The Apple iPad 2 Review
by Brian Klug, Anand Lal Shimpi & Vivek Gowri on March 19, 2011 8:01 PM ESTI asked a friend of mine, Alexander Miles, to write a bit about the properties of glass that really contribute to its overall strength after reading that the iPad 2's glass is 0.62 mm thick compared to 0.85 mm thick in the iPad 1. Hopefully this dispels some myths about glass strength and clarifies. Alex is a senior double majoring in Materials Science and Engineering and Optical Science and Engineering at the University of Arizona.
On the Strength of Glass
We usually think of things failing under compressive stress, being pushed inward from both sides until it they are crushed. Glass and ceramics, it turns out, are incredibly strong in compressive stress. Strictly by the numbers, a fire truck could be supported by a ceramic coffee cup underneath each tire, but only if the load was perfectly downward. Why then are glasses so fragile? This is because no situation causes only compressive stresses, and tensile stress (imagine pulling something from both ends) is what causes glasses to fail. One can think of tiny cracks inside the glass being pushed closed under compressive stress, but torn open under tensile stress.
A schematic illustration of crack behavior in brittle materials.
If you test the tensile strength of thousands of pieces of glass with identical processing and geometry, you will get thousands of different answers. This is markedly different from metals, where you will get nearly the same result every time. The reason being that glass and ceramic materials have a much lower fracture toughness, as much as 100 times smaller than that of a metal. Fracture toughness indicates how easily a crack can propagate, or to phrase it differently, how big a flaw will cause fracture for a given load. As the required load for normal flaw sizes in metals is enormous, metals typically do not fracture in the way glasses do. Metals usually fail in what is called plastic deformation, necking down then tearing away, long before fracture can occur. This plastic deformation is very predictable and follows the stress-strain curve for the given metal, whereas glasses are less predictable.
The question now is, how does the size of a piece of glass affect its behavior under tensile stress? It depends on the distribution of flaws within the material. If you strike a piece of glass with a hammer, a compressive stress is created right below the hammer, but a ring of tensile stress is also created around the spot you hit. You are essentially sampling the distribution of flaws, because if any of the flaws in the affected glass are big enough to widen with the stress you provided, they will rapidly propagate and the material will fracture. The stress field extends down into he material, so flaws in the volume can cause failure as well, though surface flaws are more consistently to blame as the stresses encountered there are almost always larger.
SEM image of a broken glass surface, (CC BY-NC-ND 2.0) attribution St Stev's flickr.
In glasses the distribution of failure stresses is described using Weibull statistics, giving a peak where most samples fail, and tails both on the high and low end where samples had abnormally high and low failure stresses respectively. The long and short of this is that consistently processed samples will have a more narrow distribution as the geometries are scaled down. This means that a thinner piece of glass will have fewer flaws in the bulk and far less likelihood of having a large enough flaw to cause catastrophic failure compared to a thicker piece of glass. The fact that the screens have a large aspect ratio, that is they are far thinner than they are wide or long, means that the effective stiffness will be different in the two directions. Taking this to an extreme, a very thin glass fiber is fairly flexible in bending, but very stiff axially, as its cross-section is so small that very few flaws are contained inside it. If one needs to break a glass fiber, a surface flaw is usually created by scratching it first.
| Approximate values for the strength of common soda lime glass in various conditions | |||||
| Condition | Tensile Strength (MPa) | ||||
| Theoretical Maximum (Flawless) | 9810 | ||||
| 3 Micron Fibers | 3330 | ||||
| Thin rods, fire-polished and acid etched | 3420 | ||||
| Thin rods, no special treatment | 690 | ||||
| Bulk, ion exchange tempered | 350 | ||||
| Bulk, thermally tempered | 300 | ||||
| Bulk, fire-polished and acid etched | 220 | ||||
| Bulk, no special treatment | 50 | ||||
In order to use glass screens on our devices, we would like it to be far tougher, where "tougher'' ideally means both more resistant to fracture as well as more resistant to scratching. There are two basic schemes used to strengthen glass: elimination of the surface flaws, and creation of compressive stress in the bulk of the glass. Eliminating the surface flaws by polishing, fire-polishing (heating them until surface tension flattens out the flaws), and acid-etching does indeed increase the strength, and drastically so. An increase in strength of up to one hundred times can result from such treatment, but is temporary as microscopic scratches from handling will quickly reduce the strength back to what it was before.
The second scheme for improving strength, introducing a compressive layer, works because existing compressive stress in the glass has to be overcome by the induced tensile stress before any cracks can propagate. To say it plainly, if you don't hit it hard enough with a hammer, it will not even see the type of stress that makes it fail. The down side to this method is that every force causes an equal and opposite force, meaning that a lot of compression at the surface causes tension at the center. As long as a crack does not reach the volume with the additional tensile stress imposed on it, the glass will hold together, but once it does it releases the energy kinetically and fails catastrophically (it explodes like a pumpkin with an M80 stuffed in it).
