nist 2012 mechanical reliability glaesemann
DESCRIPTION
The Strength of Thin Flexible Glass SheetsFlexible Printed Electronics MetrologySeptember 12 -13, 2012National Institute of Standards and TechnologyG. ScottTRANSCRIPT
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The Strength of Thin Flexible Glass Sheets
Flexible Printed Electronics Metrology September 12 -13, 2012National Institute of Standards and Technology
G. Scott GlaesemannScience and TechnologySullivan Park
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Flexible Electronics Applications Continuing to Emerge
Device applications include: Display (e-paper, color filter, OLED, LCD) Touch sensor Photovoltaic Lighting
Processes include: high-resolution and high-registration patterning and printing
Glass substrate opportunities progressing toward flexible sheets and web
Flexible Glass Sheets Flexible Glass Web
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Sufficient Strength to Survive ProcessingContinuous R2R ITO patterning has been demonstrated
CoolingDrum
ITO Deposition Slot Die Coating
Supply Roll Take-Up Roll
Exposure
Supply Roll Take-Up Roll
Development & Etch
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S. Garner, et al., Flexible glass substrates for continuous manufacturing, Flexible Electronics and Displays Conference, February 9, 2011.
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The strength of glass is controlled by flaws Strength is the stress on the glass at failure Function of flaw depth and fracture toughness
Glass strength is dependent on flaw depth more than mechanical properties Surface flaws from handling-induced damage are the most common Strength is primarily determined by the glass objects handling history
Strength is more likely to go down than up during the life of an object
Glass Mechanical Strength
0.01
0.1
1
10
100
Stre
ngth
(GPa
) Theoretical TypicalOptical Fiber
TypicalBulk Glass
4
=
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MaterialMaterialFracture Toughness
(MPa m)Fracture Toughness
(MPa m)Soda-Lime SilicateSilicaGlass CeramicsAl2O3PSZ AlloysWhisker Reinforced Ceramics4340 Steel
Soda-Lime SilicateSilicaGlass CeramicsAl2O3PSZ AlloysWhisker Reinforced Ceramics4340 Steel
0.5 - 0.70.75 - 0.81 - 32 - 66 - >126 - >12
90
0.5 - 0.70.75 - 0.81 - 32 - 66 - >126 - >12
90
Fracture Toughness the resistance to crack propagation
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Sharp Impact
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How Glass Responds to Sharp ContactB. Lawn, Fracture of Brittle Solids, Cambridge Univ. Press, 1993.
Median crack Lateral crack
Loading Unloading
Idealized Median cracks
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Chatter MarksChatter Marks
Sliding Contact Produces Frictive Damage
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Scratches are accompanied by lateral cracks and chipping
Scratch with Lateral Cracks
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Predicted Strength vs. Flaw Depth
IC f cK Y aV
0
20
40
60
80
100
120
140
160
180
200
0 10 20 30 40 50 60 70 80 90
Flaw Depth, microns
Pred
icte
d St
reng
th, M
Pa
Y=1.12 sqrt(pi)
Y=sqrt (pi)
Y=0.73 sqrt (pi) Newman Raju
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Printed Electronics Manufacturing
In-Device
Spool Storage and Shipping
Glass Manufacturing
Localized high stress surface and edge flexure
&Potential global bending
Short duration global bend stress (R2R)
Edge flaws: separation and guiding
Surface flaws: deposition, rollers
Long duration global bend stress
Process Step Stress Events Flaw Introduction
Initial Surface and Edge flaws created
Potential for edge and surface damage
Short duration global bend stress
Contact damage during installation
Failure Scenarios
Mechanical Failure
Initial flaws cannot survive manuf bend
stresses
Fatigue failure from previous flaw or shipping flaws
Global bend stress exceeds strength of prev and process
induced flaw populations
*Localized surface stress exceeds strength of process
induced surface flaws*Over stress edge flaws from
separation process
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Bending: Most Common Mode for Generating Stress
Bend stress can be generated by: Bulk bending Localized contact induced flexure
Maximum bend-induced tensile stress at surface, y=t/2 Dependent on thickness, modulus, and radius
RyE
bt
E2R
Compression
Tension
M Mt/2y
Youngs Modulus (approximate values)Steel 200 GPaCopper 110-128 GPaAluminum ~70 GPaGlass 50-90 GPaPolymer
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RyE
Bending induced stress
bt
E2R
Constant Bend Radius
compression
tension
M Mt/2y
Maximum bend-induced tensile stress at surface, y=t/2
0
50
100
150
200
250
300
350
400
450
500
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Bend Radius, mm
Ben
d St
ress
, MPa
1 mm
0.7 mm
0.5 mm
0.3 mm
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Reliability Design Diagram for Glass in Bending
hours
1050
900
750
600
450
300
150
seconds
700
600
500
400
300
200
100
0
50
100
150
200
250
300
350
400
0 5 10 15 20
Bend Radius, cm
Bend
Stre
ss, M
Pa
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Req
uire
d In
ert S
treng
th, M
Pa
10 micron25 micron50 micron75 micron100 micron125 micronhoursseconds125 proof
lifetime
thickness
E = 70 GPa
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Glaesemann and Gulati, Opt Engr (30) 6 1991
What is the strength of flexible glass sheets?
