0. D. von Engeln — Studies on Ice Structure. 451 



glacier ice with variously oriented crystal grains was at some 

 figure intermediate between 350 and 1000 lbs., and tended to 

 the higher rather than the lower figure. These data were 

 obtained with air temperatures between 18°F. and 20°F., but 

 variations between 10°F. and 25°F. do not seem to have a 

 notable effect on the strength of the ice. Great care was exer- 

 cised to have the pressure faces as nearly parallel planes as 

 possible and pressures were applied very slowly. In a number 

 of instances increases of the load were made only after a 

 number of hours interval ; in several cases a single test occupied 

 from 16 to 20 hours time. 



The values of the crushing strength of ice here stated are 

 notably higher than those obtained and approvingly cited by 

 H. T. Barnes in recent papers.* The average values he quotes 

 are 370 lbs. per sq. inch, pressure parallel to the principal 

 axes, 356 lbs. pressure normal to the axes. The discrepancy 

 in the first case is probably due to the following factors : (a) 

 The ice used by Barnes was river ice and because of current 

 action may not have had uniformly oriented crystals, especially 

 as large size blocks, 7 inches square, were used in the tests ; (b) 

 the tests were made at air temperatures only a few degrees 

 below freezing point and in some cases above that point ; (c) the 

 pressures were applied with comparative rapidity. The authors 

 of the second paper cited use an average figure of 400 lbs. to 

 the square inch as the crushing strength of ice in calculating the 

 linear thrust of ice sheets against dams. As such thrust 

 would be exerted at right angles to the principal axes the value 

 of the crushing strength used is probably amply high for 

 engineering practice. 



It was repeatedly noted when working with cubes under 

 pressure approaching the crushing point that cracking occurred 

 when the pressure was relieved and that if the pressure release 

 was at all rapid the ice actually broke apart. This indicates that 

 ice yields elastically to pressures slowly and carefully applied 

 and that the elastic rebound is not sufficiently rapid to enable 

 the ice to withstand quick release of pressure, hence the 

 differential stresses set up by the rapid release suffice to 

 rupture the mass. On account of this elasticity it appears 

 that there can not be any permanent deformation of ice until 

 a certain minimum stress has been applied and the experiments 

 showed that the yield-point is near the crushing strength of 

 the ice. 



If, however, pressures just below the crushing strength of 

 the. ice are applied and continued for a sufficiently long time 



* Barnes, H. T. : The Crushing Strength of Ice, Trans. Boy. Soc. Canada, 

 Third Series, vol. viii, pp. 19-22, 1914. Barnes, H. T., Hayward, J. W., 

 and McLeod, N. : The Expansive Force of Ice, ibid., pp. 22-49. 



