Non-conventional failures caused by the edge effect in cross-ply laminates made of ultra-thin plies

In a fatigue testing programme of carbon/epoxy cross-ply laminates made of ultra-thin plies, non-conventional failures are observed. These non-conventional failures consist in longitudinal cracks in the 90° layer parallel to the loading direction instead of the conventional transverse cracks in the 90° layer perpendicular to the load direction. A potential reason for this failure is the presence of a normal stress in the thickness direction. Classical Lamination Theory predicts zero values for this stress component under longitudinal loading, but the presence of a free-edge alters the stress state. The so-called “edge-effect” has been widely studied in the past, but the presence of ultra-thin plies introduces new lower limits of the thickness of the 90° layer. Additionally, the presence of a stress singularity, in the bimaterial corner generated by the 0° and 90° plies, is also investigated to check its influence on the stress state due to the low thickness values of the 90° layer. A slight geometrical modification is conceived to remove the stress singularity, isolating its effect from the pure edge effect. Numerical simulations of the stress alteration in the free edge of the samples, due to mechanical and thermal loading, are carried out, and stress distributions are numerically computed, depending on the 90° layer thickness. Detailed failure inspection by 3D tomography is also done to assess the through-the-width propagation of the non-conventional failures from the free edge. Numerical predictions and experimental 3D X-ray inspections seem to corroborate that the edge effect is responsible for these non-conventional failures.