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Safelight - A special lamp used in the darkroom to provide working visibility without affecting the photosensitive emulsion of the radiographic film.
Samarium 145 - A radioisotope of the element samarium.
Saturation (Magnetic) - A condition where incremental magnetic permeability of a ferromagnetic material becomes 1.0.
Saturation Level - The limit of indication height that is obtained as the area of the discontinuity is increased.
Scanning - Movement of the transducer over the surface of the test object in a controlled manner so as to achieve complete coverage. May be either contract or immersion method. Eddy current data can be collected using automated scanning systems to improve the quality of the measurements and to construct images of scanned areas.
Scatter to Primary Ratio - The energy of the scattered radiation (S) divided by the energy of primary beam (P) striking the same point on the imaging device.
Scattering - Random reflection of ultrasonic waves by small discontinuities or surface irregularities.
Scattering Ultrasonic - Dispersion of ultrasonic waves in a medium due to causes other than absorption.
Schlieren System - An optical system used for visual display of an ultrasonic beam passing through a transparent medium.
Scintillation Counter - A device for counting atomic particles by means of tiny flashes of light (scintillations) which the particles produce when they strike certain crystals.
Search Unit - An assembly comprising a piezoelectric element, backing material (damping), wear plate or wedge (optional) and leads enclosed in a housing. Also called transducer or probe.
Secondary Field - The magnetic field produced by induced eddy currents.
Secondary Radiation - Particles or rays produced by the interaction of any type of primary radiation with matter, e.g. Compton recoil electrons.
Seeability - The characteristic of an indication that enables an observer to see it against the adverse conditions of background, outside light, etc.
Segregation - Nonuniform distribution of alloying elements, impurities or microphases.
Segregation (Radiography) - Variation in film density which can be explained by segregation of elements of atomic numbers different from that of the matrix.
Self-Emulsifiable - The property of a liquid penetrant to combine satisfactorily with water, in either emulsion or solution form, to permit its being removed from a surface by washing (rinsing) in water. Synonymous with water-washable.
Self-Inductance - The property of an electric circuit or component that caused an e.m.f. to be generated in it as a result of a change in the current flowing through the circuit.
Semi-Infinite Conductor - A crystalline solid, such as silicon or germanium, with an electrical conductivity intermediate between that of a conductor and an insulator.
Send-Receive - The variations in the test object which affect current flow within the test object can be detected by observing their effect upon the voltage developed across a secondary receive coil.
Send/Receive Transducer - A transducer consisting of two piezoelectric elements mounted side by side separated by an acoustic barrier. One element transmits, one receives.
Sensitivity - A measure of the ability to detect small signals. Limited by the signal-to-noise ratio.
Sensitivity Panel - A plated metal panel with cracks of know depth induced into the plating; used to evaluate and compare penetrant sensitivity.

Sensitometric - The process or technique of producing images of an opaque object on photographic film or on a fluorescent screen by means of radiation.

Series Circuit - A circuit that has only one path for the electrons to take.

Shadow (Ultrasound) - A region in a test object that cannot be reached by ultrasonic energy traveling in a given direction. Shadows are caused by geometry or the presence of intervening large discontinuities.

Shallow Discontinuity - A discontinuity open to the surface of a solid object which possesses little depth in proportion to the width of this opening. A scratch or nick may be a "shallow discontinuity" in this sense.

Shear - A type of force that causes or tends to cause two regions of the same part or assembly to slide relative to each other in a direction parallel to their plane of contact. May be considered on a microscale when planes of atoms slide across each other during permanent, or plastic, deformation. May also be considered on a macroscale when gross movement occurs along one or more planes, as when a metal is cut or sheared by another metal.

Shear Fracture - Fracture that occurs when shear stresses exceed shear fractures are transverse fracture of a ductile metal under a torsional (twisting) stress, and fracture of a rivet cut by sliding movement of the joined parts in opposite directions, like the action of a the pair of scissors.

Shear Horizontal Wave - A shear wave in which the particle vibration is parallel to the incidence surface. Abbreviated SH wave.

Shear Lip - A narrow, slanting ridge, nominally about 45° to the surface, along the edge of a fracture of a ductile tensile specimen, the shear lip forms the typical “cup-and-cone” fractures. Shear lips may be present on the edges of some predominantly brittle fractures to form a “picture frame” around the surface of a rectangular part.

Shear Vertical Wave - A shear wave in which the plane of vibration is normal to the incidence surface. Abbreviated SV wave.

Shear Waves - Waves which move perpendicular to the direction the wave propagates.

