PPT: Design for Strength Mechanical Engineering Notes | EduRev

Design of Machine Elements

Mechanical Engineering : PPT: Design for Strength Mechanical Engineering Notes | EduRev

 Page 1


overview
I. Fatigue
II. How fatigue occurs
III. How  start  fatigue
IV. Fatigue life estimation
V. What is S-N curve
VI. Failure of a material due to fatigue
VII. Mechanism of Fatigue Crack Initiation
VIII.Avoidance of fatigue damage
Page 2


overview
I. Fatigue
II. How fatigue occurs
III. How  start  fatigue
IV. Fatigue life estimation
V. What is S-N curve
VI. Failure of a material due to fatigue
VII. Mechanism of Fatigue Crack Initiation
VIII.Avoidance of fatigue damage
fatigue
? in materials science, when materials fails at stresses below the yield point, 
fatigue is structural damage that occurs when a material is subjected to cyclic 
loading. 
Fatigue failure of a crankshaft
Page 3


overview
I. Fatigue
II. How fatigue occurs
III. How  start  fatigue
IV. Fatigue life estimation
V. What is S-N curve
VI. Failure of a material due to fatigue
VII. Mechanism of Fatigue Crack Initiation
VIII.Avoidance of fatigue damage
fatigue
? in materials science, when materials fails at stresses below the yield point, 
fatigue is structural damage that occurs when a material is subjected to cyclic 
loading. 
Fatigue failure of a crankshaft
How fatigue occurs
? Fatigue occurs when a material is subjected to repeated loading and 
unloading. If the loads are above a certain threshold, microscopic cracks 
will begin to form at the surface. when a crack will reach a critical size, 
and the structure will suddenly fracture. The shape of the structure will 
significantly affect the fatigue life; square holes or sharp corners will lead 
to elevated local stresses where fatigue cracks can initiate. 
Page 4


overview
I. Fatigue
II. How fatigue occurs
III. How  start  fatigue
IV. Fatigue life estimation
V. What is S-N curve
VI. Failure of a material due to fatigue
VII. Mechanism of Fatigue Crack Initiation
VIII.Avoidance of fatigue damage
fatigue
? in materials science, when materials fails at stresses below the yield point, 
fatigue is structural damage that occurs when a material is subjected to cyclic 
loading. 
Fatigue failure of a crankshaft
How fatigue occurs
? Fatigue occurs when a material is subjected to repeated loading and 
unloading. If the loads are above a certain threshold, microscopic cracks 
will begin to form at the surface. when a crack will reach a critical size, 
and the structure will suddenly fracture. The shape of the structure will 
significantly affect the fatigue life; square holes or sharp corners will lead 
to elevated local stresses where fatigue cracks can initiate. 
How  start  fatigue
? In metals and alloys, the process starts with dislocation movements, 
eventually  continuing slip bands that nucleate short cracks.
? Fatigue is a  often showing considerable scatter even in controlled 
environments.
? Damage is cumulative  Materials do not recover when rested.
Page 5


overview
I. Fatigue
II. How fatigue occurs
III. How  start  fatigue
IV. Fatigue life estimation
V. What is S-N curve
VI. Failure of a material due to fatigue
VII. Mechanism of Fatigue Crack Initiation
VIII.Avoidance of fatigue damage
fatigue
? in materials science, when materials fails at stresses below the yield point, 
fatigue is structural damage that occurs when a material is subjected to cyclic 
loading. 
Fatigue failure of a crankshaft
How fatigue occurs
? Fatigue occurs when a material is subjected to repeated loading and 
unloading. If the loads are above a certain threshold, microscopic cracks 
will begin to form at the surface. when a crack will reach a critical size, 
and the structure will suddenly fracture. The shape of the structure will 
significantly affect the fatigue life; square holes or sharp corners will lead 
to elevated local stresses where fatigue cracks can initiate. 
How  start  fatigue
? In metals and alloys, the process starts with dislocation movements, 
eventually  continuing slip bands that nucleate short cracks.
? Fatigue is a  often showing considerable scatter even in controlled 
environments.
? Damage is cumulative  Materials do not recover when rested.
Characteristics of fatigue
? Fatigue life is influenced by a variety of factors, such as temperature, 
surface finish, microstructure, presence of oxidizing or inert chemicals, 
residual stresses, etc.
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