The Studies On The Dynamic Fracture And Fragmentation Of Metal Freely Expanding Ring Driven By Electromagnetically Loading

At present, for metal expanding ring, what are essential to be clarified in the dynamic fracture and fragmentation problems are:â‘ what is difference of fracture and fragmentation mechanisms between brittle and ductile metals;â‘¡what factors do effect on necking instability; the difference of temperature between necking region and uniform region; the thermal softening effect on the necking instability, and whether the micro-voids in necking region is to grow up;â‘¢whether the current fragmentation theories are suitable for describing the fragmentation properties of brittle or ductile metal expanding rings at low and high strain rate;â‘£the physics description of the temperature which arises substantially at fracture surface of fragmentation rings;⑤whether the breakpoint on the ring circumference fracture simultaneously or sequentially, etc. According to these problems, the constitutive properties, fracture and fragmentation mechanisms of metal rings are studies in this paper by using the electromagnetically loading technique.The major researches of this paper include:1) For the theories of necking, fracture and fragmentation: with the established necking analysis model for ductile metal ring, the factors of necking instability are analyzed, and the condition of void growth up and the local temperature rise effect at the hole wall aroused by large plastic shear strain are also discussed. According to the fragmentation properties of brittle and ductile metal ring, the crack opening displacement model (COD) and the multiple-necking tensile fracture model (MNTF) are developed respectively.2) For the electromagnetically loading technique, the principle of electromagnetically loading, radial expansion motion analysis, the theoretical analysis of electromagnetism and calculation of the expanding ring, the design of electromagnetically loading system and the test techniques are also introduced.3) The studies on metal ring radial expansion motion and material constitutive properties mainly include: determining the freely expanding stage of metal rings; measuring the radial displacement or radial velocity; obtaining of flow stress and plastic strain in the metal ring at freely expanding stage; as well fitting constants of material constitutive properties.4) The experimental studies on fracture and fragmentation mechanisms of expanding ring (tube) metals include: fracture mechanisms, fragmentation properties, the explanation of the temperature and luminescence at fracture surface, fracture time sequence of the brittle and ductile metal ring, etc. The approach to attain the tensile-shear stress state of expanding tube is also explored.5) For the motion and fracture of metal expanding ring (tube), the simulation technique of method is put forth, which exerts the Poisson random fracture nucleation points on ring circumference. The expansion motion, necking instability and the process of fragmentation are analyzed for M-state TU1 and L04 rings, and the simulation of expanding tube's tensile-shear fracture was also carried out.The main conclusions obtained in this paper are:1) The theoretical analyses of necking:â‘ the larger material strain-hardening exponent is , the necking instability more difficult to initiate;â‘¡the larger stain-rate exponent is, the necking instability easier to initiate;â‘¢necking instability occurs more easily where the site has initial structural defect;â‘£within 10 to 10⁴ s^-1 strain rate range, the larger initial strain rate is, the larger initiating necking instability strain is;⑤the temperature of necking region is higher than that of uniformly deformed region;â‘¥the initial heating temperature is higher, the larger initiating necking instability strain is, and initiating necking instability time is the twice function of the initial heating temperature;⑦the void growth condition in necking might be described by using circumferential deviatoric stress criterion of the P-αspherical void model, and large plastic shear strain at the void wall can generate localized temperature rise.2) Theoretical analyses of fragmentation:â‘ the COD model is more suitable for describing the fragmentation properties of brittle metal ring;â‘¡the MNTF model is more suitable for describing the fragmentation properties of ductile metal ring, and the arrested necking formation condition is given, and the fragment size and number estimate formula are derived, which is in good agreement with experimental results.3) The power-law constitutive constants of M-state TU1 and L04 are obtained respectively. The simple hardening constitutive constants of G50 steel are also obtained.4) The fracture and fragmentation of metal rings experimental analyses:â‘ the fracture mechanism of brittle metal ring, such as Y-state TU1, is typical crack-opening pattern fracture, and residual part at fracture surface forms high resistance, and the temperature of fracture surface arises substantially (exceed 1 551 K) by residual induced current heating and luminous phenomenon occur, and the fragment properties can be well described with COD model;â‘¡the fracture mechanism of ductile metal ring, such as M-state TU1 and L04, is typical multiple necking fracture, and local temperature rise at fracture surface is generated by the large plastic shear strain at void wall (up to 1 636 K), and the fragment properties can be well described with MNTF model, and the fragment length distribution of L04 rings at the same strain rate is approximate to Weibull distribution. During the fragmentation of rings, both brittle and ductile metal have different fracture time sequence.5) The numerical simulation method which exerts the Poisson random fracture nucleation points on ring circumference can reasonably analyze the expansion motion, necking formation and fracture process of metal ring, and can be used to make fragmentation statistics.Innovation points in this paper are:a) Use the electromagnetically loading technique to implementation one-dimensional tensile stress loading for brittle and ductile metal expanding ring, and from experiment to theory establish a set of relatively complete experimental tool, and carry out constitutive property, fracture mechanism and fragmentation property research of four kinds of metals by using this tool;b) On the necking theory of expanding ring: the theoretical analysis model of necking is established; necking instability influence factors are analyzed which takes account for the material strain-hardening exponent, stain-rate sensitive exponent, initial strain rate and thermal softening effect etc.; and the temperature difference between necking and uniformly deformed region as well as the relation between initial heating temperature and necking instability initiating strain and time. The temperature rise in necking region has not been reported in detail up to now;c) On the fragmentation theory of metal expanding ring: firstly to suggest that the COD model in current fragmentation theory is suitable for describing the fragmentation properties brittle metal expanding ring, but not suitable for ductile metal ring, and put forth a MNTF model to describe that of ductile metal expanding ring;d) In the investigation of expansion fracture process of metal rings by high-speed frame photography, firstly observe luminous phenomena of two kinds of fracture mechanisms at fracture surfaces, and explain the luminous phenomena by using the crack opening to form local high resistance model, and from metallographic observation, observe the growth and coalescence of voids in the necking region and the melt phenomenon at void wall, and the fracture sequence for metal ring is studied;e) Firstly use random fracture nucleation points' method to analyze the expansion motion, necking to form and fragmentation process, and compare with experiment result and theoretical analysis result, which shows that the method of exerting random fracture nucleation points on ring circumference is reasonable.