Characterization of Marcellus Shale Fracture Properties through Size Effect Tests and Computations

TL;DR

This study investigates the size effect on Marcellus shale's fracture properties using size effect tests and numerical simulations, revealing the importance of nonlinear fracture mechanics for accurate characterization.

Contribution

The paper introduces a methodology combining size effect tests and Bazant's Size Effect Law to accurately determine fracture properties of Marcellus shale, accounting for anisotropy and size effects.

Findings

Nominal strength decreases with specimen size

SEL effectively fits the size effect data

Fracture properties vary with orientation and size

Abstract

Mechanical characterization of shale-like rocks requires understanding the scaling of the measured properties to enable the extrapolation from small scale laboratory tests to field study. In this paper, the size effect of Marcellus shale was analyzed, and the fracture properties were obtained through size effect tests. A number of fracture tests were conducted on Three-Point-Bending (TPB) specimens with increasing size. Test results show that the nominal strength decreases with increasing specimen size, and can be fitted well by Bazant's Size Effect Law (SEL). It is shown that SEL accounts for the effects of both specimen size and geometry, allowing an accurate identification of the initial fracture energy of the material, Gf, and the effective Fracture Process Zone (FPZ) length, cf. The obtained fracture properties were verified by the numerical simulations of the investigated specimens using standard Finite Element technique with cohesive model. Significant anisotropy was observed in the fracture properties determined in three principal notch orientations: arrester, divider, and short-transverse. The size effect of the measured structural strength and apparent fracture toughness was discussed. Neither strength-based criterion which neglects size effect, nor classic LEFM which does not account for the finiteness of the FPZ can predict the reported size effect data, and nonlinear fracture mechanics of the quasibrittle type is instead applicable.