Use of pultruded carbon fiber / epoxy inserts as reinforcement in composite structures
Bundy, Bryan Charles
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As polymer matrix composite based structures such as wind turbine blades get larger in span, the required stiffness is usually supplied by the increasing thickness. The fabrication of current composite thicknesses is expensive in terms of labor, mechanical components and fabrication time. The problems are compounded for very long spans. Thermosetting resins are injected into these fiber pre-forms under very high pressures so the resin can completely encase the fibers before the resin hardens. These pressures can induce waviness into the fibers that significantly reduces the critical compression properties of these large composites. Pultruded composites are a possible solution to boost the compression properties and enable thinner, more optimized blade designs. The nature of the pultrusion process creates a highly aligned and evenly spaced fiber microstructure that is difficult to move under resin infusion pressures. The issue of efficient load transfer between the pultrusion surface and a secondary resin used to bind the pultrusion in a larger composite structure is the primary focus of this study. The pultrusion surfaces were modified by abrasion, erosion, and wet chemical oxidation techniques. The interfaces of the rods after treatment were tested in two methods. First, the pullout test consisting of pulling a pultruded rod out of block of epoxy was tried. Second, a pushout test consisting of pushing out a center rod in a representative volume element of a pultruded rod based structure was used. The effectiveness measure of the surface treatments was interface shear strength The results of the interface tests indicate that erosion surface treatments enable the largest increases in interface shear strength. Chemical surface oxidation had the lowest shear strengths, less than the untreated rod coupons. Pultruded carbon fiber composites have potential as supplemental reinforcement in larger composite structures. Erosion surface treatments are concluded to be the best at short term interface shear strength boosting, however the untreated rod performed adequately with having surface damage inflicted. Chemical oxidation has the promise of enabling long term interface strength and stablility through carbon fiber energization.