| The mechanical properties of rock materials under the dynamic load are the basic parameters in the research of the propagation and attenuation law in rock mass under explosion and earthquake loading. In the last decades, domestic and foreign scholars have researched the rock constitutive characteristics under impact load and stress wave pulse propagation and boundary effect in rock mass from various angles.With the rapid development of rock mechanics, especially the damage mechanics introduced into this area, the research results of the damage evolution of the rock micro and microscopic cracks are closer to the reality. However, the rock is a complex medium, there are a lot of internal impurities, pores, cracks and micro structures, and yet the natural rock mass have joints, fractures and weak structural surfaces. As result, the combination of the rock complexity on the micro scale and the rock mass inhomogeneity and discontinuous on a macroscopic scale, makes the research of the mechanical properties of the rock medium, especially the study of dynamic ones, far behind from the research of metal materials, which research is mature both in breadth and depth. Therefore the study of the parameters of stress wave how to influence the rock material damage and destructive law has significant meaning to the construction of the national defense and economy.Through the improved impact test apparatus, this paper aim to research the granite injury threshold and the damage form, intending to find a quantitative relationship and the damage evolution equation between the stress wave parameters and rock damage, and further establishing the full damage constitutive equation, so as to correctly understand the rock damage mechanism under the stress wave, scientifically evaluate the stability of the rock mass and improve the efficiency of rock breaking, and eventually provide some theoretical basis and engineering guidance.A comprehensive analysis of the testing technology on conventional SHPB experimental system, the principle of data processing and large diameter SHPB device applied in the rock material conditions have been made which determined the specimen length-diameter ratio of granite in experiments. And the conclusion that realizing constant strain-rate loading can effectively solve the stress uniformity and the dispersion effect of the large diameter SHPB device applied to rock-like materials were also obtained. Conical-cylindric bullet evolved from the two-specimen method is an effective method to realize the constant strain rate loading was determined by means of researching on pulse shaping technique. Two principles the bullet should be followed were established through the theory of stress wave and numerical calculation which followed as, on one hand, the diameter ratio between small end-face and large end-face should be larger than 2/3. On the other hand, the loading wave rising edge time should meet the condition that the time of the stress wave propagated back and forth in the specimen three to six times, according to the calculation for the purpose of this article the loading wave rising edge of granite time should be greater than 23.67μs was adopted. Numerical analysis of different shape parameters was made by means of numerical calculation for the total length of 400 mm conical-cylindric bullet, it is concluded that the optimal diameter ratio between small end-face and large end-face is 0.7, the diameter of the end faces were 50 mm and 35 mm respectively. Further determined the bullet shape parameter to realize constant strain-rate loading of granite, the optimal cone length is 300 mm. Based on the matching principle of wave impedance, the al-mg alloy compression bar system was designed and processed. From two aspects, numerical simulation and experiments, by compared with the traditional cylindrical bullet loading waveform, the designed al-mg alloy conical-cylindric bullet could achieve the constant strain-rate loading of granite was verified.Doing impact test on 180 cube specimens by using the self-designed aluminum-magnesium alloy bullets and then the result will be statistical analyzed with the ranging impact speed from 2.6m/s to 16.3m/s by the tolerance of 0.5m/s. Analysis shows that the speed of each section have more than two pieces of granite specimen which provide guarantee for the accuracy of the test data. Through the study of the statistical analysis of the reflection wave, the concerned wave accounted for 77%. Thus shows that this experiment is in a excellent situation and the design is verified by the self-designed bullet and the constant strain rate loading had been succeed implemented. By doing analyzing the dynamic characteristics of granite, the strain rate take on a linear increasing relationship between the impact speed. Peak strain increase exponentially with the increasing strain rate and impact velocity. In a word, the granite specimen strength enhanced with the increasing impact speed. Granite is a strain rate sensitive material, and enhanced strength value of granite is 1.01 ~ 1.53. According to the impact damage form, granite specimens started to crack when the strain rate level come to 80 s-1. The specimens will be broken when the strain rate level come to 80~110. The specimens will be smash when the strain rate level exceed 110 s-1. The value of the damaged peak strain is 9000 micro strain according to the corresponding macroscopic observation. Specimen appeared transfixion crack when it comes to 9000 ~ 12000 micro strain and it was broken when it exceed 12000 micro strain. Granite longitudinal wave velocity were measured before and after the impact with ultrasonic testing technology. Granite’s damage value was represented by the changing speed of sound. the quantitative relationship between the damage and stress expression was established by fitting. And it is concluded that the damage occurred stress threshold is 50.15 MPa, at this time the corresponding strain rate is 31.7 s-1, the peak strain is 3670 micro strain.Through the comprehensive analysis of Z-W-T viscoelastic nonlinear constitutive model, and combined with granite static uniaxial compression stress-strain curve is linear in elastic phase and the characteristics of granite failure strain may be smaller, the nonlinear part of the Z-W-T constitutive was replaced by the linear. As the impact on granite test was achieved at constant strain rate loading, the Maxwell body of Z-W-T constitutive model was simplified. According to the strain rate range of the Hopkinson pressure bar, the low frequency Maxwell body of Z-W-T model was instead by elastomer. Based on the above three simplified model, the damage variable was introduced into the simplified constitutive model, and combined with the measured damage threshold of granite built the sub-type granite damage constitutive model, then the parameters of the model are determined by the experimental data. The established constitutive model and measured granite constitutive curves with different strain rate are in good agreement through the analysis which indicates that the granite constitutive model can accurately describe the dynamic mechanical properties of the pre peak.The model is extended to three-dimensional stress state by using the theory of viscoelasticity, and the incremental expression of 3D constitutive model is derived, further using FORTRAN language to compile the established constitutive model, then using ABAQUS finite element software secondary development user subroutine VUMAT material subroutine built granite dynamic model material subroutine, and apply this constitutive subroutine in the simulation of SHPB impact compression and shock splitting test of granite, the results show that the established constitutive model can describe the dynamic mechanical properties of granite peak very well. |
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