Textile composites include woven, braided, and knitted fabrics. Textile composites are considered when out-of-plane properties are also important. Textile composites generally have better dimensional stability, out-of-plane properties, and impact and delamination resistance. The natural conformability of biaxial braids makes them more cost competitive than woven fabric. These material systems are gaining popularity, in particular for the small business jets, where FAA requires take off weights of 5670 kgf or less. The vacuum assisted resin transfer molding (VARTM) process has proven to be low in cost compared to resin transfer molding (RTM). Thus, the combination of biaxial braids and the VARTM process is likely to considerably reduce overall costs. Before the braids can be confidently used in the primary structures, it is necessary to understand the performance of biaxial braided composites under various loading conditions and especially under fatigue. This will reduce uncertainty and hence reduce the factor of safety in the design This research addresses viscoelastic effects on fatigue behavior of carbon/epoxy braided composites. It is observed that braided composites exhibit creep and stress relaxation. Further it is observed that frequency in axial fatigue loading plays dominant role in fatigue life, but very little role in fatigue failure mechanisms. Rate of stiffness degradation is greatly affected by frequency. These entire phenomena such as creep, stress relaxation, frequency effect, and dependency of stiffness on rate of loading indicate the viscoelastic behavior of braided composites. In this research different tests were performed to confirm viscoelastic behavior of braided composites. Axial tension-tension fatigue tests were conducted at different frequencies and stiffness degradation was studied.

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Lakes, R. S., 1998 “Viscoelastic Solids”, CRC Press, First Edition 1998, pp 1–2.
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