Prediction of fatigue life to crack initiation in unidirectional plies containing voids

In the present work, a model is proposed to predict the life to crack initiation in porous unidirectional composite plies subjected to tensile fatigue loadings. The average values of the Local Maximum Principal Stress (LMPS) and Local Hydrostatic Stress (LHS) in a control volume of the matrix are proposed to be the driving forces to the initiation of fatigue cracks in off-axis plies. The experimental fatigue curves for the life to crack initiation of laminates characterised by different void contents are found to fall in the same scatter band once plotted in terms of the proposed driving forces, thus proving the capability of the proposed model to predict the life to crack initiation in porous laminates based on the behaviour of the void-free material. Finally, a parametric analysis is carried out to study the influence of the global void content and average void size on the reduction of the life to crack initiation and the applicability of the proposed model to different void distributions and shapes is discussed.