RESEARCH / DESIGN / TESTING / CERTIFICATION Adhesive ... · NATIONAL INSTITUTE FOR AVIATION...

John TomblinWaruna Seneviratne

Paulo EscobarYoon-Khian Yap

Pierre Harter

National Institute for Aviation Research

Wichita State University

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FAA Workshop on Key Characteristics for Advanced Material Control

September 16 – 18, 2003

RESEARCH / DESIGN / TESTING / CERTIFICATION

Adhesive Behavior in Aircraft ApplicationsAdhesive Behavior in Aircraft Applications

NATIONAL INSTITUTE FOR AVIATION RESEARCH


Program Overview• Coupon Level Testing

– Investigation of Thick Bondline Adhesive Joints• Adhesive test methods• Bondline thickness effects• Environmental effects

[FAA Report: DOT/FAA/AR-01/33]– Characteristic Shear Responses of Structural Adhesives

[FAA Report: DOT/FAA/AR-02/97]– Fatigue & Stress Relaxation of Adhesive Joints

[FAA Report: Submitted to FAA]

• Subcomponent Testing & Analysis– Box Beam Torsion Lap Shear Test– Shear Loaded Bonded Joint (SLBJ) Theory [Purdue University]

[FAA Report: DOT/FAA/AR-03/21]

Available electronically at http://actlibrary.tc.faa.gov



Research Effort

Industry Partners

Funded byFAA

Partnered withPurdue University



Motivation

• Number of certification programs involve a large range of adhesive bonding applications

• Migration from secondary to primary structure

• Limited guidance material existed• Limited experimental analytical

models that can be effectively used in design



Motivation (contd..)

• Traditional bondline thicknesses used : less than 0.010”

• Current bondline thicknesses : up to 0.140”

• Generate data regarding the effects of thick bondlines

• Long term durability of adhesive joint (fatigue/creep) needs to be addressed with respect to thick bondline joints

A

A

Section A-A

CloseoutBonded toSpar Caps

Pinned JointsThrough Spar

WebWing Skin BondedDirectly to Spar Caps

Bending andTorsion Loads



Adhesive Test Methods

0.0

1000.0

2000.0

3000.0

4000.0

5000.0

6000.0

7000.0

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Average bondline thickness (in)

App

aren

t She

ar S

tren

gth

(psi

)

ASTM D1002ASTM D3165ASTM D5656

Adhesive/Cohesive

Adhesive Adhesive

Adhesive/Cohesive

• ASTM D1002 & D3165 for joint characterization• ASTM D5656 for adhesive characterization



ASTM D5656 Test Method

• Thick adherend– Adhesive characterization

rather than Jointcharacterization

– Elastic Limit & Plastic Strain– Design & Analysis– Reduced peel stresses

• Correction for metal deformation

• Four-Pin Configuration– Reduces errors due to rotation

and slippage– Reduced scatter in data



Failure Modes

ASTM D5656



Bondline Thickness Effects

• Increasing bondline thickness resulted in reduced plastic strain and lower yield stress

PTM&W ES6292Bondline Thickness

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Shear Strain

Shea

r Str

ess

t = 0.013 int = 0.043 int = 0.083 int = 0.123 in



Environmental Effects

• Yield stress and stiffness decreased with increasing temperature and humidity

• Environmental condition affects failure mode

MGS A100/B100

0

1000

2000

3000

4000

5000

6000

7000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Shear Strain

She

ar S

tress

(psi

)

CTD

RTD

ETD 160°

ETW 160°

ETD 200°

ETW 200°

Bondline Thickness = 0.013" nom.Tg (dry) = 175°FTg (wet) = 135°F



Characteristic Shear Responses of Structural Adhesives

• 18 Adhesive Types– 6 Film Adhesives– 12 Paste Adhesives

• ASTM D5656 [4 pin holes]• Three Environmental

Conditions– Room Temp. ambient [RTD]– Elevated Temp. (180°F) dry [ETD]– Elevated Temp. (180°F) wet [ETW]

• 145 °F and 85% relative humidity for 1000 hrs

• Bondline Thickness– Film Adhesives: 0.01” – 0.03”– Paste Adhesives: 0.03” – 0.05”



Adhesive Types Investigated

Film Adhesives (6)– AF 126– EA 9628– EA 9695– EA 9696– FM 300– FM 73

Paste Adhesives (12)– EA 9309.3 NA– EA 9346.5– EA 9359.3– EA 9360– EA 9392– EA 9394– EA 9396– MGS L418– PTM&W ES 6292– 3M DP-460 EG– 3M DP-460 NS– 3M DP-820

Adhesives & Aluminum sub-panels (Phosphoric Anodized) were provided by Cessna Aircraft, Wichita, KS



Apparent Shear Strength Comparison

0

1

2

3

4

5

6

7

AF 126

AF 126

EA 9628

EA 9628

EA 9695

EA 9695

EA 9696

FM 300

FM 300

FM 73

FM 73

Film Adhesive

She

ar S

treng

th (k

si)

RTDETDETW

0

1

2

3

4

5

6

7

EA 9309.3

NA

EA 9346

.5EA 93

59.3

EA 9359.3

EA 9360

EA 9360

EA 9360

EA 9392

EA 9392

EA 9394

EA 9396

MGS L418

PTM&W

ES 62

923M

DP-46

0 EG

3M D

P-460 N

S3M D

P-820

Paste Adhesive

She

ar S

treng

th (k

si)

