The effect of bedding on deformation localization and damage constitutive modeling in coal specimens

To reveal the bedding effects on the deformation field evolution of coal bodies containing parallel bedding under static loading, the maximum shear strain field of raw coal specimens under vertical bedding loading and parallel bedding loading conditions was observed by the digital scatter correlation method, and the deformation field evolution and deformation localization characteristics of raw coal specimens containing parallel bedding during loading process under two loading conditions were quantitatively analyzed. A damage variable was defined to describe the damage of coal specimens based on the characteristic statistics, and a damage constitutive model was established to reflect the full stress–strain characteristics of coal specimens with parallel bedding. The results show that the strain field of the parallel-bedding-loaded specimens is patchily distributed during the initial compaction stage, whereas the vertical-bedding-loaded specimens form a deformation-concentrated area at the loading end. Before and after the peak strength, the strain field of the vertical-bedding loading specimen changes dramatically, and the specimen shows shear damage, whereas the strain field of the parallel-bedding loading specimen does not have large changes, and the specimen shows splitting damage, with higher crack development and degree of specimen failure. The initiation stress of deformation localization in the vertical bedding specimens is closer to the peak strength, whereas the parallel bedding specimens are more likely to show deformation localization characteristics. The damage constitutive model based on the characteristic statistics can well reflect the stress–strain characteristics of the raw coal specimens under the loading conditions of vertical and parallel bedding.