Here is a list of terms particular to stress and strain measurement with their definitions.
Glossary
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.
Normal Strains
The Lagrangian strain components of strain at a point that result in volume changes. That is, the ratio of the change in length of an object in some direction per unit original length in that same direction.
Normal stresses
The stresses at a point that are oriented perpendicular to the planes on which they act.
Opposite arms, (Wheatstone bridge)
Any two arms not sharing a common corner of a Wheatstone bridge.
Opposite corners (Wheatstone bridge)
Any two corners in a Wheatstone bridge separated by two arms.
Ordinate
In a system of rectangular Cartesian coordinates, the distance of a point along the vertical Y-axis from the horizontal X-axis. Usually the dependent variable.
Orthotropic material
Material having mechanical properties that are different in three mutually perpendicular directions at a point in a body of it, and that have three mutually perpendicular planes of material symmetry. That is, the properties are a function of the orientation at a point. <br /><br />Wood is an example of an orthotropic material. Related: anisotropic material, orthotropic material
Out-of-plane
A stress or strain inclined at some angle to the plane on which strain measurements are made.
Output (Bridge output)
Electrical output across the signal corners (S+, S-) of the Wheatstone bridge. Sometimes represented as the ratio of the output to the input, i.e., mV/V.
Pattern (strain gage)
The specific design of a strain gage, including the solder tabs, sensing grids, and endloops.
Photoelastic coating
A thin, transparent plastic of uniform thickness which, when bonded to the surface of a test part and viewed with polarized light, produces a full-field indication of the strains present in that surface under load from which stress can be inferred.
Photoelasticity
Changes in the optical properties of a transparent material when it is subjected to mechanical stress. The mechanical birefringence of certain materials enables determination of stress and strains from the interference fringe patterns they produce.
Planar rosette
Strain gage rosette in which all grids are on a common plane oriented about a common point.
Plane strain
State of strain in which one of the three principal strains is zero and the remaining two are nonzero. Synonym: biaxial strain
Plane stress
State of stress in which one of the three principal stresses is zero and the remaining two are nonzero, as in a thin sheet stressed in its own plane.
Plastic deformation
The permanent change in shape or size of a body without fracture, produced by a sustained stress beyond the elastic limit of the material.
Plate
A relatively flat, three-dimensional object for which two dimensions are large compared to the third. Usually loaded parallel to the smallest dimension (pressure) or in shear in the plane of the largest two dimensions.
Poisson bridge
half or full bridge in which strain gages are connected in adjacent arms to measure the principal strains in a uniaxial stress field.
Poisson strains
Strains created in a material by an applied normal stress perpendicular to the axis of the applied stress.
Poisson's ratio
The absolute ratio of (a) the transverse to (b) the longitudinal normal strains in a strain field produced by a uniaxial stress along the longitudinal axis. The value is always between 0.0 and 0.5 for materials having isotropic mechanical properties.
Polyimide
Flexible, temperature-resistant polymer used as a backing and encapsulation for strain gages.
Power corners (Wheatstone bridge)
The two opposite corners (P+, P-) of the Wheatstone bridge to which the input voltage (E) is applied.
Power leadwires
Leadwires between the power corners of a Wheatstone bridge and the power supply. Attenuates the input voltage of constant voltage power supplies.
Power supply (Bridge)
A source of constant current or constant voltage for providing excitation to a Wheatstone bridge.
Principal axes of stress, strain
Sets of orthogonal axes on which the greatest and least stresses and strains in each plane, respectively, are located.
Principal Strains
The maximum and minimum normal strains in a plane, always perpendicular to each other and oriented in directions for which the shear strains are zero.
Principal stresses
The maximum and minimum normal stresses in a plane, always perpendicular to each other and oriented in directions for which the shear stresses are zero.
Proportional limit
For a truly elastic material under uniaxial tensile or compressive load, the coordinate on a stress-strain curve at which the nonzero principal stress begins to vary in a nonlinear manner with the corresponding principal strain. Or in other words this is when an increase in stress no longer has a linear relationship with an increase in strain.
Protective coating
Layer of material applied over a strain gage installation to minimize the effects of moisture, chemical attack, and mechanical damage.
Pure shear
Special state of plane stress for which the sum of the two nonzero principal stresses is zero. Under this condition, there is no expansion or contraction in the direction of the zero principal stress, and consequently, no normal strain in that direction.
Quarter-bridge circuit
Wheatstone bridge circuit having a single active arm, typically the result of connecting a single strain gage to an instrument.
Range
<ol><li>The difference in the largest and smallest values of a measurand.</li><li>The largest and smallest values of a measurand.</li></ol>
Rankine failure criteria
Failure model based on the maximum or minimum principal stress required to initiate yielding.
Readout
An analog or digital device for visually indicating the sign and magnitude of a measurement.
Rectangular rosette
Three-element strain-gage rosette in which the grids are oriented at 0, 45, and 90 degrees, respectively.
Relative resistance change
The ratio of a change in resistance to the initial resistance. In strain gages directly related through the gage factor to the strain that produces it.
Repeatability
A measure of the ability of a transducer to repeatedly produce the same measurement signal for the same measurand. Repeatability is typically specified as the expected deviation in the signal, i.e., a repeatability of 1 part in 10 000 or 1:10 000, for example.
Resistive balance
State of a Wheatstone bridge for which a specific combination of bridge-arm resistances will yield no bridge output when bridge voltage is applied. A method of achieving zero balance.
Resistivity
The electrical resistance of a material to the flow of current, times the cross-sectional area of current flow and per unit length of current path. In electrical resistance strain gages, changes with strain.
Resolution
The smallest measurable increment of a measurand.
Room temperature
At or near 75 deg F (24 deg C).
Rosette
Strain gage containing two or more independent grids for making measurements of strain along each of their axes about a common point.
Rosette
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
Rosette
Rectangular rosette, Three-element strain-gage rosette in which the grids are oriented at 0, 45, and 90 degrees, respectively. Synonym: 45-degree rosette
Rosette
Tee rosette, Two-element strain-gage rosette in which the grids are oriented perpendicular to one another. Synonym: 90-degree rosette
Rosette
planar rosette, Strain gage rosette in which all grids are on a common plane, oriented about a common point.
Rosette
Strain gage containing two or more independent grids for making measurements of strain along each of their axes about a common point.
RTD, 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)
Scaling factor
A transfer coefficient or correction factor that is multiplied times one variable to yield the value of another.
Self-heating
An increase in the temperature of a strain gage installation due to the heat generated by the current passing through it when connected in a Wheatstone bridge circuit. In addition to changes in ambient temperature, a source of thermal output.