Damage Analysis of Chemically Corroded Sandstone Under Cyclic Impact and Static Axial Pressure

To explore the influence of cyclic impact and axial pressure on the damage of chemically corroded sandstone, a series of cyclic impact tests were carried out on white sandstone by using Split Hopkinson Pressure Bar. The longitudinal sections and fractures of samples were observed with the scanning electron microscope. The aim was to investigate the damage characteristics and structural changes of sandstone, that subjected to the coupling of force and chemistry. The results show that: (1) When pH of solution is 7, the total cyclic impact number and stress peak of specimens both became larger, and the rock samples responded with a significantly high resistant strength. (2) The stress wave transmission coefficient of sandstone decreases gradually with the increase of the number of cyclic impacts, while the reflection coefficient shows a tendency of" decreasing first and then increasing". (3) Cylindrical specimens with a certain axial pressure present an "X" shaped conjugate failure under cyclic impact. When axial pressure is too large or excessive impact, the "X" shaped conjugate undergoes shear to a state of broken cone. (4) The vertical section and fracture surface damage degree of white sandstone soaked in Na 2 SO 4 solution is more serious than that in NaCl solution.


Introduction 1
With the exploitation of resources and large-scale construction of infrastructures, mining industry, water conservancy and hydropower, traffic engineering and earthwork excavation are inseparable from geotechnical engineering [1]. The stability of rock is affected by static stress and external frequent vibration. Besides, in the design, construction, and management process of engineering, it is inevitable to consider the influence of complex hydro chemical field on rocks, which is caused by the discharge of domestic sewage and industrial wastewater. It is of significance to investigate the coupling of force and chemical corrosion for the long-term stability of geotechnical engineering. Recently, experts at home and abroad have carried out corresponding studies on the mechanical behavior that rock subjected to cyclic loading or chemical corrosion by theoretical derivation, simulation calculation, scientific experiments [2][3][4][5][6].
Based on the need for deep rock research, Li et al. [7] took the lead in putting forward the subject about the stability of rock under dynamic and static loading. He *Corresponding author at: School of Energy and Mechanical Engineering, Jiangxi University of Science and Technology, Nanchang, Jiangxi 330013, China. E-mail address: 1023817019@qq.com pointed out that when static axial pressure increases from 0 to 70% of the uniaxial static pressure strength, constant impact dynamic loading, the strength of rock in static loading or pure dynamic loading is less than that under the combined loading. Hu et al. [8] used a high-speed camera to record the compression fracture process of cylindrical red sandstones (d=50mm, h=50mm) subjected to the dynamic impact of Split Hopkinson Pressure Bar (SHPB). They divided the influence of cyclic loading incident energy for failure state of rock specimens into three stages: complete, crack, and broken. Gong et al. [9] put forward the tendency index for rock burst subjected to dynamic and static loading, which can efficaciously reflect static and dynamic loading on the occurrence of prompting rock burst. Jin et al. [10] performed a series of cyclic loading tests on sandstone subjected to different static loading by SHPB, and found that a combination form of static loading has influences on the failure mode of rock under cyclic impact, such as no end effect without static axial loading. Yin et al. [11] performed impact loading experiments on granite under the combined function of temperature and axial pressure loading through improved SHPB, and observed the internal structural characteristics of rock fragments with scanning electron microscope ( SEM ) . Tang et al. [12]carried out uniaxial compressive tests on three types of rocks (granite, red sandstone, and limestone) in the hydrochemical environment, and uniaxial strength reduction of rocks represents the chemical damage of rocks. The results showed that the chemical damage of rocks is directly proportional to the chemical reaction strength of water-rock. Wei et al. [13] considered the mechanism of fractal dimension of rock, and concluded that fractal dimension is linearly proportional to the damage degree of chemical corrosion. Wu et al. [14] carried out rock dynamic mechanical tests on SHPB test system, and studied the dynamic mechanism of rock burst through energy theory. Feng et al. [15]

Sample Preparation
White sandstone, a common sedimentary rock in Kunming, was selected as the test object in this experiment. The white sandstone blocks, which was selected from a good homogeneity and integrity sample, were machined into cylindrical shapes with a length of 50 mm and a diameter of 50 mm according to the relevant guidelines of International Society for Rock Mechanics (ISRM) [16]. ln the meantime, to reduce the specimen accuracy effect on the test results, both sides of the specimens were polished with the sandpapers to ensure that the unevenness and non-perpendicularity were within ± 0.02 mm. Some prepared samples that would be tested were summarized in Fig.1    The device diagram is shown in Fig. 3.

