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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al. (2019), (Vuorinen et al. (2020), (Pluvinage et al. (2019), (Nie et al. (2020), (Bragov et al. (2018)) and alloys ((Sezgin et al. (2019), (Wang et al. (2020), (Ren et al. (2020)), including dissimilar ones (Beygi et al. (2021)). The FT of composite materials are investigated in (Ramachandran et al. (2021), (Eskandari et al. (2019), (Puneeth et al. (2019), Chawla et al. (2019), Mega and Banks-Sills (2020)). The FT parameter is also applied when assessing the strength properties of concrete (Ryabchikov et al. (2020)). It is used as well for special types of concrete, such as fibre reinforced concrete (Conforti et al. (2018)). The FT was used for new printed materials (Valean et al., (2020)) Fewer publications deal with FT studies in rocks (Wang and Hu, (2017)), the boundaries between concrete and rocks (Rong et al. (2019b)), and between rocks of different types. As a rule, studies use three or four-point loading schemes when bending specimens and rectangular beams (Rong et al. (2019a), Kožar et al. (2019), Kožar et al. (2021)), half-discs (Lu et al. (2021a)), edge-notched disc specimens (for Brazilian tests) (Nazerigivi et al. (2018)), and cubes with a ‘cutout’ or a ‘notch-cutout’ (Seitl et al. (2017)). FT of interfaces between different rocks or minerals is also of interest when assessing the stability of rock masses in mining. For rocks, the methodology of FT determination recommended by the International Society of Rock Mechanics (ISRM) (Ouchterlony F., et al. (1988)) was used. The novelty of this work is the assessment of the fracture toughness of interfaces between rocks of various types.

Nomenclature K C

fracture toughness maximal bend force

P max

D a 0

diameter of chevron bend specimen chevron dip distance from specimen surface

S

distance between support points

2. Materials and methods The test uses rock specimens from the Novomoskovsk gypsum deposit (Novomoskovsk, Tula region, Russia). This deposit is of a sedimentary type with bedding angles ranging from 0 to 5 o . A complex structure characterises the gypsum-bearing strata. Its lower part consists of dolomite and gypsum and the upper part is a gypsum layer, 20-25 m thick. The thick gypsum layer in the upper part of the gypsum-bearing strata is composed of fine-crystalline (less often fibrous) gypsum of a white, light grey or, sometimes, dark grey colour. The rare interlayers of dolomite and dark grey, silty clays have thicknesses of 1 to 20 mm. In addition, single lenses of dolomite (15-20 cm) and flint (up to 5 cm) appear in the formation. There are fine-grained, cryptocrystalline silica nodules of a complex composition of sedimentary rocks. They consist of microscopic grains of quartz, chalcedony, cristobalite, and other minerals and are highly durable. The average gypsum content for the deposit as a whole is 88.78%. The specimens consisted of gypsum with vertical axis orientation. In this case, the layers in the specimens had a direction perpendicular to the axis. Testing of cylindrical, notched specimens (150 mm long) was carried out according to the 3-point scheme shown in Fig. 1 (a). The letter "a" indicates the position of the crack edge formed under cyclic loading, which corresponds to the cycle with the maximum load. The transverse cut is V-shaped (Fig. 1 (b)) and the angle θ is equal to 90 o . Fig. 2 shows a specimen example.

a b Fig. 1. 3-points scheme of notched specimen testing (a) and cross-section of a notched specimen (b).

Fig. 2. The texture of the cylindrical specimen with a notch, and marking lines.