Experimental Study on Crack Evolution Characteristics of Banded Magnetite Quartzite During Fracture Instabiligy Under Freeze-thaw Action
To investigate the impact of freezing and thawing on the crack evolution characteristics during the fracture instabiligy processes of banded magnetite quartzite,mechanical and acoustic emission tests were conducted on the rock subjected to a temperature range of-20～20℃and a maximum of 280 freeze-thaw cycles.The results indicate that the uniaxial compressive strength and modulus of elasticity decreased from 200.93 MPa and 21.67 GPa in the dry state to 106.64 MPa and 8.24 GPa after 280 freeze-thaw cycles,the reduction in strength and modulus of elasticity exhibited a tendency to stabilize during the later stages of freeze-thaw cycles,resulting in the establishment of a new dynamic equilibrium between the skeletal structure of the rock samples and their internal microcracks.Additionally,under conditions of low freeze-thaw cycles(defined as≤40 cycles),the evolution of cracks in the rock samples primarily involved the development of tensile and shear cracks,with crack rupture predominantly occurring during the accelerated expansion phase of microfracture.The high freeze-thaw cycle(defined as exceeding 40 cycles)significantly influences the crack evolution process in rock samples,predominantly resulting in tensile cracks,while shear cracks are less prevalent compared to those observed in samples subjected to low freeze-thaw cycles.Notably,as the number of freeze-thaw cycles increases,there is a discernible trend toward a reduction in the overall cracking of the rock samples.Furthermore,during the accelerated expansion phase of microfractures in rock samples exposed to low freeze-thaw cycles,high and medium frequency signals emerge slightly earlier than their medium and low frequency counterparts,the amplitude associated with the high-frequency bands was elevated.During the phase of accelerated microfracture expansion in rock samples subjected to extensive freeze-thaw cycles,there was a simultaneous emergence of middle and high-frequency signals alongside middle and low-frequency signals.Furthermore,as the freeze-thaw cycle period increased,the amplitude corresponding to the ultra-high frequency in the rock samples progressively diminished.Under the influence of a low number of freeze-thaw cycles,high-energy signals predominantly emerge during the accelerated expansion phase of microfractures,characterized by a more concentrated distribution and increased frequency.Conversely,with a higher number of freeze-thaw cycles,the distribution of high-energy signals becomes more dispersed,and their frequency diminishes as the number of cycles increases.These signals are observed throughout the entire loading process.The rock samples subjected to a low number of freeze-thaw cycles exhibit fewer microfractures during the compression stage,the elastic deformation stage,and the microfracture development stage,with no apparent correlation to the location of fracture aggregation at the point of rupture.In contrast,for rock samples exposed to a high number of freeze-thaw cycles,there is a significant relationship between the cracks formed during the initial three stages and the locations where cracks aggregate at the time of rupture.The range of acoustic emission energy and the likelihood of high-energy acoustic emission events in rock samples subjected to a high number of freeze-thaw cycles were reduced compared to those subjected to a low number of freeze-thaw cycles.This observation further suggests that rock samples experiencing fewer freeze-thaw cycles are primarily characterized by the development of large and mesoscale fissures.In contrast,those subjected to more frequent freeze-thaw cycles are predominantly influenced by the expansion,merging,and fusion of localized fissure networks.
万方数据
维普
