Face-centered Cubic A lattice with a cubic unit cell, the center of each face of which is identical in environment and orientation to its vertices. Abbr. fcc.

Fatigue The phenomenon by which a material may fracture under cyclic stresses at levels below its maximum static strength, i.e., the failure or decay of mechanical properties after repeated applications of stress. It is one of the most common forms of failure of materials under repeated and reversed stresses. Geometry, frequency, stress difference, environment, and temperature, all have a part in fatigue failure mechanisms. Fatigue endurance is the highest level of repeated loading a material can withstand in long-term use. The purpose of fatigue tests is to find that level.

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. Engineering Stress.

Fatigue limit Maximum fluctuating stress a material can endure for an infinite number of cycles. It 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. An alternate term is Fatigue Strength Reduction Factor.

Fatigue resistance A mechanical material property related to how a material responds to load.

Fatigue strength The number of cycles of stress or strain of a specific character that a given specimen sustains before failure of a specified nature occurs.

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. ASTM D-671 details a standard procedure for fatigue testing of plastics in flexure. 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.

Feedstock Any raw material.

Fiber Relatively short lengths of very small cross section of various materials made by chopping filaments.

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.

Figure A shape formed by a combination of points, lines, curves, or surfaces. Circles, squares, and triangles are plane figures. Spheres, cubes, and pyramids are solid figures.

Figurative Not literal.

Filament A variety of fiber characterized by extreme length, which permits its use in yarn with little or no twist and usually without the spinning operation required for fibers.

Flame retardant resin A resin that is compounded with certain chemicals to reduce or eliminate its tendency to burn. For polyethylene and similar resins, chemicals such as antimony trioxide and chlorinated paraffins are useful.

Flat plate formulas A flat plate of uniform thickness is used in many designs to support a load perpendicular to the plate. The illustrations give stress and deflection equations for several common plate configurations. Again, these equations are valid when working with a homogeneous, isotropic material, and when deflection is less than about one-half of the plate thickness.

Flat plate formulas – Thin-Walled Tubing The Thin Walled Tubing illustration and the equations provided can be used to calculate the stress and deformation of thin-walled tubing under internal pressure when neither end of the tubing is closed. This also applies to fairly long tubes, or in situations remote from the tube ends. As long as the wall thickness is less than about one-tenth of the radius, the circumferential or hoop stress ( 2) is practically uniform throughout the thickness of the wall, and the radial stress ( 3) is negligible. As usual, the appropriate time- and temperature-dependent modulus must be calculated for specific applications. Significant error can result if the thin-wall equations are used in calculations that involve thick walls.

Flexural (or bending) strength (1) The maximum stress that can be applied to a beam in pure bending before permanent deformation occurs. (2) 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 A-438 (cast iron), and ASTM D-86 (glass).

Flow of Force A stream of force moving from one or more sources through a material mass. Force per Unit Area Stress.

Flow stress Stress required to cause plastic deformation.

Foam A light frothy mass of fine bubbles formed in or on the surface of a liquid. For example, plastic resins in sponge form, flexible or rigid, with cells closed or interconnected.

Foaming Foaming is the process of producing a cellular material, e.g., plastic, by using foaming agents. A foaming agent is a material mixed with or dissolved in a plastic to make it foam. It includes expanding agents that produce gas on heating – such as easily volatile solvents – or chemical blowing agents that produce gas by thermal decomposition. Weight reduction is the percent volume of a foamed part that contains gas instead of resin. To get 10% weight reduction, the mold is filled 90%, then allowed to "foam" and fill that last 10%. Since only 90% of the volume is resin, the part weighs 10% less than a solid part of the same dimensions would. High weight reductions result in reduced material properties when compared to low weight reductions.

Foci Plural of focus.

Force A physical influence which tends to produce change in an object's shape and/or momentum.

Forming A general term encompassing processes in which the shape of material pieces is changed to a desired configuration.

Foundation The portion of a building that has the sole purpose of transmitting structural loads from the building into the earth.

Fracture The separation of a body caused by either physical or chemical forces, defined both as a rupture of the surface without complete separation and as complete separation of a body into two or more parts resulting in the creation of new surfaces. Brittle fracture occurs suddenly without or with little plastic deformation preceding the fracture. Ductile fracture occurs after significant plastic deformation.

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 (1) Ability of a material to resist crack propagation when subjected to shock load as in an impact test. (2) A measure of the damage tolerance of a material containing initial flaws or cracks.

Frame A stiff or rigid structure formed of joined pieces and used as a major support; the built-up superstructure in any suitable structural material to form the skeleton of a building.

Framework A skeletal structure designed to support or enclose something.

Froth An aggregation of bubbles.

Functionally graded Gradual variations in material functionality engineered throughout a structure.