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Glossary of Common Terms
Here is a list of terms particular to stress and strain measurement with their definitions.

Glossary

  • Full-bridge circuit

    A Wheatstone bridge circuit with four active arms, typically the result of connecting four strain gages to an instrument in adjacent arms.
  • Gage factor, GF, (sometimes) K

    The ratio of (a) the relative resistance change in a strain gage to (b) the Lagrangian unit strain causing the resistance change.
  • Gage length

    The nominal length of the longitudinal section of the grids in a strain gage that produce the primary resistive response of the gage to strain, usually the length of the gridlines inside the grid endloops. Synonym: active gage length
  • Gage reinforcement

    Reinforcement of a test specimen by a strain gage such that either (a) the specimen and gage both experience a global strain that is substantially lower throughout the specimen than if the gage were not present, or (b) the specimen and gage experience a substantially lower local strain that is not representative of the strain throughout the remainder of the specimen.
  • Gain (setting)

    Value of a setting of a control on strain gage instrumentation that varies the amplifier gain and facilitates scaling of the instrument output.
  • Grid

    The resistive component of a strain gage, usually constructed of thin, parallel conductors of metal wire or foil joined by endloops. Designed to maximize the response to strains along the longitudinal axis of the conductors while minimizing the response to transverse strains.
  • Grid width

    The width of the longitudinal section of grid lines in a strain gage that produce the primary resistive response of the gage to strain.
  • Haigh failure criterion

    Failure model based on the strain energy necessary to initiate yielding.
  • Half-bridge circuit

    Wheatstone bridge circuit containing two active arms, typically the result of connecting two strain gages to an instrument as adjacent arms.
  • Hardener

    An ingredient in the formulation of an adhesive or other plastic material which reacts with the resin and becomes a permanent part of the cured resin.
  • High-elongation strains

    Strains, typically 1% or more, requiring special strain gages, installation materials, and installation techniques.
  • Homogeneous material

    A material for which local variations in composition are negligible in comparison with the size of the strain gage installed on it to measure strain.
  • Hookean strains

    Idealized, linearly elastic strains that are not time-dependent.
  • hoop stresses ,tangential stresses , circumferential stresses

    Normal stresses acting on a longitudinal plane about the circumference of a thin-walled pressure vessel.
  • Hydrostatic state

    Biaxial state of stress in which the two non-zero principal stresses in a plane are equal in all directions.
  • Hysteresis (mechanical hysteresis)

    The dependence of the strain in a material not only on the instantaneous value of the stress but also on the previous history of stress.
  • In-plane

    A stress or strain within the plane on which strain measurements are made.
  • Incompressible material

    Idealized material which undergoes no change in volume when deformed by stresses, such that the Poisson's ratio is exactly 0.5 for infinitesimal strains.
  • Indicated Strains

    Strains registered by a readout of a strain gage instrument prior to subsequent adjustments, including scaling for gage factor and error correction.
  • infinitesimal strains

    Normal and shear strains, generally less than 1% in magnitude, for which their squares and products are negligible in the mathematical treatment of strain. This is the normal assumption of the theory of elasticity. (Synonym: small strains)
  • Initial imbalance

    For a Wheatstone bridge, the value of the measurement signal when the value of the measurand is zero.
  • Input voltage

    The voltage supplied to the power corners of a Wheatstone bridge.
  • Installed resistance

    The resistance of a strain gage after it has been bonded into place. May include changes in resistance from nominal due to deformation during installation, or strains induced by the contour of the surface to which it is bonded.
  • Instrument gage factor

    Value of a setting of a calibrated control on a strain gage indicator that varies the amplifier gain, readout resolution, or both, enabling the indicated strain to reflect the gage factor of the strain gage(s) being used to make the measurement.
  • Insulation resistance

    Measure of the strain gage backing and adhesive layer to electrically isolate the sensing grid from the test specimen. Typically 1000 megohms or more.
  • intrabridge leadwire resistance

    The resistance of leadwires in an arm of a Wheatstone bridge, which does not undergo a change of resistance when the strain gage is exposed to strain.
  • Intrabridge leadwires