Shear Wave Transducer - An angle beam transducer designed to cause converted shear waves to propagate at a nominal angle in a specified test medium.

Shielding - A barrier surrounding a region to exclude it from the influence of an energy field.

Shoe (Ultrasound) - A device used to adapt a straight beam transducer for use in a specific type of testing, including angle beam or surface wave tests and tests on curved surfaces. See also wedge.

Shot Peening - A carefully controlled process of blasting a large number of hardened spherical or nearly spherical particles (shot) against the softer surface of a part. Each impingement of a shot makes a small indentation in the surface of the part, thereby inducing compressive residual stresses, which are usually intended to resist fatigue fracture or stress-corrosion cracking.

Shrink (Materials) - Internal rupture occurring in castings due to contraction during cooling. Also applied to surface shrinkage cracks.

Shrinkage Cavity - Cavities in castings caused by lack of sufficient molten metal as the casting cools. A small bubble in metal that appears as dendritic, filamentary, or jagged darkened area on a radiograph film.

Shrinkage Cracks - Hot tears associated wit shrinkage cavities.

Shrinkage Porosity or Sponge - A localized lacy, or honeycombed, darkened area on a film that indicates porous metal.

SH Wave - See shear horizontal wave.

Signal (Eddy Current Testing) - A change in eddy current instrument output voltage; It has amplitude and phase.

Signal-to-Noise Ratio - Ratio between defect signal amplitude and that from non-relevant indications. Minimum acceptable ratio is 3:1.

Single-Phase Alternating Current - Simple current, alternating in direction. Commercial single-phase current follows a sine wave. Such a current requires only two conductors for its circuit. Most common commercial Frequencies are 25, 50 and 60 cycles per second.

Skin Depth - See depth of penetration.

Skin Effect - A phenomenon where induced eddy currents are restricted to the surface of a test sample. Increasing test frequency reduces penetration.

Skip Distance - In angle beam tests of plate or pipe, the distance from the sound entry point to the first reflection point on the same surface. See V-path.

Slag Inclusions - Nonmetallic solid material entrapped in weld metal or between weld metal and base metal.

Sliver (Materials) - A discontinuity consisting of a very thin elongated piece of metal attached by only one end to the parent metal into whose surface it has been rolled.

Snell’s Law - The physical law that defines the relationship between the angle of incidence and the angle of refraction.

Soak Time - The period time wherein the emulsifier remains in contact with the liquid penetrant/emulsifier is quenched with water, or completely removed by water rinsing. Synonymous with emulsification time.

Soft X-Rays - The quality or penetrating power of x-radiation; their penetrating power is relatively slight.

Solenoid - An electrically charged coil of insulated wire which produces a magnetic field within the coil.

Solidification Shrinkage - The decrease in volume of a metal during solidification.

Solvent Action - The dissolution of a fluid or solid by another material.

Solvent Cleaning - The process of removing the excess penetrant from the surface of a part by washing or wiping with a solvent for the penetrant.

Solvent Developer - A developer in which the developing powder is applied as a suspension in a quick-drying solvent.

Solvent Remover - A nonaqueous liquid employed in removal of surface penetrant from parts or for removal of unwanted background porosity indications.

Sound - Mechanical vibrations transmitted in an elastic gas, liquid, or solid.

Sound Wave Interference - When two or more sound waves from different sources are present at the same time, they interact with each other to produce a new wave.

Sound Wave Propagation - The way sound vibrations travel through different mediums.

Source (Radiography) - The origin of radiation; an x-ray tube or a radioisotope.

Source-Film Distance - The distance between the focal spot of an x-ray tube or radiation source and the film; generally expressed in inches.

Source Material (Radiography) - In atomic energy law, any material, except special nuclear material, which contains 0.05% or more of uranium, thorium, or any combination of the two.

Spalling - The cracking and flaking of particles out of a surface.

Specific Acoustic Impedance - See acoustic impedance.

Specific Activity - The activity, in curies, of 1 gram of any radioactive source.

Specific Gravity - The ratio of the density of any substance to the density of some other substance taken as standard

Specific Ionization - Number of ion pair per unit length of path of the ionizing particle in a medium, e.g. per cm of air or per micron of tissue.

Spectrum - The distribution of frequencies in a signal.

Spectrum Response - The amplification (gain) of a receiver over a range of frequencies.

Spherical Wave - A wave in which points of the same phase lie on surfaces of concentric spheres.

Spline - A shaft with a series of longitudinal, straight projections that fit into slots in a mating part to transfer rotation to or from the shaft.

Spot Examination - Local examination of welds or castings.

