Abstract
In this work acoustic emission (AE) is used as experimental evidence of the progressive nature of grain crushing. Stress controlled high pressure oedometric compression test are carried out on 1.2 mm monodisperse samples of glass beads. It was observed that the granular assembly starts to experience particle breakage at a vertical stress of about 25MPa. When this yield pressure is exceeded the glass beads start to break emitting loud impulsive sound and the vertical displacement increases rapidly. The load was increased beyond the yield stress and at each increment while the vertical stress remained constant the sample continued to emit sound. The emission of sound at a constant vertical stress indicates that crushing is a progressive failure mechanism; once the first crushing event occurs, the structure starts to rearrange causing other crushing events to occur and additional settlement. In particular, two signal processing algorithms are used on the samples of the acoustic signal to obtain two additional metrics of the crushing evolution. The first is the cumulative energy versus time. The second is the number of crushing events versus time, which is based on the automatic detection of the peaks of the sound signal envelope. There is a clear correlation between the cumulative acoustic energy emitted and the observed sample displacement. Using laser scanning, the evolution of the particle size distribution and particle shape are measured in detail so that a link between the acoustic data and the crushing intensity is established. The crushing intensity was controlled using materials with different strengths.
Highlights
It is well acknowledged that most solids emit low-level seismic signals when they are deformed or stressed [1]
The progressive nature of the crush induced deformation is well reflected in the acoustic emission (AE) analyses in terms of both Ecu and Ncr, as shown in Fig. 5 for the first yield of the glass sample G
[9] QicPic laser image analyses can be performed on the sample at the end of the experiment and the particle size distribution (PSD) and sphericity evolution can be used as another indicator of crushing evolution (Fig. 6)
Summary
It is well acknowledged that most solids emit low-level seismic signals when they are deformed or stressed [1] Such phenomenon of generating signals is termed as acoustic emission (AE) activity. Frequency and attenuation of measured acoustic events can be related to certain physical and mechanical processes of granular materials under shearing or compression. Measurement of AE are realized by detecting and monitoring the generation and the propagation of elastic waves due to the release of stored elastic strain energy during material deformation and abrupt grain interaction. Grain crushing is an elastic strain energy release mechanism that produces AE with a loudness and frequency within the range of audible sound. To control the level of crushability, the tests were run on mixtures of crushable and non-crushable spherical particles This was complimented by AE analysis of the sound emitted during compression. A simple methodology to estimate the number of crushing events occurring within the sample is proposed
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