The rate of stiffness reduction in damaged laminates with increasing transverse crack density in plies depends on two micromechanical parameters: normalized crack face opening displacement (COD) and crack face sliding displacement (CSD). A FE-based parametric study shows that the only properties that affect the CSD are the thickness ratio and the in-plane shear stiffness ratio of the damaged and neighboring undamaged layers. The dependence is described by a power function with respect to the above mentioned properties. This relationship and the previously obtained power law for COD [Lundmark P, Varna J. Constitutive relationships for damaged laminate in in-plane loading. Int J Dam Mech 2005:14(3):235-59] are used in the damaged laminate constitutive relationships [Lundmark P, Varna J. Constitutive relationships for damaged laminate in in-plane loading. Int J Dam Mech 2005:14(3):235-59], which are closed form exact expressions for general symmetric laminates in in-plane loading. The model is validated analyzing reduction in shear modulus of [Sn,90m]s laminates and comparing with direct FE-calculations. The results are excellent in case of cracks in one layer only. For laminates with two orthogonal systems of cracks, the power law underestimates the CSD. To account for interaction between both systems of cracks, which is of importance for crack face sliding, the power law is modified using the effective shear modulus of the cracked neighboring layer.