Fracture and Toughening of Brittle Structures with Designed Anisotropy

Citation

Brodnik, Neal Ryan (2020) Fracture and Toughening of Brittle Structures with Designed Anisotropy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ET0C-MK61. https://resolver.caltech.edu/CaltechTHESIS:11122019-171331857

Abstract

Despite good thermal and chemical properties, the use of ceramic materials in structural applications is limited by their inherently brittle nature. Efforts have been made to improve the toughness of ceramics through composite design, but recent developments in net shape processing such as additive manufacturing have significantly expanded this design space. Where composite topologies and morphologies were previously limited by material composition and thermodynamics, tools like 3D printing now allow for the design of composite structures of nearly any shape or arrangement.

This work seeks to understand how these processing advances might be utilized to improve the toughness of brittle composites by exploring how previously inaccessible anisotropic inclusion structures might influence fracture behavior. The study begins with the evaluation of printed photopolymer structures as model brittle materials. First, printed structures are used to explore how elastic contrast between inclusions and matrix can affect crack propagation and improve toughness. Here, anisotropy presents an opportunity to achieve similar toughness to isotropic structures at smaller volume fractions by virtue of topologies that only exhibit toughening only in a singular direction, but require significantly less material to do so. Next, the effect of anisotropic voids is explored as a means of controlling crack nucleation and growth. With consideration of both compliance and directional propagation, a "fracture diode" that exhibits controlled, predictable fracture 100% of the time can be realized.

After exploring brittle polymers, ceramics systems with similar toughness and higher stiffness are considered. First, a model layered system of mica is explored, where wedge splitting can be used achieve stable crack growth. This allows for the evaluation of how changes in compliance can improve the interlayer toughness without directly interacting with the crack. Finally, this study extends further into ceramics by exploring silicon oxycarbide (SiOC) truss structures and truss elements produced from 3D printed preceramic polymers. In addition to considering the material itself, changes in truss structure are explored as a means of changing deformation mode, and by consequence, failure strength. These model experiments suggest that if trusses are compatible, they can be interchanged to control failure of the bulk structure.

This study demonstrates how designed heterogeneities with anisotropic structure can be used to both enhance the toughness of brittle composites as well achieve a greater degree of control over both crack nucleation and propagation in brittle systems where predicting failure is otherwise difficult. Looking forward, new processing tools like additive manufacturing present major opportunities for expanding the design space of brittle composites to achieve higher toughness and better fracture control than previously available. These new techniques may be able to expand the mechanical viability of ceramics, and make them better suited to mechanically demanding applications in the future.

Thesis Files