Deformation damage characteristics and constitutive model of raw coal and briquette with central hole under asymmetric loading

Under the influence of topographic fluctuation or engineering excavation， some tunnels and mining roadways are often subjected to different degree of asymmetric loading. In order to analyze the influence of asymmetric load on the deformation and damage law of coal and rock， the raw coal and briquette coal with central holes were used as the research object， the full⁃field non⁃contact strain measurement approach of Digital Image Correlation (DIC) was employed for the real⁃time evaluation of the progression of strain⁃field localization and damage evolution of the coal specimens， and the damage and failure process of coal and rock under different loading modes was analyzed. The dam⁃ age variables were defined based on the statistical index， and the damage con⁃stitutive model was established to re⁃ flect the full stress and strain characteristics of coal and rock. The results show that the unsymmetrical load will not change the phased characteristics of the stress⁃strain curves of raw coal and briquette coal samples and the stress⁃strain curves of them still show the initial compaction stage， linear deformation stage， plastic yield stage and post failure stage. The peak stress and localized starting stress of raw coal and briquette coal samples decrease linearly with the increase of the load asymmetry degree. The greater the degree of load asymmetry， the more likely the coal sample is to be damaged. During uniform load compression， the central hole can be regarded as the initial damage of the coal sample. In the compaction stage， the raw coal is mainly characterized by the compression closure of the pri⁃ mary fracture and the deformation of the central hole， and the high strain value is scattered in the whole area. The bri⁃ quette sample is mainly characterized by the reduction of inter particle pores and the deformation of central hole caused by the relative dislocation of internal particles， and the high strain value is concentrated in the central hole. Under uniform load compression， the porous raw coal mainly occurs near⁃x⁃type shear failure， showing the hard brittle characteristics. The briquette samples are mainly subjected to tensile⁃shear composite failure， showing soft plastic characteristics. Under the action of asymmetric load， the existence of the shear action zone at the interface between loading area and non⁃loading area causes the macro shear failure of raw coal and briquette coal. The strain localization zone can well reflect the location of the main control crack on the surface. The forms of the strain lo⁃ calization band and the main control crack are affected by the relative position of the loading area and the central hole， the existence of the central hole will guide the evolution direction of the strain localization zone， and the guiding effect is controlled by the strength of coal and rock. The greater the strength of coal and rock， the smaller the influence of central hole on the forms of the strain localization band and the main control crack. The estab⁃ lished damage constitutive model can better reflect the characteristics of each stage of stress⁃strain curve of raw coal and briquette coal in laboratory test.