written 8.1 years ago by | modified 2.6 years ago by |
Mumbai University > Mechanical Engineering > Sem 4 > Material Technology
Marks: 10M
Year: Dec 2015
written 8.1 years ago by | modified 2.6 years ago by |
Mumbai University > Mechanical Engineering > Sem 4 > Material Technology
Marks: 10M
Year: Dec 2015
written 8.1 years ago by | • modified 8.1 years ago |
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:
Fatigue depends on no. of. Factors:
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:
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.
Crack nucleation:
Crack growth:
Fracture:
(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:
Factors Affecting fatigue
Fatigue cracks are nucleated in the region of such geometric irregularities. Actual stress concentrations is measured by fatigue strength reaction factor.
$\text{Fatigue strength reaction factor} = \frac{\text{Fatiue strength of a member without any stress concentration}}{\text{Fatigue strength of the same member with stress concentration}}$
Surface Roughness:
Surface Residual Stress: