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Issue link: https://ct.epubxp.com/i/178128
Source | 3TEX Work in Progress Work in Progress Source | 3TEX 3-D woven textiles are stitched through the entire thickness in the center, but only partially through at the outer edges. This allows portions of the reinforcement stack to fold at right angles to form the Pi "legs." The legs form a slot that receives the shear web, with enough additional space to allow for adhesive (light green). These preforms are easily co-infused with the blade shell halves to receive and locate the blade shear web. 3TEX confrmed I-beam test results via static and fatigue testing on 13m /42.7-ft long commercial-type wind blades. adhesively bonded to bolster the C-beam. Spacers held the bondline thickness to 3 mm, ±0.3 mm (0.1 inch, ±0.01 inch). Sharp says three conventional joint beams and fve Pi joint beams were tested at the Constructed Facilities Laboratory at North Carolina State University (Raleigh, N.C.). Te Pi joint beams failed at an average 12 percent higher load and 20 percent higher defection than the conventional beams. Tat difered slightly from SAMPE paper data: "Tose results included only three of the Pi joint tests and one of those underwent hysteresis loading. When we tested additional Pi joint cantilever beams without the hysteresis loading, the averages increased for both load and defection at failure." Notably, the failure in the Pi joint beams resulted from buckling in the shear web, not failure in the Pi joint. Te buckling forced the vertical legs of the Pi joint outward and, in turn, caused failure in the adhesive layer. Sharp asserts that increasing the shear web's buckling resistance would likely produce an even higher failure load. However, this is not the case for the conventional joint beams, in which failure occurred within the adhesive layer frst without any onset of buckling. Not least, the Pi joint beams also cost approximately 15 percent less to produce (see Table 1, below). Sharp sums up, "Manufacture using coinfused 3-D woven Pi joint preforms requires fewer steps, less fber, less resin and less adhesive than conventional joint manufacturing." Testing was extended to commercial-type construction of 13m blades. One blade with each joint type was tested under static load. Another pair was fatigue tested. Under static load the Pi-jointed blade failed at a 20 percent higher load and 25 percent greater defection than the conventional blade. Even more dramatic were the results of fatigue tests. Te test cyclically loaded the blades to 100 percent of the test load (design load plus a factor) for 1 million cycles, then it increased the load by 20 percent for each 200,000 cycles until failure. Te blade with the conventional joint failed during the 160 percent load step afer 1.45 million cycles. Te Pi joint blade Conventional Joint (C-beam with L-shaped braces) 3-D Woven Pi Joint 1. Infuse shear web in C-shaped mold 1. Infuse shear web 2. Cut and trim shear web 2. Cut and trim shear web 3. Infuse two L-brace composites in molds 3. Co-infuse spar caps and Pi joint preforms 4. Cut and trim L-brace composites 4. Cure shear webs and spar caps 5. Infuse spar caps 5. Apply adhesive layer to Pi joints 6. Cure spar cap, shear web, and L-braces 6. Insert shear web into top and bottom spar cap/Pi joints 7. Apply adhesive layer to bottom spar cap 7. Cure composite I beam. 9. Apply adhesive layer to one L-brace 10. Attach L-brace to spar cap and shear web C with L Pi 11. Cure epoxy putty adhesive Material Costs 0.17 0.16 12. Apply adhesive layer to top spar cap Epoxy Costs 0.22 0.19 13. Attach top spar cap to structure Labor Costs 0.61 0.50 14. Apply adhesive layer to one L-brace Total Costs 1 0.84 15. Attach L-brace to spar cap and shear web 16. Cure composite I-beam 20 Relative costs of each joint type Table 1: Manufacturing steps and relative costs for I-beams that use each joint type. Source | 3TEX compositesworld.com 8. Attach C-shaped shear web