The paper presents the sensor for measuring strains ranging from below 0.1% to over 50%, which is considered as the one-ply coreless inductor. The inductive sensor is installed on the tested material. Changes in dimensions of the material cause changes in the length of the sensor's coil. This brings changes in Inductance, which together with the capacity, constitutes a resonant circuit of a transistor oscillator. Change of oscillation frequency is a source of information about the deformation of a sample. This sensor in linear form is used for measuring compressibility of rock samples and other materials in triaxial cell and during triaxial test, When strains exceed ten per cent. For measuring circumferential strain of cylindrical rock sample in high pressure cell is used toroidal form of sensor. The sensor operate under hydrostatic pressure up to 1 GPa high or may be applied in normal conditions (outside a cell). Moreover the paper describes influence of temperature and high pressure on the sensor properties and influence of friction on measurement result. Some results of strains measurements of different materials are presented as well.
The sensor designed and constructed in the laboratory polish Academy of Science as toroidal coil operate, for measuring circumferential strain of cylindrical rock sample, in high pressure cell (Fig.1a). It is usually used during triaxial compression test when circumferential strain exceeds few percent. This is the case, where applying resistance sensor is impossible due to range of strains, while non-contacting methods based on resistance gauges glued on elastic blade and attached to the sample by hinges (shown e.g. in Hoque E, Sato T. Tatsuoka F. 1997) need a lot of space in the cell. For measuring compressibility the sensor operate in linear form (Fig. 1b), especially in case when it is difficult or impossible to glue an electric resistance sensor on a specimen, for example if the rock is cracked, porous, saturated with water Nurkowski J. 1998. Under such conditions the hydrostatic pressure forces the wire into cracks and pores, whereas the presence of water in rock can cause a shorting of the wire. Some problems caused by electric resistance strain sensor were described in Wawersik W. R. 1975, Attinger RO, Koppel 1. 1983, Hakami H, Aim O, Stephansson O. 1987, Linton PF, McVay MC, Bloomquist D. 1988.
During the compressibility test the strains achieve only a piece of millimeter, so main problems were reduce temperature sensitivity of the sensor and
(Figure in full paper)
recognize impact of pressure. Due to specific quality of the wire used for the coil, it is possible to reduce, to a considerable degree, the influence of the temperature on frequency. This makes possible to measure strains ranging from below 0.1% with the resolution of 0.001% (Nurkowski J. 1998). Moreover lack of direct contact sensor - specimen (the sensor is mounted by catches or by clamping rings (Fig.1: b, c), small diameter (a few millimeters) and ability to multiple use, make the sensor presented here, under certain circumstances, competitive to electric resistance gauges.