Abstract
Recently, the holding states of nanoindentation experiments have been widely used to analyze the time-dependent deformations of various rocks, and the dynamic mechanical analysis (DMA) method seems to be more applicable than the quasi-static mechanical analysis (QMA) method when the influence of creep deformation on mechanical properties of rocks was analyzed. However, the former method causes an abnormal behavior during the creep holding stages that was not clearly interpreted.2 Consequently, in this study, by amplifying the oscillation of the DMA method, the mechanical mechanism of this phenomenon was explained. Experimental results confirm that the rheological deformation of rocks consists of the creep deformation (depth increasing) and the elastic aftereffect deformation (depth decreasing) during the creep time with small oscillation; once the elastic aftereffect deformation exceeds the creep deformation, the abnormal behavior can be observed. Besides, some other abnormal behaviors might be found for other rock materials when the DMA method with different oscillations is used, which illustrates the complexity and limitation of applying this method. Thus, the QMA method was recommended to investigate the above questions in future studies.
Highlights
In the deep underground, the rheological phenomenon is the most important mechanical property of geomaterials, which negatively impacts the safety of engineering structure [1,2,3], coalbed methane (CBM) and shale gas production [4,5,6], geological disposal repository effectiveness [7], and so on
The hard minerals in shale collected from Bakken formation have much smaller creep deformation than that of softer ones [28]. e quasi-static mechanical analysis (QMA) method and the dynamic mechanical analysis (DMA) method that was regarded as two basic approaches to conduct nanoindentation creep tests [29]
In order to overcome the main limitation of the quasi-static mechanical analysis (QMA) method in which the hardness and modulus values can only be calculated at the initial part of the unloading curve, the dynamic mechanical analysis (DMA) method with quasi-static force being kept constant and small oscillation being superimposed was proposed and used [28], as shown in the left part of Figure 1(a)
Summary
The rheological phenomenon is the most important mechanical property of geomaterials, which negatively impacts the safety of engineering structure [1,2,3], coalbed methane (CBM) and shale gas production [4,5,6], geological disposal repository effectiveness [7], and so on. 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. For other geomaterials obtained in coal mines, such as coal, mudstone, and sandstone, the quasi-static mechanical analysis method was widely used to reveal their timedependent deformation phenomenon [25, 30]. This technique causes some debate in the field of rock mechanics. The rheological deformation of rock investigated by using the DMA method is very complicated, showing the limitation of using this method
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