Extreme resistance to alternating loads with optical measurement technology
If the resistance to alternating loads continues to be increased and the S-N curve moves upwards, metal strain gauges can no longer be used. The obvious alternative is optical measurement technology, based on fiber Bragg grating technology (FBG).
This technology is based on inscribing a Bragg grating into an optical fiber. The grating reflects a specific wavelength in the optical spectrum. Among other things, this wavelength is strain-dependent. This makes it possible to produce optical strain gauges.
The optical fiber has isotropic mechanical properties and essentially does not know the meaning of fatigue, such as what is typically present in metallic materials. The optical fibers can be dynamically loaded up to an ultimate strength of approximately 30,000 µm/m. In fatigue life testing, tests at ±5,000 µm/m have already reached up to 107 load cycles without failure2.
Optical fibers can also be embedded. In addition to the extreme, cyclic, high strain application already described, they can also be used where electrical strain gauges cannot be used in principle, such as in very high electromagnetic fields (transformers, high voltage switches, etc.). In fatigue life analysis, modern materials such as fiber composites demand high standards from the measurement technology with regard to resistance to alternating loads. By taking suitable action, HBM was able to increase the strain gauge resistance to alternating loads so that they can be used in most measurements. Optical measurement technology is also the appropriate tool for extreme alternating loads.
1Constantan measuring grid on a polyimide carrier foil
2 Measured with the "K-OL" optical strain gauge from HBM