## 3 Experimental Results
### Results of Dynamic Uniaxial Compression Tests on Hard Ice
#### 3.1.3 Dynamic Deformation Measurement by DSCM
According to strain contours shown in Figure 12, not all strain fields obtained before the peak load moment (moment C) are uniform; so it is not a feasible way to calculate the true strain-rate directly by these strain fields, for the strain field cannot reflect the true deformation of ice sample again when lots of cracks has formed in ice sample. Instead, we fit the pixel displacements of all reference points with respect to their coordinates by means of linear least squares approach, thus the average true strain can be obtained, and then the average true strain-rate. The average true strains in the \(y\)-direction and \(x\)-direction are shown in Figure 13. With regard to the average true strain in the \(y\)-direction, strain values before loading starts (moment A) are almost all zero. After loading starts it gradually increases, accompanied with slight fluctuation, until to time point 0.38 ms (moment B). At this moment, the average true strain is 0.8\(\%\), equal to the mean value of current local strain field (as shown in Figure 12(a2)). Thereafter, a sharp jump occurs. The average true  
Figure 12: Results of DSCM of sample HH05 mps2: (**a1**–**a4**) Strain in the \(y\)-direction; and (**b1**–**b4**) strain in the \(x\)-direction by the digital speckle correlation method (from top to bottom, figures correspond to time point 0, 0.38, 0.85 and 1.26 ms, respectively). strain reduces to a value near zero. Just after time point 0.38 ms, a large crack may form suddenly in ice sample, resulting in lots of speckle surface regions separated from the main ice body partially or totally and unloaded. According to analysis in the previous paragraph, the reference points within these regions will not continue shortening the distance between each other; instead, they restore to their original relative positions. Thus, the average true strain changes suddenly and drops down to a value near zero. As compression continues, the unloaded speckle surface regions may become loaded again, and then the average true strain increases again. The profile of average true strain versus time after time point 0.7 ms shows such scenario (as shown in Figure 13a). With regard to the average true strain in the \(x\)-direction, it is derived from pixel displacement in the \(x\)-direction. Before obvious cracks emerge, the average true strain increases slowly; once many obvious cracks emerge, it increases dramatically (as shown in Figure 13b). This is consistent with the result shown in Figure 9. Only the profile section, in whose corresponding time period detachment in ice sample has not yet occurred, are valid to deriving the average true strain-rate in the \(y\)-direction (as shown in Figure 13a, the profile section between the two blue lines is valid). The average true strain-rate of sample HH05 mps2 in the \(y\)-direction is 2.49 m\(\cdot\)s\({}^{-1}\), while the nominal strain-rate is 10.0 m\(\cdot\)s\({}^{-1}\); the former is only approximately a quarter of the latter. In-negligible deformation of loading machine itself and imperfect interfaces between ice sample and platens account for this (a detailed analysis is conducted in the Appendix A). Multiplying the average true strain-rate by the sustaining time from the initial loading moment to the moment of achieving peak load, the true ultimate strain could also be obtained. Results of average true strain-rate and true ultimate strain under different loading rates and at different temperatures are shown in Table 2.