Components Of Complex Shear Modulus Research Articles

Dynamic mechanical measurements on fresh-water ice during freezing and thawing: preliminary results

An automated measurement system for elastic (J' ) and viscous (J'' ) components of complex shear compliance, J* = J' – iJ'', and the elastic (G' ) and viscous (G'' ) components of complex shear modulus, G* = G' + iG'' = 1/J*, has been used to obtain these material parameters for fresh-water ice during freezing and thawing. The system is reviewed briefly and yields mechanical loss tangents, J''/J' = G''/G', the shear-wave velocity and attenuation, in addition to shear compliance and modulus, at sinusoidal vibration frequencies from 2 to 10 000 Hz at temperatures between –25 and 150°C. Results reported here are chiefly at temperatures from 10 to –10°C. The required sample disk pairs, which are clamped to a central drive plate, are prepared outside the apparatus for solids and gels. Liquids of known volume are inserted between the drive plate and surrounding clamps at a separation distance, h, by a syringe to form sample disks of area, A = Volume / h. Measurements at 58 frequencies between 2 and 10 000 Hz require 3.5 min; several measurements at each temperature were made to test for equilibrium. Results for both tap and distilled water above freezing revealed high values of elastic (J' ) compliance that decreased sharply at 100 Hz and higher frequencies. Tap-water samples with 4 to 6% by volume air bubbles were less compliant ("stiffer") above freezing than samples with 0 to 1% by volume air, but when frozen, the samples with the smaller volume of air bubbles were less compliant, that is, had higher modulus values than the samples with high air-bubble volumes. Dynamic mechanical property changes in the transition from water to ice are compared to changes previously found during phase transitions in other materials. Further investigation on the effects of air-bubble volumes on dynamic mechanical properties of both water and ice is planned. PACS Nos.: 62.30+d, 62.40+i