Unfolding the Early Fatigue Damage Accumulation for Cross-ply Laminates

Fatigue damage of composite laminates has attracted considerable attention from research community and industry, in view that laminated structures are inevitable to suffer from fatigue loading during their service life. It is rather complicated to understand and explain, what governs the initiation, accumulation, interaction (synergy or competition) of different damage mechanisms. Intrinsic and extrinsic scatter sources are hard to eliminate during the fatigue testing of laminates, which produce significant dispersion of laboratory data and further hinder our understanding about the progressive accumulation process of fatigue damage. Consequently, most of fatigue damage models for laminates are mathematically fitted to existing experimental data, rather than being related to the mechanisms of damage accumulation process. Considering the majority of stiffness is degraded during the early fatigue life before the final failure of laminates, the objective of this thesis is to investigate the accumulation of matrix-dominant damage, with the possible scatter phenomena taken into account. Carbon fiber/polymer laminates with cross-ply configurations were selected as the research target as they have been increasingly used for aerospace structures due to the light weight. Experimental set-up involving multiple damage monitoring systems was developed to in-situ characterize and quantify the initiation and accumulation of transverse cracks and delamination. To further investigate this scattering of crack evolution among specimens, a strength-based probabilistic model is developed. Overall, this thesis provides a clear picture about the interactive scheme of early fatigue damage accumulation of CFRP cross-ply laminates, which further enhance our understanding about the development of physics-based fatigue damage models for FRP laminates.Structural Integrity & Composite