Modeling of in-plane and interlaminar fatigue behavior of glass and carbon fiber composite materials

Abstract

This thesis presents the results of a modeling study of the fatigue behavior of fiberglass and carbon fiber reinforced epoxy composite materials intended primarily for wind turbine blades. The modeling effort is based on recent experimental results for infused glass fiber laminates typical of current blades, and hybrid carbon prepreg laminates of potential interest for future blades. There are two focus areas: in-plane performance represented by stress-life (S-N) curves, and out-of-plane ply delamination at details including ply drops and joints, based on fracture mechanics. In-plane fatigue models for both the mean performance and a statistically fit model with a 95/95 confidence limit were developed for three laminates, each representative of lower cost materials with applications in the wind turbine industry. These include polyester and epoxy resin infused glass fabrics and a hybrid carbon prepreg; two of the materials were tested in the axial and transverse directions. Models were adapted for the S-N results at several uniaxial loading conditions, including special treatment of the time dependence at high loads.

Materials are compared in terms of their fatigue exponents, constant life diagrams and in the context of a wind loads spectrum. The second part of this work contains a modeling study of delamination crack development in various composite structure detail regions using finite element analysis. Geometries include various ply joints, ply drops, and material transition areas, all using relatively thick glass and carbon fiber prepregs typical of lower cost applications. Two dimensional finite element models were used to determine the strain energy release rates, GI and GII, of delamination cracks by virtual crack closure with contact elements. Results are correlated with experimental data and approximate models where available. The model results, while static in nature, offer insight into trends observed for delamination under fatigue loading for various geometries and material variations, including a more detailed study of tapered ply drops. The results support and help explain experimentally observed trends of fatigue delamination resistance with material (glass and carbon), ply thickness, and crack locations. The influence of ply mis-orientation and ply drop location on the GI (opening mode) component is also explored.