A useful guide of technical terms to help you understand the precise concepts related to the fundamentals of strain measurement
|εs||Apparent strain||Strain gauges that are connected individually in a Wheatstone quarter bridge circuit will display an output signal if the temperature changes. This signal is called "apparent strain" or "thermal output" and is independent of the mechanical load on the test object.|
|B||Bridge factor||The bridge factor is the number that expresses the number of active strain gauges used in a Wheatstone bridge circuit. With tension and compression bars, the Poisson's ratio has to be taken into account as well. The number is between 1 and 4.|
|k||k-Gauge factor||The strain sensitivity k of a strain gauge is the proportionality factor between the relative change in resistance ΔR/R0 and the strain ε to be measured: ΔR/R0 =k⋅ε. The strain sensitivity yields a dimensionless number and is designated as the gauge factor. This gauge factor is determined for each production batch through measurement and is specified for each strain gauge package as a nominal value complete with tolerance. The gauge factors vary among the production batches by just a few per mille.|
|V||Maximum permissible effective bridge excitation voltage||A strain gauge is a resistor that converts electrical energy into heat. To prevent heating of the strain gauge, it is essential to choose a supply voltage that is not excessively high. In the strain gauge catalog, the specified excitation voltage always applies for the Wheatstone bridge as a whole. Only half of the voltage may be applied to the individual strain gauge. |
The maximum values specified are permissible only for installation on materials having excellent heat conduction characteristics (e.g. steel of sufficient thickness).
Strain gauge measurements for plastic materials and similar materials with poor heat conduction characteristics require a reduction of the excitation voltage or of a switch-on period (impulse operation).
|σ||Residual stress||“Residual” or “inherent” stresses can arise in the material due to the internal effects of force, e.g. from the non-uniform changes in volume in heat-treated parts during the hardening of steel, by non-uniform cooling of cast or injection-molded metal, or plastic objects, with welded or forged parts, through mechanical processing or, with larger objects, simply from the effect of their own weight. Residual stresses affect the material in a similar manner as loading stresses.|
|ε||Strain||Strain is a dimensional value that represents the relative change of the length of a material to its initial length.|
|σ||Stress||Mechanical stress is expressed by the quotient of the force F and the cross-sectional area A of the stressed material, σ=F/A|
|ν||Poisson's ratio||Poisson's ratio is defined by the division of the transverse strain εt and the longitudinal strain εl. For aluminum alloys, ν = 0.33, for example.|
|Temperature coefficient of the gauge factor||The specified gauge factor applies at room temperature. It changes with the change in temperature; however, with an excellent approximation, this correlation is linear. In the case of constantan measuring grids, the gauge factor is proportional to the temperature; in case of chromium-nickel measuring grids, the gauge factor is inversely proportional to the temperature. The temperature coefficient of the gauge factor and its tolerance are stated on each strain gauge package.|
|Transverse sensitivity||The transverse sensitivity is the ratio of the sensitivity of a strain gauge transverse to the measuring grid direction to its sensitivity in the measuring grid direction. The transverse sensitivity is stated on strain gauge packages.|