# Some Thoughts about Infrared Radiation Response Characteristics during Loading of Sandstone Samples
## 2 Lab Test Design
### Test Set-Up  
Figure 2a shows the uniaxial compression test system with the combined infrared radiation (IR) and strain monitoring systems. Uniaxial compression tests were performed using an MTS C64.106 hydraulic universal testing machine, which has a maximum load of 1000 kN. The strain measuring device is a TS3890N static resistance strain gauge with  
Figure 1: Scanning electron microscopy (SEM) photos at 2000\(\times\) magnification of sandstone samples. SEM Quanta TM 250 FEG was used for materials characterization. (**a**) Microcracks in sandstone. (**b**) Microvoids in sandstone.  
a resolution of 1 \(\mu\epsilon\). The infrared radiation observation equipment is VarioCAM(r) HD head 800 thermal imaging camera. It has a spatial resolution of 1240 \(\times\) 768 pixels and thermal sensitivity of 0.02 \({}^{\circ}\)C. During the test, thermal images of the sample surface infrared radiation in the wavelength range of 7.5-14 \(\upmu\)m were captured at a frequency of 25 frames per second.  
The interference of background noises is unavoidable during the experiments using infrared radiation technology. Therefore, it is necessary to use reference samples in the test system and use the infrared radiation noise of the reference sample to denoise the infrared radiation information of the loaded samples [18]. The reference samples without loading are placed in parallel on the left and right sides of the loaded sample (see Figure 2a). To ensure that the incidence rate and lithology of the reference sample and the loading sample are the same, the reference samples and the loaded sample should be of the same type. For example, A\({}_{1}\) to A\({}_{15}\) are set as loaded samples, while A\({}_{16}\) and A\({}_{17}\) are reference samples.  
The applied axial stress and volumetric strain calculated from the axial and lateral strains are used to trace the deformation and failure path of the samples. The strain gauges (three groups in total) are pasted on the two sides and back surface (relative to the side facing the infrared camera) of the test sample, as shown in Figure 2a. A thin plastic film is placed  
Figure 2: The mechanical testing system and the stress-strain behavior of the sample. (**a**) Experimental set-up; (**b**) Damage evolution during uniaxial loading of sandstone sample A\({}_{13}\). Crack initiation threshold \(\sigma_{\text{cl}}\) is the onset of crack volumetric increase that is the reversal point of crack volumetric strain; critical damage threshold \(\sigma_{\text{cd}}\) is the reversal point of the total volumetric strain.  
on the upper and lower contact surfaces of the sample and the loading plates. This can help reduce the friction and heat conduction during the test. Researchers were prevented from walking during the experiment to minimize the interference of infrared radiation from the external environment. At the same time, the laboratory windows and curtains were closed. All illuminating light sources that can generate radiation were switched off. The samples were compressed with a constant rate of 0.1 mm/min. The thermal camera was positioned at a distance of 1 m directly in front of the sample. The infrared radiation information, as well as the stress and strain variations, were simultaneously recorded until sample failure.