most drones have fixed blades , the Spark and some others have floding blades . what i do is check the blades for cracks and extend the Spark folding blades so that they ars stright on tak off and there is no stress on the motors when spinging up . my question is when shood blades be changed and if one half of a blade breaks off will the Spark still fly and how well will it fly .thank you

" if one half of a blade breaks"- One half of a half?

Actually, if that is a result of a crack before: changed to late
If it is from a hit: Motors turn off anyway

I think I have seen a report/test video, where someone cutted parts of one half blade, from the top down, to test how much of it is needed.
Not sure, doh, but I think it was ~20 % of the one tip of one propeller can be MIA and still flying

Check for signs of cracks etc on the blades and replace if any damage is visible. DJI also advise to change props every 200 flights irrespective of the condition. If a quad copter is down to three blades it will crash. As for 3.5 blades - no idea - not something I'd want to test either.

Those are the most famous spark durability test's where they tested cutting props and missing props .

Hello and good day NRRTRAINS 85. Thank you for raising this inquiry today. For safety purposes we need to check the propellers of you DJI Spark regularly and every flight to ensure that the drone is able to perform well and to avoid accidents. One the propeller of the DJI Spark is damage please make sure to replace them before flying your Drone. If you have any other inquiries or concerns please feel free to post it here at DJI Forum. We are here to help you.  

DJI Stephen Posted at 11-19 07:17
Hello and good day NRRTRAINS 85. Thank you for raising this inquiry today. For safety purposes we need to check the propellers of you DJI Spark regularly and every flight to ensure that the drone is able to perform well and to avoid accidents. One the propeller of the DJI Spark is damage please make sure to replace them before flying your Drone. If you have any other inquiries or concerns please feel free to post it here at DJI Forum. We are here to help you.

thank you and have a great day

DJI Stephen Posted at 11-19 07:17
Hello and good day NRRTRAINS 85. Thank you for raising this inquiry today. For safety purposes we need to check the propellers of you DJI Spark regularly and every flight to ensure that the drone is able to perform well and to avoid accidents. One the propeller of the DJI Spark is damage please make sure to replace them before flying your Drone. If you have any other inquiries or concerns please feel free to post it here at DJI Forum. We are here to help you.

thank you and have a great day fly safe

NRRTRAINS 85 Posted at 11-19 07:42
thank you and have a great day fly safe

You are very much welcome NRRTRAINS 85. Have a safe and happy flying always.

Szwedu Posted at 11-19 03:16
Those are the most famous spark durability test's where they tested cutting props and missing props .
https://www.youtube.com/watch?v=PAX-u8x8axI
https://www.youtube.com/watch?v=ekyvho0te-M

Wow! Never seen those videos before. That’s insane! Mine fell over 100 feet one time (don’t ask) and it still works fine. Not a propeller cracked.

Hmm. Wonder if I have enough points to buy some new props.