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Testing Terminology

The field of testing has a huge vocabulary describing the techniques and processes used to obtain test results on materials. The following definitions reference ASTM International.

Analytical Chemistry

Reference Material is a material or substance that has one or more sufficiently homogeneous and well established property values so that it may be used to calibrate an analytical instrument, to assess a measurement method, or to assign values to test samples.

Certified Reference Material has had its composition certified by a recognized standardizing agency, such as the National Institute of Standards and Technology.

Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) is a highly sensitive method capable of trace multi-element analysis, often at the parts-per-trillion level. It is used for the determination of a range of metals and several non-metals. ICP-MS also allows for routine chemical testing for trace elements in super alloys and provides ultra-trace elemental analysis for high purity alloys like those used in the Semi-Conductor Industry. Samples are decomposed to neutral elements in a high-temperature argon plasma to produce ions and are analyzed based on their mass to charge ratios. Aqueous samples are introduced by way of a nebulizer which aspirates the sample with high velocity argon, forming a fine mist. The aerosol then passes into a spray chamber where larger droplets are removed. Droplets small enough to be vaporized in the plasma torch pass into the torch body, where the aerosol is mixed with more argon gas. A coupling coil is used to transmit radio frequency to the heated argon gas, producing an argon plasma located at the torch. The hot plasma removes any remaining solvent and causes sample atomization followed by ionization.

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) is a technique that determines elemental concentrations of major, minor, and trace elements in the sample. In theory, the technique can be used to detect all elements, except argon and is used for simultaneous analysis of more than one element in metal alloys. In practice, good results are obtained for about 70 elements with detection limits at the parts per billion level. It is an argon plasma operated at atmospheric pressure and sustained by inductive coupling to a radio frequency electromagnetic field. The high temperature in the plasma causes the sample to dissolve, volatilize, dissociate, atomize and ionize. The sample is a solution of the test material that is introduced into the ICP as a fine aerosol of droplets produced by a nebulizer. The free atoms and ions generated in the plasma are electronically excited. The resulting radiation produces a spectrum of the wavelengths related to the elements in the sample. The intensity of the radiation is proportional to the quantities of each of the elements that are present in the sample.

Atomic Emission Spectrometry (AES) formerly Optical Emission Spectrometry (OES) makes use of the principle that free atoms when energized emit light at a particular narrow band of wavelength intervals. The interval or emission lines form a pattern that is characteristic of the atom that produced it. The intensities of the lines are proportional to the number of atoms that produced them and thus to the elements present. The concentration of the element can be determined by measuring the line intensities. OES is used to measure major and trace elemental constituents in metal alloys. Some elements are difficult or impossible to determine using OES. These include nitrogen, oxygen, hydrogen, halogens, and noble gases.

Wet Analytical Chemistry involves the analysis of samples without the use of instrumented methods {i.e. Atomic Absorption Spectrometry (AAA), Optical Emission Spectrometry (OES) and Inductively-Coupled Plasma Atomic Emissions Spectroscopy (ICP)}. In the area of quantitative elemental analysis, the methods include gravimetry, in which a chemical species is weighed; titrimetry, which involves volume measurement of a liquid; and a number of separation techniques that require a variety of laboratory procedures. Wet analytical methods are used to determine quantitative elemental composition, to umpire or referee check on quantitative instrumental methods, to analyze samples too small for instrumental methods, to determine coating weights and/or to determine oxidation states.

Mechanical Testing

Ductility is the ability of a metal to deform to a permanent position before it fractures.

Elastic Limit is maximum stress that a material can sustain without any permanent deformation remaining after complete release of the stress.

Elongation at Break or Total Elongation is the amount of permanent deformation determined after fracture by realigning and fitting together the broken ends of the specimen.

Elongation at Fracture or Maximum Elongation is measured at the time of fracture and includes both permanent and elastic deformation of the tensile specimen. This is usually determined by keeping an extensometer on the specimen through the final fracture.

Fatigue – progressive and localized structural damage that occurs when a material is subjected to cyclic loading.

Gage Length is the original length of that portion of the specimen over which the strain or the change of length is determined.

Hardness is the resistance of a material to deformation, particularly permanent deformation, indentation or scratching.

Mechanical properties: – Those properties of a material that are associated with the elastic and plastic reaction when force is applied.

Mechanical Testing – Mechanical properties of a metal or material relate to its behavior when subjected to a force or load during testing. In mechanical testing, the behavior of the metal is either elastic or inelastic. Elastic behavior occurs with no permanent damage to the test sample. In contrast, inelastic behavior is observed when a material is permanently deformed by the applied force, and therefore, does not return to its original shape.

Physical Testing – Physical properties are determined without the application of force to the test sample and are usually insensitive to the internal or atomic structure of the material. These properties include density, thermal and electrical conductivity, the coefficient of thermal expansion, magnetic permeability, melting temperature and freezing temperature of materials.

