F

• False Indication - A test indication that could be interpreted as originating from a discontinuity but which actually originates where no discontinuity exists.
• Faraday, Michael - a chemist in England during the early 1800's and is credited with the discovery of electromagnetic induction, electromagnetic rotations, the magneto-optical effect, diamagnetism, and many other discoveries.
• Faraday's Law - the principle saying that whenever wires are moved with an electrical current, it creates a magnetic field.
• Far Field - The zone beyond the near field in front of the transducer in which signal amplitude decreases monotonically in proportion to distance from the transducer. Also called the Fraunhofer zone.
• Fast Film - Radiographic film which has inherent graininess characteristics of a coarse nature intended to increase the relative film speed.
• Fatigue - The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum values less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stresses.
• Fatigue Crack - Crack in a material formed because of repeated stress below the maximum stress of the material, rather than because of overload.
• Fatigue Limit - The maximum stress below which a material can presumably endure an infinite number of stress cycles. If the stress is not completely reversed, the value of the mean stress, the minimum stress, or the stress ratio should be stated.
• Fatigue Strength - The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated.
• Feed-Through Coil - See encircling probe.
• Ferrite - Essentially pure iron in the microstructure of an iron or steel specimen. It may have a small amount of carbon (less than 0.02 wt%). Also called alpha iron.
• Ferromagnetic - It is a measure of coupling between the coil and test object. - Fraction of the test coil area filled by the test specimen.
• Ferromagnetic Materials - Materials that can be magnetized.
• Ferrous - Describing a metal that is more than 50% iron, such as steel, stainless steel, cast iron, ductile (nodular) cast iron, etc.
• Field Intensity - A term used to describe the strength of the electromagnetic field.
• Fillet - a radius (curvature) imparted to inside meeting surfaces; a blended curve joining an internal corner to two lateral surfaces.
• Filled Crack - A crack-like discontinuity, open to the surface, but filled with some foreign material-oxide, grease, etc.- which tends to prevent penetrants from entering.
• Film Badge - A package of photographic film worn like a badge by workers in the nuclear industry to measure exposure to ionizing radiation. The absorbed dose can be calculated by the degree of film darkening caused by the irradiation.
• Film Holder - A light tight carrier for films and screens.
• Film Speed - Relative exposure required to attain a specified density.
• Filter - A layer of absorption material that is placed in the beam of radiation for the purpose of absorbing rays of certain wavelengths and thus controlling the quality of the radiograph.
• Filters in Radiography - Filters are placed in the x-ray beam to produce a cleaner image by absorbing the lower energy x-ray photons that tend to scatter more.
• Filtration Inherent - The filtration exhibited by the walls and other materials of a radiation source through which the radiation must pass before it is utilized.
• Fine Crack - A discontinuity in a solid material with a very fine opening to the surface, but possessing length and depth greater than the width of this opening; usually depth is many time the width.
• Fission - A term defined as the splitting of an atomic nucleus into two smaller nuclei of roughly equal mass. During fission, a fissionable nucleus (such as plutonium) absorbs a neutron, becomes unstable and splits into two nuclei, releasing energy.
• Fission Products - Nuclei formed by the fission of heavy elements. They are of medium atomic weight, and almost all are radioactive. Examples: strontium 90, cesium 137.
• Fissionable Material - Any material readily fissioned by slow neutrons, for example, uranium 235 and plutonium 239.
• Fixer - A chemical solution which dissolves unexposed silver halide crystals from developed film emulsions.
• Flakes (Materials) - Short discontinuous internal fissures in ferrous metals attributed to stresses produced by localized transformation and decreased solubility of hydrogen during cooling after hot working. In a fractured surface, flakes appear as bright silvery areas; on an etched surface they appear as short, discontinuous cracks. Also called "shatter cracks and snowflakes."
• Flash Point Interference - A method used to reconstruct a flaw based on an general ellipsoid model.
• Flat Bottom Hole - A type of reflector commonly used in reference standards. The end (bottom) surface of the hole is the reflector.
• Flaw - A defect.
• Flaw Location Scale - A specially graduated ruler that can be attached to an angle beam transducer to relate the position of an indication on the cathode ray tube screen to the actual location of a discontinuity within the test object.
• Flaw Reconstruction - The process used to determine what a flaw looks like through nondestructive testing.
