Extensometer

A strain sensor is used to measure the strain in the material testing at train, pressure or bending tests. To this end, analog or incremental displacement sensors are used. From the signal of the strain is calculated using the sample dimensions. The direct method of measurement using strain gauges is, however, only very rarely applied because such strips must be glued elaborate on each individual sample.

Low strain over long lengths can be advantageously measured by fiber Bragg grating. Filter this glued optical fibers depends on their length, a very specific wavelength of light that can be analyzed by optical methods.

A distinction is made between direct and indirect measurement.

Direct measurement

When direct measurement is measured directly at the sample. A clip-on or optical strain sensor (eg Videoextensometer ) is used here. Here resolutions of up to 1 micron can be achieved. Especially in the measurement of E-modulus high precision is required.

Other variants are:

• Laser Extensometer (optical / photo-detector - evaluation of the time course of signal of a photodetector which receives the light scattered from the swept laser having a measurement marks )
• Laser Speckle extensometer (optical / camera-based - the cross-correlation of stochastic speckle interference patterns on the sample surface, the method is similar to the process of the image correlation, wherein the dot pattern to be tracked is produced by laser speckle. )
• Videoextensometer (optical / camera-based - edge tracing of measuring marks on the sample )
• Image correlation (optical / camera-based cross - correlation of stochastic patterns on the sample surface )

Measurement methods based on image correlation of surface patterns - such as laser speckle extensometer and digital image correlation - as determined from the observed flow patterns at the measuring points the "true" or logarithmic strain ε '. In contrast, identify measurement methods that are directly the distance between the measuring marks capture (eg video extensometer, laser extensometer, extensometer ), the technical strain ε a sample. The conversion of logarithmic strain in technical expansion is possible as long as the material in the observed range is uniform, especially without local necking, stretched.

From the uniform elongation, the sample itself constricts and thereby expand locally uneven. As a result, the conversion between logarithmic and technical strain is defective.

Indirect measurement

Indirect means in this case that the expansion is measured by a rotary encoder at the motor shaft. The strain is calibrated so that for each increment, for example, The sample is stretched to 0.1 mm. Often this type of strain gauge is called the crosshead. Disadvantage of this measuring method is that the " slip " in the sample holder or the elongation of the beam, as well as other deformations in the displacement measurement with enter. The indirect strain measurement is therefore not suitable for small strain values ​​.