What is the physics behind the screen protectors? Screen protectors are supposed to protect your device from nicks and scratches, and the stress of impacts which might otherwise lead to cracks. The basic idea being that screen-protector glass "gives easier", in a sense, than the glass on, say, a phone or smartwatch, acting analogously to the "crumple zone" on a car.
But, do screen protectors actually work? I've tried running the numbers a few times, however I've yet to be convinced one way or another. If anything, I'm more confused. In practice, I've observed screen protectors being shattered on both smartphones and smartwatches, while the screen was left in-tact. Did the protector actually save the display, or did it merely shatter because it is (by design) "weaker"?

We'll reduce the problem to the following:
Consider a) a phone, and b) a smartwatch, with standard "chemically strengthened" glass or equivalent (Gorilla glass, e.g.) at something like 0.1-0.3mm thick (I wasn't able to find much data on thickness).
Obviously, screen protectors aid in preventing nicks and scratches. So, instead, we'll focus on the protection they offer from cracking/shattering. Most screen protectors are 0.1-0.3mm thick, and made of either tempered glass or plastic.
How much protection do screen protectors actually offer? Is it significant enough protection to warrant their use?
I can think of a few complications: for example, smartwatches (like the Apple Watch) have curved edges, which some screen protectors extend to protect. Certainly, these edges are "weak points". Phones also require cut-outs for microphones, speakers. And, how does using plastic differ from using tempered glass as a screen-protector?
There are a couple other similar questions out there, but none with definitive (or particularly satisfying) answers. Moreover, none are quite as focused.
 A: Let us complete this question step by step.
So first of all why do screens without screen protector shatter?
They break because even a small bend in them can result in their cracking and shattering.
So the phone manufacturers should either make the frames a little bit thicker so that edges just come out of the display so that the frame absorbs the shock if the phone slips and falls down. But this would make the looks and even the use of smartphones difficult.
Or they could use toughened glass themselves but that may result in less touch precision I suppose or it can be somewhat related t their sales

Now users want two types of protections First one is the protection from scratches because replacement of screens cost a good amount and also prevention from their shattering.
So now Tempered glass is a bit thick and is made in a way that it prevents bending of the screen in case of any shock. They either break themselves or stays intact, unlike the flexible plastic protectors. They also provide protection from scratches but due to the latest bend edged screens people prefer plastic screen protector without knowing the exact protector they should use the reason is due to the thickness of tempered glass and edged screens
So in a nutshell
tempered glass provides two protections-

*

*Shock resistance

*Scratch resistance

whereas the plastic one provides only scratch resistance.
If you want a plastic protector to give you both benefits like that of tempered glass you need them to be somewhat thick and stiff or compressible.
Here is a good source to read further. They have even mentioned the third type of protectors(the compressable ones)
