Study on Dynamic Mechanical Behavior and Damage Evolution Mechanism of Fiber Reinforced Cemented Tailings Backfill

The backfilling method stands as a widely employed extraction technique in contemporary mining practices, with the performance of cemented tailings backfill (CTB) being pivotal in determining the overall quality of backfilling. Nevertheless, traditional CTB exhibits suboptimal mechanical properties, particularly crack resistance, under complex stress environments. Presently, one of the extensively explored backfill types is the fiber-reinforced cemented tailings backfill (FRCTB), with a particular emphasis on those reinforced with polypropylene fibers. In order to scrutinize the mechanical characteristics of FRCTB under intricate stress states, this study, employing a triaxial Hopkinson pressure bar experimental apparatus, investigates the dynamic mechanical behaviors, fracture damage patterns, and energy dissipation features of FRCTB under five different confining pressures (0, 1, 2, 3, 4 MPa) and various strain rates. Key findings include:

(1) Under dynamic loading, FRCTB exhibits a pronounced strain rate strengthening effect along with a notable confining pressure strengthening effect. The presence of confining pressure significantly alters the stress-strain curve of FRCTB. The peak stress and dynamic increase factor (DIF) of FRCTB linearly increase with the augmentation of both confining pressure and strain rate. The peak strain linearly increases with the strain rate, with confining pressure exerting minimal influence on the peak strain. Confining pressure substantially enhances the elastic modulus of FRCTB, while the impact of strain rate is comparatively marginal.

(2) In the absence of confining pressure, FRCTB specimens, with increasing strain rates, exhibit an outward-expanding conical failure shape. The crack volume and surface area increase in a stepwise fashion, and the hollow cylindrical polypropylene fibers undergo flattened failure. At this juncture, the polypropylene fibers endure a limit strain rate ranging from 206.5 s^-1 to 232.3 s^-1. As confining pressure gradually increases, the outward expansion tendency is progressively restrained until no discernible internal damage occurs. At this point, the polypropylene fibers manifest phenomena such as splitting, bending, and extraction. Under low (no) confining pressure conditions, the fractal dimension and porosity of FRCTB increase with the rising strain rate. In high confining pressure conditions, the fractal dimension and porosity of FRCTB are relatively similar across different strain rates.

(3) At lower confining pressures, the strain rate strengthening effect is evident in both fracture morphology and energy dissipation but diminishes as confining pressure increases. Dissipated energy density exhibits an increasing trend with the rise in confining pressure, while the energy dissipation rate shows a quadratic function decrease with increasing strain rate. The stress-strain curves of FRCTB under no confining pressure and with confining pressure can be categorized into four and five segments, respectively: elastic growth, plastic yield, post-peak energy accumulation, and post-peak failure for the former, and elastic growth, plastic damage incubation, plastic damage development, plastic damage accumulation, and post-peak failure for the latter. Under low confining pressure conditions, the fractal dimension linearly increases with the growth of dissipated energy. This trend gradually transforms into a cubic function change as confining pressure increases. There exists a notable similarity between fractal dimension and energy dissipation rate, as well as between strain rate and confining pressure.