where ε ε N , ε ε M and ε ε L are the normal vector and two strain vectors corresponding to each microplane, and N i j = n i n j , M i j = ( n i m j + m i n j ) / 2 and L i j = ( n i ℓ ℓ j + ℓ ℓ i n j ) / 2 where n i , m i and ℓ ℓ i are three mutually orthogonal vectors, one normal and two tangential, characterizing each particular microplane (subscripts i , j = 1 , 2 , 3 refer to Cartesian coordinates).