Perhaps the most familiar type of fatigue test would be classified as a "crack
initiation" process. With this method, specimens of a material (or the actual parts)
are subjected to repeated stress cycles until a crack is initiated . . .or is allowed to
continue to grow until total fracture occurs. Stress cycles can be applied axially, in
plane bending, in rotating bending, in torsion, or in combination. These test modes
utilize cycles of minimum to maximum tensile stress, or reversed cycles of tensile and
compressive stress. The resultant of the minimum stress divided by the maximum stress is
termed the "stress ratio."
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HOW IS A FATIGUE TEST CONDUCTED?
Each test specimen
is cycled over a constant load (stress) or displacement (strain) range until a crack or
total fracture occurs. The number of test cycles required to produce the desired failure
yields a single data point. Multiple specimens are tested at selected stress levels and
the number of cycles to failure of each is plotted against the applied stress
amplitude.
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HOW ARE RESULTS INTERPRETED?
The widely accepted
graphical representation of fatigue data points is the S/N diagram first introduced over a
century ago by August Wohler. This method locates the cyclic stress amplitude (S) on the
vertical axis and the number of cycles to failure (N) on the horizontal axis.
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< Typically, a
logarithmic scale is used for the horizontal axis while the vertical axis uses a linear
scale. Specimens are tested in a series of decreasing stress levels until no failure
occurs within a selected maximum number of cycles (usually 10 million cycles). The nearly
horizontal portion of the curve defines the fatigue or endurance limit for the test
material. For many nonferrous materials, however, there will not be a definite endurance
limit and the low stress portion of the curve will not have a horizontal slope.
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HOW MANY SPECIMENS ARE REQUIRED FOR AN
ACCURATE FATIGUE TEST?
The number of specimens
necessary to complete an S-N curve... or Goodman diagram... will depend on several
factors. The amount of "scatter" in the data and the desired confidence level
will often dictate how many samples should be tested. Properly prepared specimens of
homogeneous materials may require only about 10 data points to adequately describe an S-N
curve. For higher levels of confidence or for materials that exhibit variances in
composition, many more specimens may be required to yield a definitive curve.
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HOW COULD WE USE THE TEST INFORMATION?
Fatigue data can be used for a
number of purposes, including the evaluation of...
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Various manufacturing processes.
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Different material compositions.
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Variances in material lots.
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Effects of surface finish, heat treating, shot peening and
other geometric factors
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Effects of corrosive or high temperature environments.
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Optimizing designs.
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Another graphical format is the Goodman diagram, which
displays the effect of the test stress ratio. The mean stress is plotted on the horizontal
axis against the cyclic or working stress range (minimum to maximum) on the vertical axis.
Each data point represents the combined value of the mean stress and the maximum cyclic
stress which will yield infinite life, or cause a fatigue failure at a fixed number of
cycles. The apex is the point where the mean stress and the maximum cyclic stress both
equal the ultimate tensile strength of the material. The Goodman diagram is particularly
helpful in determining the effectiveness of a material or component that will be subjected
to a cyclic stress superimposed upon a non-zero mean stress. Typical examples of such an
application include bolted or riveted joints, vehicle suspension systems, aircraft
structures, turbine blades, etc.
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| WHAT ARE SOME APPLICATIONS? Specific examples of applying fatigue test results cover a wide
variety of situations:
Quality control of piston forgings
Effects of laser etching
prosthetic devices
Integrity of welding processes
Evaluating
powdered metal materials
Durability of
electronic circuit connections
Compressor valve
life expectancy
Selection of
spur gear alloys
Fastener
capability comparisons
Engine valve
treatment analysis
Wave spring
washer reliability
Suspension
bridge cable life expectancy
Trailer body
durability
Piston ring
quality
Effects of
plastic bead shot peening
Evaluation of
casting processes
Steel chain
quality control
... and MANY MORE.
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