Here is a list of terms particular to stress and strain measurement with their definitions.
Glossary
self-temperature compensation (STC)
The ability of a strain gage to minimize thermal output over some temperature range when bonded to a material having a coefficient of thermal expansion, in ppm/deg F, equivalent to the self-temperature-compensation (STC) number.
Sensitivity
The ability of a transducer to respond to the measurand, usually expressed as the ratio of the response of the output to a specific change in the measurand. For strain-gage-based transducers this is often 2 mV/V at full-scale load.
Shear Modulus (modulus of rigidity)
A material property defining the resistance to shearing stresses, equal in pure torsion to (a) the shearing stress divided by (b) the shearing strain.
Shear strains
The components of strain at a point that produce changes in shape of a body (distortion) without a volumetric change. That is, the tangent of the angular change in orientation of two initially perpendicular lines. Approximately equal in magnitude to the angle itself in radians for infinitesimal strains
Shear Stresses
Components of stress at a point that act parallel to the plane in which they lie
Shunt Calibration
The production of a calibration signal by temporarily connecting a resistor in parallel with resistive components in the arm of a Wheatstone bridge to simulate a specific decrease in resistance of those components.
SI units
Units of measurement based on International System of Units, including seconds of time, meters of distance, and kilograms of mass. From these, newtons of force and joules of work and energy are derived. Temperature is measured on the Celsius scale
Signal Conditioning Amplifier
An instrument for making strain measurements under static or dynamic loading conditions, including Wheatstone bridge power supply, completion resistors, signal filters, zero-balancing circuits, and instrumentation amplifiers for scaling the measurement signal
Signal corners (Wheatstone bridge)
The two opposite corners (S+, S-) of the Wheatstone bridge from which the measurement signal (Eo) is obtained.
Signal Leadwires
Leadwires between the signal corners of the Wheatstone bridge and the signal terminals of the instrument.
Solder tab (tab, solder)
Portion of a strain gage pattern designed for the attachment of leadwires.
Spring Constant
The ratio of the force applied to a spring to the deformation produced by that force.
Spring Element, Transducer
An elastic structure specially designed to isolate strains in a uniform field in which strain gages are bonded for purposes of measuring the force, displacement, or some other measurand associated with the production of the strains.
St. Venant failure criteria
Failure model based on the magnitude of either the maximum or minimum principal strain necessary to initiate yielding.
Stacked Rosette
Strain gage rosette constructed of grids stacked one above the other about a common point.
Static Load
A load which does not undergo a change in magnitude or direction during a measurement procedure.
Strain
A stress-induced change in length of a body in some direction relative to an undistorted length in direction, not necessarily the same. The nine possible strains about a point form a second rank tensor.
Strain at a point
The state of strain about a point in a body, defined by nine orthogonal strains (three normal and six shear) that form a second-rank tensor.
Strain Energy
The potential energy stored in a body by virtue of an elastic deformation, equal to the work that must be done to produce both normal and shear strains.
Strain Gage Bridge
Wheatstone bridge containing strain gages in one or more active arms.
Strain Gage, resistive, bonded
A sensing device, attached along its entire length to a surface so that its resistance will change as the surface is deformed.
Strain indicator
an instrument used for measuring strains under static loading conditions, typically incorporating a bridge power supply, bridge completion resistors, provisions for zero balancing, control for setting the instrument gage factor, and a readout in units of microstrain.
Strain indicator calibrator
A calibration device for strain indicators consisting of precision resistors used to simulate the effects of strain on strain gages in half- and full-bridge strain-gage circuits containing no Wheatstone bridge nonlinearity.
Strain-gage simulator
A precision variable resistor (as, for example, a decade box) used to simulate the changes in resistance produced by strain in a strain gage.
Strength ratio, failure
The ratio of the yield strength of a material to the equivalent stress calculated for a failure criterion. Failure by yielding is predicted for a value greater than or equal to unity.
Stress
The force acting across a unit area in a solid material in resisting the separation, compacting, or sliding that tends to be induced by application of forces.
Stress at a point
The state of stress about a point in a body, defined by nine orthogonal stresses (three normal and six shear) that form a second-rank tensor.
Stress concentration
Region in a stress field characterized by steep gradients of stress where failure typically occurs.
Stress field
A region of a plane in or on a body populated by an infinite number of points subjected to stresses of either uniform or varying magnitudes and directions.
Stretch
The ratio of (a) the final to (b) the initial length of a line segment subjected to strain. (The natural logarithm of stretch is natural strain.)
Stretch
The ratio of (a) the final to (b) the initial length of a line segment subjected to strain. (The natural logarithm of stretch is natural strain.)
Surface Forces
Forces applied over the surface of a body. They, along with body forces, are of the two kinds that can act on a body.
Switch-and-balance unit
A device connected to a strain indicator for connecting a number of strain gage installations to the strain indicator sequentially and for zero-balancing the installations individually.
Tab, solder
Portion of a strain gage pattern designed for the attachment of leadwires.
Tee rosette
Two-element strain-gage rosette in which the grids are oriented perpendicular to one another.
Temperature coefficient of gage factor
The ratio of the unit variation of gage factor to the temperature variation.
Temperature coefficient of resistance (TCR)
The unit change in resistance (per degree of temperature) when the temperature of a resistive device changes.
Temperature compensation
The process of making some characteristic of a circuit or device relatively independent of changes in ambient temperature.
Temperature sensors, resistance
Resistive sensors, constructed much like a strain gage but with nickel or nickel/manganin grids, used to measure the surface temperature of test specimens to which they are bonded. Synonym: resistive temperature detectors (RTD)
Tensile strain
Deformation along a line segment that increases in length when a load is applied along that line.
Tensile Stress
Normal component of stress about a point that is positive.
Terminals (Wheatstone bridge)
Junctions at the corners of the Wheatstone bridge. Depending upon the leadwires, each may be physically located at the instrument or remotely at the sensor site.
Thermal Output
Portion of the measurement signal caused by a temperature-induced change in resistance of a strain gage that is independent of, and unrelated to, the mechanical strain in the test part. (2) The reversible part of the temperature-induced strain of a strain gage installed on an unrestrained test specimen when exposed to a change in temperature.
Thermal Stresses
Stresses generated in a body that, having undergone a temperature change, is constrained in some way that free thermal expansion or contraction is impeded.
Thin-Walled
Description of a pressure vessel when the ratio of the wall thickness to the radius is so small that the distribution of normal stresses on a plane perpendicular to the surface is essentially uniform through the thickness of the shell. Usually taken to mean a vessel with a ratio of 0.1 or less.
Three-Wire Circuit
A quarter bridge circuit incorporating three leadwires. Of these, one lead is connected between one solder tab and the power terminal (usually P+) of the instrument. The other two leads are connected between the other solder tab and the signal terminal (usually S-) and the dummy terminal (usually D), respectively, to improve temperature compensation in the leadwires and reduce leadwire attenuation errors.
Time-dependent strains
Strains in solid materials that change with time at a fixed load, including elastico-viscous (Maxwell), firmo-viscous (Kelvin), and combined behaviors.
Torsion
A twisting deformation of a body about an axis in which straight lines that are initially parallel to the axis become helixes.
Transducer
Any device or element for converting an input signal into an output signal of a different form, commonly a device for converting a physical variable (force, pressure, displacement, etc.) into electrical signals.
Transfer Coefficient
A constant that is multiplied times one variable to yield the value of another. The gage factor of a strain gage, for example, is a ~ for obtaining the relative resistance change of a strain gage from the strain producing it.