This document discusses fatigue resistance testing. It defines fatigue as failure that occurs in structures subjected to fluctuating stresses over time, usually from repeated stress cycling that causes crack initiation and propagation. There are three types of cyclic stresses: reversed, repeated, and random. Fatigue resistance testing involves subjecting test specimens to these different stress cycles to determine their fatigue life. Two common fatigue testing machines are described: rotating bending and axial loading types. An example fatigue test on LDPE is also mentioned.
2. What IS a FATIGUE ?
2
Fatigue is a form of failure that occurs in
structures subjected to dynamic and fluctuating
stresses (e.g., bridges, aircraft, machine
components).
3. 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.
4. 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.
5. Cyclic Stresses
reversed stress cycle
repeated stress cycle
randomly Stress Cycle
Reversed Stress
Cycle:
The amplitude
is symmetrical
about a mean
zero stress
level.
Repeated Stress
cycle:
The maxima and
minima are
asymmetrical
relative to the
zero stress
level.
Random Stress
Cycle:
The stress level
may vary
randomly in
amplitude and
frequency
9. Fatigue Resistance Testing
The sequence of stress amplitude. The nature of the test-piece.
Constant-
amplitude
test
Variable-
amplitude
tests
Specimens Component
Routine
life test
Short
life test
Long
life test
12. 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.
13. 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.
15. 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.
