FATIGUE

G.GopinathAssistant Prof - Mechanical

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

Characterization of Fatigue

• fatigue failurestatic failure

• 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

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• 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

Fatigue Fracture

• Beach marks

Final ductile fracture

Rubbing – Brittle propagation

Factors required to cause fatigueThree 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

Other factors..

Stress cycles

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StagesOccurs 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

SEM image of fatigue striations

S-N Curve

• Method of presenting the fatigue data

• S = Stress a, max, min

• 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

S-N Curve

CN pa

Basquin equation

Endurance limit for some NF alloys

Endurance limit (S) @ N = 108 or 5 x 108 Cycles

Types of Fatigue• High-cycle fatigue (HCF)

– Stress controlled, >104cycles• Low cycle fatigue (LCF)

– Strain controlled, <104 cycles• Thermal fatigue• Surface fatigue

– Rolling contact– Pitting, spalling, cracking (wear)

• Impact fatigue• Corrosion fatigue• Fretting fatigue

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

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”

R.R.Moore Rotating Beam Test (HCF)

Load = 5 Kg, 2 Kg, 1 Kg, 0.5 Kg …

RPM = 500 – 10,000

600,000 Cycles / hour14,400,000 Cycles / day

PROCEDURE• Most methods for completely reversed cycles

mean = 0

• S = Stress a, max, min

• Determined for a specific value of mean, 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

• 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)

Fatigue Limit

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

• Heat treatment (material condition)

• Welding

• Geometrical discontinuities

• Surface finish and surface treatment

• Size

• Mean stress