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

Glossary

  • Compensating Gage

    A strain gage installed external to an instrument and used for both bridge completion and temperature compensation, but not for strain measurement.
  • Compliance Voltage

    The maximum voltage that can be generated by a constant-current power supply.
  • Composite Material

    A material, usually man-made, that is a three-dimensional combination of at least two chemically distinct materials, with a distinct interface separating the components, created to obtain properties that cannot be achieved by any of the components acting alone.
  • Compressive Strain

    Deformation along a line segment that decreases in length when a load is applied along that line.
  • Compressive Stress

    Normal component of stress about a point that is negative.
  • Constant-current power supply (Wheatstone bridge)

    Power source that, when attached to the power corners of a Wheatstone bridge, produces the same current for all values of bridge resistance.
  • Constant-voltage power supply (Wheatstone bridge)

    Power source that, when attached to the power corners of a Wheatstone bridge, produces the same voltage for all values of bridge resistance.
  • Constantan alloy

    A malleable alloy of nominally 60% copper and 40% nickel that, possessing a gage factor of very nearly 2.0 and a relatively low temperature coefficient of resistance is ideally suited for manufacturing foil strain gages.
  • Constitutive Relationships

    The cause-and-effect relationships between stress and strain, related through the elastic and shear moduli and the Poisson ratios, of which Hooke's law is a special case for isotropic materials.
  • Conversion Cycle

    The three sequential steps of finite duration repeated by an analog-to-digital (A/D) converter, namely sampling, converting, and waiting for the next conversion cycle to begin.
  • Corner (Wheatstone bridge)

    An electrical connection within a Wheatstone bridge of two adjacent arms and either a voltage-input (P+, P-) or voltage-output (S+, S-) leadwire.
  • Count

    The smallest unit in a digital display, representing an analog signal having a magnitude equal to the least significant bit of the analog-to-digital converter providing the digital data to the display.
  • Creep

    Time-dependent strain produced in materials following a change in loading conditions.
  • Customary (English )units

    Traditional system of measurement units used primarily in the United States and based on pounds of mass and inches or feet of length.
  • Cyanoacrylate

    A one-component fast-curing adhesive used with bondable strain gages for short-term testing. Sensitive to moisture and certain solvents and subject to embrittlement over time.
  • Deadweight Loads

    Constant load achieved by supporting a mass or masses in a gravitational field such that the load is unaffected by time, temperature, or any variable other than location in the gravitational field.
  • Deformation

    Change in the shape of a body.
  • Delta Rosette

    Three-element strain-gage rosette in which the grids are oriented at 0, 60, 120 degrees, respectively. Synonyms: 60-degree rosette, equiangular rosette
  • Differential gage factor

    The slope of the relative resistance change with strain at a specific strain.
  • Differential-Input Voltage

    The maximum voltage that can be applied across the input terminals of a strain gage instrument without causing damage to the instrument.
  • Digital Data

    Integer data resulting from the analog-to-digital conversion of an analog measurement signal.
  • Displacements

    Vectors in three-dimensional space, reflecting the movement of the endpoints of an elastic line, on which the mathematical concept of strain is based.
  • Distortional Energy

    The potential energy (on which the von Mises failure criterion is based) that results from the change in shape (but not any accompanying change in volume) of an elastically deformed body.
  • Drift, Zero Drift

    Gradual, normally permanent change in the resistance of an installed strain gage, bridge completion resistor, or leadwire that is unrelated to strain.
  • Ductile Materials

    Materials that experience significant plastic deformation before the onset of fracture.
  • Dummy resistor (Wheatstone bridge)

    A bridge completion resistor, usually built into an instrument but sometimes installed externally, located in an inactive arm of the Wheatstone bridge adjacent to an active arm.
  • Dynamic Loads

    Loads that vary significantly with time as measurements are being made.
  • Elastic Aftereffects

    Time-dependent strain produced in materials (especially glass) following a change in loading conditions. Synonym: creep
  • Elastic Deformation

    Stress-induced change in the shape of a body that is fully reversible when the stress is removed.
  • Elastic Limit

    The coordinate on the stress-strain curve for a ductile material corresponding to the onset of plastic deformation in the material, typically higher than the proportional limit and lower than the yield point.
  • Elastic Modulus/ Modulus of Elasticity/ Young's Modulus

    A materials property defined as the ratio of (a) the uniaxial stress along an axis of the material to (b) the accompanying normal strain along that axis.
  • Electrical Resistance Strain Gage

    A passive sensor containing a strain-sensing grid made of carbon, metal wire, metal foil, vapor-deposited metal film, or semiconductor material that enables relative resistance changes in the grid to be related to the strains associated with the changes.
  • Elongation

    An absolute strain in tension.
  • Encapsulation

    A top layer of insulation bonded over the grid and backing of a strain gage.
  • Endloops

    Low-resistance electrical connections between the ends of gridlines in a strain gage pattern. Not intended to measure strain.
  • Engineering

    The application of science and mathematics by which the properties of matter and the sources of energy in nature are made useful to people in structures, machines, products, systems, and processes.
  • Engineering gage factor

    The ratio of the relative resistance change to strain for a small, but finite, deformation. May be significantly different from the differential gage factor at any strain over the finite range.
  • Epoxy

    A synthetic, thermal-setting resin having excellent adhesion to a wide variety of materials, good resistance to chemical attack and water penetration, outstanding electrical properties, low or moderate curing temperatures, low shrinkage on curing, and good optical properties that is widely used in a variety of formulations for strain gage backings, adhesives, and protective coatings as well as photoelastic adhesives and model materials.
  • Equiangular rosette (delta rosette)

    Three-element strain-gage rosette in which the grids are oriented at 0, 60, 120 degrees, respectively.
  • Equivalent stress

    Numerical representation of a failure criterion, in units of stress, achieved through rearrangment of the mathematical expression for the criterion in such a way that the results for a specific stress state can be compared directly to the yield strength of a material.
  • Error

    A known discrepancy between a measured quantity and the true value of that quantity that can be corrected by calculation or calibration.
  • Eulerian Strain

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

    A device used to measure absolute strain, the change in length of a line segment.
  • Extrabridge leadwires

    Leadwires, including those for power supply, signal, or remote sense, that are attached to the corners of a Wheatstone bridge but are not a resistive component in an arm of the bridge.
  • Failure criterion

    A mathematical model used to predict the failure of a material subjected to combined stresses, based on the yield strength(s) of that material measured in a uniaxial stress state. Synonym: failure model
  • Finite Strains

    Normal and shear strains, generally greater in magnitude than 1%, for which their squares and products are not negligible in the mathematical treatment of strain. (Synonym: large strains)
  • Foil strain gage

    Strain gage containing a grid that has been fabricated from a thin metal (foil), typically a few hundred microinches thick.
  • Free surface

    Surface of a body with no normal stress perpendicular or shear stresses parallel to it such that a strain gage or photoelastic coating can be applied for measuring strains in the plane of the surface.
  • Free thermal expansion

    The dimensional change of an unconstrained specimen subject to a change in temperature.
  • Frequency response

    A measure of the effectiveness of an instrument to transmit signals applied to it in terms of their frequency, typically as the fraction of the maximum power obtained in the output signal at a specific frequency.

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