## 3 Experimental Results
### Results of Dynamic Uniaxial Compression Tests on Hard Ice
#### 3.1.3 Dynamic Deformation Measurement by DSCM  
It can be seen in Figure 8 that the nominal ultimate strain corresponding to peak stress varies from 0.5 to 1.2% and these values are obviously greater than the true ultimate strain values of ice under uniaxial compression. In order to measure the true deformation of an ice specimen, the non-contacting digital speckle correlation method is used for deformation measurement. In the following paragraph, the sample named \"HH05 mps2\" is taken as an example to elaborate the deformation measurement results by means of DSCM. The label \"HH05 mps2\" means the hard ice sample which is loaded with a 0.5 m\(\cdot\)s\({}^{-1}\) loading rate along the horizontal direction, and its serial number is 2. Figure 9: \(Cont.\)  
The stress versus nominal strain-rate profile of sample HH05 mps2 is shown in Figure 10. Labels A, B, C, and D correspond to four time points of loading process. A is the moment of starting loading and this moment is regarded as the initial time point, so A corresponds to time point 0 ms. B is the moment when a certain load is reached, but the load can continue increasing. It corresponds to time point 0.38 ms. C is the moment when peak load is reached, and corresponds to time point 0.85 ms. D is the moment when the ice specimen is nearly total fail, corresponding to time point 1.26 ms. A reference point set of 41 points in vertical direction and 44 points in horizontal direction is built, and the spacing between the adjacent. Reference points are three pixels (as shown in Figure 11). The size of the subset zone of each reference point is 21 by 21 pixels, and reference point locates at center. The pixel displacement of each reference point is calculated firstly, and then the strain. In order to be intuitive, results are showed in terms of strain, instead of pixel displacement. Local strain results of the speckle surface at A, B, C, D four different moments are shown in Figure 12, figures in the left column correspond to the strain in the \(y\)-direction and right correspond to strain in \(x\)-direction. With regard to the strain in the \(y\)-direction, it should be compression strain all the time and increase with time; while, it is not exactly the same as what is expected. At starting moment A, strain in the \(y\)-direction is uniform (as shown in Figure 12(a1)); strain values are less than 0.06%, as ice specimen is not loaded. At moment B, load increases to approximately half of the peak load; strain in the \(y\)-direction also increases obviously. Strain values are in the range of 0.6% to 1.0%, with an average value of 0.8% (as shown in Figure 12(a2)). It is approximately uniform, though not as uniform as that at moment A. Actually ice specimen experiences almost uniform deformation in the \(y\)-direction. However, a small difference in \(y\)-direction relative displacements (i.e., displacement of interest reference point  
Figure 9: Failure processes of hard ice at moderate rates: (**a,b**) the horizontal loading failure processes of hard ice; and (**c,d**) the vertical loading failure processes of hard ice. The recording rate of high-speed camera is 50,000 fps. with respect to that of its furthest front reference point) may lead to an obvious difference in strains. As the strain field is derived from displacement field, numerical errors exist during derivation and the difference may be amplified. At moment C, the peak load is reached; while strain in the \(y\)-direction does not increase when compared with that at moment B. On the contrary, strain values in most of surface regions reduce to less than 0.1\(\%\), local strain even is in tension form in some regions. There are also regions in which strain values increase up to \(\sim\)1.4\(\%\) (as shown in Figure 12(a3)). The strain field is considerable uneven. How does this happen? In fact, this can be associated with damage occurring in ice sample. When ice sample is divided into several parts by cracks, integrity of the speckle surface is destroyed.