Use of laboratory indentation tests to study the surface crack propagation caused by various indenters
Introduction
Indentations, widely used to study the breakage and mechanical properties of rocks, can form internal cracks [1], [2], [3], [4], [5], [6], [7]. Many geological and machinery factors may substantially affect internal crack propagation. For geological factors, many studies have shown that increasing the confining stress can increase the deflection angle of an internal crack [6], [8], [9], [10], [11]. Bejari and Hamidi, Liu et al., Zhai et al. and Zhu et al. stated that internal cracks frequently propagate along joint surfaces [12], [13], [14], [15]. However, the laboratory and numerical tests performed by Zhang et al. indicated that internal cracks may propagate through the joints [16]. A similar phenomenon was reported by Goldstein and Osipenko [17]. For machinery factors, Li et al. proposed that internal cracks tend to propagate along the indentation direction for small wedge angles [18]. Moon and Oh stated that the tip width may affect the internal crack connection and further influence the optimal spacing/penetration ratio [19]. Souissi et al. proposed that the increase in the spacing between indentations may restrain the internal crack connection [20]. Han et al. stated that the spacing and the indentation depth substantially affect the internal crack propagation between indentations [2]. The numerical and laboratory study by Liu and Wang indicated that the indentation sequence may influence the internal crack connection mode between adjacent plastic zones and further affect the indentation efficiency [21].
The above studies substantially contribute to understanding that internal crack propagation is critical to rock breakage by indentations because internal crack connections between indentations form at the bottom surface of the chip. However, a chip containing bottom and lateral surfaces may fail to form despite the successful internal crack connection. Surface cracks forming after indentations [9], [22], [23], [24] may be responsible for the lateral surfaces of the chips. Laboratory tests using constant cross section (CCS) indenters (Fig. 1(a)) can verify the importance of surface cracks. The rock breakage is poor (Fig. 1(b)) despite the successful internal crack connection between indentations (Fig. 1(c)) [4]. Similarly, geological and machinery factors can influence surface crack propagation. For instance, to investigate the influence of the bedding plane orientation on rock breakage, the laboratory test by Liu et al. clearly showed that thick chips failed to form without incisions due to surface cracks (restrained by the bedding planes) when internal cracks successfully connected adjacent plastic zones [4]. Additionally, the variations in the spacing and indentation depth substantially affect the surface crack incision and further influence the indentation efficiency [2], [9].
Clearly, the above studies show that the CCS indenter can form internal crack connections between indentations. In addition, successful surface crack incisions between indentations are critical to effective rock breakage. However, studies on how to promote surface crack incision are insufficient. In addition, in the practical excavation process, the geological conditions are approximately fixed. To promote surface crack propagation and rock breakage, we may make efforts in the machinery aspect. The laboratory tests by Liu et al. and Xiao et al. may shed some light on how to promote surface crack incision [9], [22]. The indentation tests using the CCS indenter clearly show that surface cracks frequently initiate from the corner of the contact area between the CCS indenter and the rock (Fig. 2(a)). More interestingly, by applying the Vickers indenter (Fig. 2(b)), surface cracks initiate from the juncture of the adjacent surfaces of the indenter. This valuable phenomenon has also been reported by Rickhey et al. [25]. Thus, in the present article, we prefabricated 4 built-up indenters based on the commonly used CCS indenter and the Vickers indenter. Then, the analysis of rock fracture and indentation efficiency were conducted to verify the feasibility of these built-up indenters.
Section snippets
Indenter prefabrication
First, according to the CCS indenter size (Fig. 3(a1) and 3(a2)) reported by Han et al. and Liu et al. [2], [4], we prefabricated a CCS indenter (Fig. 3(a1)) and two indenters W1 and M1 (Fig. 3(b1) and 3(c1)). Fig. 3(b2) and 3(c2) present the geometric sizes of the cutting teeth with constant cross sections. In addition, by combining the Vickers indenter and the CCS indenter (Fig. 2(b)), we prefabricated Indenters M2 and M3 (Fig. 3(d1) and 3(e1)) to try to promote surface crack propagation.
Internal crack propagation
Previous studies on indentation tests indicated that the propagation of internal cracks (including median and lateral cracks) between adjacent indentations was critical to chip formation [2], [9]. Thus, we first analyzed the typical internal crack propagation (Fig. 5). Clearly, when the indentation depth was 4 mm for Indenter M1, two reversed triangle-shaped plastic zones formed after the indentations (Fig. 5(a)). Similar triangle-shaped plastic zones were observed in previous studies [2], [9],
Indentation efficiency evaluation
Chip masses (or groove volume) and the indentation energy are two critical indexes of the indentation efficiency [2], [9] because the specific energy can be written as:where SE, W and M are the specific energy (energy required to cut through a unit mass of rock), the indentation energy and the chip mass, respectively.
Clearly, the indentation efficiency negatively relates to the value of the SE. The following sections mainly discuss the chip mass and the indentation energy.
Conclusions
Internal and surface crack development caused by indentations are critical to the breakage between indentations. The usual CCS indenters can ensure sufficient internal crack connection between indentations for relatively high indentation depths. In addition, surface cracks frequently initiate from the corner of the contact area (between the CCS indenter and the rock) and the junctures of the adjacent surfaces of the Vickers indenter. Thus, to ensure sufficient surface and internal crack
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge these financial supports: Projects (51804110, 51774132, 51774131) supported by the National Natural Science Foundation of China, Project (2020JJ5101) supported by the Natural Science Foundation of Hunan Province and Scientific Research Foundation of Hunan Province Education Department (18B391, 20A118, 19A114).
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