Connections between micro/nano scale heterogeneity of mechanical properties of coals and their propensity to outbursts and crushing

The outbursts of coal and gas is one of the main coal mining hazards, therefore, for the coal mining industry, studying of mechanisms and predisposing factors for these events is of the utmost importance. It is demonstrated here that the micro/nano scale structure of coal samples is one of predisposing factors for the coal propensity to outburst. The same is related to the coal propensity to crushing and formation of fine powder (dust). The results of micro/nanoindentation experimental studies of heterogeneity of spatial distribution brittleness and mechanical properties of coals at micro/nano scales are presented for samples taken from both hazardous (outburstprone) and non-hazardous strata (packs) of the same coal seam. The experiments were performed on both ‘as received’ coal samples and ones after sorption treatment by dimethylformamide. The latter treatment allowed to partially discharge the internal stresses that exist in the coal samples. The mapping the indentation results enabled us to reveal the actual heterogeneity of distribution of mechanical properties at nanoscale. It has been confirmed that hardness of coals at microand nanoscale is not an informative parameter for characterization of their propensity to destruction. It was established that higher heterogeneity of stiffness could be a reason to formation of multiple cracks at coals after microhardness tests. The part of energy spent for the irreversible changes in the material structure within the total work of indentation is the parameter indicating clearly the propensity of coal samples to crushing and formation of fine powder (dust). Coal samples from the non-hazardous packs have a low ratio of inclusions prone to irreversible changes of the structure and those prone to keeping their structural integrity, while the ratio is about a unity for samples from the hazardous packs. Thus, there is a natural distinction of the mechanical properties between two coal samples having similar origin and rank but different in their proneness to instantaneous outbursts

An inverse problem for adhesive contact and non-direct evaluation of material properties for nanomechanics applications

We show how the values of the effective elastic modulus of contacting solids and the work of adhesion, that are the crucial material parameters for application of theories of adhesive contact to nanomechanics, may be quantified from a single test using a non-direct approach (the Borodich-Galanov (BG) method). Usually these characteristics are not determined from the same test, e.g. often sharp pyramidal indenters are used to determine the elastic modulus from a nanoindentation test, while the work of adhesion is determined from a different test by the direct measurements of pull-off force of a sphere. The latter measurements can be greatly affected by roughness of contacting solids and they are unstable due to instability of the load-displacement diagrams at tension. The BG method is based on an inverse analysis of a stable region of the force-displacements curve obtained from the depth-sensing indentation of a sphere into an elastic sample. Various aspects related to solving the inverse problem for adhesive contact and experimental evaluation of material properties for nanomechanics applications are discussed. It is shown that the BG method is simple and robust. Some theoretical aspects of the method are discussed and the BG method is developed by application of statistical approaches to experimental data. The advantages of the BG method are demonstrated by its application to soft polymer (polyvinylsiloxane) samples