Work in Progress
Pi Potential
Sharp believes the 3TEX-tested Pi joint design could reduce the
weight, cost and need for repairs in wind blades. "By increasing the
ultimate strength and fatigue life, this type of joint should permit
designers to reduce the material in the loaded sections of the blade,
lowering material and manufacturing costs." Indeed, 80m to 100m
(262-f to 328-f) long blades now under development for ofshore
turbines might soon beneft from Pi joints (see "Learn More").
Further, integrating Pi joints into blade construction methods
appears to be a solution because increasing blade lengths tests the
limits of current bond technology. During their discussion of design drivers and expected failure modes in future longer blades at
the 2012 Sandia Wind Turbine Workshop (May 31-June 1, Sandia National Laboratories, Albuquerque, N.M.), representatives
of Bladena (Ringsted, Denmark) pointed out that when the blade
length surpasses 60m/197 f, fatigue failure in the bondlines and
failure of the shear web become a critical failure mode, a reality
that could be addressed with Pi joints. Bladena, a commercial spinof of wind energy research institute Risø DTU (Roskilde County,
Denmark), also observed a nonlinear crushing pressure phenomenon that increases in longer blades because they bend more. Te
Pi-jointed beams in the test blades exhibited increased stifness,
which would counter the crushing pressure. Finally, Bladena iden-
tifed interlaminar failure in the load-carrying spar caps and shear
web fanges as a risk that increases with blade length. Orthogonally
woven 3-D textiles are inherently resistant to delamination. Tus,
they could improve performance not only in the shear web-to-blade
shell joint but also in the spar cap. Terefore, Sharp believes Pi joint
preforms could provide benefts elsewhere, especially in very large
composite structures, such as those in ships. | CT |
Senior Editor
Ginger Gardiner is a senior editor on the
staf of Composites Technology, based in
Washington, N.C.
ggardiner@compositesworld.com
Read this article online | http://short.compositesworld.com/G669Zx6I.
Guest columnist David Russell described the multiple advantages of Pi joints
in "The Composites Affordability Initiative, Part I" | HPC March 2007 (p. 9) |
http://short.compositesworld.com/DaWuNQ7U.
One potential destination for Pi joint technology is a U.K.-based Energy
Technologies Institute (ETI) blade design project contracted to Blade
Dynamics (Isle of Wight, U.K.) | "Blade Dynamics receives investment for
new wind blade design" | http://short.compositesworld.com/qC2iQSpC.
CT oCTober 2013
surpassed this, withstanding more than 700,000 cycles at the 180
percent load step; it failed afer a total of 2.5 million cycles.
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