3. Fatigue failure

Definition of fatigue failure

Failure of material under cyclical load (load that varies over time)

Stress level fatigue occur

can occur below yield stress

Dangers of fatigue stress

• no warning →no deformation, necking

• sudden, catastrophic fracture

4 Methods where crack appear

• manufacturing defects

• fatigue processes

• corrosion

• creep

Types of fatigue failure (in terms of cycle and presence of cracks)

Cycle:

1. Low cycle fatigue

• low no. of cycle

• occur above σ_y

2. High cycle fatigue

• million cycles to fail

• occur below σ_y

Crack:
1. Crack-free

2. Pre-existing cracks - non destructive testing method used to spot cracks)

High cycle load cyclic stress-no. of cycle relationship (Stress-number (SN) curve)

for crack-free specimen

• for any cyclic load, follow SN curve of σ_m (mean stress)

Benefit for SN curve

• material comparison

• early design estimate

• high cycle fatigue regime

Limitations of SN curve

• remaining life of cracked component

• crack growth behaviour

• high scatter in experiemental data

• long test required

• most of lifetime used to initate defect

• IGNORES CRACK SIZE

Method to solve limitation of SN curve

find one curve for certain stress value

shift curve up and down for different stress

estimate but useful

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Calculation of fatigue life (by number of cycle)

N_i: crack initiation

N_p: crack propagation

• depends on crack size & stress intensity range

Definition of Paris Law

rate of fatigue crack growth

relate crack growth to range of stress intensity factor

occur AFTER crack propagation

Methods to form defect(crack initiation) (Low vs high cycle)

Low:

• plasticity causes slip steps on surface

• act as stress concetrator to start fatigue

High:

• no gross plasticity but homogeneties in atomic level raise local stress and initiate fatigue

Design method to reduce fatigue

• reduce section stress (raise cross-section)

• remove stress concentrators(add fillers, welding)

Environmental method to cope with fatigue

• consider corrosion-fatigue relationship

Surface treatment to cope with fatigue

• locally harden surface as crack usually initates there

• case hardening, work hardening, shot peening (locally inducing elastic compressive stresses)

3 ways material is used to cope with fatigue

• remove porosity ( at source or via hot working)

• employ 2 phases (to deflect crack path)

• employ composites(blunt crack as it propagates)

Maintenance method to cope with fatigue

• employ non-destructive method(NDE) to detect cracking

• repair, remove replace or review