Influence Mechanism Of Grain Size On The Strain Rate Dependence Of Strength And Fracture Parameters Of Sandstones

The rock parameters exhibit an obvious loading rate correlation,which is closely related to the mesoscopic structure of the rock.The grain size as an important parameter can reflect how the rock can be formed from the mesoscale.Three kinds of sandstone from Jurassic formation of Xiaojihan Coal Mine,Shanxi province in China,were used to study the strength and fracture parameters and discuss the dependence of loading rate,and it is of great value to theoretical research.The author first characterized the particle distribution of the selected sandstone samples,and then used the integrated method of physical tests,mesoanalysis technology and numerical simulation to study the mechanical behaviors of three kinds of sandstone under multiple strain rates and analyze the influence of grain size on the rate dependence of strength and fracture toughness.This paper includes:(1)The creep tests of three kinds of sandstones were carried out under strain rates in the range of 10-8 s-1 to 10-5 s-1 using the rock rheology-impact test machine designed by NEU.Then,the uniaxial compressive tests were conducted by the servo-controlled machine under the quasi-static loadings in the range of 10-5 s-1 to 10-3 s-1.According to the followed results,the influence of grain size on the rate dependence of compressive strength can be analyzed when the strain rate is lower.(2)The dynamic compressive and Brazilian tests of three kinds of sandstones were carried out to measure the dynamic strength under intermediate strain rate using the experimental apparatus of the split hopkison bar(SHPB)driven by a pendulum hammer.Moreover,the combined methods of optical technology and numerical simulation were used to analyze the effect of grain size on the crack evolution under dynamic loadings,which can explain the mechanism of the influence of the grain size on the rate dependence of strength.(3)The physical test and numerical simulation of three point bending were conducted to analyze the fracture process of sandstones with three kinds of grain size and discover the influence mechanism of grain size on the fracture toughness.Fractal theory was used to characterize the particle distribution from the mesoscropic quantitatively and further build the relationship between fictitious growth length and grain size.Combined with the previous fracture model,the fracture toughness of sandstones with different grain sizes can be calculated.(4)The application of the application of the modified fracture model on the calculation of fracture toughness of semi-circle bend(SCB)samples were confirmed by the comparative analysis of the stress field at the pre-cracked tips of straight-cut bending samples and SCB samples.And then SCB samples were used to conduct the dynamic fracture tests of three point bending and calculate the fracture toughness with modified fracture model proposed in this paper.