PSI - Issue 46

A.S. Voznesenskii et al. / Procedia Structural Integrity 46 (2023) 155–161 A.S. Voznesenskii et al. / Structural Integrity Procedia 00 (2019) 000–000

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Eq. (1) was used to calculate the fracture toughness:

K C = A min P max / D

1.5 ,

(1)

A min = [1.835+7.15a 0 /D+9.85(a 0 /D) (2) where K C – fracture toughness; P max – maximal bend force; D – diameter of chevron bend specimen; a 0 – chevron dip distance from specimen surface; S – distance between support points. The results of the experimental determination of the fracture toughness of gypsum, flint, carbonaceous clay interlayers, and of the interfaces between them are considered. The tests used the apparatus shown in Fig. 3. 2 ]S/D,

Fig. 3. Exterior view of the test setup and apparatus.

The basis of the testing is the ASIS test complex (NPO Geotek LLC, Penza, Russia). Three-point testing of the specimens used the International Society for Rock Mechanics (ISRM) recommendations. A device allows the minimisation of friction in the nodes of the circuit and gauges measured the deflection directly on the specimen. The ASIS software made it possible to set up a cyclic testing program with an indication of the number of cycles, the maximum and minimum deformations of the testing machine frame, and the speed of movements, with their increase and decrease. Two LVDT displacement transducers with a measuring range of 10 mm were installed directly on the specimen. These converters had a connection to an 18-bit ADC of the QMBox measuring system (R-Technology, Moscow, Russia). The testing program for sequentially testing in cycles was set to maximum displacements of 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 mm. The minimum displacements were 0.05 mm. The rate of both the increase and decrease in the load in the cycles was 0.2 mm / min. A simultaneous decrease in the load with increasing deformations automatically led to the end of the tests. The measuring system recorded the parameters, i.e. frame displacements, the resulting load on the specimen, and the specimen deflection at the midpoint during deformation. The frame movement and the specimen deflection did not coincide with each other. Therefore, we took into consideration the displacements measured directly on the specimen. 3. Results The figures 4 and 5 below show sample of the test results. Homogeneous areas of the specimens and interfaces between rocks of different types have notches. Accordingly, the specimens were designated as:  gypsum stone (GG);  flint (KK);  gypsum-flint interface(GK);