Mechanical, structural and scaling properties of coals: depth sensing indentation studies

This paper discusses special features of mechanical behaviour of coals discovered using depth-sensing indentation (DSI) techniques along with other traditional methods of material testing. Many of the special features are caused by the presence of multiscale complex heterogeneous internal structures within the samples and brittleness of some coal components. Experimental methodology for studying mechanical properties of coals and other natural extreme materials like bones is discussed. It is argued that values of microhardness of bituminous coals correlate strongly with the maximum load; therefore, the use of this parameter in application to coals may be meaningless. For analysis of the force-displacement curves obtained by DSI, both Oliver–Pharr and Galanov–Dub approaches are employed. It is argued that during nanoindentation, the integrity of the internal structure of a coal sample within a small area of high stress field near the tip of indenter may be destroyed. Hence, the standard approaches to mechanical testing of coals should be re-examine

The estimation of the kinetic parameters of low-temperature coal oxidation

Coals self-heating reasoned by their oxidation processes often could cause spontaneous combustion that may occur during all the stages of coal production, storage, transportation and utilization. The steady-state method enables determination of the critical ambient temperature, above which spontaneous combustion occurs, as a function of the reactor’s size. These critical ambient temperatures are used to calculate the kinetic constants of oxidation coal. The transient method (or Chen method) is applied to directly estimate the rate of oxidation by determining the crossing point, when the thermal conduction term near the center of a cylinder becomes zero. A modified method is proposed for determining the kinetic constants of coal oxidation from the steady-state heating temperature at the center of the reactor. The method is based on the dependence of the dimensionless temperature on the Frank-Kamenetskii parameter. The kinetic constants have been calculated from the results of a numerical experiment with a cylindrical reactor

Evaluation of elastic modulus and hardness of highly inhomogeneous materials by nanoindentation

The experimental and numerical techniques for evaluation of mechanical properties of highly inhomogeneous materials are discussed. The techniques are applied to coal as an example of such a material. Characterization of coals is a very difficult task because they are composed of a number of distinct organic entities called macerals and some amount of inorganic substances along with internal pores and cracks. It is argued that to avoid the influence of the pores and cracks, the samples of the materials have to be prepared as very thin and very smooth sections, and the depth-sensing nanoindentation (DSNI) techniques has to be employed rather than the conventional microindentation. It is shown that the use of the modern nanoindentation techniques integrated with transmitted light microscopy is very effective for evaluation of elastic modulus and hardness of coal macerals. However, because the thin sections are glued to the substrate and the glue thickness is approximately equal to the thickness of the section, the conventional DSNI techniques show the effective properties of the section/substrate system rather than the properties of the material. As the first approximation, it is proposed to describe the sample/substrate system using the classic exponential weight function for the dependence of the equivalent elastic contact modulus on the depth of indentation. This simple approach allows us to extract the contact modulus of the material constitutes from the data measured on a region occupied by a specific component of the material. The proposed approach is demonstrated on application to the experimental data obtained by Berkovich nanoindentation with varying maximum depth of indentation

The estimation of the kinetic parameters of low-temperature coal oxidation

Coals self-heating reasoned by their oxidation processes often could cause spontaneous combustion that may occur during all the stages of coal production, storage, transportation and utilization. The steady-state method enables determination of the critical ambient temperature, above which spontaneous combustion occurs, as a function of the reactor’s size. These critical ambient temperatures are used to calculate the kinetic constants of oxidation coal. The transient method (or Chen method) is applied to directly estimate the rate of oxidation by determining the crossing point, when the thermal conduction term near the center of a cylinder becomes zero. A modified method is proposed for determining the kinetic constants of coal oxidation from the steady-state heating temperature at the center of the reactor. The method is based on the dependence of the dimensionless temperature on the Frank-Kamenetskii parameter. The kinetic constants have been calculated from the results of a numerical experiment with a cylindrical reactor

Indentation of bituminous coals: fracture, crushing and dust formation

Bituminous coals are still the main source of energy in the world. However, these brittle porous materials are prone to crushing under action of industrial tools. Our early depth-sensing nanoindentation tests of bituminous coals showed that even if the depth of indentation is within the nanoscale, these brittle coals are no longer continuous elastic media within the indentation zone but rather fine powders of crushed particles irrespectively to the coal maceral. Dust formation of the materials is a problem of great practical importance for the mining industry. Indeed, the powders of coal particles formed during crushing may cause not only explosions in mines but they also contaminate the environment around the roads of the coal transportation. In this study, the crushing of coals due to action of rigid conical indenters and formation of small coal particles is investigated. The studies are based on development of the Galanov-Grigoriev (GG) model of crushing of brittle porous materials and the model adjustment to specific features of coal fracture. Using the indentation tests, we estimate the size distribution of the dust particles formed within the region of fully crushed material. Results are presented for coals of three different stages of metamorphism

Evaluation of elastic modulus and hardness of highly inhomogeneous materials by nanoindentation

The experimental and numerical techniques for evaluation of mechanical properties of highly inhomogeneous materials are discussed. The techniques are applied to coal as an example of such a material. Characterization of coals is a very difficult task because they are composed of a number of distinct organic entities called macerals and some amount of inorganic substances along with internal pores and cracks. It is argued that to avoid the influence of the pores and cracks, the samples of the materials have to be prepared as very thin and very smooth sections, and the depth-sensing nanoindentation (DSNI) techniques has to be employed rather than the conventional microindentation. It is shown that the use of the modern nanoindentation techniques integrated with transmitted light microscopy is very effective for evaluation of elastic modulus and hardness of coal macerals. However, because the thin sections are glued to the substrate and the glue thickness is approximately equal to the thickness of the section, the conventional DSNI techniques show the effective properties of the section/substrate system rather than the properties of the material. As the first approximation, it is proposed to describe the sample/substrate system using the classic exponential weight function for the dependence of the equivalent elastic contact modulus on the depth of indentation. This simple approach allows us to extract the contact modulus of the material constitutes from the data measured on a region occupied by a specific component of the material. The proposed approach is demonstrated on application to the experimental data obtained by Berkovich nanoindentation with varying maximum depth of indentation