Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M

Year: Dec 2015

Fatigue:

A component subjected to repeated loading develops a characteristic behavior fundamentally different from the behavior of a metal part subjected to steady loads.

This behavior is called fatigue.

Fatigue is marked by:

• Loss of strength
• Loss of ductility
• Increased uncertainty in both strength & service life.

Fatigue depends on no. of. Factors:

• Nature of loading (repeating/reversing)
• Magnitude of maximum load
• No.of. cycles to failure
• Conditions of metal in the test piece
• surface finish of test piece
• Temperature
• Atom conditions

It will occur without any warning. It is very important all the materials including.

Metals, Plastics, rubber & concrete:

All rotating parts such as axles & crankshafts are subjected to alternating stress, Aircraft wings are subject to repeated gust load, floor beam of bridges, piping are subject to fatigue through temperature variations and consequent cycling of thermal stress.

Fatigue Testing:

Endurance limit is defined as the definite stress at and below which failure by fatigue approximately does not take place.

For clean steel this value is 0.46% of the Ultimate strength.

This Machine consists of:

• Electric motor is capable running at 10,000 rpm.
• Large bearing is giving support to the specimen
• Collect to hold the specimen
• Rotating lever arm, subjected to downward force, for make the specimen bending.

Upper surface of the specimen remains in tension while the lower surfaces are in compression. While the specimen rotating that rotates in between maximum tensile stress & minimum compressive stress.

The maximum value of the stress is usually lower than the yield strength of material. The test is continued i.e., the cycles of the stress are applied until the specimen fails (or) until a limiting no. of cycles has been reached. If a soft steel specimen able to bear 5,00,000 cycles without failures. Then no. of same material are fatigue tested at various stress levels and finally plotted in the logarithmic scale.

Stages of fatigue failure:

The specimen subjected to alternating tensile/compressive stresses. It has 3 stages.

1. Crack nucleation:

   • During first few cycles of loading, localized changes takes place in structure. By using the sub-microscope we can identify the faults. It occurs only surface of the specimen.

2. Crack growth:

   • Submicroscopic cracks formed grow as the cycle of loading continue and become microscopic cracks.

3. Fracture:

   • When critical size is reached the crack propagates. Area of cross section will reduce. So fracture finally occurs.

(i) Reversed: When stress is zero stress amplitude is equal to the maximum stress.

(ii) Alternating/ Fluctuating: Mean stress is no longer zero., Maximum stress = Mean stress + Stress amplitude.

(iii) Repeated: It is the result of push-pull type arrangement can observe in lab testing.

(iv) Irregular: It can be found in aircraft, where the stress variation during flight is random.

Fatigue Properties:

1. Fatigue life: It is the property of the individual specimen and is carried at after testing a no.of specimens at the same stress.
2. Fatigue strength: Strength of a material for a particular fatigue life.

Factors Affecting fatigue

1. Temperature: Fatigue higher at low temperature.
2. Effect of stress concentration:

Fatigue cracks are nucleated in the region of such geometric irregularities. Actual stress concentrations is measured by fatigue strength reaction factor.

$\text{Fatigue strength reaction factor} = \frac{\text{Fatiue strength of a member without any stress concentration}}{\text{Fatigue strength of the same member with stress concentration}}$

1. Surface Roughness:

   • All fatigue cracks nucleate at the surface of the members the conditions of the surface such as surface roughness and surface oxidations (or) corrosion are very important. Smoothly polished specimens have higher fatigue strength.

2. Surface Residual Stress:

   • It arises during casting (or) during cold working when the plastic deformation not uniform throughout the part.