FEA Project: iPhone Drop Test

Introduction

Smartphones have become an essential part lives around the world. Consequently, smartphones have exhibited a rapid increase in design quality. Manufacturers compete to build the lightest and thinnest phone on the market. However, given their heavy use and price tag, people also expect phones to be durable enough to withstand probable drops from chest and pocket height.

Apple has responded to both the calls for aesthetics and practicality, by creating new manufacturing techniques and patented a 7000 series aluminum alloy.
“7000 series aluminum alloys may provide aluminum alloys with high tensile yield strength, high extrusion speed, and/or low thermal conductivity. In certain variations, the alloys are press quenchable, allowing processing without additional subsequent solution heat treatment while not compromising the ability to form an aluminum alloy having a high tensile yield strength.”

Apple has chosen materials that minimize damage to the body and the screen, which can easily crack. How well will these materials perform under probable drops from chest and pocket height? Drop tests will be simulated with an Abaqus model of an iPhone 7 impacting a firm surface. Due to a limitation of Abaqus knowledge, only the body and screen were modeled. The method used to model the problem is via a 3D elastic solid with triangular elements and ~15,000 nodes in total between the iPhone display and the iPhone body.

FEM Solution

In reality, an iPhone under the simulated conditions would not deflect to the degree shown in the figures above because it would first break, or shatter, transferring energy away from the remains of the iPhone. However, this is not represented in this elastic model, which is C0 continuous, because all the energy stays contained in the model (it is not clear why the tie constraints break). An example of this is shown in Figure 4, where the stress propagated upward, from the impact location to the top of the phone is clear, causing a ripple of deflection. The model also deforms more than a real iPhone would because inertia is not modeled in this Abaqus simulation, due to a limitation of Abaqus knowledge.

Failure in the simulation is regarded as a recorded bending stress higher than the true fatigue stress, because that means the iPhone did not entirely survive the drop. Based on post-processing results, the iPhone body fails each case. The fatigue strength of Aluminum 7525 is 160 MPa, much less than the bending stress of both cases in Table 3, and experiences significant deflection. The body is expected to undergo significant deformation in a real-life test case with the same conditions. The fatigue strength of Gorilla Glass 4 is unknown. However, it is a reasonable assumption, that since the body fails each case, the screen should also fail each case, because aluminum is expected to have a higher fatigue strength than glass. The screen is expected to shatter in a real-life test case with the same conditions.

The full paper with FEM derivation, BCs, loading conditions, additional test cases, and complete discussion can be accessed here.

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