Measurement of the grain-boundary tensile strength in columnar freshwater ice

Abstract

A new procedure for measuring the tensile strength of grain boundaries in a columnar ice sample is presented. In this experiment, an ice coating is grown on an Al surface with its columnar grains normal to the interface. The sample is loaded by a compressive stress wave of about 20ns duration, generated by impinging a Nd-doped yttrium aluminium garnet laser pulse on the back side of the Al substrate. The compression stress wave propagates through the ice coating turns tensile after reflecting from its free surface and loads the ice-Al interface normally. Because of the Poisson effect, this interface-normal stress is accompanied by an interface-parallel biaxial stress of a somewhat lower (33% of the axial stress) magnitude. Because the interface strength is much larger than the tensile strength of grain boundaries, this latter stress, even though small, is sufficient to cause the onset of cracking in some of the favourably oriented grain boundaries. The threshold laser energy corresponding to this event is determined by observing the sample under an optical microscope, after each loading. Finally, this threshold laser energy is converted to the local interface stress, causing boundary separation using an elastic wave simulation. Using this procedure, an average across-column grain-boundary tensile strength of 47MPa was determined at-10oC. Since the boundaries are loaded at a strain rate of almost 107 s-1, all inelastic deformations during the boundary decohesion process are essentially suppressed, and thus the measured values are essentially intrinsic depending only on the structure and chemistry of the grain boundary, including the defects, if any.