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- Lack of Fusion - Discontinuity due to lack of union between weld metal and parent metal.
- Lamb Wave -A type of ultrasonic wave propagation in which the wave is guided between two parallel surfaces of the test object. The mode and velocity depend on the product of the test frequency and the separation between the surfaces.
- Lamb Wave Generation - Lamb waves are similar to longitudinal waves but are bounded by the sheet or plate surface causing a wave-guide effect.
- Lamellar - Plate-like; made of a number of parallel plates or sheets. Usually applied to microstructure. The most common lamellar microstructure is pearlite in ferrous metals.
- Laminations - Metal defects with separation or weakness generally aligned parallel to the worked surface of the metal. May be the result of pipe, blisters, seams, inclusions or segregation elongated and made directional by working. Lamination defects may also occur in metal-powder compacts.
- Lap - A surface defect, appearing as a seam, caused by folding over hot metal, fins or sharp corners and then rolling or forging them into the surface, but not welding them.
- Latent Image - The invisible image on exposed film which has not been developed.
- Latitude (Radiographic) - The range of thickness of material that can be recorded on the radiograph within the useful reading range of film density. A high contrast film has little latitude and conversely a low contrast film has great latitude.
- Leak Test - A test on sealed sources to assure that radioactive material is not being released.
- Leak Testing (LT) - Techniques are used to detect and locate leaks in pressure containment parts, pressure vessels, and structures.
- Leakage Field - The magnetic field formed outside of a magnet when there is a crack in the magnet.
- Left-hand Rule - If you place your left hand around the conductor and close your fist on it with your thumb pointing toward the direction of current flow.
- Lenz's Law - An induced current has a direction such that its magnetic field opposes the change in magnetic field that induced the current.
- Licensed Material - Source material, special nuclear material or byproduct material received, possessed, used or transferred under a general or special license issued by the Atomic Energy Commission.
- Life-Off - Distance between the coil of a surface probe and sample. It is a measure of coupling between probe and sample.
- Light Metal - One of the low-density metals such as aluminum magnetism, titanium, beryllium or their alloys.
- Linear Accelerator - A particle accelerator designed to move charged particles at high velocities along a straight path to a target.
- Linear Elastic Fracture Mechanics - method of fracture analysis that can determine the stress (or load) required to induce fracture instability in a structure containing a crack-like-flaw of known size and shape.
- Linearity, Amplitude - A measure of the proportionality of the signal input to the receiver and the amplitude of the signal appearing on the display of the ultrasonic instrument or on an auxiliary display.
- Linearity, Area - Constant proportionality between the signal amplitude and the areas of equal discontinuities located at the same depth in the far field. Necessarily limited by the size of the ultrasonic beam and configuration of the reflector.
- Line Focus Principle - The process of making the angle between the anode face and the central ray such that the effective focal spot is small in relation to the actual spot size.
- Line of Force - The lines that can be produced by a permanent magnet or a use of a sheet of paper and iron filings are called "magnetic lines of force". These lines are imaginary lines, but the concept is convenient for describing a magnetic field. A magnetic field has an intensity of so may lines per square inch. These lines have a definite direction and always form closed loops and return back onto themselves.
- Linear Accelerator (linac) - An apparatus used to accelerate electrons to high velocities by means of a high frequency electrical wave traveling along a tube in the linear direction of the electron beam.
- Linearity -The characteristic of an instrument that is revealed by a linear change in reflected signal amplitude. The vertical linearity is determined by plotting the change in ratios of signal amplitude from two reflecting areas. The horizontal linearity is determined by plotting the distance the signal is displaced along the sweep against the change in material thickness.
- Lipophilic Emulsifiers - Oil-based emulsifiers that are able to mix with penetrants regardless of the concentration.
- Liquid Penetrant Inspection (LPI) - A method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw.
- Load - A device that uses electrical power.
- Logarithmic Decrement - The natural logarithm of the ratio of the amplitudes of two successive cycles in a damped wave train.
- Longitudinal - Lengthwise, or in an axial direction.
- Longitudinal Direction - The principal direction of flow in a worked metal.
- Longitudinal Magnetization - Magnetization of a material in such a way that the magnetic flux runs substantially parallel to the long axis of the part, the flux path completing itself through the air outside the material. It is sometimes called bipolar magnetization, because at least two external poles exist in longitudinal magnetization.
- Longitudinal Waves - Commonly used term for compressional wave.
- Longitudinal Wave (Velocity) - Rate of propagation of a wave parallel to the direction of motion of the particles.
- Loop - A single turn of wire or cable used to carry electric current. It is used for magnetizing and demagnetizing purposes.
- Loss of Back Reflection - Absence or significant reduction of an indication from the back surface of the test Object.
- Low-Cycle Fatigue - Fatigue that occurs at relatively small numbers of cycle, or stress applications. The numbers of cycles may be in the tens, hundreds, or even thousands of cycles. There is no exact dividing line between low- and high- cycle fatigue, but for practical purposes, low-cycle fatigue may be accompanied by some plastic, or permanent, deformation.
- Lox-safe penetrant - A penetrant material or system specifically designed to be compatible with or nonreactive in the presence of liquid oxygen.
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