Fatigue behavior of glass fiber reinforced composite materials for wind turbine blades

Glass fiber reinforced composite materials are used in a variety of applications such as wind turbine blades where resistance to fatigue (repeated loading and unloading) limits the service lifetime of the part. The fatigue process in these materials includes the gradual accumulation of stable damage prior to actual failure, with associated changes in material stiffness. In this thesis, a characterization of the cyclic. fatigue behavior of [0&deg/±45°]4 glass/polyester composite materials is presented in terms of the following parameters: static properties; S-N relationship; damage initiation and growth; fatigue modulus change; edge effects; and effects of triaxial reinforcement variations on fatigue properties. The stiffness reductions resulting from fatigue damage were measured for laminates at various intervals during the cyclic life in order to determine changes in stiffness due to fatigue loading. The fatigue damage (matrix cracks, delamination and fiber breakage) was examined by edge replication and optical microscopy. Consistent with literature observations, the results suggest four distinct stages: undamaged, damage initiation and accumulation, crack interactions and delamination, and the final failure process. Fatigue failure criteria have been examined for the same laminates. A secant modulus criterion was not found to be consistent with the data for this class of laminates. A cumulative secant modulus criterion was introduced. This criterion does appear to be valid for the limited materials and load conditions tested in this thesis. The effects of free (machined) test coupon edges on the fatigue resistance were investigated by conducting a series of tension-tension fatigue tests on two material systems. One system was a conventional laminate with machined edges, while the other was fabricated by wrapping the laminate edges during fabrication. The results show that edge effects on the fatigue resistance are not significant. The effects of triaxial reinforcement variations on the fatigue properties were also investigated. Of two cases studied, Material W with relatively loosely stitched structure performed significantly better under fatigue loading, consistent with expectations from finite element modelling. No significant effect of stitching on static strength was observed. FATIGUE BEHAVIOR OF GLASS FIBER REINFORCED COMPOSITE MATERIALS FOR WIND TURBINE BLADES

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