    Leadwires in an arm of a Wheatstone bridge.
  • Iso-Elastic alloy

    Alloy of 36% nickel, 8% chromium, 4% manganese-silicon-vanadium, and 0.5% molybdenum in iron and used to make grids of strain gages with a high gage factor (approximately 3.2) and good fatigue life. The alloy exhibits high thermal output which typically limits the use to gages made of it to dynamic testing applications.
  • Isotropic material

    A material which has the same mechanical properties in all directions.
  • Junction

    The points (P+, P-, S+, S-) in a Wheatstone bridge circuit where two arms and either a signal or power supply lead are connected together. Related: anisotropic material, orthotropic material
  • Karma alloy

    Alloy of 20% chromium, 3% aluminum, and 3% iron in nickel and used to make grids of electrical resistance strain gages for applications at both cryogenic and elevated temperatures. It also possesses good fatigue characteristics.
  • Lagrangian strain

    A unit strain defined as the ratio of (a) the change in length of a line segment to (b) its initial length.
  • leadwire

    A conductor in a circuit, usually of low, but finite, resistance. In arms of Wheatstone bridge circuits, leadwires between the terminals and the resistive components contribute to the attenuation of the measurement signal. Shunt calibration is often used to eliminate the measurement error resulting from leadwire attenuation.
  • Leadwire attenuation

    A decrease in the output from a Wheatstone bridge circuit resulting from the presence of leadwires in an active arm which do not undergo a change in resistance when the strain gage is exposed to strain. Synonym: leadwire desensitization.
  • Local gage factor

    The differential gage factor at a specific strain level.
  • Lot, strain gage

    The designation for a group of strain gages with grid elements from a common melt, subjected to the same mechanical and thermal processes during manufacturing.
  • Manganin alloy

    Alloy of 4% nickel and 12% manganese in copper that has low sensitivity to strain but high sensitivity to hydrostatic pressure. A grid material for gages designed to measure shock waves.
  • Manufacturer's gage factor

    Gage factor of a strain gage as supplied by the manufacturer and determined in accordance with ASTM and OIML standards from measurements of longitudinal strains in a biaxial strain field produced by a uniform uniaxial stress in a material having a Poisson's ratio of 0.285.
  • Matrix

    The physical size of the strain gage backing and any encapsulation, as matrix length and matrix width.
  • Maximum (algebraic maximum)

    The most positive number in a set. Example: The number 3 is more positive than 2, and -2 is more positive than -3.
  • Measurand

    A physical parameter being quantified by measurement.
  • Mechanical hysteresis

    The dependence of the strain in a material not only on the instantaneous value of the stress but also on the previous history of stress.
  • Modulus of rigidity , shear modulus

    A material property defining the resistance to shearing stresses, equal in pure torsion to (a) the shearing stress divided by (b) the shearing strain.
  • Mohr's circles

    Graphical method of representing stress or strain about a point. Drawn as a circle in a Cartesian coordinate system, the abscissa of any point on the circle is the normal stress or strain. The ordinate of that point for stress circles is the shear stress. For strain circles it is half the shear strain. The algebraically maximum and minimum abscissas are the principal stress and strains. All angles between stresses and strains are doubled in the graphical representation.
  • Natural frequency

    The frequency at which a structure having a specific spring constant vibrates with a specific mass (including self-mass) applied to it.
  • Natural Strain

    Strain expressed as the natural logarithm of the ratio of (a) the final length of a line segment to (b) its initial length.
  • Natural strain

    Strain expressed as the natural logarithm of the ratio of (a) the final length of a line segment to (b) its initial length.
  • Noise

    Interfering and unwanted portion of a measurement signal produced by a source unrelated to the measurand.
  • Nominal resistance

    The intended resistance of a strain gage as designed. The actual resistance may vary due to manufacturing tolerances and effects of installation.
  • Nonlinearity (Wheatstone bridge)

    Deviation of the signal produced by unbalanced Wheatstone bridge from direct proportionality with strain-induced changes in resistances in one or more arms.

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