Acoustic emission responses and damage estimation of coal with carbon fiber-reinforced polymer confinement under uniaxial compression

Fiber-reinforced polymer(FRP)wrapping is a potential technique for coal pillar reinforcement.In this study,an acoustic emission(AE)technique was employed to monitor coal specimens with carbon FRP(CFRP)jackets during uniaxial compression,which addressed the inability to observe the cracks inside the FRP-reinforced coal pillars by conventional field inspection techniques.The spatiotemporal fractal evolution of the cumulated AE events during loading was investigated based on fractal theory.The results indicated that the AE response and fractal features of the coal specimens were closely related to their damage evolution,with CFRP exerting a significant influence.In particular,during the unstable crack development stage,the evolutionary patterns of the AE count and energy curves of the CFRPconfined specimens underwent a transformation from the slight shockemajor shock type to the slight shockesub-major shockeslight shockemajor shock type,in contrast to the unconfined coal specimens.The AE b-values decreased to a minimum and then increased marginally.The AE spatial fractal dimension increased rapidly,whereas the AE temporal fractal dimension fluctuated significantly during the accumulation and release of strain energy.Ultimately,based on the AE count and AE energy evolution,a damage factor was proposed for the coal samples with CFRP jackets.Furthermore,a damage constitutive model was established,considering the CFRP jacket and the compaction characteristics of the coal.This model provides an effective description of the stressestrain relationship of coal specimens with CFRP jackets.

Experimental and numerical study of coal mechanical properties during coalification jumps

The mechanical characteristics of coal reservoirs are important parameters in the hydraulic fracturing of coal.In this study,coal samples of different ranks were collected from 12 coal mines located in Xinjiang and Shanxi,China.The coal ranks were identified with by the increased Maximum vitrine reflectance(Ro,max)value.The triaxial compression experiments were performed to determine the confining pressure effect on the mechanical properties of coal samples of different ranks.The numerical approaches,including the power function,arctangent,and exponential function models,were used to find the correlation between coal elastic modulus and the confining pressure.The fitting equations of compressive strength and elastic modulus of coal ranks were constructed under different confining pressures.The results showed that the coal compressive strength of different ranks has a positive linear correlation with the confining pressure.The coal elastic modulus and confining pressure showed an exponential function.Poisson’s ratio of coal and confining pressure show negative logarithmic function.The stress sensitivity of the coal elastic modulus decreases with the increase of confining pressure.The coalification jump identifies that the compressive strength,elastic modulus,and stress sensitivity coefficient of coal have a polynomial relationship with the increase of coal ranks.The inflection points in coalification at Ro,max=0.70%,1.30%,and 2.40%,are the first,second,and third coalification jumps.These findings provide significant support to coal fracturing during CBM production.

Study of a damage constitutive model for water-bearing coal measures sedimentary rock with nonlinear deformation during compaction stage

The problem of repeated immersion-induced fatigue damage in engineering coal measures sedimentary rock,including coal-rock pillars,reservoir bank slopes,and water-rich tunnels at the boundary of coal mine underground reservoirs,has profound implications for their stability,safety,and operation,and can even lead to geological disasters.To address this issue,this paper aims to construct a constitutive model that accurately captures the comprehensive process of deformation and failure in water-bearing coal measures sedimentary rock.It explores the deformation characteristics of these formations and provides a theoretical foundation for numerical simulations of geological disasters induced by water-rock interaction.This study integrates the deformation mechanisms of void and matrix deformation in coal seam sedimentary rocks,while considering the influence of immersion cycles.Subsequently,it formulates a segmented constitutive model to depict the entire process of deformation and failure in cyclically immersed water-bearing coal measures sedimentary rock under uniaxial compression.The proposed model's accuracy and rationality are validated through comparisons with experimental research findings and existing theoretical curves from similar models.The results demonstrate the model's effectiveness in describing the deformation behavior of non-dense water-bearing coal measures sedimentary rock under uniaxial compression or low confining pressure before reaching peak stress,although further refinements may be necessary to precisely capture post-peak deformation characteristics.Model parameters,including the deformation caused by voids(γ0)between voids,increase exponentially with immersion times,while the elastic modulus(Ev)of voids and the parameter(F0)related to the average strength of microelements decrease exponentially.The homogeneity degree(m)exhibits no discernible pattern.These research outcomes provide valuable insights for the stability control of engineering coal measures sedimentary rock under water-rock interaction and the mitigation of related geological disasters.