Plastic strain ratio – The ratio of the true strain in the width direction to the true strain in the thickness direction of a sheet material loaded beyond yield.

Reduction of Area is the difference between the original cross sectional area of the tension test sample and the area of its smallest cross section after fracture. Reduction of area is expressed as a percentage of the original cross section.

Strain Hardening Exponent “n” is a measure of the increase in hardness and strength caused by the permanent deformation of a tensile sample. “n” is the calculated exponent of the power curve obtained by mathematically approximating the shape of the stress strain curve between yield and ultimate stress.

Ultimate Tensile Strength is the maximum tensile stress that a material is capable of sustaining and is calculated as the maximum load during a tension test carried to rupture, divided by the original cross sectional area of the sample.

Yield Strength is the engineering stress at which the permanent elongation of the sample has begun.

Yield by Extension under Load is the engineering stress at which the permanent elongation of the sample has begun as determined by constructing a line perpendicular to the strain axis of a stress strain curve at a strain that is specified as a percentage of the gage length.

Young’s Modulus of Elasticity is the ratio of the stress to the corresponding strain below the proportional limit and the greatest stress that the sample can sustain without deviation from a linear relationship of stress to strain.

Metallurgical Examination

Austenite Grain Size is the grain size of a steel sample which exists or existed in the austenite phase at a given temperature. ASTM Test Method E112 discusses methods to determine grain size of steel samples.

Banded Structure is characterized by alternate bands parallel to the direction of rolling or forging that produces elongation of segregated areas. Usually it is shown by alternate layers of light and dark etching regions.

Brightfield Illumination causes surfaces normal to the axis of the microscope to appear bright during reflected light microscopy.

Cold Worked Structure is a microstructure resulting from permanent deformation of a metal or alloy. It is characterized by elongated grains in the material.

Decarburization is the carbon loss from the surface of a steel part due to the reaction with one or more chemical substances during the heat treatment of the part. Partial decarburization is where the carbon content is less than the unaffected interior portion of the piece. Complete decarburization is where the carbon content is less than the solubility limit of carbon in ferrite so that only ferrite is present at the surface of the piece.

Duplex Grain Size is the presence of two grain sizes, in substantial amounts, with one size significantly larger than the other.

Etching is the controlled preferential attack on a metal surface for the purpose of revealing structural details such as grain size or second phases such as carbides.

Ferrite Grain Size is the grain size of the ferrite in predominantly ferritic steel.

Flow Lines are the resulting fiber pattern observed in a hot or cold worked material, showing the manner in which the metal flowed during the forming process.

Inclusions are foreign material in the structure of metals. They are usually referred to as non-metallic particles, such as oxides, sulfides, silicates and aluminates.

Knoop Hardness is a type of diamond hardness measurement that is used to determine the depth of hardened areas and the hardness of small and/or thin samples. The resulting indentation is elongated along one axis.

Macroetch is the controlled surface etching of a metallic sample to reveal a structure that is visible at low magnification, usually no greater than 10 times. It is used to show flow lines and ingot patterns.

McQuaid-Ehn Grain Size is the austenitic grain size in steels from carburizing at 1700°F followed by slow cooling. The method is described in ASTM E 112 and is used to determine if the steel was made to fine grain practice.

Metallography is the branch of science which relates to the constitution and structure of metals and alloys, and their relationship to the properties of the material.

Microstructure is revealed by examining a suitably etched metal sample with a microscope.

Photomacrograph is a photograph taken at low magnification to show the macrostructure of a suitably etched sample.

Photomicrograph is a photograph taken at a relatively high magnification (50 times or greater) to show the microstructure of a suitably polished and etched sample.

Polishing is a mechanical, chemical, and/or electrolytic process used to prepare a smooth reflective surface free from artifacts or damage introduced during sectioning or grinding and suitable for microscopic examination.

Recrystallization is the formation of a new grain structure through nucleation and growth that is produced by subjecting a rolled or formed metal to elevated temperature for a time long enough to accomplish the change in the structure.

Segregation is the concentration of alloying elements in specific regions in a metal which causes variations in the etching characteristics of the sample.

Vickers Hardness is a type of diamond hardness measurement similar to Knoop hardness, but with an indenter that has equal length sides and looks diamond shaped.

Nondestructive Testing

Defect is any discontinuity that does not meet the specified acceptance criteria.

Discontinuity is an intentional or unintentional interruption in the physical structure or configuration of the material or component.

False indications are NDT indications caused by conditions such as part geometry (blind holes, keyways), magnetic writing, localized hardening and scratches. They are considered non-relevant.

Indication is the response or evidence from a nondestructive examination.

Interpretation is a determination of whether indications are relevant, non-relevant or false.

Leakage field is the magnetic field that leaves or enters the surface of a part as a result of a discontinuity or change of section.

Non-relevant indication is an NDT indication that is caused by a condition or type of discontinuity that is not rejectable.

Relevant indication is an NDT indication that is caused by a condition or type of discontinuity that requires evaluation.