Characterisation of Partial Melting and Solidification of Granite E93/7 by the Acoustic Emission Technique
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Characterisation of Partial Melting and Solidification of Granite E93/7 by the Acoustic Emission Technique Lyubka M. Spasova, Fergus G.F. Gibb and Michael I. Ojovan Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK ABSTRACT The acoustic emission (AE) technique was used to detect and characterise the processes associated with generation of stress waves during melting and solidification of granite E93/7 at a pressure of 0.15 GPa. The AE signals recorded as a result of partial melting of the granite at a temperature of 780 ºC and subsequent solidification during cooling were distinguished from the equipment noise and their parameters used to characterise the AE sources associated with the phase transformations during melting and solidification of the granite. The mechanisms generating AE during granite melting were differentiated by AE signals with their highest peaks in the frequency spectrum at 170 and 268 kHz. The transformation of the liquid into glass during solidification of the partially melted granite generated AE waves in an essentially broad range of frequencies between 100 and 300 kHz. This preliminary work demonstrates the potential of the AE technique for use in applications related to deep borehole disposal of radioactive wastes. INTRODUCTION
Over the last 40 years non-destructive methods for testing and evaluation, such as acoustic emission (AE), have been extensively developed and applied in industry for assessment of the mechanical performance of various structures under conditions varying from compression or tensile loading to thermal shock and hydraulic tests [1,2]. In the field of materials science AE has been shown to be a valuable tool, not only for characterising the mechanical performance of materials under stress in relation to their microstructure, but also for detecting microstructural processes active during phase transformations in metals, alloys, ceramics and glasses [3-7]. AE is a naturally occurring phenomenon associated with the release of stored elastic energy within a material in the form of transient waves with frequencies typically in the range 20 kHz to 1.2 MHz [1]. The AE technique has been successfully used to study phase transformations due to its high sensitivity to micro-scale changes in the shape or volume of a material and its capability for application in-situ and in real time under extreme experimental conditions such as high temperature (>1000 ºC) or pressure. The latter is usually achieved by attachment of waveguides to the samples or with advanced approaches such as the development of a non-contact laser AE technique used during processing of ceramic materials [8]. In this work a new application of the AE technique to monitor and characterise the processes of granite melting and solidification during cooling has been explored. The study has focused on granites due to their potential role as a host environment for the disposal of high level radioactive waste (HLW) packages [9]. The proce
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