Research On Expansion, Deformation, Instability And Fracture Of Metal Ring And Cylindrical Shell Under High Strain Rate Loading

In this dissertation,the explosively driven expanding ring and electromagnetical-ly driven expanding ring experimental platforms were used to investigate the expanding fracture phenomenon of metal rings and cylindrical shells.The Doppler Pins System,high speed camera and scanning electron microscope were adopted to diagnose the expanding fracture processes.In order to accurately describe the dynamic mechanical behavior at uniform expansion stage,the instability stage and the fracture stage in the process of the expansion fracture processes of metal rings and metal cylindrical shells,the influence of initial loading process on the deformation behavior of materials,the instability phenomenon associated with the strain rate,fracture mode transition and fracture characteristics were studied.The main work and conclusions are as follows:1.The plastic deformation behaviors of metal rings and cylindrical shells under high strain rate loadinga)A new method considering the deformation inhomogeneity of shock stage is developed,which can obtain the stress-strain relation with more accuracy.b)A method based on the expanding ring experiments is proposed to obtain the constitutive equation under constant strain rate.Based on this method,the constitutive equation of 20#steel between the strain rate 3.0 × 103 s-1-5.5 × 103 s-1 is obtained.2.Plastic instability of expanding rings under high strain ratesa)The constitutive equation and loading history co-dominate the critical instability strain.b)A new method for obtaining the critical instability strain under different load strain rates is developed.The results show that the critical instability strain rate of 7075A1 ring increases with the increase of load strain rate.When the critical instability strain rate is greater than 1000 s-1,the instability strain tends to be constant.c)The Doppler pins system with frequency modulation is used to accurately obtain the velocity rebound of expanding ring,and two kinds of velocity oscillation phenomena are explained.3.Fracture behaviors of metal rings and cylindrical shells under high strain rates loadinga)Firstly,the concept of failure,fracture and fragmentation of materials and structures are systematically discussed.Secondly,The concept between fracture of a material and fracture of a structure are distinguished.Based on these understandings,the fracture strain in the ring and cylindrical shell is decomposed reasonably,which can be used to describe the different characteristics of the expansion fracture at different stages.b)The non-continuous fracture phenomenon of one-dimensional ring is studied theoretically,and the method of using average fracture strain and local fracture strain is developed to describe the one-dimensional ring fracture.The study shows that the average fracture strain is the superposition of the continuous change in the uniform deformation stage and the non-continuous amount of N after the instabil-ity stage.However,the local fracture strain is only related to loading strain rate and stress history,and has no direct correlation with the number of fragments.c)The results show that fracture strain of TU1 rings and cylindrical shells increases with the increase of strain rate.Near the strain rate of about 104 s-1,fracture patterns change,and at the same time,reduction of cross section area changes from 100 to 102.Under this strain rate,the fracture mode is dominated by diffusion necking,while at higher strain rates,and the fracture mode is mainly dominated by the sheet necking.d)In order to describe the dynamic fracture behavior of materials under high strain rate,in addition to considering the loop average strain as the fracture strain,it is necessary to introduce the reduction of the area of the section describing the degree of necking.so that the fracture characteristics can be more completely described.