Session: 13-11-02: Friction, Fracture, and Damage II

Paper Number: 166880

Crack-Parallel Stress Effect on Fracture Properties of Eagle Ford Shale Identified From Gap Test 

In recent years, interest in the study of shale rocks has increased significantly due to their crucial role in various energy fields including oil and gas production, CO2 sequestration and geothermal energy extraction. Of particular importance to field applications is the fundamental understanding of fracture properties of shale rocks through the study of crack initiation and propagation. Previous studies demonstrated that the measured fracture toughness of shale according to the Linear Elastic Fracture Mechanics (LEFM) vary with the shape and the size of the specimens. This size and shape dependence of the LEFM fracture properties suggests a quasi-brittle fracture behavior in shale characterized by a non-negligible size of the fracture process zone (FPZ), thus violating the LEFM assumption that the fracture occurs in one mathematical point. Using the nonlinear fracture mechanics approach, in particular the Bažant's Size Effect Law (SEL), the quasi-brittle fracture properties such as the initial fracture energy and the effective fracture process zone length can be uniquely identified as material properties. However, a recently developed fracture test, called the gap test, reveals that in concrete, composites, aluminum and probably all quasi-brittle materials, the effective mode I fracture energy depends strongly on the crack-parallel normal stress, either in-plane or out-of-plane. The reason why this effect has been neglected until now is that, in all standardized fracture test specimens, the crack-parallel stresses are negligible. The gap test consists of a simple modification of the standard three-point-bend test in which a pair of plastic pads with a near-perfect yield plateau is placed next to the notch mouth to generate a constant crack-parallel compression, and the end supports are installed initially with a gap that closes only after the pads yield. This modification allows the specimen to transit from one statically determinate loading configuration to another, making the evaluation unambiguous. In this research, the gap test is adapted to study the effect of crack parallel stress on fracture of shale. Since the shale is transversely isotropic, the specimens are prepared in three different configurations, referred to as arrester, divider, and short-transverse, in which the notches are aligned with one of three principal orientations with respect to the plane of isotropy. For each configuration, gap tests are performed on specimens with increasing size under various crack-parallel stresses. From the measured peak loads of the gap test, the fracture properties of shale are calculated by using the SEL. The fracture properties are found not only to be anisotropic but also depend strongly on the crack-parallel normal stress.

Presenting Author: Anh Nguyen Northwestern University

Presenting Author Biography: Anh is from Hanoi, Vietnam. He attended SUNY Korea in South Korea where he earned a BE/MS in Mechanical Engineering. At SUNY Korea, he studied configurational mechanics under the supervision of Dr. Y. Eugene Pak. He then joined Professor Zdeněk P. Bažant research group in Spring 2022. His research here focuses on computational fracture mechanics of quasibrittle material.

Authors:

Anh Nguyen Northwestern University
Yang Zhao Northwestern University
Hongshun Chen Northwestern University
Horacio D. Espinosa Northwestern University
Zdenek P. Bazant Northwestern University