Mechanism of anisotropic characteristics of layered limestone and a constitutive model for different bedding angles based on a Brazilian tensile test

Abstract

Layered limestone contains a large number of bedding planes, which leads to the anisotropy of mechanical properties. In this research, a Brazilian tensile test was conducted on limestone samples with seven different bedding dip angles to study the effect of different bedding angles on the tensile mechanical properties. The relationship curves for the horizontal tensile stress and strain were plotted. These curves underwent three stages of compaction, elasticity, and destruction. Furthermore, the variation law of the tensile strength and the tensile modulus with the bedding dip angles were studied, and a directivity index was proposed to quantitatively evaluate both the spatial directionality and the magnitude variability of the tensile strength of laminated rock samples. Additionally, the fracture patterns could be divided into three types, including “perpendicular splitting” in a roughly straight shape (the bedding angle ranged from 0 to 30°), “shearing and splitting” in an arc shape (30 to 75°), and “parallel splitting” in a smoothly straight shape (75 to 90°). The curves of the acoustic emission (AE) ringing counts with the bedding angles of the rock samples exhibited obvious three-stage patterns, and the rock samples had a large variation in the cumulative amplitude of the AE ringing counts because of the anisotropic characteristics. Based on the elastic theory, the anisotropic constitutive model was established to calculate the expression for the axial stress and strain during the test. The theoretically calculated curves could be well matched with the test values. These research results indicated that the tensile strengths, tensile moduli, and fracture patterns of the laminated limestone samples all displayed distinct anisotropy and directionality, which implied the necessity of considering anisotropy in the study of failure mechanisms, the theoretical calculation, and the engineering application of laminated limestone rocks.