Damage characterization of red sandstones using uniaxial compression experiments

This paper proposes calculation formulae for damage variables that are characterized by four methods based on acoustic emission (AE), crack volume strain, a damage statistic constitutive model, and dissipation energy. Damage variables characterized by the four abovementioned methods are positively correlated to each other. An obvious inflection point exists in the curve of the damage variable characterized by AE and the strain, and the damage strength point is located before the inflection point. Both the high and low frequencies of the AE signals increase after the damage strength point, and the high-frequency signals mainly appear after the damage strength point, indicating that the damage strength point is located before the starting point for the formation of a large quantity of cracks. No obvious inflection point exists in the curve of the damage variable characterized by the crack volume strain and the strain; the curve is approximately a straight line before the peak stress point, and the damage strength point is located after the starting point of the approximate straight-line segment. Damage variable initially changes very slightly and begins to increase nonlinearly before the damage strength point; the curve is approximately a straight line after the damage strength point, and no obvious inflection point exists in the curve. Damage variable fluctuates slightly before the damage strength point and increases sharply in an approximately linear manner after the damage strength point. The damage strength point is located near the inflection point at which the damage variable begins to increase sharply, and the dissipation energy is the most effective method to identify the damage strength point. The value of the damage variable characterized by the crack volume strain is the largest whereas that characterized by the dissipation energy is the smallest. The value of the damage variable characterized by the crack volume strain is highly consistent with that of the damage variable characterized by AE, indicating that the crack volume strain is the most closely related parameter to AE.