what are the main causes  for shell to crack on p3a p3p and can they be prevented

Very few P3s developed shell cracking and the explanation that fits the evidence best is that there was a bad batch of plastic way back in the manufacturing process.
Reports of cracking shells are now infrequent.
The best way to prevent cracking is to buy a shell that isn't going to crack - and that's just about all of them these days.

I got a new p3s last Wednesday. Checked the phantom out, there were stress marks in the holes, and little gray marks from the screws hitting the sides during manufacturing, however I have no cracks. I do have the new shell however, but also backed the screws out by 1/4-1/2 a turn.

Since the dawn of aviation, materials that take flight are constantly loaded and unloaded with aerodynamic stress and vibration. For airworthiness, even metal aircraft are regularly inspected for cracks and fatigue.

Our Phantoms are not immune to these same stresses and if you fly your P3 long enough and subject it to enough stress, then it will eventually crack. As one person's opinion, those that fly the longest without cracks, do several things:

1) Fly reasonably conservatively. Try to avoid extreme braking and really hard maneuvering except in emergencies.
2) Try to reduce battery thermal loading by removing the battery after the P3 lands.
3) Plan their flights before taking flight. This will help to reduce unexpected extreme control movements and reduce aerodynamic stress.
4) Have four or more batteries and tend to fly more frequent but shorter duration flights, reducing higher temperature battery levels and allowing the motors to cool between flights.
5) Store the P3 (when not flying) in a temperature stable environment (not the boot/trunk of an auto)
6) Don't crash into things


p.s. As labroides points out, the new shell has been redesigned to be stronger and last longer!!!

Thanks for your answer very helpful

Terrylewis gave some good advice.What's more, if you find any crack on the drone , i'd suggest you to contact our NA office for the repairing. Here is the contacting info:
Email: support.us@dji.com
Phone: +1 (818) 235 0789  
Mon-Fri 9:00 am - 5:00 pm (PST)

terrylewis Posted at 2016-4-3 19:26
Since the dawn of aviation, materials that take flight are constantly loaded and unloaded with aerod ...

I showed in a thread a long time ago that this problem was a manufacturing defect. No precautions can keep them from cracking.

DJI- Paladin

tell me whether you fix the problem and whether you find the causes of cracking shells on the upper side . I am my copter 4 month old sent for repair and dont  like to do it again !!!

  

Just to argue the development of new shells (" - and that's just about all of them these days.")
I've got mine since December, 18hs, 138Km, 130 flights. Yesterday I found 1 crack in the lower and 1 in the upper shell.
I must admit I'm disappointed...
I tend to agree with Elmer on the manufacturing issue

Here is what I did on all my phantom 3s. Cheap, simple, effective and almost invisible. If the zip ties are on tight enough this will take care if any potential cracking of the two arm screw holes. Now this still leaves the motor mount area susceptible to cracking but the new shells can take a little more abuse.

edbighi@gmail.c Posted at 2016-4-11 10:52
Here is what I did on all my phantom 3s. Cheap, simple, effective and almost invisible. If the zip t ...

But most P3s never develop any cracking anyway.
Trying to prevent something that probably isn't going to happen doesn't make much sense.
It's a bit like taking anti-vampire precautions.

labroides@yahoo Posted at 2016-4-11 09:03
But most P3s never develop any cracking anyway.
Trying to prevent something that probably isn't go ...

I don't see the hurt of doing it however. If it prevents a crack in your shell, which could lead to a crash then why not do it. I've also read posts about brand new phantoms this year producing cracks, the problem is still out there.

MrPinaColada Posted at 2016-4-11 12:35
I don't see the hurt of doing it however. If it prevents a crack in your shell, which could lead  ...

Still the numbers showing cracking is a very tiny fraction.
But there's no evidence it will prevent cracking and even if cracking developed, that doesn't cause a crash.
This is more about feeling that you are doing something rather than actually achieving anything.

My 2nd of 3 psadvance just developed a crack. All three drone were purchase within the first two months of release and the two that cracked were bought from the same store. This is a real problem.

ssmulinari Posted at 2016-4-10 16:42
Just to argue the development of new shells (" - and that's just about all of them these days.")
I' ...

I bought mine the end of January and found cracks in two places like yours, on one the lower shell below a motor and one on the upper shell near a different motor.This was last  month and I contacted DJI and they sent me an RMA to send it in for repair. I sent it to them and had it back within two weeks with new shells. So very happy with their service. I found a different issue after getting it back with the tilted gimbal issue happening randomly while flying. So just sent it back to have that fixed.

My P3S bought Nov 2016 new developed under arm near motor cracks on screw hole. 8hrs with 70 flights, all gentle no crashes.