# A Novel Carbon Fiber Composite Material for the Simulation of Damage Evolution in Thick Aquifers
## 3 Damage Self-Sensing Performance of CFSSM  
### Experimental Design and Specim Preparation  
To analyze the influence of carbon fiber on the damage self-sensing performance of similar simulation materials, this study selects a composition of sand, calcium carbonate, and gypsum as a similar material. Depending on the simulation requirements for different rock lithologies, the material ratios vary. For simulating fine sandstone, the mass ratio of sand to calcium carbonate to gypsum is 4:0.3:0.7 [19,36]. Based on this ratio, the experiment  
Figure 1: Principles of damage self-monitoring in the carbon fiber composite materials. further explores the effects of carbon fiber. We incorporate different mass fractions of carbon fiber into similar materials, with gradients of 0% (control group), 0.5%, 1%, 1.5%, and 2%. The carbon fibers used were 6 mm long chopped carbon fibers (from Toray Industries, Inc. in Tokyo, Japan) whose main parameters (sourced from the productive parameters of the manufacturer) are given in Figure 2.  
The preparation of CFSSM includes the following steps:  
(1) Mix sand, calcium carbonate, and gypsum in a mixer for 3 min. (2) Due to the hydrophobic nature of carbon fibers, they are difficult to disperse in water. Therefore, adding 1.6 wt% hydroxyethyl cellulose (which contains more polar groups) can significantly improve their dispersion [37]. Hydroxyethyl cellulose forms more hydrogen bonds with the carbon fibers through its polar groups, enhancing the hydrophilicity and wettability of the carbon fibers, thereby achieving the best dispersion effect. The mixture should be thoroughly stirred, and the mixture from Step 1 added to prepare CFSSM. (3) Apply lubricant evenly on the inner walls of a mold sized 4 cm \(\times\) 4 cm \(\times\) 16 cm and fix a set of carbon fiber cloth at both ends of the mold to serve as electrodes. Then, pour CFSSM into the steel mold and compact it repeatedly with a rubber mallet to simulate the compaction process in similar material models, enhancing the density and uniformity of the composite material. (4) Place the mold in a constant humidity room at a temperature of 23 \({}^{\circ}\)C and relative humidity of 100% for 2 days of curing. After curing, CFSSM should be in a slightly moist state without further water seepage, simulating the state before removing baffles in similar models. (5) Allow the sample to dry naturally in a similar material simulation laboratory for 5 days. Once the sample has fully set, remove it from the mold, and continue drying for 21 days, simulating the state after the removal of baffles in similar models. (6) To screen and polish the specimens after curing, strictly follow _The ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 2007-2014_. For each concentration group, three specimens were selected for testing. Subsequently, a three-point bending test was conducted. The experimental setup comprised loading equipment, resistivity acquisition equipment, and acoustic emission (AE) monitoring equipment (Figure 3). The loading equipment was the MTS C64.106 rock mechanics testing system manufactured by MTS Industrial Systems (China) Co., Ltd. (Shanghai, China). The loading process was conducted in the displacement loading mode (loading rate of 0.3 mm/min). Figure 2: Parameters of chopped carbon fiber material. The resistivity acquisition equipment adopted the LCR developed by the Yisheng Victor Tech Co., Ltd. in Shenzhen, China. To avoid short-circuits, an insulating paper was placed between the specimens and metal parts of the testing machine to separate the specimens from the testing machine. To investigate resistivity changes caused by fracture development within the CFSSM during uniaxial compression tests, the resistivity change rate (\(RCR\)) of the CFSSM was defined as:  
\[RCR=\frac{R_{n}-R_{0}}{R_{0}}\times 100\% \tag{1}\]  
where \(R_{n}\) is the resistivity measured at the \(n\)-th time, and \(R_{0}\) is the resistivity measured before starting a test.