Strain gauges are measuring elements that convert force, pressure, tension, etc., into an electrical signal.  They are the most universally used measuring devices for electrical measurement of mechanical quantities.  A strain gauge is a resistive elastic sensor whose resistance is a function of applied strain (unit deformation).

Many types of strain gauges depend on the electrical resistance to strain.  These types include: piezoresistive or semiconductor, carbon-resistive, bonded metallic wire, and foil gauges.  In these gauges the electrical resistance varies linearly with strain.  The resistance of an electrically conductive material changes with dimensional changes that take place when the conductor is deformed elastically. When such a material is stretched, the conductors become longer and narrower, which causes an increase in resistance. A Wheatstone bridge then converts this change in resistance to an absolute voltage. The resulting value is linearly related to strain by a constant called the gauge factor.  Capacitance devices, which depend on geometric features, can be used to measure strain. Changing the plate area, or the gap can vary the capacitance. The electrical properties of the materials used to form the capacitor are relatively unimportant, so capacitance strain gauge materials can be chosen to meet the mechanical requirements. This allows the gauges to be more rugged, providing a significant advantage over resistance strain gauges.

In a photoelectric strain gauge a beam of light is passed through a variable slit, actuated by the extensometer, and directed to a photoelectric cell. As the gap opening changes, the amount of light reaching the cell varies, causing a varying intensity in the current generated by the cell.  Semiconductor or piezoelectric strain gauges are constructed of ferroelectric materials. In ferroelectric materials, such as crystalline quartz, a change in the electronic charge across the faces of the crystal occurs when the material is mechanically stressed. The piezoresistive effect is defined as the change in resistance of a material due to an applied stress and this term is used commonly in connection with semi conducting materials.  Optical strain gauge types include photoelastic, moiré interferometry, and holographic interferometry strain gauges.  In a fiber optic strain gauge the sensor measures the strain by shifting the light frequency of the light reflected down the fiber from the Bragg grating, which is embedded inside the fiber itself.

Gage Pattern

The gage pattern refers cumulatively to the shape of the grid, the number and orientation of the grids in a multiple-grid (rosette) gage, the solder tab configuration, and various construction features that are standard for a particular pattern.  Arrangement types include uniaxial, dual linear, strip gauges, diaphragm, tee rosette, rectangular rosette, and delta rosette.  Specialty applications for strain gauges include crack detection, crack propagation, extensometer, temperature measurement, residual stress, shear modulus gauge, and transducer gauge.

Specifications

The three primary specifications when selecting strain gauges are: operating temperature, the stat of the strain (including gradient, magnitude, and time dependence) and the stability required by the application.  The operating temperature range is the range of ambient temperature where the use of the strain gauge is permitted without permanent changes of the measurement properties.  Other important parameters to consider include the active gauge length, the gage factor, nominal resistance, and strain sensitive material.  The gauge length of a strain gauge is the active or strain-sensitive length of the grid. The end loops and solder tabs are considered insensitive to strain because of their relatively large cross-sectional area and low electrical resistance.  The strain sensitivity, k, of a strain gage is the proportionality factor between the relative changes of the resistance.  The strain sensitivity is a figure without dimension and is generally called gage factor.  The resistance of a strain gage is defined as the electrical resistance measured between the two metal ribbons or contact areas intended for the connection of measurement cables.  The principal component that determines the operating characteristics of a strain gage is the strain-sensitive material used in the foil grid.

Common features of strain gauges include ribbon leads, lead wires, solder tabs, cryogenic use, embeddable, encapsulated, waterproof, and weldable.

Many types of strain gauges depend on the electrical resistance to strain.  These types include: piezoresistive or semiconductor, carbon-resistive, bonded metallic wire, and foil gauges.  In these gauges the electrical resistance varies linearly with strain. 

The three primary specifications when selecting strain gauges are: operating temperature, the stat of the strain (including gradient, magnitude, and time dependence) and the stability required by the application.