Test Scheme
Before the impact tests started, a uniaxial compressive strength test by the RMT-150C rock mechanics testing machine is necessary to provide a reference for the axial pressure of subsequent impact tests. The physical and mechanical parameters are shown in Table 2.

Test Principle
To obtain the dynamic stress-strain curves, adopting the three-wave theory is widely used owing to its accuracy [19].Combined with the one-dimensional stress wave theory, the stress , strain , and average strain ratė of the specimen can be calculated in the light of the following equations: where Ae and As are the cross-sectional area of elastic rod and rock specimen; Ls is the length of the specimen; Ee is the elastic modulus of the elastic bar; Ce is longitudinal wave velocity; ( ), ( ), ( ) denote the incident, reflected, and transmission pulse signals, respectively.

3.Test Results
A group of representative specimen parameters and test data are listed, as shown in Table 3.

Wave propagation characteristics in white sandstone under cyclic impact loading
Stress wave characteristics of No. w-033 sample is taken as an example. Fig.5 shows the waveform superposition diagram of the cyclic impact process. From The transmission coefficient is defined as the peak ratio transmitted wave and incident wave in the waveform, and so is reflected wave [20]. Therefore, Eq.
(4) 、 (5)can be simplified as follow:   where R and T are the reflection coefficient and transmission coefficient of stress wave propagating at the nonlinear joint；K0 is the initial stiffness of the joint; is the frequency of stress wave; = is the wave impedance of the specimen; = is the ratio of joint closure to maximum joint closure.

The strength characteristics of white sandstone under cyclic impact loading
Stress peak is an important indicator for measuring the strength characteristics of rock and can be applied to characterize the anti-compressive ability of rock under cyclic impact loading [21]. As shown in Fig. 7, sixteen typical specimens were tested and the trend of stress peak can be obtained during cyclic impact process. The stress  a great influence on the mode and degree of specimen failure [24]. As shown in Fig. 8, the states of the specimens after impact can be divided into two categories, namely, "X" hyperbolic conjugate state and broken cone state. The "X" hyperbolic conjugate state is defined as an "X" shaped specimen that can withstand the loading after impact, as shown in Fig. 9 (a)and(b). of the cracks. Secondly, "X" shape is a non-holonomic regular solid, and the angle deviation will be produced when the load is applied instantaneously. (h) is broken, and the rock is disordered, but there is a smooth surface locally. This is due to their mineralogical composition, which result in high strength against shear.
The type of chemical solution is closely related to the damage of rock microstructure [26]. Comparing the micro-surface of (a), (b) with (c), (d), it is found that the corrosion damage degree of Na2SO4 solution is greater than that of NaCl solution. Main performance: (a), (b) surface is rougher and better integrity than(c), (d). In addition to the hydraulic dissolution of sandstone soluble components, SO4 2increase the pore water pressure of white sandstone specimen, deform and displace the rock particles to damage structure. The chemical reaction of some reaction zones to the interior of white sandstone is promoted, causing cracking damage [27]. In pore water, pore water chemical effect of Cl is small, and the damage degree is small. 2) With increased the number of cycle impact, the transmission coefficient of stress wave propagating in the specimen gradually decreased under the same axial pressure. However, the reflection coefficient showed a trend of "first decrease and then increase".
3) Under the axial pressure of 6.3Mpa, the stress peak of sandstone resisting cyclic impact showed a trend of "stable in the early stage and then sharp decline ".
Compared with axial pressure of 6.3Mpa, the stress peak of the sandstone was lower upon dynamic loading under an axial pressure of 12.6Mpa and 20.5MPa; when the sandstone had axial pressure of 26.8Mpa, the cyclic impact times of sandstone was few and the dynamic stress peak value was small.     Trend chart of transmission and re ection coe cient