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Fatigue Resistance Test
2
What IS a FATIGUE ?
Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses (e.g., bridges, aircraft, machine components).
Fatigue
This type of failure normally occurs after a lengthy period of repeated stress or strain cycling.
The process occurs by the initiation and propagation of cracks, and typically the fracture surface is perpendicular to the direction of an applied tensile stress.
Cyclic Stresses
The applied stress may be axial (tension–compression), flexural (bending), or torsional (twisting) in nature.
Three different fluctuating stress–time modes are possible.
Cyclic Stresses reversed stress cycle repeated stress cycle randomly Stress Cycle
Reversed Stress Cycle:
The amplitude is symmetrical
about a meanzero stress
level.
Repeated Stress cycle:The maxima and minima
are asymmetrical
relative to the zero stresslevel.
Random Stress Cycle:
The stress level may
vary randomly in amplitude
and frequency
Reversed Stress Cycle
Repeated Stress Cycle
Random Stress Cycle
Fatigue Resistance Testing
The sequence of stress amplitude. The nature of the test-piece.
Constant-amplitude test
Variable-amplitude tests
Specimens Component
Routine life testShort
life test
Long life test
Fatigue Testing Machines:
Rotating Bending Testing Machine
Axial loading (push-pull) type fatigue tester
Rotating Bending Testing Machine
Rotating Bending Testing Machine
• This type of machine gives S-N curve• The motor is turning at a constant
revolution per minute or frequency• To create a failure on the specimen, a
constant-stationary force is applied on the specimen, which creates a constant bending moment.
Working Principle
A stationary moment applied to a rotating specimen causes the stress at any point on the outer surface of the specimen to go from zero to a maximum tension stress, back to zero and finally to a compressive stress.The process is repeated.
Axial loading (push-pull) type fatigue tester
Axial loading (push-pull) type fatigue tester
In this type the specimen is not exposed to bending but to pure axial (tensile or compressive) loading. Specimen is held at two ends and loaded cyclically between two extreme (maximum and minimum) values.
Fatigue Test for LDPE
Fatigue Test for LDPE
Thank You