Damage characterization of shale under uniaxial compression by acoustic emission monitoring

Understanding the damage behavior and cracking mechanism of brittle shale is crucial for hydraulic fracturing design.In this research,uniaxial compression tests are conducted on shale samples with different bedding plane orientations,and acoustic emission monitoring is implemented synchronously.The results indicate that the apparent elastic modulus increases with increasing bedding orientation.For the bedding orienta-tions of 45°and 90°,the lateral deformation is anisotropic due to the bedding structure,revealing the anisotropic Poisson effect.A shear failure surface and tensile failure surfaces form parallel to the bedding plane for bedding orientations of 45°and 90°,respectively.For the bedding orientation of 0°,shear failure mainly occurs through the bedding planes.Additionally,the damage mechanism of shale is investigated by crack classification based on AE parameters.It is found that crack initiation is induced by the generation of shear cracks for the bedding orientation of 45°,whereas by the generation of tensile cracks for other bedding orientations.According to damage attributable to different type cracks,shear cracks dominate the damage behavior for bedding orientations of 0°and 45°,whereas tensile cracks dominate the damage behavior for bedding orientation of 90°.Finally,the information entropy is calculated by AE energy.A low value of information entropy,approximately 0.36,predicts failure with a low degree of instability for the bedding orientation of 0°,whereas a high value of information entropy,more than 1.5,predicts failure with a high degree of instability for other bedding orientations.This finding indicates that the failure behavior is gradual progressive damage for bedding orientation of 0°,whereas sudden damage dominates failure behavior for other bedding orientations.

Effect of dry-wet cycles on dynamic properties and microstructures of sandstone:Experiments and modelling

Underground pumped storage power plant(UPSP)is an innovative concept for space recycling of abandoned mines.Its realization requires better understanding of the dynamic performance and durability of reservoir rock.This paper conducted ultrasonic detection,split Hopkinson pressure bar(SHPB)impact,mercury intrusion porosimetry(MIP),and backscatter electron observation(BSE)tests to investigate the dynamical behaviour and microstructure of sandstone with cyclical dry-wet damage.A coupling FEM-DEM model was constructed for reappearing mesoscopic structure damage.The results show that dry-wet cycles decrease the dynamic compressive strength(DCS)with a maximum reduction of 39.40%,the elastic limit strength is reduced from 41.75 to 25.62 MPa.The sieved fragments obtain the highest crack growth rate during the 23rd dry-wet cycle with a predictable life of 25 cycles for each rock particle.The pore fractal features of the macropores and micro-meso pores show great differences between the early and late cycles,which verifies the computational statistics analysis of particle deterioration.The numerical results show that the failure patterns are governed by the strain in pre-peak stage and the shear cracks are dominant.The dry-wet cycles reduce the energy transfer efficiency and lead to the discretization of force chain and crack fields.

Identification of damage mode in AZ31 magnesium alloy under tension using acoustic emission

In order to characterize different damage modes, real-time detection of the tensile cracking process for AZ31 magnesium alloy was performed using acoustic emission （AE） technique. Results showed that elastic deformation, plastic deformation, microcracking, stable and unstable propagation occurred during crack damage. Four damage modes were determined using AE multiparameter analysis. Dislocation motion signals with amplitudes 〈70 dB and twinning signals with 70-100 dB were found. Microcrack signal energy was concentrated from 2400 aJ to 4100 aJ, mainly at a rise time of less than 800 gs. A stable crack propagation signal had high peak to counts in the 20 to 50 range, whereas its ring count was in the 20 to 2000 range. The average frequency of unstable propagation signals was approximately 100 kHz, with duration from 2000 gs to 10s gs. The damage mechanisms and AE resources from different crack propagation steps were discussed. Various damage modes could be characterized by different AE signal parameters when they appeared simultaneously during crack propagation.

