Mesoscopic Characterization Of Dynamic Damage Of The Different Microstructures Of High Purity Aluminium

The strength and performance of materials in extreme environments are of scientific interest because these two parameters determine a wide spectrum of uses ranging across defense and industrial applications. Understanding the dynamic performance of materials is important, because it helps not only to improve the research and design level of various materials, but also to improve the strength and performance of the civil-defense equipments and civilian materials.The fracture process of materials under dynamic tensile effect is a very complicated physical and mechanical problem. Beginning in the17th century, this problem has been researched scientifically and in the20th century, the theories of fracture has been established systematically.However, up to now, the actual fracture strength of metals is still smaller2or3orders of magnitude than the real one, and we still cannot forecast when and where the material will damage accurately by a theoretical model. The key to solve this problem is to understand the relationship between material microstructure and damage evolution.The microstructure can affect the damage evolution behavior of materials intensively. Grain size and boundary are considered as two important parts of microstructure. Grain size can affect dynamic strength of materials by coordinating the deformation and decreasing the stress concentration. Grain boundaries may act as defects or sources of dislocations, or result in a particular stress concentration through the lattice mismatch, or alert the development of dislocations during compress stage, which can change the mechanical response characteristic of materials. Therefore, it is significant for researching the effect of grain size and boundary on the damage evolution behavior of materials to understand the damage evolution mechanism and laws.In this thesis, one-dimensional strain impact experiments were carried out in a light gas gun for ultrapure aluminum samples from the rolling aluminum bar cut along different orientations. The sample is induced incompletely spall by controlling the loading pressure. Based on the measurement of free-surface velocity profile and the "soft-recovery" of the shocked specimen, the characteristics of damage distribution inside sample have been analyzed in detail. The main work and innovation of the thesis are summarized as follows.1.The samples with same thickness cut along three different directions of a rolling aluminum bar have been impacted by a light gun. During the same experiment, the impact velocities are nearly same and samples are in incompletely spall state. The effect of material microstructure on its strength has been analyzed according to the free surface velocity profile. Under lower impact velocity, the free-surface velocity profile of samples is very different; however, under higher impact velocity, the free-surface velocity profiles nearly have no difference. The effect of material microstructure on the dynamic damage is higher under lower velocity, and when the pressure reaches a critical value, the difference of microstructure will not play a main role during damage evolution of materials.2.Based on the statistical method of damage distribution and the analysis method of material metallographic microscopic, the characteristics of damage distribution and the feature of the nucleation and growth during the origin of damage evolution have been analyzed. Under lower shock pressure, the region of damage in the "cross-cutting" sample is found to be wider than that in the "longitudinal-cutting" sample and the damage coalesces in the shock direction. In contrast, the region of damage in the "longitudinal-cutting" sample is narrow, and damage coalesces in the direction perpendicular to the shock loading. The damage distributions of samples with different cutting directions are almost the same under high pressure. The effect of microstructure on the maximum damage is relatively negligible. In addition, under tensile stress, the microvoid almost nucleates at the grain boundary for all samples, especially at those grain boundaries with larger curvatures. The nucleated voids grow along the grain boundary toward one side of the grain.