Effect of voids on the crack formation in a [45/-45/0]s laminate under cyclic axial tension

In the present work, the influence of manufacturing induced voids on damage mechanisms at the microscopic scale was analysed on [45/-45/0](s) laminates subjected to tension fatigue loading. Microscopic observations of the top surface of the 45 ply revealed that the first event of damage at the microscopic scale was the initiation of multiple micro-cracks in the matrix between the fibres, located preferentially in correspondence of the voids in that layer. The subsequent coalescence of these micro-cracks gave rise to the formation of a crack propagating in the 45 fibres direction. This is qualitatively the same scenario observed in void-free specimens in a recent work by the authors, thus confirming that the same crack initiation criterion can be applied in the absence and presence of voids. In addition, the micro-scale damage is shown to evolve faster and therefore off-axis cracks to initiate earlier and in a larger quantity in the presence of voids. (C) 2016 Elsevier Ltd. All rights reserved.