One shouldn't be surprised to see wind turbine blades fail in a hurricane!
Kinda voids the warranty.
Still, I was curious enough to read the manufacturing guide you mentioned.
I found a copy of the blade manufacturing manual here:
http://www.engineeringforchange.org/static/content/Energy/S00019/blade_manufacture_guide.pdfMy reading of it is that Hugh had little to do with its writing or design, he simply inspired the designers in creating a copy of his wooden blade shape, with fiberglass material.
I applaud their noble goal, but I see errors in the design as I read through it. The authors know a lot about making parts with fiberglass, but I also have some criticism there.
Design:
Fiberglass is a layered material. It forms a shell-like part, therefore many loads applied to the part become stresses in its skin. Those stresses cannot distribute themselves through the thickness of the part because the core of it is a light foam filler material. While that foam can support a bit of shear and compression, it cannot resist much tension at all. Furthermore, the foam can only do so much to maintain the "cross-section" of the part. Any force on the part that could cause the cross-section to change could cause the part to collapse.
To be more specific, the blade is subjected to bending when the thrust of wind load is applied to it. The bending pushes the tips backward, putting the wind-ward face in tension, and the back face in compression. For this to be successful, the tension side must not have the ability to stretch. But it does - there is an elbow near the root where the airfoil cross-section ends and the root cross section blends in. This blend is rather sharp, however. This will be forced to straighten out under the radial tension from rotation and windward face tension from thrust. The other skin in compression is fairly uniform, but it's not reinforced by any internal structure other than the "stringer". If the back skin is even slightly wavy, the compression could cause the skin to buckle and then it would abruptly collapse.
Fabrication:
The lay-up procedure is well detailed and relatively complete. However the lay-up misses an important factor known as "volume fraction" of fiber to resin. In the lay-up, one should take care to press the layers together with a roller so that excess resin between the layers is squeezed out. If this is not done, the layers are too far apart, floating here and there in a sea of glue, so they lose their ability to work together. The final step of pouring all of the left-over resin into the mould is just about the worst thing you could do.
The "stringer" is a beneficial feature, and this is done well enough given the materials and tools available. What has been neglected are "ribs". There are none! This is a major problem. The blade changes cross-section a number of times but there are no ribs inside to define what the cross-section should be. Rather, stiffness of the shells is relied upon to enforce the cross-section, but experience proves that this is not adequate. The elbows at the roots will "unfold" under the bending stress applied by wind thrust, causing the blade to flex substantially more than a wooden blade would, furthermore putting a sharp flex at the inside corner of that elbow. I think you mention some cracking in this area, and that's why. Those ribs are also crucial in preventing the skin from buckling when it is in compression, as I mentioned above.
Test:
The first test looks like a good approach, but not nearly enough information is provided to allow a builder to do it correctly.
The second test seems so irrelevant that I cannot begin to see the point of it. 10 Newtons?
The thrust load on a wind turbine rotor can be many pounds per square foot. On a 12-foot rotor, without furling, that may be 200-300 pounds. With correct and complete furling this can be reduced by a fair fraction, however the rotor is now spinning its wing-like blades edge-on to the wind, so the problems don't actually stop, they just change. Rather than rely on brute strength, the blades must also be stiff to handle this kind of swinging about, otherwise their flexibility will provoke either fatigue or a tower strike.
None of the tests shown in the builder's manual address this well enough to show that the blades should survive.
So... It's a good guide, but it doesn't leave you with a part guaranteed to last for long (even if a hurricane doesn't blow through).
The authors should take some of these failures to heart, and produce a revision that introduces ribs, make more gradual transitions in shape at the elbows, and squeeze out the excess resin from the parts.