Transferring momentum : Novel drop protection concept for mobile devices

Abstract

Dropping a tablet (or mobile phone) can be extremely costly, as loss of functionality, visible body damage, screen delamination and failure are all too familiar outcomes. This paper discusses the analysis led design of a novel passive protection concept, capable of isolating a device from the primary impact, and is also insensitive to impact angle and device dependent features. A high fidelity finite element model of an iPad Air was used to develop the BLOKTM protection concept, which utilises different grades of elastomer, optimised internal castellation geometry and a high stiffness backplate. Sensitivity studies include the influence of glass properties, screen bonding and impact angle on the robustness of the numerical predictions, whereby quantitative comparisons with experimental data in terms of metal body damage (location, size) and accelerometer data were used. Explicit finite element analysis verifies the effectiveness of decoupling the tablet from the impact loads, as resultant acceleration for unprotected versus protected was reduced by ∼76% (2152 g vs 509 g), and consistent with ∼74% reduction observed through testing (1723 g vs 447 g). For the protected tablet, simulation predicted displacements within 6%, with peak acceleration overestimated by 14%, and attributed to overestimating elastomer stiffness at full compression and its subsequent unloading. Final validation demonstrated device independence by protecting an iPad Air 2™ (with significantly different internal structure to Air™), against corner and short edge impacts. The concept developed resulted in a product to market with a mass of 165 g (∼36% tablet mass), providing protection from a 1.8 m drop onto concrete, far exceeding MIL-STD-810G requirements.