The phenomenon of the same fracture spacing exists extensively in seismic and geological field

which belongs to strain localization.The phenomenon can not be predicted and explained by classical macroscopical continuum theory.In the paper

the strain gradient term is introduced into the yield function to analyze the phenomenon of the same fracture spacing.The model of strain gradient theory is established in plane strain state.The field of plastic strain in rock specimen is analyzed on the condition of horizontal stress acting on the edges of the specimen.It is found that the distribution of plastic strain exhibits the phenomenon of the same spacing.The pattern and the magnitude of the distribution of plastic strain are influenced by the size of specimen

the compressive stress acting on the edges of the specimen

and the softening modulus

etc.The present conclusion agrees with some previous experiments.The gradient plastic theory can approximately describe the phenomenon of the same fracture spacing.The present discussion offers a new thinking that contributes to further investigate fracture spacing.