Study of Strain Rate and Thermal Damage of Dholpur Sandstone at Elevated Temperature

Excerpt

With the increasing demand of natural resources and its existence at higher depth, rock mechanics is facing a completely new challenge of rock mass behavior at higher temperature. The majority of rock masses for instance found in nuclear waste burial sites, geothermal energy extraction, enhanced and natural geothermal system and underground development of deep mine, oil and gas storage cavern encounters high temperatures and therefore rock strength and deformation characteristics under high temperature need to be studied in detail. During the last few years, some investigations have been carried out in this field (Alm 1985; Brede 1993; Alshayea et al. 2000; Xi 1998; Chen et al. 2005; Liu et al. 2001; Verma et al. 2010, 2013). Under the influence of high temperature below melting point of sandstone, its micro-structures may be re-arranged, consequently new micro-cracks develop due to the pre-existing ones which become widespread and widen. Meanwhile, various physical and petrographic changes take place in the boundary of minerals matrix. After cooling down to air dried temperature, thermally induced changes become permanent to some part of matrix of sandstone. The effect of temperature on the physical and mechanical properties of rock like sandstone has become an important area in rock mechanics. The results of this study will be helpful for restoration and redesign of fire-damaged sandstone buildings and monuments. Wide scale application of building sandstone material can be seen indifferent monuments, temples, and buildings in India, i.e., the Red Forts of Delhi and Agra, Places and buildings in Fatehpur Sikri (Cramer 1986; Anna et al. 2013; Shi and Jinyu 2014; Qiu-hua et al. 2007; Yang et al. 2015). In recent years, however, the behavior of rock is very complex under thermo-mechanical (TM) coupling (Dwivedi et al. 2008; Hudson et al. 2005; Zhou et al. 2011) with measurements of basic physico-mechanical parameters; including modulus of rock deformation, Poisson’s ratio, tensile strength, compressive strength, cohesion, internal friction angle, viscosity and thermal expansion coefficients etc. with temperature(Heuze 1983; Lau et al. 1995; Du et al. 2004; Wu et al. 2005; Zhang et al. 2007; Johnson et al. 1978; Malkowski et al. 2012; Vishal et al. 2011). In this paper, the mechanical properties of Dholpur sandstone from upper Bhander subgroup has been cooled down after heating and then variation of mechanical and thermal damage with temperature have been carried out. Therefore, these results do not always replicate the necessary characteristics of rock masses at high temperature. In this paper, sandstones were collected from five locations Fig. 1. Rock samples were cored and NX size rock specimens were prepared as per ISRM (1979) standard. Average mechanical properties, stress–strain behavior, peak stress, peak strain, strain rate and elastic modulus of Dholpur sandstone of five locations, each location having nine rock specimens with temperatures ranging from air dried to 950 °C were obtained and a thermal damage equation is proposed. This paper investigates at continuum damage a mechanics law which describes brittle-plastic behavior of Dholpur sandstone above critical temperature. In particular, thermal sensitivity of the mechanical properties related to rock behavior such as strength, brittle-to-ductile transition, strain hardening and softening and dilatancy, etc. are dealt with at elevated temperature (Byerlee 1978; Hueckel and Baldi 1990; Dong and Shan 1999; Zhang et al. 2014). The results indicated that the temperature had great influence on macro and micro fracture mechanism of Dholpur sandstone, and that large plastic deformation occurred at higher temperature. The mechanical properties of Dholpur sandstone was studied using universal testing electro-hydraulic system and programmable high temperature furnace up to 1000 °C. The rock specimens were then heated in a high temperature furnace, which were 123 mm in length and 50 mm in diameter with L/D ratio of approximately 2.46. 45 specimens were divided into five groups and rock specimens in each group was heated to temperatures 25, 100, 250, 450, 550, 650, 800, 850, 950 °C individually.

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