The formation and effect of outer-ply microcracks in cross-ply laminates: A variational approach

Abstract The microcracking process in laminates which have outer-ply 90° plies (e.g. [90 m /0 n ] s ) has some important differences from the microcracking process in laminates which lack outer-ply 90° plies (e.g. [0 n /90 m ] s ). Foremost among the differences is the characteristic damage state. [90 m /0 n ] s laminates form antisymmetric or staggered microcracks while [0 n /90 m ] 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 [[90 m /0 n ] s laminate having staggered microcracks. The new analysis and a previous analysis of [0 m /90 n ] 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 [[90 m /0 n ] s laminates than in the corresponding [0 n /90 m ] s laminates. (2) Although [[90 m /0 n ] s laminates will initiate microcracks at lower loads, the corresponding [0 n /90 m ] s laminates will develop more microcracks after continued loading. (3) A bending effect present in [90 m /0 n ] s laminates but not in [0 n /90 m ] s laminates promotes mode I delamination in [90 m /0 n ] s laminates.

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