Study On The Feature Of Nucleation And Growth Of Damage In Ultrapure Aluminum Under Dynamic Tensile Loading

Under dynamic loading, tensile stress will be produced due to the rarefaction wave interaction inside the material and tensile fracture in materials will start from the microvoids or cracks caused by tensile stress. The location and quantity of macrovoids or cracks are closely related to microstructure of the material, such as dislocations, grain boundaries and inclusions. In-depth understanding of mechanism and laws of the microstructure dynamic evolution, exploring and establishing a failure theoretical model with predictive ability which can connect different scale, achieving the transition from “observation” to “control” for the material, not only is necessary for raising the level of material research and design, but also is significant for improving the strength and performance of defense equipment and civilian materials.The microstructure can affect the damage evolution behavior of materials intensively. Grain boundary is the most important part of microstructure. Previous studies stated that voids or cracks in the materials were easy to appear on grain boundaries and expand along them, and the grain boundaries were relatively weak microstructures in the materials. In our work, polycrystalline samples and double-crystal samples are cut from the rolling aluminum bar. one-dimensional strain impact experiments were carried out in a light gas gun for ultrapure aluminum samples.Soft-recovered samples have been analyzed by metallographic microscopy,electron back scattering diffraction(EBSD) and synchrotron radiation x-ray tomography technology, the feature of nucleation and growth of damage in ultrapure aluminum under dynamic tensile loading were analyzed. The main work and innovation of the thesis are summarized as follows.1.Plate-impact experiments were conducted to study the features and mechanisms of void nucleation and growth in the pure aluminum under dynamic loading. Soft-recovered samples have been analyzed by metallographic microscopy,electron back scattering diffraction(EBSD) and synchrotron radiation x-ray tomography technology. It was found that most of the void nucleation in grains was near the boundaries of “weak-orientation” grains and grew toward the grain boundaries with fractured small grains around the boundaries. This was mainlycaused by the accumulation and interaction of slip systems in the “weak-orientation”grains. In addition, the micro voids were nearly octahedron because the octahedral slip systems were formed by 8 slip planes in the polycrystalline of pure aluminum.The EBSD results are agree with the three-dimensional structure observed by synchrotron radiation x-ray.2.We characterize spall damage in shock-recovered ultrapure Al with metallography and x-ray tomography. The measured damage profiles in ultrapure Al induced by planar impact at different shock strengths, can be described with a Gaussian function,and showed dependence on shock strengths. Optical metallography is reasonably accurate for damage profile measurements, and agrees within 10%-25%with x-ray tomography. Full tomography analysis showed that void size distributions followed a power law with an exponent oft ssnn??0( t??2.05.1), which is likely due to void nucleation and growth, and the exponent is considerably smaller than the predictions from percolation models.