THE FORMATION AND EFFECT OF OUTER-PLY MICROCRACKS IN CROSS-PLY LAMINATES - A VARIATIONAL APPROACH

The microcracking process in laminates which have outer-ply 90-degrees plies (e.g. [90m/0n]s) has some important differences from the microcracking process in laminates which lack outer-ply 90-degrees plies (e.g. [0n/90m]s). Foremost among the differences is the characteristic damage state. [90m/0n]s laminates form antisymmetric or staggered microcracks while [0n/90m]s laminates form symmetric microcracks. To explain observed differences, this paper presents a variational mechanics analysis of the stresses and the energy release rate in a [90m/0n]s laminate having staggered microcracks. The new analysis and a previous analysis of [0m/90n]s laminates are used to assess the effect of laminate structure on the mechanical properties and failure properties of cross-ply laminates. The findings are as follows. (1) A given level of microcracking damage causes a greater amount of degradation in mechanical properties in [90m/0n]s laminates than in the corresponding [0n/90m]s laminates. (2) Although [90m/0n]s laminates will initiate microcracks at lower loads, the corresponding [0n/90m]s laminates will develop more microcracks after continued loading. (3) A bending effect present in [90m/0n]s laminates but not in [0n/90m]s laminates promotes mode 1 delamination in [90m/0n]s laminates.