Some problems in the paper---- Haversian cortical bone model with many radial microcraks: An elastic analytic solution.

Dear everyone,

Recently I have read the paper--Haversian cortical bone model with many radial microcraks: An elastic analytic solution. In this paper, I can't understand the dislocations Burgers vectors (bt and bn), the following words and the corresponding expressions(6a) and (6b). The paper is attached.   

Now, if it is assumed that bt and bn are continually distributed

over the microcracks, the shear and normal stresses could be

formulated as (6a) and (6b).

 Waiting for your help. Thanks.

 Sincerely yours,

Xu Wang