Fracture process zone : microstructure and nanomechanics in quasi-brittle materials

Abstract

Cracks begin (and end) at a crack tip; the "Fracture Process Zone" (FPZ) is a region of damage around the crack tip. The context of this research is the FPZ in quasi-brittle materials, which is characterized by cracking at various scales. This study focuses on crack propagation and FPZ development at a fundamental material scale: the scale of the grain. With regard to the FPZ, the study seeks to understand how the FPZ develops and manifests in quasi-brittle material, what the physical and mechanical structure of the FPZ is, and how pre-existing material microstructure influences the developed FPZ. The attainment of several research objectives marks the course of the investigation: the development of a multi-disciplinary technique to assess both intact and FPZ regions of quasi-brittle material, the assessment of the fundamental properties (microstructure, small-scale mechanical properties) of intact and FPZ quasi-brittle material, and a conceptual model of FPZ development in quasi-brittle material. In pursuit of these objectives, the study uses nanoindentation to probe the nanomechanical properties of the FPZ for two marbles of varying grain size, and microscopy to probe the structure of the FPZ at the grain scale. The marbles are from Carrara, Italy (typical grain size 300 m), and Danby, Vermont (typical grain size 520 m). Grids of nanoindentations and microscopy were placed within the FPZ regions of Danby and Carrara marble specimens. Both marbles exhibited lower nanomechanical properties near the crack tip and/or near the area of future wing-crack formation, i.e. the FPZ. However, the Danby marble exhibited this trend over a larger distance, and thus nanomechanically supports the increase of the FPZ with grain size. The microscopy investigations suggested increased microcracking near FPZ regions, and increased microcrack density with decreased grain size. Ultimately the study provides four contributions to the study of fracture of quasi-brittle materials: an algorithm for the automatic assessment of microcracking from ESEM micrographs, new nanomechanical information on the two marble types, validation of the use of nanomechanics as a tool for identifying damage in quasi-brittle materials, and a quantitative assessment of the role of grain size in the damage of quasi-brittle materials.