Study On The Relationship Between Type Ⅱ Size Effect And Strain Rate Effect

The physical properties of quasi-brittle materials(rocks,concrete)are related to the loading rate of structural members and the size of the components,rather than the ideal values of classical elasto-plastic mechanics.The properties of the quasi-brittle material are related to the structural characteristics of the material and are related to the magnitude of the load applied to the component and the size of the structural specimen.That is,in the time and space dimensions,the physical properties of the material have a certain impact.However,there are few studies on the relationship between structural dimensions and dynamic loads such as rocks and concrete,and the correlation between the two is not adequately studied,resulting in a number of practical problems.For example,the process of structural damage is not sufficient and lead to the design of the structure cannot be more safe and reliable.The type I(no crack sample)size effect and strain rate effect were studied by professor Qi Chengzhi,the theoretical relationship between them,and its dynamic size effect of the physical mechanism were found by professor Qi.In this paper,we focus on the structural level of rock,consider the existence of fracture process in the crack tip,and then use the study of the size effect and strain rate correlation of type I(no crack specimen)of Professor Qi,apply Hu,Duan’s boundary effect formula and study the relationship of type II size effect and strain rate effect.The main research results include:(1)According to the dynamic crack tip stress field and Mohr Strength Criterion,the fracture process zone size of the crack is solved.Based on the stress field of the variable velocity dynamic crack tip and the Mohr Strength Criterion,the shape and size of the crack fracture process zone are solved.It is concluded that the fracture process zone of the crack tip increases with the increase of the strain rate during the crack initiation process.During the crack propagation process,the fracture process zone of the crack tip increases with the crack propagation velocity increase;crack propagation process compared to the crack initiation process,the fracture process area increases the amplitude even greater.(2)Based on the boundary effect formula of Hu,Duan,the relationship between the fracture toughness and the loading rate of the cracked body,the relationship between the propagation toughness and the crack velocity is deduced,and the relationship between the size effect and the strain rate effect is deduced.It is concluded that the fracture toughness of the specimen with cracked quasi-brittle material increases with the increase of the strain rate.During the propagation of the crack,the fracture toughness of the specimen increases with the crack velocity Increase,the propagation toughness of the specimen is greater than the fracture toughness.The theoretical results are in agreement with the experimental results.(3)Based on the energy release rate of the crack tip,the relationship between the size of the fracture zone and the crack velocity is deduced.The relationship between the fracture toughness and the crack velocity is calculated.In the energy angle,the previous theoretical results are validated.That is,with the increase of the crack speed,the increase of the crack length and the energy release rate of the crack tip increases,and it can be found that the area of the fracture process area increases with the crack velocity,and the resistance of the material to crack propagation increases,which shows that the fracture toughness of the specimen increases with the increase of crack velocity.