Subject: Machine Design -I

Topic: Basic properties of Machine design and theories of failure

Difficulty: Medium

• Failure of material under fluctuating stresses at a stress magnitude which is lower than the ultimate tensile strength of the material called fatigue failure.
• A fatigue failure always starts as a small crack and grows in size under repeated loading of the stress.
• It is generally occurs in turbines, aircraft etc.
• It particularly occur without any warning.
• Fatigue result is brittle appearing fracture, without any deformation at the fracture.

Stages of fatigue failure are

(a) Crack nucleation.

(b) Crack growth

(c) Fracture

(a)Crack nucleation: Fatigue failure begins with the formation of a small crack. This is generally found on external surface of the surface

(b)Crack growth: The crack form on external surface is then developed slowly into the material in the direction roughly perpendicular to the main tensile axis. That results in weakening the strength of material.

(c)Fracture: A fatigue crack advances a small amount during each stress cycle and each incremental of advance is shown on fracture surface as a multiple ripple line.