Samarium sulfide (SmS) based sensors of internal stress in plastic, composite and concrete design
Dubrovin P.V., Kaminski V.V., Vasiliev L.N., Soloviev S.M., Shpeizman V.V.
A.F. Ioffe Physico-Technical Institute, 194021 St Petersburg, Russia

Tensoresistive sensors for the measurements of three mutually perpendicular strains (stress) components and also bulk compression pressure in the critical points of designs under their loading are proposed. For this purpose the sensors are placed into a material of designs of concrete, plastic or composite (carbon-, glassfibre-reinforced plastics) during their manufacturing and can produce an information during both bench tests of designs and their operation.

The sensitive elements of the sensors in concrete are semiconducting thin-film tensoresistors of the special form with an active layer of SmS based material. They have tensosensitivity factor K=30100 and electroresistive sensitivity for bulk compression 13% at 10 MPa (100 atm.). Tensoresistors are efficient up to bulk compression pressure of 10³ MPa and up to temperatures about 400°C, have long-time stability (it was experimentally checked during 12 years) and have record radiation stability among known semiconductors. They withstand all magnitudes of strains, which are available for concrete. Their output characteristics are practically linear with a strain, pressure and temperature in all effective ranges. An output sensor signal is up to 20 mV without amplification according with maximum accessible strains in concrete. The relative strain resolution power is not worse than 10^-6. The parallel tests of concrete specimens, carried out by means of SmS based sensors, on the universal test machine "Instron 1342", have established a good correspondence of the results. However, SmS based sensors note more distinctly the boundaries of deformation mechanism change, in particular, crossover from the elastic zone to the plastic one. The nearest analogs of such sensors are the enclosed designs based on metallic tensoresistors and magnetoelastic sensors. The last ones below essentially in compare with SmS based sensors in all operational parameters and exceed them in some orders in dimensions, that limits their application area. The sensors for concrete are protected by the copyright certificate of the invention N 1717946. Such sensors were used for elaboration of strengthening of oil wells, for the stress state control in conditions of a ground moving, and also for the definition of limited negative temperatures, for which the realization of concrete works (definition of the concrete paste hardening temperature) is possible. They can be used for long-term monitoring of mechanical stresses while bridges, tunnels, mountain pits strengthening etc. may operate. High radiation stability allows using SmS based sensors for concrete designs monitoring under radiation.

The sensors for plastics and composites are similar to those for concrete and differ from them by the fact of the active tensoresistor layer is deposited on a special substrate, which does not bring distortions in the stress field of the researched object. The tensoresistors withstand strains up to ~1.5%. An output signal of the sensor in the effective range of strains is up to 0.5 V without amplification. The information reliability, which is obtained by sensors located into composites and plastics, was also checked up by testing them on "Instron 1342". The tensoresistors with bases from 0.5 mm up to 10 cm can be fabricated. High tensosensitivity allows measuring stresses arising in plastic designs not only under loading, but also due to temperature change. It seems perspective to manufacture composite materials with beforehand built-in SmS based tensoresistors (plates, rods and so on) for the responsible details in space, shipbuilding, automobile etc. industries. There are no analogs for such materials yet.