Characterization of the Microstructure and Mechanical Behavior of Intact and Damaged Alumina

  • Author / Creator
    Lo, Calvin
  • Advanced ceramics such as alumina serve a critical role as a structural material in many applications (e.g., light-weight turbine blades, ultrasonic cutting, and body armor). To improve their performance in these applications, it is important to understand how microstructural features affect the mechanical response of ceramics under loading. X-ray computed tomography (XCT) is well-suited to characterizing the microstructure of advanced ceramics since it can non-destructively visualize micron-scale internal features. By coupling XCT characterization to mechanical testing, this thesis aims to investigate the effects of processing induced pores and internal cracks on the material behavior of intact and damaged alumina.       In the first part of this thesis, XCT characterization is combined with uniaxial compression experiments to study intact alumina, where the major processing-induced defects are pores. Quantitative XCT measurements of pores in AD85 alumina showed low variability across a range of pore characteristics, with median pore size values ranging from 16.0 to 17.2 micrometer over ten samples. In terms of spatial distribution, analytic comparison against randomly distributed points showed that the spacing between pores was highly regular. Voronoi tessellation showed that while spacing is mostly independent of pore size, smaller pores exhibited greater variability in spacing. Mechanical testing results were found to vary depending on strain rate, with greater scatter at quasi-static rates than dynamic strain rates. The coefficient of variation for compressive strength and failure strain decreased from 10.28% and 10.23% at quasi-static to 5.20% and 4.17% at dynamic rates. Based on the low scatter in general pore characteristics between the 10 samples, the differences in variability between quasi-static and dynamic properties are attributed to variability in testing conditions (e.g. misalignment of platens) and the activation of a greater number of pores in dynamic compression.        In the second part of this thesis, a similar approach was used to study the behavior of pre-damaged alumina. XCT was used to characterize internal crack networks in pre-damaged alumina, focusing on crack characteristics such as surface area and crack orientation. Quasi-static and dynamic compression experiments showed that crack damage led to a reduced stiffness and rate of lateral expansion during the early stages of loading. With further compression, elastic properties were found to increase towards intact levels. Shortly before failure, the crack growth was found to increase the lateral strain rate and reduce the stiffness. Digital image correlation (DIC) coupled with high-speed imaging was used to observe the localized deformation mechanisms activated during loading, including lateral crack closure, axial crack opening and closing, and inclined crack sliding. The dynamic compressive strength of the damaged alumina decreased with increasing crack surface area, however, quasi-static strength results did not differ significantly between intact and damaged specimens. In-situ visualization of the dynamic compression experiments revealed that the damaged specimens exhibited a mixed fracture mode that included axial splitting and failure along pre-existing cracks.           In summary, this thesis presents: 1. Improved characterization of microstructural defects through XCT for the design, manufacturing, and modeling of advanced ceramics; and 2. Insights into the evolution of mechanical properties, deformation mechanisms, and fracture behavior of damaged ceramics.

  • Subjects / Keywords
  • Graduation date
    Fall 2020
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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