Effect of Uniaxial Compression on Coal Nanostructure as Measured by Small Angle X-ray Scattering

Abstract

To study the effect of uniaxial compression on coal nanostructure during uniaxial compression, in situ synchrotron radiation small angle X-ray scattering experiments were carried out on four coals with different ranks under uniaxial compression. According to the scattering data during the uniaxial compression process, the fractal characteristics and the variation feature of fractal dimension with stress were obtained. Four coals with different ranks all possess two fractal characteristics: pore fractal occur in the smaller pore range (7–17 nm) in the high q value range, and surface fractal occur in the larger pore range (17–52 nm) in the low q value range. For two low rank coals, with increasing stress, the pore fractal dimension DP decreased and the surface fractal dimension DS increased, respectively; the variation trends of DP and DS were obvious. This indicates that with increasing stress, the heterogeneity and complexity of the pores decrease, the surface roughness of the pores increases, and stress has a significant effect on the nanopore structure. The smaller pores are more susceptible to stress, and the influence range of stress on low rank coals is larger than that on high rank coals. The change rate of fractal dimension (RD) has a poor relationship with compressibility during uniaxial loading and is related to coal rank. The RD per unit stress for high rank coals is larger than that for low rank coals. Nanostructure response to uniaxial compressive stress is more significant in low rank coals than in high rank coals. Compared with low rank coals, high rank coals have strong aromatization and molecular structure, and the nanostructures are less susceptible to failure under uniaxial stress.

Evolution of Acoustic Emission and Microcrack in Jointed Coal under Loading and Their Correlations

“…It can be seen the peak stress values (PSV) of samples A and B were averagely 27.6 and 13.9 MPa, respectively, indicating that the strength of sample A that was compressed in the direction perpendicular to joint planes is much higher than that of sample B whose loading direction is parallel to joint planes. This result is consistent with many previous studies. ,,, Both samples experience compaction, elastic deformation, yielding and postpeak deforming stages, corresponding to the stress ranges of 0–20% PSV, 20–85% PSV, 85–100% PSV and after stress peak. Figure also illustrates the fracture morphology of the cracked samples.…”

“…This result is consistent with many previous studies. 16,23,45,46 Both samples experience compaction, elastic deformation, yielding and postpeak deforming stages, corresponding to the stress ranges of 0−20% PSV, 20−85% PSV, 85−100% PSV and after stress peak. Figure 4 also illustrates the fracture morphology of the cracked samples.…”