Fatigue Resistance

What is fatigue?

• The fracture of a material after repeated application of stress

• occurs at a stress that is smaller than the instantaneous stress needed to cause fracture

Why are fatigue failures dangerous?

They often occur catastrophically with little or no warning.

What types of engineering systems commonly suffer fatigue failure?

Aircraft, railway lines, wind turbines, crankshafts, railway axles and machine components.

What caused the Hatfield rail crash?

Fatigue cracks in the rail caused by repeated wheel loading led to rail fracture and derailment.

What is gauge corner cracking?

Microscopic fatigue cracks that form in rails due to repeated wheel contact stresses.

What are the three common forms of fatigue loading cycles?

Fully reversed sinusoidal

asymmetrical tension-compression

random loading cycles

What is a fully reversed stress cycle?

A fatigue cycle where tensile and compressive stresses are equal in magnitude.

What is the stress range, amplitude and mean stress in fatigue loading?

σ_r = σmax​−σmin​

σ_a = σ_r/2

σ_m = (​σmax​ + σmin) / 2

What is the stress ratio?

R = σmin/σmax

What are the three stages of fatigue failure?

Crack initiation, crack propagation, and fast brittle fracture.

Where do fatigue cracks usually initiate?

On the surface at stress concentration sites.

Give examples of stress concentrators.

Voids, weld defects, sharp corners, keyways, threads, scratches, corrosion and holes.

What are beachmarks?

Macroscopic concentric rings on a fatigue fracture surface caused by interruptions in loading.

What are striations?

Microscopic fatigue crack growth markings

What does the lighter thumbnail region on a fatigue fracture surface represent?

Slow incremental crack growth.

What does the darker region on a fatigue fracture surface represent?

Fast brittle fracture

How is a fatigue test usually performed?

• Apply a sinusoidal stress cycle to a specimen

• record number of cycles to failure

• repeat many times

• test over a range of amplitudes

• frequency of 5-10 Hz

What type of loading is commonly used in fatigue testing?

Fully reversed loading with zero mean stress.

Why are many specimens tested at each stress amplitude?

Fatigue data show significant experimental scatter.

Why are fatigue tests usually started at around 2/3 of UTS?

Failure occurs quickly at this stress level.

What is typically plotted on an S-N diagram?

Stress amplitude versus log(number of cycles to failure)

What is a fatigue limit?

A stress amplitude below which fatigue failure will not occur regardless of cycle number.

Which materials commonly show a fatigue limit?

Some ferrous and titanium alloys.

Do aluminium alloys usually exhibit a fatigue limit?

No.

What is fatigue strength?

The stress amplitude causing failure at a specified number of cycles.

What is fatigue life?

The number of cycles to failure at a specified stress amplitude.

On an S-N diagram, what do points below the curve represent?

Survival (no fatigue failure).

What is a typical safety factor range in stress for fatigue design?

1.5–3.0

What is a typical safety factor range in life for fatigue design?

5–20

Why are safety factors in life larger than safety factors in stress?

Due to experimental uncertainty and flattening of the S-N curve at high cycle counts.

Name factors that affect fatigue resistance.

Mean stress, surface condition, scratches, design features, and environmental effects.

Why can higher testing frequencies be problematic in fatigue testing?

They may heat the specimen and invalidate results

Why does fatigue crack growth eventually lead to sudden fracture?

The remaining uncracked section becomes too small to carry the load.