Experimental investigation on the viscoelastic properties 1 of constituents in mudstone 2

In deep underground engineering, the creep behaviors of soft rocks have been widely investigated 10 to help understand the mechanism of the time-dependent large deformation and failure of underground 11 engineering structures. However, rocks were used to be regarded as homogeneous materials and there are 12 limited studying results about the time-dependent properties of constituents in them to reveal their creep 13 mechanism. In this context, the targeting nanoindentation technique (TNIT) was adopted to investigate the 14 viscoelastic characteristics of kaolinite and quartz in a two-constituent mudstone sample. The TNIT consists 15 of identifications of mineralogical constituents in mudstone and nanoindentation experiments on identified 16 constituents. After conducting experiments, the unloading stages of the typical indentation curves were 17 analyzed to calculate the hardness and elastic modulus of constituents in mudstone. And the 180 s load-holding 18 stages with the maximum load of 50 mN were transformed to the typical creep strain-time curves for fitting 19 analysis by using the Kelvin model, the standard viscoelastic model and the extended viscoelastic model. 20 Fitting results show that the standard viscoelastic model can perfectly express the nanoindentation creep 21 behaviors of both kaolinite and quartz and fitting constants are suitable to be used to calculate their creep 22 parameters. The creep parameters of kaolinite are much smaller than that of quartz, which drives the time- 23 dependent large deformation of the soft mudstone. At last, the standard viscoelastic model was verified on a 24 sandstone sample.


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Creep property, a time-dependent behaviour, is the inherent attributes of rocks, soft mudstone in 29 particular(J. Sun, 2007). In deep underground engineering, the creep deformations of rocks become more 30 common due to the high stress (He et al., 2005). The time-dependent deformations of rocks negatively affect the 31 mining safety(X. Li

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Nanoindentation technique, also called the depth-sensing indentation, was first proposed and used by 48 Kalei in 1968 in Russia (Kalei, 1968). This technique has proven to be an effective and convenient method 49 for determining the elastoplastic mechanical properties of solids based on a small rock sample, most 50 notably elastic modulus, hardness (Oliver and Pharr, 1992) and fracture toughness (Zeng et al., 2019).

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As shown in Fig. 1(a), the coal mine named Tongting is located in Anhui province, China; and the sampling 86 layer is above the 7# coal strata deposited at the Permian layer, where the stratigraphic column is presented in 87 Fig. 1

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For the elastic element in Fig. 6, the stress ( ) is proportional to the strain (  ), while for the viscous 209 element, it is proportional to the strain rate ( & or  Pharr, 1992, 2004). As shown in Fig. 7(a), a 228 typical load-depth curve can be obtained as the indenter enters into and exists from the surface of mudstone.

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The curve consists of loading segment (o-a), load-holding segment (a-b), and unloading segment (b-c); also, 230 segment b-d is the curve tangent of unloading segment (c) at its initial portion. In Fig. 7(b), a typical Berkovich     In this section, the proposed nanoindentation creep viscoelastic models in Fig. 6 will be used to fit the

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Fitting curves by using the Kelvin model did not yield desirable results. As show in Fig. 9 (a) and

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According to the fitting results by using the standard viscoelastic model, the creep parameters of both 325 kaolinite and quartz in mudstone can be obtained, as shown in Table 2. 326

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According to the fitting results in Fig. 12(a) Table 4. It also shows that the mechanical properties of quartz in sandstone are slightly 358 larger than that of quartz in mudstone. For example, the mean elastic modulus of quartz in sandstone is 85.81 359 GPa and that in mudstone is 66.54 GPa. The reason for the smaller mechanical properties of quartz in mudstone 360 is probably due to the influence of soft kaolinite as matrix in mudstone.

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In this paper, the viscoelastic characteristics of kaolinite and quartz in mudstone were investigated by 364 using the targeting nanoindentation technique. Conclusions are as follows.

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(1) The soft mudstone sample studied in this studied are composed of kaolinite and quartz, in which 366 kaolinite is soft and as matrix with hard quartz embedded in it.

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(2) For broken soft rocks that cannot provide intact standardized samples, the nanoindentation method can

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(4) The mechanical properties of quartz in mudstone and sandstone are a slightly different, which may be 380 due to the influence of soft clay minerals in mudstone that soften the mechanical performance of quartz in it.