Issue 29

R Massabò, Frattura ed Integrità Strutturale, 29 (2014) 230-240; DOI: 10.3221/IGF-ESIS.29.20 233 when the reference surface coincides with the mid-surface of the bottom layer, define the generalized displacement components of its points. The third term in Eq. (5a), with summations on the n-1 of interfaces, supply the zig-zag contributions, 2  k , [9,10], which are continuous in 3 x but with jumps in the first derivatives at the interfaces, 0 3 C , and are necessary to satisfy continuity of the shear tractions at the interfaces in plates with arbitrary stacking sequences; the fourth term, with summations on the n-1 interfaces, define a discontinuous field and supply the contribution of the relative displacements (jumps) at the cohesive interfaces. Eq. (5) define a first order model, since the displacements are piecewise linear functions of 3 x . Higher order models have been proposed in the theories in [11,13], which however have no advantages over I order models in the presence of imperfect interfaces, as it was proven in [18]. The linear infinitesimal nonzero strain components at the coordinate 3 x within layer k are derived using Eq. (5):     1 1 22 02 2 2 2 3 2 2 3 3 2 2 1 1               , , , , ˆ k k k j j j j j v x x x v (6a)   1 23 0 2 2 2 1 2          , k k j j w (6b) where the comma followed by a subscript denotes a derivative with respect to the corresponding coordinate. Figure 1 : (a) Composite plate showing discretization into layers, imperfect/cohesive interfaces and delaminations. (b) infinitesimal element of layer k showing stress resultants and couples and interfacial tractions. Homogenized displacement field The n-1 unknown zigzag functions 2 2  ( ) k x for k = 1..n-1 in Eq. (5a) are determined as functions of the global displacement variables and displacement jumps by imposing continuity of the shear tractions, Eq. (2), across the layer interfaces, which yields: 1 23 3 23 3     ( ) ( ) ( ) ( ), k k k k x x for k =1..n-1 . ( 7) Through Eq. (7), (6) and (1) the kth zig-zag function is defined in terms of the global displacement variables:     1 2 0 2 2 22      ; , k k w (8) where :   22 55 55     ; ( ) i j i j C A and 1        ( ) ( ) k k k ij ij ij A A A . (9) Once the functions 2 2  ( ) k x , for k =1...n-1 , have been defined, the relative displacement at each cohesive interface, 2 ˆ k v , is defined through the constitutive law of the interface, Eq. (4b), using Eq. (1a), (2), (6b) and (8). The expression for the displacement jump at the  S ( ) k interface at the coordinate 3 k x in terms of the global displacement variables is: x 3 k-1 x 3 k p ( x 2 , t ) k th layer cohesive interfaces delaminations x 1 x 3 x 2 h L 2 L 1