Influence of the layer thickness on the very high cycle fatigue behaviour of composite materials

icaf2023 Tracking Number 71

High-efficiency structures like blades of wind turbines and helicopters undergo over 108 load cycles during their lifetime. For composite materials, a lack of research in this so-called very high cycle fatigue (VHCF) regime may lead to conservative designs or fatal failures. One aspect known for its strong influence on static and fatigue crack initiation is the layer thickness of the composite plies. As the thickness decreases the crack initiation stress grows. The effect of the layer thickness under VHCF promises higher lifetimes and enhanced structures. To investigate this effect, the two cross-ply layups (902/02)s and (90/0)2s with 0.5 mm respectively 0.25 mm layer thickness are tested in a specialised VHCF four-point bending test system. For every layup a test series with up to seven comparable load levels is conducted and evaluated. Cross-ply layups are favourable because of lower damage interactions, compared to quasi-isotropic layups. They develop transverse cracks in the outer layers and delaminations grow from the crack tips along the ply interface. The composite material consists of the fatigue-optimised fibre SE2020 from 3B-fibreglas and the epoxy system RIM135. The material characterisation shows a strongly improved fibre-matrix adhesion. Due to the glass fibre and the manufacturing in the RTM process the laminate is of very high quality and highly transparent. Therefore, transmitted light photography can be used to determine the two important fatigue damage parameters crack density and delamination area ratio. Also, general damage mechanisms and the flexural modulus degradation are assessed to identify the influence of the ply thickness. Comparing the photographs of both test series, a higher crack density is immediately noticeable for the thin layers. Quantitative data determined from the photographs by a specially developed damage detection software confirms this and shows nearly similar delamination area ratios. With a finite element model, the influence of the crack density and delamination area ratio on the damage development is calculated. Plotting the surface strain and the stress intensity factor over the damage parameters, the influence of ply thickness is directly visible and the development of a “damage path” and different ratios between the damage parameters can be understood.