Abstract

A procedure Is described for monitoring the micro-fracturing of ice plates subjected to constant loads. Sample time records of fresh water ice plate deflections as well as corresponding total acoustic emission activities are presented. The linear elastic as well as viscoelastic response for a simply supported rectangular ice plate is given. Acoustic emission methods have been utilized with success in a number of investigations involving the deformation of rocks, soils, and metals. By way of contrast little information on acoustic emission from ice appears in the published literature. In the present investigation acoustic emission methods are used to study the micro-fracturing activity in polycrystalline ice subjected to flexural loads. Experimental results obtained in the laboratory indicate that the acoustic emissions recorded from ice are important in describing the deformation and fracture of ice. The relationship between acoustic emissions and the time dependent inelastic flexural deformation in ice is studied. Furthermore the influence of the magnitude of the applied load and the rate of deformation on cracking activity is explored.

Introduction

The stress or pressure waves which are generated during dynamic processes in materials are referred to as acoustic emission. Most materials emit sounds or stress eaves as they are deformed and these sounds provide information on the deformation characteristics and micro-fracture and also warn of impending failure. The term acoustic emission has been generally accepted to describe low level sounds or pressure waves emitted by materials subjected to thermal and/or mechanical loadings.1 The small-scale stable fractures which accompany the deformation of some materials under certain loading conditions are referred to as micro-fractures to distinguish them from failure of the materials as a whole. Micro-fractures radiate elastic energy and consequently it is possible to study their occurrence and growth using acoustic emission techniques. The relationship between acoustic emission and time dependent inelastic deformations in the field of rock mechanics has been investigated by a number of authors2,3,4,5 for both uniaxial and triaxial loading conditions. The results of these investigations suggest a strong correlation between creep strain and acoustic emission. Thus, a nearly linear relationship was found to exist between axial creep strain and accumulated acoustic emission activity for three types of rock under uniaxial compression. 4 By way of contrast a limited number of studies have been carried out on the acoustic emission characteristics of ice during time dependent deformations. 6,7,8,9,10,11

Creep deformations studies in ice and their associated mechanisms are reported in references.12,13,14 Gold 6,7,8,9 has studied the micro-cracking activity In ice during creep under constant compressive loadings. He reports two stages of cracking. The rate of micro-crack formation during the first stage decreased very markedly as the creep rate approached a constant value. Above a certain stress the creep rate continuously increased with time. Under this condition the second stage of cracking was observed. Thus, the sudden transition reported by Brown15 in the internal cracking activity of ice deformations during creep. A fairly uniform distribution of micro-cracks was observed during the first stage of creep. Micro-cracks tends to form in planes that were about 45° applied stress and parallel to the grain boundaries after an initial cracking period during the second stage of the creep.

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