There seems to be a lot of buzz around the IPhone 6+ getting bent (literally) in owner’s pockets (Bentgate!). Lewis from Unbox Therapy even followed up by performing a series of bend tests with the IPhone 6+, the Galaxy Note 3, the HTC One M8, Moto X, iPhone 5S and a Nokia Lumia 1020 (Video Below). Out of the large number of phones with varying screen size being put through a manual bend test, the IPhone 6+ was the only one deformed permanently when bent.
uBreakiFixCo has also posted a video of their bend test on their Youtube channel with a four point bending test rig and 100 lbs of force. At the same time, Engadget has also covered the case of the bendable iPhone 6+ and the series from Unbox Therapy in their article.
Materials Issue? Maybe not…
The overall feeling seems to be that the blame is on the use of an aluminum body panel instead of the usual polymer (often polycarbonate) body on the “lower-end” phones. There is this believe that aluminum is a naturally soft metal, and is softer when compared to the polycarbonate body structure of the other phones. This, in fact, is not true in most normal cases. We can compare the materials strength of the two materials and be able to tell which is easier, requiring less force, to bent or deform permanently.
The following table show each materials yield strength in MPa. To put things in to perspective, the (yield) strength is essentially the amount of force a material can support per unit area without significant permanent deformation and should be able to return to its initial shape and size after the load is removed. For example, a material with a strength of 9.81 MPa can support 1 kg of weight under the influence of Earth’s gravity with cross-sectional dimensions of 1 mm² (1 mm x 1 mm if the cross-sectional area is a square). A material with twice that strength can support twice the weight with the same cross-sectional dimensions.
You can see that aluminum have at least a similar strength if not higher strength than the polymers shown here, the strength of some aluminum can even be similar to those of steel (the workhorse of our world).
There wasn’t any information available on which aluminum alloy is used for the IPhone 6+, but my guess is the 1xxx series aluminum (properties are expected to be similar to the 1050 aluminum alloy above). 1xxx series aluminum are often used for applications where it is not expected to carry a large load (several hundreds to thousands kg for example) and requires good corrosion resistance. This seems fitting for a phone; our sweaty palms amongst other things like water and humidity in the air can eventually corrode some other materials like irons and steels.I should point out that we don’t generally use these 1xxx aluminum series in its softest form. Normally these aluminum alloys are cold worked leading to a typical strength of ~135 MPa. Compare that to the typical polymer used for cellphones. I am not sure what they are using for modern mobile phones, but my old HTC Nexus One says polycarbonate on the casing so I can only assume the current phones use a similar material. The strength of polycarbonate peaks at around 75 MPa compare to that of aluminum typically with a strength of 135 MPa. At most, the aluminum and polycarbonate can be said to have similar strength (dependent on the process and treatment) but likely the aluminum will have a higher strength.
So what is really going on with the “bendable” IPhone 6+ then? I am going to take a stab in the dark that the design engineers was overzealous with keeping the phone’s sleek and slim profile and also trying to keep the weight down.
The bending resistance of a material is highly dependent on the thickness dimension and the shape of the structure. The thickness dimension in this case is defined to be the direction of which we are applying the bending load. In engineering this is taken into account with the second moment of area term.
Since polycarbonate has a lower density (~1.2 g/cm³) than aluminum, the engineers can design this to be thicker without adding on a significant amount of weight. Moreover, engineers can add in features, like extra ribs on the inside, to further increase the bending resistance.
In the case of aluminum though, the density (~2.7 g/cm³) is more than twice that of polycarbonate. This imposes a design constraint on the thickness to be at most 1/2 that of the polycarbonate structure in order to maintain the same overall weight. More over it is more difficult to manufacture aluminum parts with added features to improve the bending resistance compare to the case of the polycarbonate. It is do-able but most likely not cost-effective.
The Real Root Cause?
Firstly, I should state that I don’t own an IPhone 6 or IPhone 6+ nor am I planning to get one. I can only analyze what I have seen so far on the internet make an educated guess.
It would seem that bending issue isn’t exactly the issue with using aluminum as the structure material. The issue is more with the mechanical design to accommodate for using aluminum for the structure. The aluminum structure is likely designed to be as thin as possible (to maintain its light weight) with very little overhead (safety factor) to accommodate for unforeseen loading issues.
Aluminum (even those high strength alloys) being ductile (deform without breaking) is not new to any one. The interesting thing is that the screen did not crack even when the phone itself has been deformed out of shape. Glass, as we are used to, shatter very easily in bending; this can only mean that the Gorilla Glass 3 is chemically toughened. The treatment is likely similar to the case of this video below that shows the capability of normal glass versus that of chemically toughened glass.
Bonus Content 2:
The reason why some phones in the tests performed by uBreakiFixCo deflected more than the IPhone 6+ but were able to return to its original shape without permanent deformation is likely due to the different internal structure of each of the phones. In this case, the deflection is not an ideal way to quantify their strength. But since uBreakiFixCo performed a proper 4 point bending rig with a known load and a known deflection, we can actually estimated, with some assumptions, to an extent the bending force necessary to bend the IPhone 6+ (as well as the other phones) permanently. I will post something on this end when I have time.