Study On The Relationship Between Type ? Size Effect And Strain Rate Effect Of Quasi-brittle Materials

Energy transformation plays a fundamental role in the process deformation and fracture of rock.The study and determination of energy consumption and its relationship with strength in the process of rock failure will be helpful for understanding the change of rock strength and the essential characteristics of the failure as the whole under the dynamic loading.In this study,the Hopkinson pressure bar device(SHPB)was used to dynamically load semi-Brazil disc rock specimens.The effects of loading rate,length-to-diameter ratio of specimen and specimen size on dynamic characteristics of rock materials such as dynamic initiation toughness,dynamic propagation toughness,fracture energy and kinetic energy were studied.The main results are as follows:(1)At present,kinetic energy analysis in dynamic fracture energy analysis,is still lacking.Combined with high-speed camera technology,the kinetic energy analysis data is carried out.The analysis results show that kinetic energy can not be ignored under medium and high strain rate loading conditions.(2)When the size of the test piece is constant,the dynamic toughness,the absorption energy,the kinetic energy and the fracture energy increase with the increase of the loading rate.It shows significant rate effect,and the larger the specimen size is,the more significant the rate effect is.(3)When the loading rate is constant,the dynamic initiation toughness increases with the increase of the aspect ratio of the test piece,and the absorpted energy decreases with the increase of the aspect ratio.The influence of the aspect ratio on the kinetic energy is quite weak.(4)In order to avoid the interaction between the size and strain rate of the test piece under multi-size conditions,the impact energy factor is introduced.The analysis shows that under the single-size condition,the loading rate and the impact factor can represent the dynamic loading very well,but under multi-size test piece conditions the impact factor has the advantage of being more intuitive and objective.(5)When the impact factor is constant,the dynamic toughness,absorpted energy and kinetic energy increase with the increase of the size of the test pieces;the dynamic iniation toughness increases with the increase of the sample size;The change of absorpted energy and kinetic energy decrease with the increase of the size of the test piece.(6)By improving the available model,it is found that the size of the dynamic crack tip fracture process zone increases with the increase of the impact factor;and as the sample sizeincreases,the size of the fracture process zone increases.But when the test pieces size is further increased,the area of the fracture process zone remains approximately unchanged,that is,it is no longer affected by the size of the test pieces.