NDT Glossary

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D

• Damping - Limiting the duration or decreasing the amplitude of vibrations.
  
• Damping Capacity - A measure of the ability of a material to dissipate mechanical energy.
  
• Damping Material - A highly absorbent material used to cause rapid decay of vibration. The material bonded to the back of the piezoelectric element of a transducer to limit the duration of vibrations.
  
• Dark Adaptation - The adjustment of the eyes that occurs over time when one enters a darkened area. This adjustment permits the eye to see better in the dim lighting conditions.
  
• Dead Spot - An area where destructive waves interference occurs to such an extent that the net effect is zero sound energy in the area.
  
• Dead Zone - In ultrasonic testing, the interval following the initial pulse where the transducer ring down prevents detection or interpretation of reflected energy (echoes). In contact ultrasonic testing, the area just below the the surface of a test object that can not be inspected because of the transducer is still ringing down and not yet ready to receive signals.
  
• Deburring - Removing burrs, sharp edges or fins from metal parts by filing, grinding, or tumble or vibratory deburring.
  
• Decarburization - The loss of carbon from the surface of a ferrous alloy usually resulting from heating the material in certain environments.
  
• Decay - The gradual reduction of the quantity of some substance or energy form to zero.
  
• Decay Radioactive - The spontaneous change of an atomic nucleus and the emission of a particle or a photon.
  
• Decay Rate - The speed at which radioactive decay occurs. For a definite quantity of a nuclei, the rate of decay is usually expressed in terms of half-life.
  
• Decibel - A logarithmic unit for expressing power relationships. n = 10 log₁₀(I₁/I₂) where n is the difference of decibels of intensities 1 & 2.
  
• Decontamination - The removal of radioactive contaminants from surfaces.
  
• Deep Drawing - The forming of deeply recessed parts by means of plastic flow of the material.
  
• Deep Etching - Severe etching of a metallic surface for examination at a magnification of ten diameters or less to reveal gross features such as segregation, cracks, porosity or grain flow.
  
• Defect - A discontinuity or other imperfection causing a reduction in the quality of a material or component.
  
• Definition - The sharpness of features on a radiograph that correspond to boundaries from thickness or material density changes in the radiographed component.
  
• Deflection - Deformation within the elastic range caused by a load or force that does not exceed the elastic limit of the material. Temporary deformation such as that of a spring.
  
• Delamination - A laminar discontinuity such as an area of unbonded materials.
  
• Delay Line - A material (liquid or solid) placed in front of a transducer to cause a time delay between the initial pulse and the front surface reflection.
  
• Delta Effect - The scattering or reradiation of energy from a discontinuity. The reradiated energy may include waves of both the incident mode and converted modes (longitudinal and shear).
  
• Demagnetization - The process of removing existing magnetism from a part.
  
• Dendrite - A crystal that has a tree-like branching pattern, being most evident in cast metals slowly cooled through the solidification range.
  
• Densitometer - An instrument used to measure the degree of darkening of developed radiographic film.
  
• Density - The mass of a substance per unit volume.
  
• Density Gradient - The change in density of a radiographic film as a function of position. The maximum density gradient of a film is usually called gamma.
  
• Depth Compensation - See distance amplitude correction.
  
• Depth of Fusion - The depth to which the base metal melted and fused during welding.
  
• Depth of Penetration (Standard) - The depth to which the eddy current density has decreased to 1/e or 36.8% of the surface density. Also known as skin depth.
  
• Descalling - Removal of the thick layer of oxides formed on some metals at elevated temperatures.
  
• Destructive Interference - A reduction in wave intensity that occurs due to the interaction of waves that are out of phase.
  
• Detector - A device that determines the presence of or measures the amount of energy, such as radiation.
  
• Deuterium - An isotope of hydrogen having one neutron and one proton with an atomic weight = 2.014.
  
• Deuteron - The nucleus of a hydrogen-2 (deuterium) atom consisting of one neutron and one proton. A deuteron is considered a subatomic particle with "unit" positive charge.
  
• Developer (Penetrant) - A finely divided powder applied over the surface of a part to help bring out penetrant indications.
  
• Developer (Radiographic) - A chemical solution which reduces exposed silver halide crystals to metallic silver.
  
• Developing Time - The time between the application of the developer and the examination of the part for indications.
• Diamagnetic Materials - Materials that have all paired electrons in the atoms and thus have no net magnetic moment. The magnetic permeability of diamagnetic materials is usually very close to 1.
  
• Differential Probe - A probe having two sensing coils located side-by-side allowing it to convert a floating signals to a low voltage ground referenced signal to be displayed on a ground referenced oscilloscope.
  
• Diffraction - Any redistribution in space of the intensity of waves that results from the presence of an object causing variations of either the amplitude or phase of the waves.
  
• Diffraction Mottling - A diffuse diffraction pattern on a radiograph that sometimes results when thin sections of crystalline material are radiographed.
  
