) Relates stress intensity to crack propagation rate

Fatigue Technology The Three Basic Methods

• Also known as Stress-Life and Total Life Method

• Estimates the total fatigue life to catastrophic failure

• Fatigue life computed from log stress vs log cycles (S-N) curve.

• Method is appropriate for long life fatigue problems where there is little plasticity since the method is based on nominal elastic stress

• There is a large amount of scatter in the S-N curve.

S-N Method

N o t c h e d S h a f t

U n n o t c h e d S h a f t

Stre

ss A

mpl

itud

e

Life in Cycles

S-N Method- Similitude

The life of this . . . . . . . . . . . . . . . . is the same as the life of this . . . . .

if both are subject to the same stress

σ

Nominal stress σ

5

Because of This The Fatigue Life Can easily Be Estimated Once the Stress Range, Number of Cycles

and Stress-Life Curve Are Known

Stre

ss R

ange

Allowable cycles to failure N

Counted number of cycles n

•  Is also called the local strain approach, crack initiation method, and strain-life approach

• E-N method is one of the most common life prediction methods used in the automotive industry

• Practically, crack initiation means that a crack of around 1-2 mm has developed. This is often a high proportion of the component life.

• Many automotive components are designed to survive some significant plastic strains in use (especially on the test track!). The E-N method will handle these better than the S-N method which basically ignores plasticity.

E-N Method

The crack initiation life here . . . . . is the same as it is here . . . . . if both experience the same local strains

E-N Method- Similitude

ε Local strain stress ε

ε

N

Plastic (Low CycleFatigue Line)

Elastic (High CycleFatigue Line)

Strain-Life (E-N) Curve

•  Also known as “Low Cycle Fatigue” or “Local Strain Approach”.

•  Local strains can be elastic or plastic hence its suitability for low cycle fatigue.

Crack Propagation (LEFM) Method

• What remnant life is there after initiation?

• What is the safe life or inspection schedule for a component that is or may be cracked?

• The crack growth method is based on the principles of Linear Elastic Fracture Mechanics (LEFM)

•  It relates stress intensity factors to crack growth rates

•  It uses cycle-by-cycle calculations to predict lifetimes

•  It is frequently used in Aerospace, Offshore, and Power Generation industries

This crack . . . . . . . grows at the same rate as this one if both experience the same stress intensity factors

Crack Propagation Method - Similitude