On Relating Physical Damage to Mechanical Response in Advanced Ceramics

Abstract

This thesis is concerned with the experimental mechanics of brittle failure in advanced ceramics. It seeks to bridge global damage accumulation to local failure phenomena. Alumina serves as the primary model material. Other advanced ceramics (silicon carbide and boron carbide) and cermets are included for the purposes of comparison against alumina. Microstructural features such as grain boundaries, grain sizes, inclusionary bodies, and internal pores and cracks are obtained for alumina and compared to mechanical responses. For all brittle materials, experiments used to probe the mechanical response and failure behaviors include quasi-static compressive testing, Kolsky pressure bar dynamic compressive testing, and impact testing. Failure behaviors are captured using ultra-high-speed imaging, which also allows for the determination of 2D strain fields via digital-image-correlation. With 2D strains, contributions derived from this research are made to better model damage evolution in all materials during loading and catastrophic failure. Damage evolution is expanded to include the changes to axial-lateral response as well as stress-strain response. Experiments demonstrate non-linear and non-monotonic changes in behavior. Further refinement of mechanical response tracking demonstrates the importance of shear modulus to compressive failure. Internal local phenomena like crack volume changes can be inferred from global phenomena, such as simultaneous changes in apparent Young’s modulus and Poisson’s ratio. This thesis constitutes a major contribution to the field of mechanics of brittle materials, in general, and advanced ceramics, in particular. The understanding of physical failure processes is greatly improved, along with which characteristics are of significance to dynamic fracture and failure.