Avalanches Dynamics And Its Application In Mining Materials Under Uniaxial Compression Basted On Acoustic Emission

Avalanches occur in many physical systems including magnetization,phase transitions,neuronal networks even decision-making processes.The collapse of porous materials presents typical avalanches behaviors.These behaviors of the porous materials are not smooth and continuous as classically expected for elastic or plastic materials but instead occurs as a sequence of avalanches.Experimentally,this jamming of microstructures can be detected by measuring the acoustic emission(AE).So in this thesis,acoustic emission selected to be the tool to investigate the avalanches dynamics of geo-materials(coal and sandstone).The doctoral thesis is organized as follows:Chapter 1 is the background of avalanches in porous materials and summary of the rest chapters.In this chapter,a review of avalanches studies including mean-field theory,computer simulation and lab tests is presented.Especially,the recent acoustic emission experiments during the compression of porous materials for avalanches analysis has been carefully reviewed.Chapter 2 introduces the fundamental knowledge of statistical methods for avalanches which has been widely used in this thesis.Gutenberg-Richter law,waiting time,after shock and Bath's law have been introduced respectively.Coal and sandstone samples under uniaxial compression with acoustic emission capture tests have been carried.The probability density function of absolute AE energy meets power low well both coal and sandstone samples.The maximum likelihood method has been used to estimate the exponents for avalanche process more reliable.Moreover,the result of Bath's analysis is excellent agree with the theory.Chapter 3 tries to use DEM,namely fiber bundle model,study the dynamics of avalanches in rock failure process.The failure of fiber elements in fiber bundle model was analogous to the acoustic emission phenomenon.The results showed that,at low stress stage,FBM ignored the occasional avalanche processes,but FBM characterized AE characteristics well in high stress stage.The distribution of avalanche sizes and AE events both had double power-law behavior,and the probability distribution function(PDF)of simulations and tests share the similar power exponents and inflection points.Simulation and experimental results shows a behavior analogous to the second-order phase transitions,and branching ratio could be as a suitable order parameter to describe this transition.Moreover,near to the critical point,the evolution of branching ratio came from FBM and AE tests were consistent.Chapter 4 shows the not fix points behavior of avalanche statistics for coal and sandstone by using much larger samples and higher stresses.AE change their spectral parameters from a non-critical steady state to an all-important precursor regime before samples finally collapse.The AE energy distribution always follows a power law but that the exponents are different in the two regimes.The energy exponents for the randomly distributed events are generally larger than those of the highly correlated events(uncorrelated 1.7 vs.correlated 1.5 for sandstone and uncorrelated 1.5 vs.correlated 1.3 for coal).Super-jerks are defined and the final collapse is one such super-jerk but other ‘near collapse' events equally qualify.In this way a very large data set of events is reduced to a sparse sequence of super-jerks(21 in our coal sample).We argue that the reduction of the energy exponent is not a continuous process but that two competing fix-point like mechanisms are at play: randomly-distributed isolated collapse avalanches lead to the higher exponent while dense,correlated bursts of energy release show the smaller exponent.The apparent continuous shift of the exponent is most likely due to the superposition of the two mechanisms.Chapter 5 presents three applications of avalanche analysis in rock mechanics tests: The effect of supercritical CO2 on mechanical properties of sandstone,and sandstone acoustic emission absolute energy statistical properties during uniaxial creep.All experimental results showed that the avalanche statistics analysis agreed with the mechanical behaviors,NMR characteristics,mineral analysis and wave velocity changes of rock samples in damage process.Finally,chapter 6 describes the main conclusions from Chap.2 to Chap.5.In this chapter,some future researches like the mixing of exponents,avalanche of rock sample in triaxial condition and the relation between avalanche behavior and microstructure,etc.also have been discussed.