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Issue link: https://ct.epubxp.com/i/178128
Work in Progress Work in Progress Wind blades Pi preforms increase shear web failure load Easily co-infused structural joint increases ultimate strength and fatigue life, offering solutions for designers as blades get longer and move offshore. W ind blades are commonly made by infusing top and bottom blade shells separately, then adhesively bonding them together around a prefabricated shear web. Te current state of the art leaves much to be desired. Te typical construction is essentially an I-beam, in which a C-shaped shear web provides an area for adhesive bonding to spar caps that are co-infused with the blade shell. Typically, the C-beam must be braced with additional L-beams during assembly (see lef image, top of p. 25). Afer epoxy paste adhesive is applied to the bottom blade half and the shear web, their surfaces are mated, and sufcient time must be allowed for cure. Ten, adhesive is applied to the top blade half, the upper surface of the shear web and the leading and trailing edges. Again, the adhesive surfaces are mated, and time is required for cure. Tis multistep process requires signifcant labor and time. Additionally, when the top half of the blade is lowered onto the bottom half, the joints are mated blindly, that is, without visual or other access to the bonding areas. Industry presentations and patents list other shortcomings and clearly state that blade designs would beneft from an improved bond confguration between the shear web and spar cap. Textile manufacturer 3TEX (Cary, N.C.) began work on such a confguration several years ago as part of a Small Business Innovation Research (SBIR) project awarded by the U.S. Department of Energy (DoE, Washington, D.C.), using the Pi joint concept that was demonstrated in the Air Force Research Lab's (AFRL, Dayton, Ohio) Composites Afordability Initiative (see "Learn More," p. 29). Composite shell Spar Supporting spar Adhesive layer Composite shell 16 Wind blade structural elements are typically a shaped shear web bonded to spar caps co-infused into top and bottom blade shell halves. The thick and uneven adhesive layers limit the strength of the joint and create reliability and repair issues. Source | Linxia Gu, University of Nebraska-Lincoln compositesworld.com Why Pi? 3TEX cites two reasons for its use. First, the Pi joint (so named because its primary component takes the shape of the Greek letter π) have been shown in the aerospace industry to provide superior strength in pull-test loads, compared to conventional laminated joints. AFRL's Dr. John Russell says the joint "provides symmetrical loads to the adhesive area and acts as a double lap-shear joint, increasing the surface area for bonding." Also, the primary load in the adhesive bonds is located farther away from the area of maximum strain. Tese factors combine, says Russell, to make the Pi joint a more efective design. Pi joints also have demonstrated high tolerance of several manufacturing defects associated with