The effect of complex inclusion geometries on fracture and crack coalescence behavior in brittle material

Abstract

This research study investigates the cracking processes in a brittle material associated with inclusions of varying shape, orientation and materials. Specifically, this study summarizes a series of uniaxial compression tests on gypsum specimens with varying inclusion materials, shapes and pair configurations using high speed imagery to determine cracking behavior. The inclusions in the study consisted of differing materials, of contrasting Young's Modulus (higher and lower than the matrix), shapes (hexagon, diamond, ellipse), and relative pair orientations (bridging angle). In addition, single ellipse inclusions were tested to investigate the cracking behavior associated with an ellipse inclusion in a brittle material. Similar to previous research regarding the coalescence of cracks propagating from inclusion pairs, the inclusion material did not affect the coalescence patterns. The coalescence behavior trended from indirect or no coalescence, to direct shear coalescence, to combined direct tensile-shear coalescence as the inclusion bridging angle was increased, similar to past studies on circular and square inclusion pairs, as well as flaw pairs. An analogy was proposed relating the debonded inclusion interfaces to corresponding flaw pairs to compare coalescence behavior. Although the general coalescence trends regarding the effect of bridging angle on inclusion pairs were comparable to those of flaw pairs, the coalescence based on a debonded interface representation did not appear to be similar to that of corresponding flaw pairs. Along with previous work conducted by the MIT rock mechanics group, this research provides detailed experimental observations regarding both the cracking and coalescence behaviors of inclusions in a brittle material.