Spread Spectrum - A method of testing material using a correlation of continuous signals rather than pulse-echo or pitch-catch techniques.

Spurious Echo - A general term used for any indication that cannot be associated with a discontinuity or boundary at the location displayed.

Squint Angle - The angle by which the ultrasonic beam axis deviated from the probe axis.

Squirter - See water column.

Stable Isotope - A nuclide that does not undergo radioactive decay.

Stacked Crystal - Several crystals cemented together with the daces of the same polarity in the same direction.

Stain Ratio - The ratio of the change in length to the original length.

Standard - A reference object used as a basis for comparison or calibration. - A concept established by authority, custom or agreement to serve as a model or rule in the measurement of quantity or the establishment of a practice or procedure.

Standing Wave - A wave in which the energy flux is zero at all points. Such waves result from the interaction of similar waves traveling in opposite directions as when reflected waves meet advancing waves. A particular case is that of waves in a body whose thickness is an integral multiple of half-wavelengths, as in resonance testing.

Stepped Wedge - A device which is used, with appropriate penetrameters on each step, for the inspection of parts having great variations in thickness or a complex geometry. The stepped wedge must be made of material radiographically similar to that being radiographed.

Stop Bath - A mild acetic acid solution used to arrest film development.

Storage Container - A device in which sealed sources are transported or stored.

Straight Beam - An ultrasonic wave traveling normal to the test surface.

Strain - A measure of relative change in the size or shape of a body. “Linear strain” is change (increase or decrease) in a linear dimension. Usually expressed in inches per inch (in. /in.), or millimeters per millimeter (mm/mm).

Stress - Force per unit area, often thought of as a force acting through a small area within a plane. It can be divided into components, perpendicular and parallel to the plane, called normal stress and shear stress, respectively. Usually expressed as pounds per square inch (psi), or megapascals (MPa).

Stress Concentration - Changes in contour, or discontinuities, that cause local increases in stress on a metal under load. Typical are sharp-cornered grooves, threads, fillets, holes, etc. Effect is most critical when the stress concentration is perpendicular (normal) to the principal tensile stress. Same as notch or stress raiser.

Stress Corrosion - Preferential attack area under stress in a corrosive environment, where such an environment alone would not have caused corrosion.

Stress-Corrosion Cracking - Failure by cracking under combined action or corrosion and a tensile stress, either external (applied) or internal (residual). Cracking may be either intergranular or transgranular, de pending on the metal and the corrosive medium.

Stress Intensity - A measure of the magnitude of the $1/\sqrt{r}$ stress singularity ahead (beyond) the crack tip.

Stress Measurement - A measurement of the stress on a given object.

Striations -Microscopic ridges or lines on a fatigue fracture that show the location of the tip or the fatigue crack at some point in time. They are locally perpendicular to the direction of growth of the fatigue crack. In ductile metals, the fatigue crack advances by one striation with each load application, assuming the load magnitude is great enough. Must not be confused with beachmarks, which are much larger and are formed in a different way.

Stringers - In metals that have been hot worked, elongated patterns of impurities, or inclusions, that are aligned longitudinally. Commonly the term is associated with elongated oxide or sulfide inclusions in steel.

Subatomic Particles - Particles that are smaller than the atom (protons, neutrons, electrons are the three main ones).

Subionization - When low energy electrons interact with an atom giving the orbital electrons a little excess energy.

Substrate - Layer of metal underlying a coating, regardless of whether the layer is basis metal.

Subsurface Discontinuity - Any discontinuity which does not open onto the surface of the part in which it exists. Not detectable by liquid penetrant inspection.

Surface-Breaking Cracks - Cracks formed on the surface of an object.

Surface Probe - A probe for testing surfaces, which has a finite coverage. The coil is usually pancake in shape.

Surface Waves - See Rayleigh wave.

Survey (Radiography) - An evaluation of the radiation hazards incident to the production, use, release, disposal, or presence of radioactive materials or other sources of radiation under a specific set of conditions. When appropriate, such evaluation includes a physical survey of the location of materials and equipment, and measurements of levels of radiation.

Suspension - Liquid bath in which a solid is suspended in liquid.

SV Wave - See shear vertical wave.

Sweep (Ultrasonics) - The uniform and repeated movement of a spot across the screen of the cathode ray tube to form the horizontal baseline.

Sweep Delay - A delay in time of starting the sweep after the initial pulse. Also denotes the control for adjusting the time.

Sweep Length - The length of time or distance represented by the horizontal baseline on an A-scan.

Swept Frequency Technique - An eddy current techniques the involves collecting data at a wide range of frequencies and looking for signal changes as a function of frequency.