The Ballistic Resistance Of Double-Layered Metallic Target And The Deformation & Fracture Of Taylor Rod

Due to the objective existence of local conflicts and terrorist attacks, high speed impact dynamics has recently been one of the most active research fields in explosion mechanics. The design of shield structures with high ballistic resistance and penetrators with high penetration ability is the final aim of the research in this field. Metallic plates are widely used in construction of military and civilian ballistic protective structures. In recent years, the possibility of replacing a monolithic target with a double-layered one to obtain a safer, more lightweight ballistic protective structure has been reported. What should be noticed is that both projectile and target should be included in the research of high speed impact dynamics. In the sub-ordnance impact velocity range, projectiles are often suffered from deformation, fracture and even fragmentation as more and more advanced materials are used to build protective components. Investigation on the ballistic resistance of double-layered metallic targets (DLMTs), deformation and fracture behavior of kinetic energy rods would provide a reference for the attack and protection design.At present, the understanding on ballistic resistance of DLMTs is far from systemic, although there have been some relevant reports. Especially, both the ballistic resistance of DLMTs with large gap space and the ballistic resistance of dual hardness DLMTs are poorly known. At the same time, few reports can be found in the open literature on either the deformation and fracture of Taylor rod or the fracture mechanisms of Taylor impact fracture. Thus, the deformation and fracture modes of Taylor rod is not clear enough.Based on the background mentioned above, this dissertation focused on DLMTs and kinetic energy rods. The mechanical behavior of 7A04-T6 aluminum alloy, the ballistic resistance of DLMTs with large gap space and dual hardness DLMTs in the subordnance impact speed regime, and the deformation/fracture behavior as well as the fracture mechanisms were investigated by conducting experiments and numerical simulations.As the base of the numerical simulations in this dissertation, the mechanical behavior of 7A04-T6 aluminum alloy was investigated, and both a constitutive relation and a fracture criterion were built. By using a universal testing machine and a torsion testing machine, the quasistatic mechanical behavior of 7A04-T6 aluminum alloy from room temperature to 250℃was obtained. The effect of strain rate on the yield strength and fracture strain was calibrated by numerical simulations on several Taylor impact tests. Experiments and numerical simulations showed that both the strain and strain rate hardening effect of 7A04-T6 aluminum alloy is moderate while the fracture strain increases with temperature and decreases with stress triaxiality and strain rate. Based on the test result, a modified J-C constitutive relation and a modified J-C fracture criterion were built. Model parameters were calibrated from the test data and with the help of numerical simulation.For the ballistic resistance of DLMTs, the ballistic limits of DLMTs in contact and with large gap space, and dual hardness DLMTs were investigated. Firstly, blunt projectiles were fired against Q235 steel DLMTs in contact and with large gap space, from which the ballistic limit of either DLMTs was driven. The mechanisms of the effect of large gap space on the ballistic resistance of DLMTs was revealed by analysis on the high speed photography and the recovered targets as well as numerical simulations. Experiments showed that there are two ballistic limits for DLMTs with large gap space due to the existence of two typical impact scenarios, and that the overall ballistic resistance of DLMTs in contact is superior to that of DLMTs with large gap space. Numerical simulations showed that finite element method can not only predict target deformation and fracture patterns in accordance with experiments but also give close predictions on the ballistic limit. Especially, numerical simulations proved the effect of the two impact scenarios on the ballistic resistance of DLMTs with large gap space. Afterwards, blunt and ogival projectiles were fired against"Hard+Soft"and"Soft+Hard"DLMTs composed of Armox 560 T steel and Weldox 700 E steel. Finite element model was built for penetration of dual hardness DLMTs by blunt projectiles, and the mechanisms of the effect of hardness configurations on the ballistic resistance was revealed. Experiments showed that both projectiles suffered from fragmentation during the penetration of"Hard+Soft"targets while both projectiles can relatively keep intact, i.e., the ballistic resistance of the former is bigger than the latter. Numerical simulations showed that the two hardness configurations of dual hardness DLMTs result in different fracture region in the blunt projectiles and finally offer distinct ballistic resistance.Deformation and fracture, and the fracture mechanisms of Taylor rods of two ductility regimes were investigated to analyze the deformation and fracture behavior of kinetic energy rods. Taylor impact tests were conducted on projectiles of 7A04-T6 aluminum alloy and 38CrSi steels of different ductility with the same geometric dimensions. Corresponding finite element model was built. In the finite element calculation, fracture criteria with good predictive capacity were found, and the fracture mechanisms of different fracture modes was presented. Experiments showed that with increasing impact velocity less ductile projectiles of 7A04-T6 aluminum alloy and 38CrSi steel suffer from mushrooming, shear cracking and fragmentation, and that high ductile projectiles of 38CrSi steel give birth to mushrooming, shear cracking and petalling. Numerical simulations on Taylor impact tests with 7A04-T6 aluminum alloy projectiles showed that, the modified J-C fracture criterion with corporation of low stress triaxiality cut-off and the C-L fracture criterion can predict deformation and fracture modes observed in the tests, and that the predicted impact speed range corresponding to each deformation/fracture mode is in close agreement with that obtained in experiments. Numerical simulations on Taylor impact tests with 38CrSi steel of high ductility showed that C-L fracture criterion give prediction on deformation and fracture modes in good agreement with the experimentally obtained. Analysis on the typical fractured elements showed that, for projectiles of high ductility, shear cracking is caused by tension-shear while petalling by tension, and that, for less ductile projectiles, the fracture in the central region of the projectile head is caused by compression and compression-shear while shear cracking at the critical velocity resulted in shear cracking is due to pure shear and the cracking in the projectile head periphery is initiated by tension-shear but driven by compression and compression-shear.The result of this thesis on the ballistic resistance of DLMTs with large gap space and DLMTs of dual hardness would provide a reference for metallic target design, while the result on the deformation and fracture modes of Taylor rod as well as the fracture mechanisms would support the design of kinetic energy rods.