• Diffusion - (1) the movement or atoms or molecules to new sites within a material. (2) Spreading of a constituent in a gas, liquid or solid, tending to make the composition of all parts uniform.
  
• Dimensional Threshold - The minimum thickness that a layer of a fluorescent penetrant must reach so that it will fluoresce when exposed to UV light.
  
• Dimpled Rupture Fracture - A fractographic term describing ductile fracture that occurs by the formation and coalescence of microvoides along the fracture path. Seen at high magnification as tiny cups, or half-voids.
  
• Direct Current (DC) - Electrical current that flows in only one direction in a circuit.
  
• Directional Properties - Properties whose magnitude vary depending on the relationship of the test axis to a specific direction within the material. The variation results from preferred orientation or from fibering in the structure. Anisotropy.
  
• Discernible Image - Image capable of being recognized by sight without the aid of magnification.
  
• Discontinuity - a break in the continuity of a medium or material.
  
• Disintegration, Nuclear - A spontaneous nuclear transformation (radioactivity) characterized by the emission of energy and/or mass from the nucleus.
  
• Dislocation - A discontinuity in the structure of a crystal. Two basic linear types are recognized, but combinations and partial dislocations are most prevalent. An "edge dislocation" corresponds to the row of mismatched atoms along a straight edge formed by an extra, partial plane of atoms within the body of the crystal, that is, by a parallel section through the crystal. A "screw dislocation" corresponds to the highly distorted lattice adjacent to the axis of a spiral structure in a crystal, the spiral structure being characterized by a distortion that has joined normally parallel planes together to form a continuous helical ramp winding about the dislocation as an axis with a pitch of one interplanar distance.
  
• Dispersion - The variation of phase velocity with frequency. In general, any process separating radiation into components having different frequencies, energies, velocities, or other characteristics.
  
• Dispersive Medium - A medium in which the propagation velocity depends on the wave frequency.
  
• Distance Amplitude Correction (DAC) - Compensation of gain as a function of time for difference in amplitude of reflections from equal reflectors at different sound travel distances. Refers also to compensation by electronic means such as swept gain, time corrected gain, time variable gain and sensitivity time control.
• Distance-Amplitude Blocks - A set of ultrasonic reference specimens in which each specimen has a different metal path length to a equal-sized reflector. The specimens are used to develop distance amplitude response curves.
  
• Distance-Amplitude Response Curve - A curve showing the relationship between signal amplitude and equal-sized reflecting surfaces at various distances from the transducer. Standard blocks are used to obtain such curve.
  
• Distorted Field - A magnetic field that does not follow a straight path or have a uniform distribution due to the irregular in shape of the magnetized test object. The direction of a magnetic field in a symmetrical object will be substantially uniform if produced by a uniformly applied magnetizing force, as in the case of a bar magnetized in a solenoid.
  
• Distortion - A Change in the shape of a part due to the action of mechanical forces.
• Divergence - An improper term used to describe the spreading of ultrasonic waves beyond the near field. It is a function of transducer diameter and wavelength in the medium.
• Domain - A substructure in a ferromagnetic material within which all the elementary magnets (electron spins) are held aligned in one direction by interatomic forces; if isolated, a domain would be magnetically saturated.
  
• Doppler Effect - The change in the observed frequency of an acoustic or electromagnetic wave due to relative motion of the source and the receiver.
  
• Dose - The amount of ionizing radiation energy absorbed per unit mass of irradiated material at a specific location, such as a part of the human body. Measured in reps, rems per hour.
  
• Dosimeter - A device that measures radiation dose, such as a film badge or ionization chamber.
  
• Drain Time - The period of time allowed for excess liquid to gradually flow off of a part after application of either the penetrant or emulsifier material.
  
• Dry Developer - A powder that is applied dry to the test component when performing a penetrant inspection.
  
• D/T Ratio - The working distance for the X-ray tube in relation to the film distance. The working distance, d, and the specimen thickness, t, are both measured with reference to the source side of the specimen.
  
• Dual Element Transducers - An ultrasonic transducer that has two active elements in one case. One of the elements send the sound wave and the second receives the reflected sound energy. Near surface resolution is improved since there is no dead-zone issue to content with.
  
• Ductile – Permitting plastic (or permanently) prior to eventual fracture.
  
• Ductile Crack Propagation - Slow crack propagation that is accompanied by noticeable plastic deformation and requires energy to be supplied from outside the body.
  
• Ductility - The ability of a material to deform plastically without fracturing of area in a tensile test or by other means.
  
• Dwell Time - The period of time wherein penetrant or developer is in contact with the surface of the part. Drain time is considered to be a portion of the dwell time. Synonymous with penetration time or emulsifier time.
  
• Dynamic - Moving, or having high velocity. Frequently used with impact testing of metal specimens. Opposite of static, or essentially stationary, testing or service.
  
• Dynamic Creep - Creep that occurs under conditions of fluctuating load or fluctuating temperature.
  
• Dynamic Range – The ratio of maximum to minimum reflective areas that can be distinguished on the cathode ray tube at a constant gain setting.
  

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