Stress-strain curve of brittle rock has been well studied in the past fifty years. Researches have been focused on the imperfect loading induced by loading machine and the imperfectly machined rock specimen. In this study, uniaxial tests of brittle shale from Longmaxi Formation were conducted. Nonuniform loading induced by flatly seated upper loading platen was analyzed by comparing the data from two linear variable differential transformers. Particle flow code 2D was used to simulate the bedding down region of stress-strain curve and the distortion of loading platen. A new explanation about bedding down region of stress-strain curve was proposed. Conclusion shows that: (1) The simulation results indicated that the distortion of load train leaded to a non-uniform loading that was not ignorable. (2) Both experiments and simulation results showed that besides the compression of the pores inside the rock, the ‘bedding down’ region in stress-strain curve could also be induced by the contacting process between the upper loading platen and the imperfectly machined rock specimen. The study provides a better understanding on the uniaxial/tri-axial tests of brittle geo-materials.

1. Introduction

Stress-strain curve of rock compression test has been well studied in the past fifty year. Stress-strain curve was first separated into type I and type II in 1970 by Wawersik and Fairhurst (Wawersik and Fairhurst, 1970). Plenty of uniaxial and triaxial rock test were conducted by Wawersik and Brace, 1971 to study the post-peak failure. Suggested method for the complete stress-strain curve for intact rock in uniaxial compression was compiled by Fairhurst and Hudson (1999). This method is being widely used in rock compression testing and is also a guidance for design of testing machine.

The nonlinearity of the initial phase in stress strain curve of compression test shows as an increase in tangent modulus as the loading increases. The modulus of the initial phase in stress-strain curve is called initial modulus (Fjar et al., 2004). This nonlinearity phenomenon was first interpreted by Bieniawski (1967) as ‘closure of micro cracks’. Since then, the interpretation of ‘closure of micro cracks’ was accepted by many other researchers (Li et al., 1998). The bend of stress-strain curve in its initial phase could also be found in most related literatures (Wawersik and Fairhurst, 1970; Rummel and Fairhurst, 1970; Hudson et al., 1972) involving stress-strain curve of cock compression test. It was named ‘bedding down’ region by Fairhurst and Hudson (1999). The dynamic response was also investigated by Chen et al. (2018, 2019). In this paper, bedding down region is studied in terms of interaction between upper loading platen and rock specimen, and an alternative interpretation was proposed.

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