Macroscopic And Mesoscopic Trans-scale Failure Mechanism In Polyacrylonitrile Fiber Reinorced Cement Backfill

To implement the development concept of " Clear waters and green mountains are as good as mountains of gold and silver " and to promote the goal of "emission peak and carbon neutrality".This study,supported by the National Natural Science Foundation of China(No.51764013)and Natural Science Foundation for Distinguished Young Scholars of Jiangxi Province(No.20192ACBL21014),investigated the damage mechanism of polyacrylonitrile fiber tailings cemented backfill(P-FRB)from macro and mesoscopic scale levels,with a view to providing a reference for further research on the mechanism of fiber to backfill material enhancement.Cemented filling technology is an important means to reduce tailings pollution,achieve comprehensive utilization of industrial solid waste and promote sustainable development of the mining industry.Ordinary cement tailings backfill(CPB)can hardly cope with the complex stress environment under deep mining operation conditions,and there are certain safety hazards in the mining process,such as collapse of the mining area and subsidence of the roof.In this study,the initial microstructure,fine damage mechanism and macroscopic damage mechanism of P-FRB are investigated through indoor tests and theoretical analysis,revealing the mechanical lifting mechanism of polyacrylonitrile fiber on the backfill material,defining and dividing the fiber pullout process,and discovering the acoustic emission damage precursors of P-FRB for the first time.The study results can provide a reference basis for the adoption of P-FRB support in mines,and also provide theoretical support for promoting the resourceful recycling of mine solid waste.The main work of this study is as follows.(1)The initial microstructure observation and initial pore distribution determination of P-FRB specimens were performed by scanning electron microscopy technique and nuclear magnetic resonance technique.The backfill material inevitably produces a large number of initial pores and fractures during the hydration and hardening process,and these initial defects are the basis for the expansion of the backfill fractures,which have a significant connection with the mechanical properties of backfill.The study shows that the polyacrylonitrile fiber can effectively optimize the initial microstructure of the backfill material,reduce the initial pores and fractures of the backfill,and lay the foundation for the improvement of the mechanical properties of P-FRB specimens.(2)Uniaxial compression tests were conducted on P-FRB specimens with different cement tailing ratios using MTS electronic universal testing machine,and the mechanical enhancement mechanism of polyacrylonitrile fibers on the backfill material was discussed.The polyacrylonitrile fibers inhibit the expansion of internal fracture development in the backfill through the crackblocking effect and bridging effect,thus enhancing the compressive strength and post-peak damage toughness of the backfill,while the crack-blocking effect and bridging effect of the fibers depend on the fiber pull-out process.In this study,the fiber pull-out process is defined and divided into three stages: microslip stage,slip stage and detachment stage.(3)The acoustic emission response characteristics and acoustic emission damage precursors of P-FRB specimens during different damage processes were compared and discussed through acoustic emission monitoring tests.It was found that the acoustic emission parameters of the CPB specimens without fiber doping showed a decreasing trend before the damage,while the acoustic emission parameters of the P-FRB specimens showed a surge after the specimens reached their stress peaks under the effect of fiber cracking resistance.In addition,there is a significant relationship between the ratio of the backfill material and its acoustic emission parameters: the acoustic emission energy parameters of CPB specimens and P-FRB specimens increase with the decrease of cement tailing ratio.(4)The macroscopic failure mechanism and surface strain characteristics of P-FRB specimens were studied by digital image correlation(DIC)method.The research shows that the polyacrylonitrile fiber can effectively reduce the lateral displacement of the crack monitoring point on the surface of the backfill,and make the P-FRB specimen maintain good integrity after failure.By analyzing the DIC strain cloud images of the CPB and P-FRB specimens,it was found that the red strain concentration area of the CPB specimen showed a significant tendency,while the P-FRB specimen had a higher degree of dispersion in the red strain concentration area under the action of polyacrylonitrile fibers.