L.J. Weber and W.A. Nixon , Iowa Institute of Hydraulic Research,, Department of Civil, Environmental Engineering, University of Iowa, Iowa City, Iowa 52242-1585
Abstract
Experiments were conducted on both columnar S2 freshwater ice and randomly oriented polycrystalline freshwater ice to determine the hysteretic (or internal friction) response of the ice under fatigue loading. The ice was loaded in four-point bending, and had a single crack placed at the center of the span, such that the crack was opening in a model manner. The major loading span was 300 mm, with a minor span of 150 mm. Tests were conducted at a temperature of -5℃, and the beams had a square cross section of 100 by 100 mm. Crack lengths were approximately 50 mm in all cases (precise measurements were taken for each sample tested). The columnar ice had a mean grain column diameter of 3.6 + 0.8 mm, while the polycrystalline ice had a mean grain diameter of 1.5 + 0.3 mm.
Two series of tests were conducted. In the first, the applied stress intensity amplitude (△ K) was held constant at 80 kPa√m for the columnar ice and 110 kPa√m for the polycrystalline ice, with an R-ratio (ratio of the minimum to the maximum applied K value) of 0.05 in both cases. For this series, the loading frequency was varied to determine the optimal conditions for fatigue crack growth testing, with tests being conducted at frequencies of 0.001, 0.01, 0.1, 1 and 5 Hz. In the second series of tests, the effect of strain amplitude on the internal friction was examined. Samples were loaded for ten cycles at a n initial AK of 60 kPa√m They were then subjected to further groups of ten cycles at AK ranges which increased by 20 kPa√m at each step (i.e. at 80 kPa√m m for 10 cycles, then at 100kPa√m for 10 cycles and so on) until failure occurred.
The results from these two sets of experiments were examined first from the point of view of the variation of internal friction, φ. The internal friction results are compared with t hose of Cole (1990) for fully reversed loadings on uncracked freshwater ice. Further, using Granato-Lucke theory to analyze the test results, some insights into possible dislocation mechanisms at the crack tip are presented.
