Mixed-mode I/II delamination fatigue strengthening of polymer composites using z-pins

An experimental investigation is presented into the improvement to the delamination fatigue resistant properties of z-pinned carbon fibre-epoxy composite under mixed-mode I/II cyclic interlaminar loading. Delamination fatigue tests are performed on unpinned and z-pinned composites under different mixed-mode ratios spanning mode I to mode II interlaminar cyclic conditions. The fatigue resistance and fatigue strengthening mechanisms induced by the z-pins is dependent on the cyclic mixed-mode ratio. The threshold critical strain energy release rate needed to initiate delamination growth in the z-pinned composite increases with the G I -to-G II ratio. The fatigue crack growth rate slows considerably and the critical strain energy release rate for fast fatigue fracture increases with the G I -to-G II ratio. The delamination fatigue strengthening induced by the z-pins increases with the G I -to-G II ratio due to a transition in the crack bridging toughening process from pin pull-out under mode I dominated loads to combined tensile and shear fracture under mixed-mode loads to pin shear rupture under mode II dominated loads. It is also found that the effect of the G I -to-G II ratio on the fatigue properties is greater for the z-pinned composite compared to the unpinned laminate due to the high fatigue sensitivity of z-pins to the mixed-mode ratio.