Failure Analysis of a Tempered Glass Screen Protector AME 60646: Failure of Materials Beth Wendelberger 9 December 2015
Product Information iPhone 6 Juce Peel Glass Screen Protector Hardness rating: 8-9H Oleophobic coating Tempered glass Thickness Optically clear adhesive = 0.4 mm PET fjlm Shock absorbent silicon 1
Description of Failure Possible Failure Circumstances: • Mechanism 1: Dynamic fracture from drop impact • Mechanism 2: Initiation from fmaw and propagation in bending Crack of interest Crack Path: • Spans width of screen • Initiation behind branching • Change of path direction 2
Toughening Mechanisms Glass Tempering Ion Exchange 3
Toughening Mechanisms Glass Tempering Ion Exchange Heat glass to uniform temperature ~650°C Fast quenching with cold air jets Severe temperature gradient induces residual stress distribution σ + Thickness - - 4
Toughening Mechanisms Glass Tempering Ion Exchange K+ K+ K+ Heat glass to uniform Dip glass into hot molten temperature ~650°C potassium bath Na+ Na+ Na+ K+ K+ Na+ Na+ K+ K+ Fast quenching with cold Potassium ions displace K+ Na+ K+ air jets sodium ions Na+ K+ K+ Na+ K+ K+ Na+ Na+ Severe temperature K+ gradient induces residual Close packing of stress distribution ions induces surface σ compressive force + Thickness - - 5
Tempering Equations Griffjth’s Law for Tempered Glass [1] : σ app = Applied stress σ f = σ app + σ res σ res ) Y -1 C 1/2 σ res = Residual Stress K Ic = ( σ app + σ f = Failure Stress K Ic = Fracture Toughness Negative compressive residual stress increases the fracture toughness in tempered glass. 6
SEM: Initiation Circular Area: Possible mirror region Possible half penny crack Debris: Outside surface of glass Indication of initiation area Likely that impact initiated a crack in the glass 7
SEM: Propagation Striations Possible sign of fatigue Show direction of propagation: lat - erally outward from this area 8
Mechanism 1: Dynamic fracture from impact (c) (b) (a) (a) Mirror, (b) Mist, and (c) Hackle regions SEM of Sample in a dynamic glass fracture [1} . 9
Mechanism 1: Dynamic fracture from impact σ f R m/m = A m/m [2] R m/m = fracture mirror radius A m/m =2.08 MPa√m (Pyrex) If smooth circle in SEM is mirror region, the mirror radius is approximately 0.25 mm. A fracture mirror radius of 0.25 mm corresponds to a fracture stress of 131.55 MPa. 10
Mechanism 2: Flaw plus bending Initiation: Penny crack from impact Propagation: Bending 11
Product effectiveness Hardness Material (Mohs scale 1-10) Gorilla Glass [5] 6-6.5 Quartz [5] 7 Screen Protector [6] 8-9 Sapphire [5] 9 Diamond [5] 10 Gorilla glass vs. Screen Protector: elasticity vs. fracture toughness Catastrophic failure: absorbs energy through crack propagation and branching Prevents surface damage Conclusion In this case, it is likely that the screen protector acted more as preventive barrier for sur- face damage than to absorb energy from impact. A simple fjlm or plastic screen protector may have been just as effective and would not have needed replacement. 12
Sources [1] Bradt, R. The Fractography and Crack Patterns of Broken Glass. Journal of Failure Analysis and Prevention 11:79-96, 2011. [2] Redner, A., Mognato, E., Schiavonato, M. Correlation between Strength and Measured Residual Stress in Tempered Glass Products. Journal of ASTM International, Vol. 2, No. 3, 2005. [3] Shen Ye. Smartphone Futurology: The Science behind Smartphone Glass. Windows Central. 2015. [4] Shutov, A, Popov, P., Bubeev, A. Prediction of the Character of Tempered Glass Fracture. Glass and Ceramics Vol. 55, Nos. 1-2, 1998. [5] “A Two-Horse Race: Gorilla glass versus Sapphire.”ZTE. ZTE Corporation. Web. 20 Nov. 2015. [6] “Juce Peel Glass Screen Protector for iPhone 6 – Clear.” Juce Mobile. Juce Mobile. Web. 20 Nov. 2015. 13
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