The way this layer is created varies based on the application. For car windshields, they are thermally tempered by chilling the outer surface while the center is still hot, as the surface remains solid while the center is still busy shrinking, which leaves the surface in compression. Similarly, coating the glass object in a second type of glass with a lower thermal expansion will cause the same effect, as the outer surface shrinks the center is shrinking faster. The multiple-glass approach has the additional benefit that cracks have difficultly moving from one type of glass to the next, leading Corning to produce some glasses with as many as 7 layers.
The final method, and most relevant to our discussion, is ion-exchange. Ion-exchange refers to removing small ions, like sodium, from the glass, and replacing them with larger ions like potassium, all at a temperature that prevents the structure of the glass from adjusting itself to these new bigger ions. The way this swap is actually done is by immersing the glass in a molten salt solution containing the ion we want to substitute in, and allowing it to diffuse in over time, while the smaller, more mobile, ion diffuses out. Depending upon the type of glass, the ions being exchanged, and the desired depth, this process can take as long as several days.
The iPad 2 and previous iPad both utilize Corning Gorilla Glass. This type of glass is an alkali-aluminosilicate, being primarily silica and aluminum with an alkali metal, along with other unspecified components mixed in to tweak its properties. The biggest benefit of alluminosilicate glasses, aside from being relatively tough to start with, is the fact that the rate of ion exchange is fairly high even at temperatures low enough that the structure cannot react, meaning it can be processed quickly and create deep protective layers in the glass. The iPad 2 has a modest reduction in the thickness of the glass (about 23% thinner, for those interested) compared to the first iPad, and the question of increased fracture risk has been posed. Given the identical surface quality between the two generations, the reduction of thickness should create no palpable change in toughness for the typical user. That is to say, a drop that would shatter the screen on the original iPad would likely do the same for the new model. That being said, several other design changes appear to account for the change, and might yield better performance in this department.
Where its predecessor used small metal clips to retain the glass screen, the current iteration uses a ring of adhesive around the entire perimeter that not only distributes the load around the glass and prevents scoring at the glass-metal interface, but better couples the stresses into the more compliant aluminum frame. Both of these measures should improve the performance; either way, drop-testing new electronics is generally not recommended.
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Shadowmaster625 - Monday, March 21, 2011 - link
If that is true then why not focus on that? I still have yet to read anything on anandtech about remote desktop on a tablet. How powerful does a tablet need to be to stream 1-2 mbps compressed images of my desktop over wifi? I can skin my desktop to make it look more like a mobile OS. Touch commands can be fairly easily emulated and transposed into mouse commands. Audio doesnt require much bandwidth. Only video would present a problem, but even then it really doesnt take all that much hardware to play a video. Yet all I see are horrible implementations of remote desktop.marc1000 - Monday, March 21, 2011 - link
i remember reading here at AT that the Nokia N8 could be used to drive an HDMI display and hooked to an keyboard/mouse, to "simulate" a pc-like experience . and now the Ipad2 does the same thing, but 1 year later.IMHO all that apple does is beautifull, but they charge the price for the "beautifullness" of their products.
well, I just want a tablet/smartphone that works, so I will wait until these prices drop... until then I will live with my Htc HD (1st gen.. ugly!)
Watwatwat - Monday, March 21, 2011 - link
almost feels like i learned something:DJayman30 - Monday, March 21, 2011 - link
Maybe the iPad doesnt fit into a busy techbloggers "workflow" but as a consumer device it's an incredible gadget. My ipad 1 is used daily and has never sat on a shelf unused for more than a few hours. I reach for it first instead of my iPhone or clunky Gateway laptop for 90% of my daily computing needs. It has incredible performance, portability, battery life, and overall utility.I can buy & read books, download and play great games, movies and music quickly and easily. Great Email and web surfing are just icing on the cake! Works great for me!
FYI. iPad 1 costs $399 now.
cotak - Tuesday, March 22, 2011 - link
I am with the staff at anand. I use an iPhone 4. It's nice fo getting a little reading of news etc done. And I have looked at the iPad and I have consider getting one. And likely I will have one but not for myself. It'll be too limiting for me to use. For my fiancee though it's perfect since on a regular week she can go 5 6 days without powering up her computer.So is there s market for tablets? Maybe but I don't think it's big as various peope like to think it could be. And the fall out from that might be pretty big. Nvidia's betting on it, moto has only recently make it back into the black, and rim seems to have concentrated on the playbook over improving their last series of phone release. So maybe some of these players might not be be best stocks to hold for the medium term...
