Strain Gauge Applications
Strain gauges are used to record changes in shape (expansion/compression) of the surface of components. They enable the experimental determination of mechanical stresses and thus the stress on the material. This is important both in cases where these stresses cannot be determined with sufficient accuracy by standard software simulation,since assumptions have to be made and boundary conditions applied for every calculation. If these do not correspond to reality, despite precise calculation, the result is incorrect. In these cases, the measurement with strain gages is used to check the theory.
Areas of application for DMS are the strain measurement on machines, components, wooden structures, supporting structures, buildings, pressure vessels, etc. They are also used in pickups (sensors) with which the load of electronic scales (load cells), forces (force pickups) or torques (torque pickups), accelerations and pressures (pressure transmitter) can be measured. Static loads and loads changing over time can be recorded.
Structure and forms
The typical strain gauge is a foil strain gauge, which means that the measuring grid foil made of resistance wire (3-8 µm thick) is laminated onto a thin plastic carrier and etched out and provided with electrical connections. Most strain gages have a second thin plastic film on their upper side, which is firmly glued to the carrier and mechanically protects the measuring grid. The combination of several strain gauges on a carrier in a suitable geometry is referred to as rosette strain gauges or strain gauge rosettes.
For special applications, e.g. in the high-temperature range or for very large strain gauges (measurements on concrete), strain gauges made from a thin resistance wire (Ø 18–25 µm) are laid in a meandering pattern.
During production, a distinction is made between strain gauges for experimental stress analysis and strain gauges for transducer construction. The strain gauges are optimized differently for each area.
In principle, the measuring grid can consist of metals or semiconductors. Semiconductor strain gages (silicon) utilize the piezoresistive effect that is characteristic of semiconductors, i.e. the change in specific resistance that occurs when the semiconductor crystal is deformed. The change in resistance due to changes in length and cross-section plays only a minor role in semiconductor strain gauges. Due to the pronounced piezoresistive effect, semiconductor strain gauges can have relatively large k-factors and, accordingly, much higher sensitivities than metal strain gauges. However, their temperature dependence is also very large and this temperature effect is not linear.
Constantan or NiCr compounds are mostly used as materials for metallic foil strain gauges. The shape of the measuring grid is varied and is based on the different applications. The length of the measuring grid can be manufactured over a range of 0.2…150mm. With strain gages for everyday measurement tasks, the measurement uncertainties are currently between 1% and about 0.1% of the respective full scale value. With more effort, however, the uncertainties can be reduced to 0.005% of the full scale value, whereby the achievement of such uncertainties is not just a question of the transducer technology, but also requires the manufacturer to have the appropriate test equipment available.
The carrier foils of the strain gauges are made of acrylic resin, epoxy resin or phenolic resin or polyamide, among other things.
The history of the DMS
Simmons and Ruge are considered the fathers of the DMS, but they had no contact with each other and worked independently of each other. From today's perspective, however, Edward E. Simmons invented a force transducer using the strain gauge principle, while Arthur C. Ruge, then employed at the Massachusetts Institute of Technology (MIT), invented the "SG" type of sensor used today as a strain gauge in stress analysis. The principle of the DMS was already described in 1856 by William Thomson, who later became Lord Kelvin. Since Simmons had already submitted a patent when Ruge wanted to market his DMS in 1940, the patent was bought out without further ado in order to avoid patent disputes (patent granted by Simmons: August 1942, patent granted by Ruge: June 1944). The first (wire) strain gauges therefore bore the designation SR-4: Simmons, Ruge and 4 others. 1938 is considered to be the year of birth of the DMS because Simmons was published and Ruge's main work was in this year.