Effects Of Annealing On Spallation Properties Of High-purity OFHC

Spallation is one of the typical tensile fractures under dynamic loading. When a compression shock is introduced into a material by a high explosive or plate impact, the interaction of rarefaction waves can generate a tensile pulse. If the magnitude of the tensile exceeds a certain value, the process of spallation can begin. The spallation process in the ductile metals is of multiple scales, including microscopic, mesoscopic and macroscopic scales. It is widely accepted that the mechanism for spallation in materials is controlled by the nucleation of defects, the growth, and eventually the coalescence of these defects to form a rupture surface within the material. If voids and defects nucleate and grow, but not coalesce leading to full separation, that is called incomplete spall. When voids and defects link together, the completed spall comes forth. Current researches showed a clear correlation between the process of spallation and many factors, such as:material properties, loading conditions and so on. On the bases of previous studies, a series of experiments were conducted to study the effects of annealing on spallation properties of high-purity OFHC copper at incomplete spall and complete spall in this thesis. The main work and innovation points of this thesis are summarized as follows:1. In the thesis, multi-points measurement technique was used to measure the free surface velocities of the different regions of the sample. Multi-points measurement was designed by array and circle Doppler Pins System (DPS); it can measure the velocities in a spatially and temporally resolved manner. At the same time, a new equipment which could drive a planar flyer via network detonation technique to generate a low impact velocity was designed. The velocity and planarity of the flyer and other equipment parameters were analyzed in detail by experimental measurement and numerical simulation. It can be used to conduct the spallation experiment. It can not only measure the free surface velocity but also recycle the sample loaded by shock wave.2. One-dimensional strain impact experiments were performed for the annealed and unannealed OFHC (99.99%) by the devise of network detonation driving and one-stage gas gun. With the impact velocity varying from114m/s to336m/s, the rear free surface velocity profiles have been measured with the multi-points DPS. The features of the free surface velocities of two series of experiments have been analyzed. Results indicate that there are some different in velocities between the annealed and unannealed OFHC. The pullback velocity of the annealed OFHC is higher than the unannealed one’s, and the spall strength for completed spall too. The processes of recovery, recrystallization and grain growth will be happened on annealing, we conjecture that processes will make the density of dislocation of crystals, defects between crystals and others down, leading to the nucleation points decreasing under the tensile stress. So the annealed OFHC will be harder to spall than the unannealed. Besides, the elimination of work hardening and the decrescence of residual stress after annealing will also affect the process of spallation.3. Velocities that were got from multi-points DPS measurement have been analyzed. The results indicate that there are significant differences at different points of the same sample. Analyses conclude that this phenomenon might be relevant to the anisotropic properties of the material, such as heterogeneousness of grain sizes, grain orientations, impurities, micro-defects, etc. Experiments suggest that the pullback velocity is not a single-valued function, and spall strength for completed spall samples too. By fitted to Weibull failure statistics, the results indicate a similar mean value and variability for the Pullback velocity and the spall strength for OFHC. On the one hand, with the increase of impact stresses, the Weibull modulus of pullback velocity statistics also increases gradually. The Weibull modulus "m" is large for a uniform material failure response and small if there is a wide range of spall properties. So the variability of incomplete spall is larger than the complete spall in pullback velocity. In addition, the higher the impact stress is, the larger the pullback velocities will be. On the other hand, compared the unannealed OFHC with the annealed one, it suggests that the disperse of spallation properties of the former is smaller than the latter, that’s to say, the process of spallation of the unannealed OFHC is more uniform than the annealed one.