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Glass screen protectors supposedly protect your phone from impact. A youtube comment by McZidanne sums up this idea pretty clearly:

The thing about tempered glass cover isn't the protection they offer. It's the fact that they give in easier than the screen, so when a surge of energy is released into the phone as a result of a fall, the tempered glass absorbs that energy and protects the phone from having to absorb it.

However logically with my limited understanding of Physics this does not seem to be a phenomenon I can understand. A thin hard object shouldn't be able to reduce the force applied on the screen nearly at all, or can it? At least a plastic screen protector is soft, and can thus (theoretically) change the angle of the force partially (even if too insignificantly to be useful), but something hard as glass seems to me to be even less capable of protecting against impact. Am I missing something about the material properties of tempered glass?

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  • $\begingroup$ I hope this question falls under the "Explanations of observed physical or astronomical phenomena" on-topic category. $\endgroup$ – David Mulder Aug 30 '18 at 15:20
  • $\begingroup$ And secondly, I do get why tempered glass is used over normal glass, and I get that a glass screen protector can give a phone a more premium feel when the only goal is to protect against scratches. The thing I am asking about is just the relation between hardness and ability to protect from drops. (Same thing as hard cases for example) $\endgroup$ – David Mulder Aug 30 '18 at 15:22
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The thing about tempered glass cover isn't the protection they offer. It's the fact that they give in easier than the screen, so when a surge of energy is released into the phone as a result of a fall, the tempered glass absorbs that energy and protects the phone from having to absorb it.

To say that the glass "gives in easier" is a bit misleading, but this is the basic concept. To protect the real screen of the phone, one wants a protector that will absorb the shock from any impact, rather than that shock breaking the real screen of the phone. So, we want a material that is hard and tough so that it will withstand large impacts (i.e. dropping the phone from your hip on concrete). Tempered glass is harder and tougher than plastics, see figure 1 in the first citation below.

A thin hard object shouldn't be able to reduce the force applied on the screen nearly at all, or can it?

Yes, certainly! Appearances can be deceiving in nature, eh? For instance, a given weight of spider silk is stronger than steel!

Regular glass has about the same (see fig. 1) fracture toughness as commercial plastics. But tempered glass has a much higher fracture toughness than regular glass (that's the whole point of it), especially if they're chemically tempered rather than thermally tempered. Note that glasses generally have much larger hardness than plastics as well, so toughness is the dominant contributor to the material's ability to survive a fall, no matter how it falls.

So, tempered glass has a larger resistance to fracturing (it is tougher) than plastic, meaning that the tempered glass can absorb more energy from an impact than the plastic. Thus, for a given impact force, the tempered glass is less likely to break than the plastic.

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  • $\begingroup$ Absorbed energy is energy turned into heat? And let's put it this way, I would expect a cushion in front of a display to help a lot more than a piece of glass... why does it matter at what point the glass fractures (unless it actually changes a non-negligible amount of impact force into heat)? Isn't the only important thing how "squishy" it is to allow changing the angle of the impact force partially? 🤔😵 $\endgroup$ – David Mulder Aug 30 '18 at 16:22
  • $\begingroup$ No, changing the angle of the impact force is not the only important thing. IF it was, then at what angle would you prefer to hit the ground at after falling from the top of a building? If wrapped in squishy stuff, the impact will go thru the squishy stuff and hit you. Something hard will absorb the impact. And it matters at what point the glass/plastic fractures, because your question is about which one is better to survive under impact, which the impact will cause the material to fracture.... So you want the material that's least likely to fracture. $\endgroup$ – N. Steinle Aug 30 '18 at 16:24
  • $\begingroup$ @DavidMulder Are you considering something hitting the phone directly into the screen, or see you thinking about the case where you drop your phone and it hits the ground on it side or it's corner? $\endgroup$ – Aaron Stevens Aug 30 '18 at 17:07
  • $\begingroup$ Ummm, what's all this about plastic vs. glass? Phone screens are glass, Gorilla Glass normally. $\endgroup$ – Maury Markowitz Aug 30 '18 at 19:33
  • $\begingroup$ @MauryMarkowitz Yes, but the question from the OP is about screen protectors over the real glass of the phone itself $\endgroup$ – N. Steinle Aug 30 '18 at 19:36
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I believe that the property of a glass protector, most relevant to the protection of the screen (as opposed to protecting itself from scratches or breakage) and separating it from a plastic protector is its stiffness or rigidity, i.e., its ability to resist deformation.

As pointed out by JMac in the comments, this (stiffness) helps spread an impact over a large area reducing the stress on the screen.

An easily deformable plastic protector would pass a localized stress right through and this is exactly the kind of a stress that is likely to break the screen.

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How much energy can a material absorb (per unit volume) is given by the material's toughness, which is the area under the strain-stress curve up to fracture:

enter image description here

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    $\begingroup$ Is there supposed to be more to this? Currently this doesn't answer the question. $\endgroup$ – JMac Aug 30 '18 at 15:56
  • $\begingroup$ I'm responding to this: "The thing I am asking about is just the relation between hardness and ability to protect from drops." - How much of that energy generated by the free fall can the glass absorb, well, you'll have to look for the corresponding stress curve and compute the toughness. $\endgroup$ – user3408085 Aug 30 '18 at 16:02
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    $\begingroup$ Even the question you're talking about, they asked about hardness and you answered about toughness. Those are not the same. I also don't think you're even touching the core of the question, which is why one might choose a glass protector over a plastic one. $\endgroup$ – JMac Aug 30 '18 at 16:08
  • $\begingroup$ Well, the hardness wont have any effect on how much energy the material can absorb. About the "core of the question", you can look for the strain-stress curve of any plastic and see the difference on the amount of energy that can be absorbed. $\endgroup$ – user3408085 Aug 30 '18 at 16:10
  • $\begingroup$ And toughness doesn't explain why someone would choose a glass screen over a plastic one. I think OP may actually have been onto something when he specifically mentions hardness; because that will have a relationship to how the impact can actually propagate through the phone. I suspect that the glass is able to absorb the energy more uniformly across it's surface compared to a plastic screen. Regardless, this definitely isn't an answer to what was asked. $\endgroup$ – JMac Aug 30 '18 at 16:22

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