## 4 Summary of Results and Discussion
### Dynamic Deformation of Ice Samples at Moderate Rates
For soft ice, they are 2.5 MPa and 4.5 MPa, respectively. At \(-10\)\({}^{\circ}\)C, for hard ice, they are 7.0 MPa and 8.0 MPa, respectively, while, for soft ice, they are 4.5 MPa and 6.5 MPa, respectively. The compressive strength of hard ice at \(-10\)\({}^{\circ}\)C is 1.75 times that at \(-5\)\({}^{\circ}\)C when loading horizontally, and 1.60 times when loading vertically. For soft ice, strength at \(-10\)\({}^{\circ}\)C is 1.80 times that at \(-5\)\({}^{\circ}\)C when loading horizontally, and 1.44 times when loading vertically. Even though the ices have different air porosities, both are affected by temperature significantly, no matter whether loading horizontally or vertically. [PERSON] et al. [8] also investigated ice compressive strength at \(-10\)\({}^{\circ}\)C and \(-40\)\({}^{\circ}\)C at about 10 s\({}^{-1}\) strain-rate. Study of [PERSON] et al. showed that the average strength of ice was 4.70 MPa at \(-10\)\({}^{\circ}\)C and 5.81 MPa at \(-40\)\({}^{\circ}\)C (excluding strength data of bonded-end samples). The latter is 1.2 times the former. By comparison with the result of [PERSON] et al., ice compressive strength seems to be more sensitive on temperature in the higher temperature range. Actually, under quasi-static  
Figure 16: Relationship between the true deformation and the nominal deformation under dynamic loading conditions: (**a**) scatter plot of true strain-rate versus nominal strain-rate; and (**b**) scatter plot of true ultimate strain versus the nominal ultimate strain. conditions, similar observation is also found [33; 34]. However, quantitative analysis on the rate of change of the compressive strength at moderate strain rates with respect to temperature in different temperature ranges needs further investigation. When loaded horizontally, the average strength of hard ice is 1.6 times that of soft ice at \(-5\)\({}^{\circ}\)C, and 1.7 times at \(-10\)\({}^{\circ}\)C. While when loaded vertically, the average strength of hard ice is only 1.1 times that of soft ice at \(-5\)\({}^{\circ}\)C, and 1.2 times at \(-10\)\({}^{\circ}\)C. Air porosity weakens ice strength and has a greater influence on the horizontal strength than on the vertical strength. [PERSON] [5] conducted uniaxial compression tests on both bubble-free freshwater ice and saltwater ice over the strain-rate range of 10\({}^{-1}\)-10 s\({}^{-1}\) at \(-11\)\({}^{\circ}\)C. In his study the vertical compressive strength of freshwater ice was found to be 1.3 to 1.5 times that of saltwater ice. The corresponding factor obtained in this work is 1.2, slightly lower than the value obtained by [PERSON]. As bubble-free freshwater ice is generally stronger than natural lake ice, while saltwater ice is generally weaker than natural lake ice, so it is reasonable that the factor we obtained is slightly lower. Effect of air porosity on compressive strength at moderate strain rates is similar to that under quasi-static conditions. [PERSON] and [PERSON] [35; 36] derived equations for the uniaxial compressive strength of first-year sea ice based on results of 283 quasi-static compression tests, and the equations showed that uniaxial compressive strength of first-year sea ice decreased with increasing total porosity. According to their equations, the horizontal compressive strength of ice with a total porosity of 0.1% is 1.29 times that of ice with a total porosity of 2.0%, and the factor for the vertical compressive strength is 1.36. At low strain rates, effect of total porosity or air porosity on vertical compressive strength is a little greater than that on horizontal compressive strength. While a greater air porosity effect is found on horizontal strength in the present study. This discrepancy may result from significant texture difference between sea ice and freshwater ice. In most cases, freshwater ice contains lower air porosity, so the effect of air porosity on strength of freshwater ice does not cause adequate attention.