关键词: 单轴拉伸, 应变软化, 塑性区, 尺寸效应, 破坏过程, 应力-应变曲线, 失稳

Abstract: Due to the fact that there is a great deal of experimental and analytical difficulty in investigating the failure process and the size effect of rock specimen in uniaxial tension, in the present paper these problems are modeled numerically by FLAC. In elastic and strain-softening stages, the adopted constitutive relations of rock are linear elastic and linear strain-softening due to tensile failure, respectively. To ensure that elements subjected to tensile failure are localized into a narrow band within the specimen, two null elements at two edges of specimen are prescribed to model two notches. It is found from numerical results that the complete tensile stress-tensile strain curve exhibits pre-peak and post-peak branches. Prior to the peak stress, tensile stress concentration appears near the two notches so that two elements close to the two notches due to tension yield. The number of yielded elements is increased with axial tensile strain and the new plastic elements are even closer to the center of specimen until a horizontal and longer plastic zone is formed. Tensile stress distribution undergoes three distinct stages: upward concave stage, stage of downward concavity with two peaks and stage of downward concavity with one peak. The first stage corresponds to the elastic stage of complete stress-strain curve; however, the latter two stages correspond to the inelastic stage including strain-hardening and strain-softening stages. Softening branch of complete stress-strain curve becomes steeper so that the instability of rock specimen easily occurs as the height of specimen is increased or the width of specimen is decreased. Peak stress of complete stress-strain curve increases with the width of specimen, which is due to the fact that the stress state within the specimen is not a simple uniaxial state of stress for wider specimen. For thinner specimen, the needed time steps form the initial formation of yielded elements to the full development of a plastic zone are less than those for wider specimen. This reflects that the precursor for thinner specimen is less apparent. Brittle failure without obvious precursor is concerned with seriously natural hazards, such as rockburst and earthquake.

Key words: uniaxial tension, strain-softening, plastic zone, size effect, failure process, stress-strain curve, instability