| Wave impedance is an inherent property of materials,which reflects the ability of media to prevent waves from passing through.It is an important physical quantity to characterize the dynamic properties of media.At the same time,wave impedance,as a whole characterization parameter of rock lithology and microstructure,can distinguish and reflect the physical and mechanical properties of rock to a certain extent.It is of great significance to study the influence of wave impedance on the dynamic mechanical response of rock in the field of rock breaking and protection.In this paper,combined with the split Hopkinson pressure bar,digital image correlation and acoustic testing,the influence mechanism of wave impedance on dynamic mechanical response of rock under impact load was studied by theoretical analysis and laboratory test.According to the test results,the definition of rock damage variable,the establishment of constitutive model and brittleness evaluation method under impact load were studied based on wave impedance.The main contents and conclusions are as follows:(1)Combined with the basic principle of SHPB and the theory of elastic wave,the function relationship between the non dimensional stress of transmitting(reflecting)wave and the wave impedance of the sample was derived theoretically.The method of equivalent simplification of the incident wave by step was proposed,and the influence of the wave impedance of the sample and the rising time of the incident wave on the transmitting and reflecting wave was obtained.The results show that the smaller the sample wave impedance is,the larger the reflection wave is,and the longer the duration is;the smaller the transmission wave is,and the longer the time required to reach the amplitude is.The influence of wave impedance on the transmission and reflection wave is not affected by the rise time of incident wave.(2)The impact compression tests of cement mortar samples with wave impedance gradient change and granite samples with wave impedance gradient change under different impact velocities were carried out.The dynamic mechanical behaviors of rocks with different wave impedance of the same lithology were compared from three aspects of stress wave propagation characteristics,dynamic stress-strain relationship and impact fracture degree.The impact compression and splitting tensile tests of three kinds of rocks with different wave impedances under different impact velocities were carried out.The dynamic mechanical behaviors of rocks with different lithology and wave impedance were compared from the aspects of stress wave propagation characteristics,dynamic stress-strain relationship,dynamic tensile mechanical properties,dynamic evolution process of surface strain field,energy dissipation law and fracture degree.The results show that the dynamic mechanical response of rock under impact load is affected by both wave impedance and strain rate.(3)Based on the theoretical analysis and experimental results,the influence mechanism of rock wave impedance on dynamic mechanical response in SHPB experiment was studied.The influence mechanism of wave impedance on dynamic mechanical response of rock can be considered from three aspects: material properties,dynamics and energy.For the same lithology,there is a strong correlation between the micro structure and damage deterioration degree of rock and its wave impedance,the smaller the wave impedance of rock is,the more the micro defects are and the greater the degree of damage deterioration is.For rocks with different lithology,wave impedance can be regarded as a whole characterization parameter of solid phase and microstructure,which can reflect the inherent properties of rock materials to a certain extent.Under the action of the same incident wave,the transmission and reflection wave,strain rate history,stress history and transmission(reflection)energy of rock samples with different wave impedances show obvious differences.The smaller the rock wave impedance is,the larger the reflection wave is,the larger the strain rate is,the smaller the loading rate is,and the larger the reflection energy is.There is a progressive relationship among wave impedance,stress wave propagation,strain rate/stress history/reflection energy and dynamic mechanical response.(4)Based on the Taylor model of meso damage mechanics,the functional relationship between P-wave velocity and density of rock was derived.Both the theoretical calculation and the measured data show that the P-wave velocity of rock is closely related to its density,and there is a good positive correlation between the two parameters.It was demonstrated that wave impedance is superior to P-wave velocity in characterizing rock damage degree.The relationship between the degree of rock damage deterioration and its wave impedance was studied by combining the electron microscope and ultrasonic testing technology.The results of microstructure observation and timefrequency analysis of ultrasonic wave signal show that there is a strong correlation between rock damage deterioration and wave impedance.The feasibility of using wave impedance to characterize the damage degree of rock was demonstrated.Furthermore,a new method of defining rock damage variable by wave impedance was proposed,and its rationality was verified by test results.(5)A statistical damage constitutive model based on rock wave impedance under impact load was established,and the relationship between the parameters of the model and rock wave impedance was studied.The results show that: the established constitutive model can comprehensively characterize the strain rate response,initial damage response and damage evolution response of rock materials under dynamic load.It can reflect the stress-strain relationship of rock samples with different wave impedance at different strain rates.The constitutive model contains few parameters and has clear physical and mechanical significance.There are obvious functional relationships between each parameter and rock wave impedance.It can predict the stress-strain relationship of rock at a certain strain rate according to the established constitutive model and wave impedance value.(6)The relationship among rock brittleness characteristics,constitutive model parameters,damage evolution law and wave impedance under impact load was studied.The results show that: the smaller the initial damage degree of rock,the larger the peak value of damage evolution rate and the smaller the strain corresponding to the peak value of damage evolution rate,the greater the degree of rock brittleness.Thus,the evaluation method of rock brittleness based on wave impedance and damage evolution rate under impact load was established.The established evaluation method of brittleness characteristics has certain accuracy and applicability for quantitative comparison of brittleness degree of rock with different wave impedance under impact load,and the established evaluation index is reasonable.(7)The dynamic characteristics of layered composite rock mass under the condition of stress wave "from high wave impedance to low wave impedance" and "from low wave impedance to high wave impedance" were studied comparatively.The mechanical characteristics and strength conditions of composite rock mass were analyzed theoretically.Meanwhile,the failure characteristics of composite rock mass were studied with DIC test system.The results show that: at the same impact velocity,the dynamic characteristics of composite rock mass in the case of the stress wave propagating from the hard to the soft are obviously different from those in the case of the stress wave propagating from the soft to the hard due to the matching relationship of the wave impedance.However,the difference gradually decreases and tends to be zero with increasing the impact velocity.It is also shown that the failure degree and failure form of the two parts of composite rock are obviously different.The damage degree of the red sandstone with relatively small wave impedance is more severe than that of the gray sandstone.Red sandstone is dominated by shear failure,and the red sandstone at the interface layer is damaged after being compared with the red sandstone in other areas.Gray sandstone is mainly tensile failure,local shear failure occurs at high speed,and the gray sandstone at the interface layer is destroyed before the gray sandstone in other areas. |
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