UDATE - 8/5/2017: - Upped the temperature to 210 for PLA. This seemed to do the trick. The blade only lasted about 30 minutes, BUT...this time, I was clearing brush in an area with lots of rocks, some concrete, and a cedar fence. The estimated count was ~5 firm bumps on the cedar fence, ~10 rocks (be careful of flying rocks, BTW), and 3 really bad bashes against the concrete; it was a sloped hill, and I couldn't see the chunks of concrete buried underneath the brush. There were some gouges in the plastic where it ate the impact without breaking. If you want to see, find the picture with the dirty yellow plastic.
UPDATE - 7/22/2017: - By request, uploading V7 with blades that are 2 cm longer. Further updates, anything but the first two (V6 and V7) are on here by request.
UPDATE - 7/19/2017: - https://youtu.be/5b0-aYOpFo8
Let's begin with: USE AT YOUR OWN RISK!!!!
This is version 6 of my brush cutter. It attaches to a Ryobi 18V String Trimmer (THIS ONE). It is designed to cut down thicker brush that may grow in your yard, and this version seems to be able to handle large weeds about the diameter of my pinky finger (average adult male). I did break it when I hit the cedar fence, but it's not meant to take that amount of force.
I recommend printing this at 100% infill. As long as you don't abuse it, it probably won't break. You can then sharpen it by sanding it if it starts getting dull. Also, 100% infill adds to the rigidity of the design, and you want that.
BONUS: Here's a video of Version 5, which definitely broke: https://youtu.be/4e7GXS0hwms
The version 6 brush cutter blades are twice as thick, and it tore through those weeds.
7/18/2017: After several tests, I think the design is okay, but PLA is the wrong material for this. They all run for about 10 minutes before breaking. I have some PETG and ABS (both of which I've never printed in), and I'll be testing this print in both of those. If I'm not happy with those, I'll try nylon filament. Let me know if you guys have any other ideas.