Experimental investigation on the nanoindentation viscoelastic constitutive model of quartz and kaolinite in mudstone

Abstract

The creep behaviors in deep underground engineering structures, especially in soft rocks, have a remarkable impact on the long-term stability of the excavations, which finally leads to the high risk and failure of it. Accordingly, it is essential to recognize the time-dependent deformation through the investigation of this phenomenon. In this study, the creep behaviors of soft rocks have been widely examined to help understand the underlying mechanism of the extended time-dependent deformation. Due to the limited results about the time-dependent properties of the constituents of the rock that reveal their heterogeneity, the targeting nanoindentation technique (TNIT), was adopted to investigate the viscoelastic characteristics of kaolinite and quartz in a two-constituent mudstone sample. The TNIT consists of identifications of mineralogical ingredients in mudstone with nanoindentation experiments on each identified constituent. After conducting experiments, the unloading stages of the typical indentation curves were analyzed to calculate the hardness and elastic modulus of both elements in mudstone. Additionally, the 180 s load-holding stages with the peak load of 50 mN were transformed into the typical creep strain-time curves for fitting analysis by using the Kelvin model, the standard viscoelastic model, and the extended viscoelastic model. Fitting results show that the standard viscoelastic model not only can perfectly express the nanoindentation creep behaviors of both kaolinite and quartz but also can produce suitable constants used to measure their creep parameters. Furthermore, the creep parameters of kaolinite are much smaller than that of quartz, which causes the considerable time-dependent deformation of the soft mudstone. Eventually, the standard viscoelastic model was also verified on the quartz in a sandstone sample.

Modeling the Viscoelastic Behavior of Quartz and Clay Minerals in Shale by Nanoindentation Creep Tests

“…Compared with the other two models, it consists of five fitting parameters, and has the highest calculation cost for the regression of the nanoindentation data. However, some of the depth-time curves could not be fitted or the obtained regression parameters were unreasonable for this model, similar to the result of extended viscoelastic model with two Kelvin models in series that was used to fit quartz nanoindentation creep data [49]. Consequently, it appears that this model is unstable.…”

“…However, Figure 8-10 show that the viscoelastic parameters were variably influenced by the peak load. In previous studies, different peak loads, such as 4.8 mN [15, 57], 8 mN [21], 12.5 mN [22], and 50 mN [49], were used to characterize the deformation behavior of single mineral phases in shale or sandstone. Consequently, the effect of peak load on creep was not discussed in these studies.…”

Effects of Thermal Treatment on Mineral Composition and Pore Structure of Coal

“…Mineral Composition after Thermal Treatment. XRD tests are commonly used to investigate the mineral composition of coal and rock materials [20,39,46,53,54]. Therefore, XRD tests were carried out on coal powder after different temperature treatments (25, 50, 75, and 100°C).…”

Explanation on the Abnormal Behavior during the Nanoindentation Holding Stages by Amplifying Oscillation

“…Recently, the holding states of nanoindentation experiments have been widely used to analyze the time-dependent deformations of biomaterials [10], metals and alloys [11,12], ceramics [13,14], polymers and composites [15][16][17][18], types of cement [19,20], and even crystals [21,22]. Besides, the estimation of the macrofracture toughness of minerals and rocks is given in [23], while the analysis of the micromechanical properties of mineral crystal grains is becoming more and more extensive [24][25][26]. All those above mean that the experimental technique was a robust way to conduct creep tests.…”