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Measurement of Glass strength Ring-on-Ring
Ring-on-Ring test (ASTM C1499) Maximum stress in region below inner ring on other side
Limitations: Thin glass (with deflection greater than half thickness at failure) shows significant non-linear effects
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Ring-on-Ring Testing and Large Deflections
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Gulati et al., Overview of Strength Tests for LCD Substrates and Panels
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Large Deflections during Ring-on-Ring Strength Test
Stress(MPa)
Load (N)
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Ball on Clamped Ring
0 200 400 600
0
500
1000
Stre
ss (M
Pa)
Load (N)
Stress under ball
Membrane Stress
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Ball on Clamped Ring Contact Area
4 mm
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0 100 200 300 400 500 600-200-100
0100200300400500600700800900
1000110012001300140015001600
Stre
ss (M
Pa)
Load (N)
Measured Stress Under Ball
0.3mm
MaximumBend-InducedStress
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Wrapping around Ball
Membrane
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Glass Surface Strength is Independent of ThicknessNew test methods developed for flexible substrates
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G.S. Glaesemann, et al., The Strength of Thin Fusion Drawn Glass Sheets, 11th ESG Conference 2012, Maastricht, The NetherlandsStrength, MPa
Failu
re P
roba
bilit
y, %
100 1000010001.E-1
5.E-1
1
5
10
50
90
99
1.E-1
100 Pm
200 Pm
300 Pm
Strength (MPa)
Failu
re P
roba
bilit
y (%
)
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Measurement of Glass strength 4 point bend
Four point bend strength (ASTM C158) Maximum stress in region between loading points (opposite
surface) Edges are often weaker than surface Effectively tests only two bottom edges
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Two Point Bend Concept
Upper Platen
Lower Platen
Glass Specimen
Upper Platen
Lower Platen
Glass Specimen
Bend a piece of glass between two platens
Elastic Beam Theory describes basic behavior
Concept is not new (its been around for decades)
Used extensively in the fiber industry
New concept: Use high speed video to capture failure location
Upper Platen
Lower Platen
Glass Specimen Failure Location
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Two Point Bend Test Setup
Specimen goes here
CameraLight Source
Mirror
Load Frame
Upper Platen
Lower Platen
Camera Rail
Screen
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Strength, MPa
Failu
re P
roba
bilit
y, %
10 10000100 10001.E-1
5.E-1
1
5
10
50
90
100
1.E-1
Edge Strength after Mechanical Score and Break of 100 m thick Glass
Scoreside
Breakside
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Surface failures as suspensions
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The Fracture Surface
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Branching
Mirror Mist Hackle BranchingMirror Mist Hackle Branching
Mirror
Mist
Hackle(points to origin)
Flaw Origin
The Fracture Surface
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Velocity Mist / Mist Hackle
Mist hackles occur beyond the mirror region and appear as a gray-matte surface. These markings occur when the fracture approaches terminal velocity. This feature is not present on low stressed breaks.
The shape of the hackle is indicative of the stress that was associated with the break. The above sketches are the extremes - the shape of the mist region can vary.
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Origin
Bending Mist / Velocity Hackle
Bending Mist Hackle
Secondary Wallner Lines
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540 MPa 320 MPa
Strength Determined from Mirror Radius
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250 MPa 210 MPa
Return to Mirror
Strength Determined from Mirror Radius
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Glass Mechanical Reliability Key Principles
Every glass object has strength-limiting flaws Machining, polishing Glass chips scratching surface Flaws potentially introduced at every handling step
Strength is not a pure, material property Controlled by flaws Determined by manufacturing and handling history Statistical in nature
Strength is always a characterization of a flaw population We never measure flawless glass Strength is always measured after some damage has been done
Surface flaw due to contact
Edge flaw due to cutting
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Strength Testing Flexible Glass
Match strength testing to the failure mode of interest Failure mode 1st
Strength testing 2nd
Novel strength testing methods Surface Strength Ball on Clamped Ring Edge Strength Modified 2 point Bend
Fractography Cause of failure Foundational method for improving strength
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