Failure Analysis of a Tempered Glass Screen Protector AME 60646: - - PowerPoint PPT Presentation

Failure Analysis of a Tempered Glass Screen Protector

AME 60646: Failure of Materials Beth Wendelberger 9 December 2015

Product Information

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iPhone 6 Juce Peel Glass Screen Protector

Optically clear adhesive Tempered glass Shock absorbent silicon PET fjlm Oleophobic coating Thickness = 0.4 mm Hardness rating: 8-9H

Description of Failure

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Crack of interest Possible Failure Circumstances:

• Mechanism 1: Dynamic fracture from drop impact
• Mechanism 2: Initiation from fmaw and propagation

in bending Crack Path:

• Spans width of screen
• Initiation behind branching
• Change of path direction

Toughening Mechanisms

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Glass Tempering Ion Exchange

Toughening Mechanisms

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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 +

Toughening Mechanisms

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Glass Tempering Ion Exchange

Heat glass to uniform temperature ~650°C Dip glass into hot molten potassium bath Potassium ions displace sodium ions Close packing of ions induces surface compressive force Fast quenching with cold air jets Severe temperature gradient induces residual stress distribution

σ Thickness +

• K+

Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+

Tempering Equations

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σf = σapp + σres KIc = ( σapp + σres ) Y

• 1 C1/2

σapp = Applied stress σres = Residual Stress σf = Failure Stress KIc = Fracture Toughness Griffjth’s Law for Tempered Glass [1] : Negative compressive residual stress increases the fracture toughness in tempered glass.

SEM: Initiation

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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

SEM: Propagation

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Striations Possible sign of fatigue Show direction of propagation: lat- erally outward from this area

Mechanism 1: Dynamic fracture from impact

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(a) Mirror, (b) Mist, and (c) Hackle regions in a dynamic glass fracture [1}. SEM of Sample

(a) (b) (c)

Mechanism 1: Dynamic fracture from impact

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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. σf Rm/m = Am/m [2]

Rm/m = fracture mirror radius Am/m=2.08 MPa√m (Pyrex)

Mechanism 2: Flaw plus bending

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Initiation: Penny crack from impact Propagation: Bending

Product effectiveness

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Hardness (Mohs scale 1-10) 6-6.5 7 8-9 9 10 Material Gorilla Glass[5] Quartz[5] Screen Protector[6] Sapphire[5] Diamond[5]

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.

Sources

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[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.