## 2 Experimental Methods
### Digital Speckle Correlation Method for Deformation Measurement
\(x_{i}\) and \(x^{\prime}_{i}\) are the coordinates of the \(i\)th pixel of the reference subset zone and the deformed subset zone in \(x\) direction, respectively. \(y_{j}\) and \(y^{\prime}_{j}\) are coordinates of the \(j\)th pixel of the un-deformed subset zone and the deformed subset zone in the \(y\) direction, respectively. \(x^{\prime}_{i}=x_{i}+\mathrm{u}\) and \(y^{\prime}_{j}=y_{j}+\mathrm{v}\). \(\mathrm{u}\) and \(\mathrm{v}\) are integer pixel displacement in the \(x\) direction and in the \(y\) direction, respectively. \(C\) is the correlation coefficient and it depends on and v. Given a reference point \(\mathbf{P}\), the value (u\({}_{0}\) and v\({}_{0}\)) letting \(C\) obtain a peak value is the integer pixel displacement of point \(\mathbf{P}\) in the current deformed image. After obtaining the integer pixel displacement, we also need to determine sub-pixel displacements of reference points in order to improve the deformation measurement accuracy. Recently, there exist many effective sub-pixel displacement calculation methods, such as the sub-pixel interpolation algorithm, correlation coefficient surface fitting method, coordinate rotation method, and the Newton-Raphson iterative method [31]. In this work, sub-pixel displacement calculation is carried out by use of a correlation coefficient surface fitting method which holds a strong anti-noise ability, high resolution, and high computational efficiency. The digital speckle correlation method requires an image acquisition system consisting of high-intensity light sources and a high-speed camera (as shown in Figure 4). Two 300-W Raikage lights are arranged on both sides of the specimen. The highest imaging frequency of the high-speed camera is up to 100,000 frames per second (fps) and an imaging frequency of 50,000 fps is adopted in the present experiments. In order to obtain images with good quality, the centerline of the camera lens is perpendicular to the specimen speckle surface. The high-speed camera is triggered by the electrical signal of the loading control system. The resolution of the digital speckle correlation method depends on various factors, such as image speckle quality and the correlation algorithm chosen [32]. Speckles, carrying the displacement information, are divided into natural speckles and artificial speckles. Different kinds of minerals contained in granite rock, which show different colors, consist of typical natural speckles. Materials without natural speckles, i.e., ice, should be pretreated to construct artificial speckles on their surface in order to use DSCM. Constructing artificial speckles include the following steps: (a) polishing the surface or zone that is to be observed; (b) spraying paint toward the interested specimen surface at a certain distance, ensuring that paint mists scatter to the surface to form speckles; and (c) when paint speckles on the surface are dry, repeating Step 2 with another color paint to form a multi-colored mixed speckle field. Generally, a combination of white paint and black paint is enough. In practice, it is found that speckle construction on the surface of ice specimen is much more difficult than that on the surface of rock and wood, because the former should be operated at subzero temperatures. Under such conditions the paint adhesion is worse than at room temperature. Through a trial-and-error method, we obtain some useful skills. Oil paint is more suitable than water-based paint for speckle field construction. First, we spray the whole surface with white paint, then spray paint of another color. Speckles on the surface of the specimen can be dried after 24 h; it is suggested that taking the specimen out and carrying out testing as soon as possible, otherwise small holes may emerge on the surface of the specimen which results in poor adhesion between paint and ice.