Delamination onset in composite materials due to fatigue loading

An experimental program has been carried out for the evaluation of the delamination onset condition of various composite material systems, under the application of constant amplitude cyclic loading. Standard Double Cantilever Beam End Notch Flexure and Mixed Mode Bending specimens were used; the assessment of resistance to fatigue delamination onset has been performed by means of procedures defined in similitude with the standard static test procedures, introducing appropriate variants, where necessary. Three materials systems have been investigated: two carbon epoxy unidirectional materials and a carbon epoxy 5-harness satin. Normalised curves show a common trend and allow the identification of onset values for the evaluation of defects tolerance in the "no growth" approach. A similitude with the interlaminar fracture toughness measured in static tests, as a function of mode-mix ratio, has been highlighted with a higher influence of the mode I component. A modification of the Benzeggagh-Kenane relationship is proposed, that captures more faithfully the trend of the onset results.