I was reading about Thermoplastic charastics recently and found it is very flexible, but when chilled it can become quite brittle.

My P3 shell was bent in a crash early on, but I was able to effectively straighten it with a heatgun. No cracks have ever resulted.

Do you suppose cracks result durring cold weather flying? Mine doesn't seem disposed to crack at average temperatures.

I'm not saying the P3 is even made of thermoplastic. I'm just wondering about the cold thing.

I fly during many cold days (20 F or so) last Winter and did not see any cracks on my P3A.

If it was simply cold weather, every Phantom flown in sufficiently cold conditions would have cracked but they haven't.
And some in warm climates have.
The key to understanding the cause is that it only affects a very small number and there is no common factor.
Most of the explanations proposed (tight screws, braking settings, poor shell design, hard flying, heat, cold etc) are unable to explain what has been observed.
ABS is a very strong and resilient plastic which should easily be able to supply the strength required and for most Phantoms it works perfectly and never cracks.
The only explanation that fits the observations is the use of sub-standard material or moulding technique in a few batches.