Early stage damage in off-axis plies under fatigue loading

The very early stages of damage evolution under fatigue loadings were investigated by testing [45/-45/0]s glass/epoxy specimens. Microscope observations revealed that the first damage event was the initiation of micro-cracks in the 45° plies within the inter-fibre region. Off-axis cracks then formed by the coalescence of these micro-cracks and propagated along the fibres direction with the same mechanism. The orientation of the micro-cracks was proven to be normal to the direction of the local maximum principal stress in the matrix. The results presented here are the first published evidences of fatigue damage initiation at the inter-fiber scale and represent a further validation of the fatigue crack initiation criterion recently presented by the authors.