List of Strain Measurement Related Terms
|εs||Apparent strain||Strain gauges that are connected individually in a Wheatstone quarter bridge circuit will show 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 number which express how many active strain gauges are used in an Wheatstone bridge circuit. At tension and compression bar the Poisson's ratio as to be taken into account as well. The number is between 1 and 4.|
|k||k-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 gauge factor. This gauge factor is determined for each production batch by measuring and is specified an each strain gauge package as a nominal value complete with tolerance. The gauge factors vary between the production batches by just a few thousandths.|
|V||Maximum permissible effective bridge excitation voltage||A strain gauge is a resistor, converting 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 catalogue 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 featuring excellent heat conduction characteristics (e.g. steel of sufficient thickness).
Strain gauge measurements on plastic materials, and similar materials with poor heat conduction characteristics, require a reduction of the excitation voltage or switch-on period (impulse operation).
|σ||Residual stress||“Residual” or “inherent” stresses can arise in material due to the internal effects of force, e.g. from 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 similarly to loading stresses.|
|ε||Strain||Strain is a dimensional value that represents the relative changes of length of a material to its initial length.|
|σ||Stress||The 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||The 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 as the temperature change; however with an excellent approximation, this correlation is linear. In the case of constant 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.|