Numerical and experimental analysis of ultra-thin plies in carbon/epoxy cross-ply laminates subjected to bending after tensile cyclic loading

As a result of a fatigue testing campaign in cross-ply laminates with different thicknesses in the 90 degrees ply block (conventional and ultra-thin), different cracking patterns were observed. While laminates with conventional 90 degrees ply thickness show a damage pattern with transverse cracks in the 90 degrees ply block and delaminations along the 0 degrees/ 90 degrees interface, laminates with ultra-thin plies show paths of debonds inside the ultra-thin 90 degrees ply block both transverse and longitudinal to the loading direction. In order to see the influence that these different fatigue damage patterns could have on the mechanical behaviour of the laminates, static bending tests have been carried out after cyclic loading, and apparent flexural stiffness loss (concerning the pristine state) was measured. Nu-merical simulations of the bending test, including the cyclic damage pattern (and detailed non-linear frictionless contact conditions) were also carried out for a deeper understanding of the experimental evidence. Although the morphology of the fatigue damages encountered is different for the two studied 90 degrees ply block thicknesses, their flexural behaviours are quite similar, in terms of apparent flexural stiffness loss, for both cross-ply laminates, independently of its 90 degrees ply block thickness.