Experiments of strain localization processes of coal specimens in uniaxial compression based on the digital image correlation method

To further reveal the strain localization processes of coal specimens in uniaxial compression,the authors re- cord the images of a surface of each specimen by use of a camera. The digital image correlation method is used to ob- tain strain fields. The authors arrange monitored lines orthogonal each other according to positions of clear strain locali- zation bands. The relation between the variation coefficient of the maximum shear strain and the longitudinal stress is established. The study found that ① the subset size and spacing of monitored points do not have an influence on the developing tendency of the relation above;② the distribution of the maximum shear strain in the normal direction of the strain localization band is not strictly symmetric,which is related to heterogeneities of coal,and the maximum shear strain in the tangential direction is not uniform,reflecting the strain localization band propagation;③ with an increase of the longitudinal strain,the thicknesses of strain localization bands become narrow and the profile of the maximum shear strain in the normal direction becomes steep;④ the rapid increase of the variation coefficient is due to the fact that unclear strain localization bands are converted into clear bands,which corresponds to the ratio of the longitudinal stress to the uniaxial compressive strength of 0. 56.