I've run the 6.8 blades for a couple years to see how they would hold up, they did fine. Last fall I put up an 8.5 ft turbine and they are doing quite well also. I just ordered a couple sets of the 10.6 blades and suspect they will work fine as well.
I believe they have a recomended maximum rpm which shouldn't be exceeded. As you said you didn't have a furling system on the turbines which could have easily exceeded the maximum rpm in high winds or gusts. Even though the blades were stalling in 15 mph winds they could easily overcome the alternator load in higher winds especially not being furled. In a 30 mph wind your pushing 3.5kw through the alternator and in a 40 mph gust your looking at 8.5kw with those blades. My guess would be they experienced a few different forces that caused them to fail. Firstly an overspeed condition, possibly trying to yaw while in the overspeed creating a massive flex and lastly their most likely, being fiberglass, a bit more brittle in the cold weather less likely to flex as well. So the combination lead to the failure.
A furling system most likely would have saved the blades. I'm just speculating by what you've said and there could very well have been some manufacturing defects that could have added to the situation.
They aren't the best blades by any means, wood blades are much more durable. They don't perform as well as my hand carved blades but the difference in cost ( price vs labor ) makes a quick inexpensive way to get a turbine in the air. The do require a little work and definately need to be "re balanced" even though they say they are balanced.
I wouldn't recommend using them in an area where there are people or livestock in close proximity to the turbines.
As with anything, do all you can to build a safe machine and the chances of failure is considerably reduced.
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