Abstract
The macroscopic response of a geomaterial is entirely determined by changes at the particle scale. It has been established that particle crushing is affected by particle size, shape and mineral composition and initial density; and the initiation of breakage has often been related to the onset of yielding. Encouraged by the success of X-ray tomography in revealing particle-scale mechanisms of deformation, we present our findings regarding the onset of particle breakage, deriving from our study of 3D images of a dry granular assembly undergoing crushing. We propose two bespoke image analysis algorithms, which allow us to track breakage and identify contacts prior to breakage. The combination of the two algorithms, along with the high resolution of the 3D images enables us for the first time to track breakage of individual particles, identify different breakage modes for each particle and simultaneously study the effect of particle morphology and coordination number on breakage. Three different breakage types are identified: chipping, splitting and fragmentation. We have found that particle heterogeneity and sphericity mainly contribute to fragmentation, whereas the coordination number also affects chipping. The confining stress state within the particles with high coordination number made them more resistive to fragmentation, whereas particles with low coordination number mainly undergo fragmentation. The shearing of the particles at their contact points, leads to local stress concentrations resulting into surface chipping. Finally, we discuss the relation between the initiation of breakage and yielding, showing that some breakage occurs before the point where yielding is traditionally defined.
Highlights
In the study of grain breakage phenomena in particulate matter, standard quantitative measurements are usually limited to an initial and a final sieving analysis, and to the externally-measured response of a specimen to applied loading
Particles break with increasing applied macroscopic stress, and particles with lower coordination number and larger particle size are found to
McDowel et al [5] explained that crushing tensile strength is expected to fall with increasing particle size due to the increased likelihood of internal flaws existing in the larger particles, yet smaller particles tend to break as increased coordination number outweighs the effect of particle strength solely due to particle size
Summary
In the study of grain breakage phenomena in particulate matter, standard quantitative measurements are usually limited to an initial and a final sieving analysis, and to the externally-measured response of a specimen to applied loading. Numerical analyses [5] and experimental findings supported by DEM evidence using crushable agglomerates [6] show that coarse particles survive in a granular material undergoing crushing [4] This is because in a polydisperse assembly a decrease in coordination number and an increase in particle size are two opposing conditions. Coop and Lee [14] performed oedometric compression tests on loose specimens of three different sands and showed that the stiffness reduced due to particle crushing and attributed yield to the onset of crushing They found that the yield stress depends on the strength of the soil particles and the relative density of the specimen. We discuss the relation between the initiation of breakage and yielding, as it has been observed in this particular set of experiments
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