On the characterization of mechanical properties of alumina ceramics for high speed application

• Main Author: Wang, Zhiyong
• Other Authors: Li Peifeng
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Materials::Ceramic materials; DRNTU::Engineering::Materials::Testing of materials
• Online Access: https://hdl.handle.net/10356/65531
• Institution: Nanyang Technological University

Ceramics have been widely used under impact conditions due to the notable features such as light weight and high compressive strength. The high speed applications require better understanding of mechanical properties of ceramics at various length scales. This work aims to characterize the mechanical properties of alumina ceramic from microscopic up to macroscopic scales. Dynamic experiments and finite element simulations were developed to investigate the underlying deformation and fracture mechanisms of alumina. The dynamic response of alumina ceramic was determined in a modified split-Hopkinson pressure bar test. The analysis of the wave signals included wave dispersion correction and a direct analytical solution of elastic wave propagation in the specimen to align the measured signals. It was found that equilibrium is established after approximately five wave propagation cycles within specimens, thus supporting the assumption of stress field uniformity. Post-test scanning electron microscopic examination reveals that the failure modes of alumina are strain rate sensitive. A predominant intergranular fracture can be observed at low loading rate, while a combination of intergranular and transgranular fractures can be found in the dynamic test. To support the observations from the quasi-static test at the grain level, micro-cantilever beams with equilateral triangle cross-section were subjected to a low rate load at the free end. A 3$D$ FE model of polycrystalline alumina was established based on the experimental observations. The simulation results show that the localized stress distribution is affected by the material orientations. The crack initiates at the bottom of the cantilever beam at the vicinity of the fixed end, and then propagates along the grain boundaries to the top. Finally, an analytical model was developed to describe the penetration behavior based on quasi-static spherical cavity expansion theory. A damage evolution function was proposed in the crack zone. The analytical solution shows that the penetration resistance is influenced by both material properties and damage evolution parameters. The penetration resistance and penetration velocity can be obtained for the process of penetrating the thick ceramic material by long rod projectiles. This work demonstrates both theoretical and practical importance on the understanding of the behavior of ceramic materials, especially at different length and time scales. The consideration of deformation and failure mechanisms over a wide range of strain rates ensures that the development of a physically realistic constitutive model is suitable for engineering application.