Strain sensor

from Wikipedia, the free encyclopedia

A strain sensor is used in the material testing in tensile , compressive or bending tests , the elongation to be measured. For this purpose, analog or incremental displacement sensors used. The strain is calculated from their signal with the aid of the specimen dimensions. The direct method of measurement using strain gauges , on the other hand, is only rarely used, since such strips have to be affixed to each individual sample in a laborious manner.

Small strains over great lengths can advantageously be measured using fiber Bragg gratings . Depending on their length, these glued-on optical fibers filter a very specific light wavelength that can be analyzed using optical methods.

A distinction is made between direct and indirect measurement.

Direct measurement

With direct measurement, measurements are taken directly on the sample. A clip-on transducer or optical strain sensor (e.g. video extensometer ) is used here. Resolutions of up to 1  µm can be achieved here. Particularly when measuring the modulus of elasticity , high precision is required.

Other variants are:

Measurement methods based on an image correlation of surface patterns - such as the laser speckle extensometer and gray value correlation - determine the “true” or logarithmic expansion ε ' based on the pattern flow observed at the measuring points . By contrast, determined measuring method which detect the distance between the measuring marks directly (eg. B. Videoextensometer , laser extensometer , clip-on ), the technical elongation ε of a sample. The conversion of logarithmic elongation into technical elongation is possible as long as the material in the observed measuring range is elongated evenly, in particular without local constriction .

   

From the uniform expansion, the sample constricts and is thereby locally expanded unevenly. As a result, the conversion between logarithmic and technical expansion is flawed.

Indirect measurement

In this case, indirect means that the strain is measured by a rotary encoder on the motor axis . The elongation is calibrated so that per increment z. B. the sample is stretched by 0.1 mm. Often this type of extensometer is referred to as a traverse transducer. The disadvantage of this measuring method is that the " slip " in the specimen holder or the expansion of the crossbeam as well as other deformations are included in the path measurement . The indirect strain measurement is therefore not suitable for small strain values.

See also

  • Tasimeter a historical measuring method from Edison

Web links