Study on damage characteristics of iron ore under variable line density charge structure blasting

Blasting test research was conducted on iron ore specimens with variable line density charging structures.Computer tomography(CT),digital image processing,and three-dimensional model reconstruction techniques were used to analyze the damage characteristics of iron ore specimens after blasting based on the calculated number of box dimensions.The results show that increasing the variable line density section charge uncoupling coefficient reduces the overall damage to the specimen by up to 1.73%,indicating that the overall damage size negatively correlates with the size of the variable line density section charge uncoupling coefficient.The damage characteristics of iron ore specimens from different layers(uncoupled charging section,transition section,coupled charging section)have some variability;when the uncoupling coefficient of the uncoupled charging section was reduced,the uncoupled section of the center of the damaged layer increased and then reduced.In contrast,the transition section shows a trend of increase,and the coupled section shows a minor difference,fully demonstrating the change in the variable line density section of the uncoupling coefficient of the specimen blasting damage effects.This study concludes that in the actual blasting project,choosing a reasonable variable line charge density structure can make the release of explosive blast energy more uniform to efficiently and thoroughly use explosive power to improve the iron ore crushing effect.

Computational multiscale methods for granular materials

The fine-scale heterogeneity of granular material is characterized by its polydisperse microstructure with randomness and no periodicity. To predict the mechanical response of the material as the microstructure evolves, it is demonstrated to develop computational multiscale methods using discrete particle assembly-Cosserat continuum modeling in micro- and macro- scales,respectively. The computational homogenization method and the bridge scale method along the concurrent scale linking approach are briefly introduced. Based on the weak form of the Hu-Washizu variational principle, the mixed finite element procedure of gradient Cosserat continuum in the frame of the second-order homogenization scheme is developed. The meso-mechanically informed anisotropic damage of effective Cosserat continuum is characterized and identified and the microscopic mechanisms of macroscopic damage phenomenon are revealed. c 2013 The Chinese Society of Theoretical and Applied Mechanics. [doi： 10.1063/2.1301101]

Assessment of two quasi-static approaches to mimic repeated impact response and damage behaviour of CFRP laminates

Full impact damage tolerance assessment requires the ability to properly mimic the repeated impact response and damage behaviour of composite materials using quasi-static approximations.To this aim,this paper reports an experimental investigation evaluating two quasi-static methods for mimicking repeated impact response and damage behaviour of Carbon Fibre Reinforced Polymer(CFRP)composite laminates.In this study,an 8.45-J single impact was repeated 225 times and mimicked with 225 times 6.51-J quasi-static(energy equivalent)indentations and with 225 quasi-static(force equivalent)indentations following the recorded impact peak force variation.Results show that the loading rate and the inertial effect are the two major factors affecting the responses of the composite laminates under out-of-plane concentrated loading.Both the energyand force-equivalent quasi-static indentations failed to reproduce the impact responses greatly associated with high loading rate and inertial effect.The force-equivalent quasi-static indentations were performed in a semi-automatic way and induced damage states more similar to those of the repeated impacts than those of the energy-equivalent quasi-static indentations,whereas the latter can be better automated and has better reproducibility compared to that of the repeated impact responses,as it is less dependent on high loading rate and inertial effect.

Low-velocity impact response and infrared radiation characteristics of thermoplastic/thermoset composites

The low-velocity impact response and infrared radiation characteristics of composites have rarely been focused on simultaneously.This study aims to investigate the low-velocity impact response and infrared radiation characteristics of the glass fiber reinforced thermoplastic polypropylene and carbon fiber reinforced thermosetting epoxy resin laminates wildly used in the aircraft industry.The impact tests were conducted at five energy levels.Characterization parameters such as impact load,displacement,and absorbed energy were measured.The damage evolution and damage modes of the laminates were analyzed through active and passive thermography,ultrasonic C-scan,and optical microscope.The results indicate that Thermosets(TS)laminates exhibit better impact resistance,while Thermoplastics(TP)laminates show higher delamination ductility,and the maximum contact force of TP laminates is much smaller than that of the TS laminates under lowvelocity impacts,but the low bending stiffness and low ductility of the TP matrix cause the difference in energy absorption level between the two not significant.The temperature characteristic changes of passive infrared thermography heat maps could characterize the damage mode of the laminates.The correlation between the heat maps and the impact characteristic curves is explained;the fluctuation of the impact characteristic curves is directly related to the hot spot characteristics changes of the heat maps.More frequent curve fluctuations correspond to a larger and brighter hot spot on the heat map,which peaks at the maximum impact load after the impact force versus time curve fluctuation cutoff point,the maximum center displacement of the impact force versus displacement curve,and the maximum absorbed energy of the absorbed energy versus time curve.