What is metal fatigue?

Answer 1

Metal fatigue is caused by repeated cycling of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe.

The process of fatigue consists of three stages:

• Initial crack initiation
• Progressive crack growth across the part
• Final sudden fracture of the remaining cross section

Fundamentals of Metal Fatigue

Stress life method -- plots of alternating stress, S, vs. cycles to

failure, N.

• ignores true stress and strain (assumes elastic strains) which may be significant since initiation of fatigue cracks is plastic deformation
• stress life methods should not be used to estimate lives below 1000 cycles

Endurance Limit: stress for which material has "infinite" life

(> 1x106 cycles)

• existence due to interstitial elements (pin dislocations and prevent slip)
• can disappear due to periodic overloads, corrosive environments or high temperatures
• Most nonferrous alloys do not exhibit endurance limit (some use value at 5x108 cycles or some other number much higher than the design life)

Metal Fatigue (Definition - The phenomenon leading to fracture under repeated or fluctuating stress. Fatigue fractures are progressive beginning as minute cracks and grow under the action of fluctuating stress.)

Metal breaks down over time and extended use. (They get bent and twisted over and over). The particles in the metal move further apart and have less attraction to each other. When a crack develops it weakens the metal - metal fatigue - and can eventually fail enen if a small force is applied.

Answer 2

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