• Fluorescence, Radiographic - The emission of electromagnetic radiation by a substance as the result of the absorption of electromagnetic or corpuscular radiation having greater unit energy than that of the fluorescent radiation. Fluorescence is characterized by the fact that it occurs only so long as the stimulus responsible for it is maintained. The characteristic x-radiation emitted as a result of absorption of x-rays of higher frequency is a typical example of fluorescence.
• Fluorescence, Penetrant - Property of emitting light as the result of, and only during the absorption of radiation from some other energy source.
• Fluorescent - A bright vivid color that glows under a black light.
• Fluorescent Dye - A dye which becomes fluorescent giving off light, when it is exposed to short wave radiation such as ultraviolet or near ultraviolet light.
• Fluorescent Dye Penetrant - A highly penetrating liquid used in performance of of liquid penetrant testing and characterized by its ability to fluoresce under black light.
• Fluorescent Minerals - Minerals that glow when exposed to sunlight.
• Fluorescent Screen - The coating of material in cathode ray tubes, which glows under electronic bombardment from the cathode.
• Flux, Neutron - The intensity of neutron radiation. It is expressed as the number of neutrons passing through 1 square centimeter in 1 second.
• Flux Density - The number of flux lines per unit of area, measured at right angles to the direction of the flux. It is the measure of magnetic field strength.
• Flux Leakage - Flux, or lines of force, leaking from pole to pole outside a magnet.
• Focal-Film Distance (ffd) - The distance in inches between the focal spot of the x-ray tube, or the radiation source, and the film.
• Focal Zone - The distance before and after the focal point in which the intensity differs a specified amount (usually 6db) from the focal intensity. Also called depth of field or depth of focus.
• Focused Beam - A sound beam that converges to a cross section smaller than that generated by the transducer.
• Focused Transducer - A transducer that produces a focused sound beam.
• Focusing - Concentration or convergence of energy into a small beam.
• Fog - A darkening of the film resulting from chemical action of the developer, aging, scattered secondary radiation, pre-exposure to radiation or exposure to visible light.
• Foil - Metal in sheet form less than 0.006 inch in thickness.
• Forward Scatter - Radiation scattered in approximately the same direction of the primary beam.
• Foucault Currents Method - In France the eddy current method is known as the "Foucault Currents" method.
• Fractograph - Descriptive treatment of fracture, especially in metals, with specific reference to photographs of he fracture surface. Macrofractography involves photographs at low magnification, microfractography at high magnification.
• Fracture - A break, or separation, of a part into two or more pieces.
• Fracture Mechanics - A quantitative analysis for evaluating structural behavior in terms of applied stress, crack length, and specimen or machine component geometry.
• Fracture Toughness - A generic term for measures of resistance to extension of a crack. The term is sometimes restricted to results of fracture mechanics tests, which are directly applicable in fracture control. However, the term commonly includes results from simple tests of notched or precracked specimens not based on fracture mechanics analysis. Results from tests of the latter type are often useful for fracture control, based on either service experience or empirical correlations with fracture mechanics tests.
• Fraunhofer Diffraction - A form of diffraction in which the light source and the receiving screen are in effect at infinite distances from the diffracting object, so the wave fronts can be treated as planar rather than spherical.
• Free Electron - An electron that is produced when the valence electron in any atom gains sufficient energy from some outside force and then breaks away from the parent atom.
• Frequency - The number of waves that pass a given point in a specified unit of time.
• Frequency, Fundamental – In resonance testing, the frequency at which the wavelength is twice the thickness of the test material.
• Frequency, Pulse Repetition - The number of pulses per second.
• Frequency Response - The range of frequencies over which a device operates as expected.
• Frequency, Test - The nominal wave frequency used in a test.
• Fretting Wear - Surface damage to a metal part resulting from microwelding due to slight movement in a nearly stationary joint. Also called fretting corrosion.
• Front Surface - The first surface of the test object encountered by the ultrasonic beam. See interface.
• Full Wave Rectified Single Phase AC- Rectified alternating current for which the rectified is so connected that the reverse half of the cycle is "turned around," and fed into the circuit flowing in the same direction as the first half of the cycle. This produces pulsating dc, but with no interval between the pulses. Such current is also referred to as single-phase full-wave dc.
• Full Wave Rectified Three Phase AC - When tree-phase alternating current is rectified the full-wave rectification system is used. The result is dc with very little pulsation in fact only a ripple of varying voltage distinguishes it from straight dc.
• Function Generators - A device that generates a function wave such as a sine wave or square wave.
• Fusion - This is a particular process, also known as charged particle bombardment, that yields radioisotopes that are not readily available by the neutron bombardment or fission process.