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Fatigue. Permanent structural change that occurs in a material subjected to fluctuating stress and strain. However, in the case of glass, fatigue is determined by long-term static testing and is analogous to stress rupture in other materials. In general, fatigue failure can occur with stress levels below the elastic limit.

Fatigue life. Number of cycles of fluctuating stress and strain of a specified nature that a material will sustain before failure occurs. Fatigue life is a function of the magnitude of the fluctuating stress, geometry of the specimen and test conditions. An S-N diagram is a plot of the fatigue life at various levels of fluctuating stress.

Fatigue limit. Maximum fluctuating stress a material can endure for an infinite number of cycles. It is usually determined from an S-N diagram and is equal to the stress corresponding to the asymptote of the locus of points corresponding to the fatigue life of a number of fatigue test specimens. An alternate term is endurance limit.

Fatigue notch factor. Ratio of fatigue strength of a specimen with no stress concentration to fatigue strength of a specimen with a notch or other stress raisers. Fatigue notch factor is usually lower than the theoretical stress concentration factor because of stress relief due to plastic deformation. An alternate term is strength reduction ratio.

Fatigue ratio. Ratio of fatigue strength or fatigue limit to tensile strength. For many materials fatigue ratio may be used to estimate fatigue properties from data obtained in tension tests.

Fatigue strength. Magnitude of fluctuating stress required to cause failure in a fatigue test specimen after a specified number of cycles of loading. Usually determined directly from the S-N diagram.

Fatigue strength reduction factor. An alternate term for fatigue notch factor.

Fatigue test. A method for determining the behavior of materials under fluctuating loads. A specified mean load (which may be zero) and an alternating load are applied to a specimen and the number of cycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with identical specimens and various fluctuating loads. Loads may be applied axially, in torsion or in flexure. Depending on amplitude of the mean and cyclic load, net stress in the specimen may be in one direction through the loading cycle or may reverse direction.

Data from fatigue testing often are presented in an S-N diagram which is a plot of the number of cycles required to cause failure in a specimen against the amplitude of the cyclical stress developed. The cyclical stress represented may be stress amplitude, maximum stress or minimum stress. Each curve in the diagram represents a constant mean stress.

Most fatigue tests are conducted in flexure, rotating beam or vibratory type machines. Fatigue testing is generally discussed in “Manual on Fatigue Testing,” ASTM STP 91-A and “Mechanical Testing of Materials,” A. J. Fenner, Philosophical Library Inc. ASTM D-671 details a standard procedure for fatigue testing of plastics in flexure.

Fiber stress. Stress through a point in a part in which stress distribution is not uniform. For example, the stress in a beam under bending load varies from compression to tension across the beam. It is more meaningful in determining the properties of the beam material to consider the maximum stress generated in the outer fibers of the beam. Similarly, stress in a beam under twist loading is a maximum in the material furthest from the axis of twist.

File hardness. Simple determination of the comparative hardness of a metal. It is a statement as to whether a file does or does not bite into a material.

Firestone flexometer test. Method for determining compression fatigue characteristics of rubber. A pyramidal rubber specimen is subjected to an oscillating compressive load and the number of load cycles required to produce a specified deflection is reported. (ASTM D-623).

Flare test. Method for determining ductility of tubing material. It is similar to an expansion test and a pin test.

Flattening test. Measure of the ductility of metal pipe. A short section of pipe is crushed diametrically between parallel plates to a specified extent and examined for failure.

Flex resistance. Ability of foam rubber to sustain repeated compressive loads without damage to cell structure. (ASTM D-1055).

Flexural modulus of elasticity. Alternate term for modulus in bending.

Flexural strength. Maximum fiber stress developed in a specimen just before it cracks or breaks in a flexure test. Flexural yield strength is reported instead of flexural strength for materials that do not crack in the flexure test. An alternate term is modulus of rupture.

Flexure test. Method for measuring behavior of materials subjected to simple beam loading. It is also called a transverse beam test with some materials. Specimen is supported on two knife edges as a simple beam and load is applied at its midpoint. Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted in a stress-strain diagram, and maximum fiber stress at failure is flexural strength. Flexural yield strength is reported for materials that do not crack. Standard test procedures are given in ASTM D 790 (plastics), ASTM C-328 and ASTM C-369 (fired whiteware), ASTM D-797 (elastomers), ASTM A-438 (cast iron) and ASTM C-158 (glass).

Flow stress. Stress required to cause plastic deformation.

Fluting diameter. Smallest diameter about which sheet metal can be bent to form a smooth curve rather than a series of planes with a fluted appearance.

Fracture stress. True stress generated in a material at fracture.

Fracture test. Visual test wherein a specimen is fractured and examined for grain size, case depth, etc.

Fracture toughness. Ability of a material to resist crack propagation when subjected to shock load as in an impact test.

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