Anyhow tablets are useful for light use. But for people who use their devices/technology enought to justify spending regularly on technology, it might be too small and too slow. And really isn't the repeat buyer the important one I the market? My fiancee could get a iPad 1 and happly use it for the next 6 years without considering buying a new one. If people like that are the market, it's not going to be as big as some people in wall street seems to think.
vshin - Tuesday, March 22, 2011 - link
The one thing I'm surprised that isn't getting as much coverage is that small text is actually harder to read on the Ipad 2 than on the Ipad 1. Some folks chalk this up to being "spoiled" by the Iphone 4's retina display but it's actually because Ipad 2 implements heavy use of anti-aliasing, which can't be configured or turned off. I find my eyes tiring very easily as I browse the web or read on iBook in portrait mode.speedkills - Tuesday, March 22, 2011 - link
I noticed over and over in the review people came back to the point that the current app switching functionality is a very poor user experience. It's too bad Apple isn't enabling the multi-touch gestures for multi-tasking by default as they work very well and while they are not for general release it would be nice to see Anandtech weigh on them to see if they feel they should be included in iOS 5 or if they feel a better implementation is needed.Personally both my girlfriend and I have been using them and absolutely loving them. A four finger swipe up shows the multi-tasking bar instead of having to use the double-tap but 95% of the time a simple four finger swipe left or right is sufficient swapping between your most recently used apps. It makes it very easy to do something like copy a bit of text out of a web page, swipe over to Evernote and make a note, then swipe back to Safari and continue surfing.
It's really too bad Apple chose to hide them by default (my guess is they want to save them for the iPhone release to make iOS 5 look like a bigger jump in functionality) because they are a great and much needed addition to the iPad 2.
Mac Ike - Tuesday, March 22, 2011 - link
First of all this was great and extensive review,thank you very much! I have used Windows computers everyday at Work since Windows 3.1,I enjoy all Computers,but Macs are more stable more elegant,easier-to-use,and hold their value longer! I'm not trying to make Windows-users/lovers change to my preferences. I have not purchased an ipad yet,even though I can see many uses for it,and enjoy using iPads. I want ALL OF THE POWER I could possibly need with me,at all times,so I carry my 17-inch MacBook Pro for those times; when not practical I carry my iphone 4. Serious workloads might send me to my imac,it deprnds on ehere I am and what i need to do or what I can do in the environment that I'm in.Since I enjoyed the ipad 1,it only stands to reason that I would enjoy ipad 2,because the speed increase,cameras,weight-reduction,...can only enhance the experience! I am always amazed by the rude,idiotic,self-centered opinions and insults expressed on these Forums! If YOU don't want or need an ipad (or iPhone/Mac),then buy what YOU like! Why insult someone else,for THEIR DECISIONS or PURCHASES?? it's almost always a cheaper or alternative choices you can make. I give less than a Damn if you like another brand more,or feel that Apple's prices are too high!! If you want a cheaper computer,don't need a tablet,can't see the value of design/workmanship/elegance of OS &Hardware,hardware-software integration,good,buy your low-priced,cheap shit,and leave the rest of us alone,so that we can enjoy what WE LIKE! I don't ask any Apple-Haters or Fence-Sitters to purchase my products for me,so it's just amazing how people feel that you're an Apple-Fan Boy or Girl if you prefer Apple products! If I want a $60,000 Car,and you feel it's worth it(V-8 or not),and you can get a car for $20,000,why should your choice determine mine?? If you Windows-trolls don't like Apple-gear,why come to Apple-topics to complain about SOMEONE ELSE's choices?? Rude,immature,and Stupid!
rice2999 - Tuesday, March 22, 2011 - link
Dear anandThere is nothing about screen brightness setting to go with the battery life testing. Did you use 50% default brightness? Xoom screen is dimmer than ipad/ipad2. Though it can be argued, it is not exactly fair to compare battery life while the screen is on different brightness, since screen is probably the one uses the most battery.
I forget where I saw it, someone else actually used a meter to measure the brightness and adjusted the brightness to the same before testing the battery life.
Sabresiberian - Wednesday, March 23, 2011 - link
Every editor said what I've been saying all along about tablets - they are coolz toyz, but when people start trying to use them, they will find they aren't as handy as either a laptop or a smartphone and they'll be returned or be gathering dust somewhere. Except for niche uses, such as book readers and possibly media device controllers in the home, something like that, tablets are a fad - in my opinion, of course.That being said, they do have innovative technologies I think notebook manufacturers should pay strong attention to and add to their products (netbooks and notebooks). There are things the netbook/notebook can do a tablet's form factor prohibits (add a keyboard to a tablet and it really isn't a tablet any more now, is it?), but there is no reason the notebook should lack any feature a tablet has (except to save money on very low-priced budget models).
As to this thing having enough power to replace a "mainstream" computer if attached to a monitor and keyboard - right. I'll run out right now and buy one to play World of Warcraft on - or isn't that "mainstream" enough for you?
Won't be long now and a device this size will be able to replace what we consider to be a power-house PC these days - but then what will the power-house PC build be like then?
;)