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Correlating metals fatigue and fracture to materials properties requires extensive
analysis and testing. The control of fracture and fatigue has become a high priority
of designers and materials researchers.
Outlined below are two ASTM Test Methods,
E 466 and E 606,
some
key factors in fatigue testing performance, low cycle fatigue systems and thermal-mechanical fatigue testing.
ASTM Test Method E 466 - Constant Amplitude Axial Fatigue Test
Attributes of the E 466 ASTM test method of axial load fatigue testing:
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- Guides in the selection of materials for use under cyclic stress conditions
- Evaluates the effects of material shape, surface condition, stress level, and related
factors on fatigue life of metallic materials
- Covers tests of materials in the regime where the strains are mostly elastic during
critical loading and throughout the test
- Limited to axial notched and unnotched specimens that are subjected to a periodic
function of constant cyclic amplitude
Examples of Important Fatigue Applications Subjected to Cyclic Loads
- Rotating shafts or axles
- Aircraft wings
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Key Factors in Performing Fatigue Tests
- Specimen alignment
- Specimen surface condition
- Precision in setting and maintaining cyclic loading parameters and test frequency
Any misalignment of a specimen will produce bending, causing premature failure and
giving erroneous results. Strain gauges are recommended to measure and thus eliminate
bending strains which could be induced in the specimen during a test.
Repeatability of
gripping alignment is
also critical to a successful test program because of the time required for realignment.
Precise, dynamically rated extensometers enable measuring the axial deformations.
ASTM Test Method E 606 - Constant Amplitude Low-Cycle Fatigue
Test
- Broadly applied method for constant amplitude low-cycle fatigue testing, typically
of homogeneous metallic materials
- Uniform though not necessarily ambient, temperature, pressure, humidity, etc., is
applied to the test specimen.
- Tests are conducted in constant amplitude stress or strain cycling (without hold
times)
Such tests are useful in developing data from mechanical design, materials R&D,
process and quality control, product performance, and failure analysis. Much valuable
information can be obtained regarding the stability of materials under cyclic loading,
or whether changes occur due to cyclic plastic straining, and when cracks begin
to form.
Low Cycle Fatigue (LCF) Testing
The test system should be free of backlash in the loading train since through-zero
loads are applied in tension and compression.
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image to enlarge
Thermal-Mechanical Fatigue (TMF) Testing
The system measures the components of strain resulting from the thermal expansion
as well as mechanically induced deformation and, therefore, can control total strain
on the specimen to match a required test profile. Thermal-mechanical fatigue testing
requires specially designed water-cooled grips and extensometers so that the test
system does not fail before the specimen.
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