RTDETDETW

Film Adhesive

Paste Adhesive



Shear Modulus Comparison

0.00

0.03

0.05

0.08

0.10

0.13

0.15

0.18

0.20

AF 126

AF 126

EA 9628

EA 9628

EA 9695

EA 9695

EA 9696

FM 300

FM 300

FM 73

FM 73

Film Adhesive

Shea

r M

odul

us (M

si)

RTDETDETW

0.00

0.03

0.05

0.08

0.10

0.13

0.15

0.18

0.20

EA 9309

.3 NA

EA 9346.5

EA 9359

.3EA 93

59.3

EA 9360

EA 9360

EA 9360

EA 9392

EA 9392

EA 9394

EA 9396

MGS L418

PTM&W

ES 62

923M

DP-46

0 EG

3M D

P-460 N

S3M D

P-820

Paste Adhesive

She

ar M

odul

us (M

si)

RTDETDETW

Film Adhesive

Paste Adhesive

FAA Final Report: DOT/FAA/ARFAA Final Report: DOT/FAA/AR--02/9702/97



Fatigue of Thick Bondline Adhesive Joints

Modified ASTM D3166-99[Aluminum Adherend of 0.375”]

• Three Adhesives– PTM&W [0.060” & 0.160”]– Loctite [0.032”]– EA9696 [0.02”]

• Three Stress Levels– 103, 104 and 105 cycles

• Three Frequencies– F=2 Hz, 5 Hz and 10 Hz

• Three Environmental Conditions

– RTD, RTW– CTD (-40°F)



Stress Level Determination

Based on the initial SN Curve

y=-3.227*ln(x)+100.96

100000Cy SL1≈65% UL ≈183% LL10000Cy SL2≈72% UL ≈202% LL1000Cy SL3≈78% UL ≈220% LL

Note: For RTW and CTD, %UL are different

0

10

20

30

40

50

60

70

80

90

100

0 200000 400000 600000 800000 1000000 1200000

Number of Cycles

% o

f Ulti

mat

e

Loctite (RTD)



Loctite Stress Levels

Fatigue life in a range below knee point and above linear limit point.

Loctite RTD

0

10 0 0

2 0 0 0

3 0 0 0

0 0 . 1 0 . 2 0 .3 0 . 4 0 .5 0 . 6

S h e a r S t ra in (in / in )

2 3 0 6 p s i2 117 p s i19 18 p s i

Lin e a r Limit Lo a d

3 9 3 lb s (1 04 8 p si)

Loctite CTD

0

10 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6

S h e a r S t ra in (in /in )

Lin e a r Lim it Lo a d5 7 6 lb s (15 3 6 p s i)

3 3 7 9 p s i3 10 2 p s i2 8 10 p s i

Loctite RTW

0

5 0 0

10 0 0

15 0 0

2 0 0 0

2 5 0 0

3 0 0 0

0 0 .1 0 .2 0 . 3 0 . 4 0 . 5 0 .6

S h e a r S t ra in (in /in )

Lin e a r Lim it Lo ad2 2 5 lb s (6 0 0 p s i)

13 2 4 p s i12 15 p s i110 1 p s i



Fatigue Behavior of Adhesives

2 Hz 5 Hz 10 Hz

40

45

50

55

60

65

70

75

80

85

90

95

100

100 1000 10000 100000 1000000 10000000

Number of Cycles

% o

f Ult

imat

e

RTDRTWCTD

SL1

SL1

SL1

SL2

SL2

SL2

SL3

SL3

SL3

40

45

50

55

60

65

70

75

80

85

90

95

100

100 1000 10000 100000 1000000 10000000Number of Cycles

% o

f Ult

imat

e

RTDRTWCTD

SL1

SL1

SL1

SL2

SL2

SL2

SL3

SL3

SL3

40

45

50

55

60

65

70

75

80

85

90

95

100

100 1000 10000 100000 1000000 10000000Number of Cycles

% o

f Ult

imat

e

RTDRTWCTD

SL1

SL1

SL1

SL2

SL2

SL2

SL3

SL3

SL3



Stress Relaxation of Adhesive Joints

• Applied stress gradually decreases to a stable value over time • Elastic strain that appears during initial rapid loading is slowly

replaced by creep strain, with the total of the two being constant • Steady-state creep and linear viscoelastic material behavior

0t

σ

Ge ε’

G1 ε’

σ0

Δσ

G2 η2

σ

G1

ε



Modified ALCOA Stressing Fixture

σ(t) = μ • 2 δ (t)

Calibration for each environmental condition

Test Results Format

δ

P

Load Cellδ

P

Load Cell

δ (t)

σ (t)

μ

δ (t)

σ (t)

μ

Time

σ

Time

σ



Stress Level DeterminationSh

ear

Stre

ss

Shear Strain

25% YS

15% YS

10% YS

Test Temperatures

150°F

180°F

210°FYield Stress



Creep Deformation

• Loctite– 25% YS– 180 °F– 167 hours

~18°[50X magnification]



Loctite Stress Relaxation Results

0

50

100

150

200

250

300

350

400

450

0 10000 20000 30000 40000 50000

Time (sec)

Stre

ss (p

si)

10% YS

15% YS

25% YS

0

50

100

150

200

250

0 10000 20000 30000 40000 50000

Time (sec)

Stre

ss (p

si)

25% YS

10% YS

15% YS

0

20

40

60

80

100

120

0 10000 20000 30000 40000 50000

Time (sec)

Stre

ss (p

si)

10% YS

15% YS

25% YS

150 °F

180 °F

210 °F



Box Beam Lap Shear Torsion Test

Load

Loading Plate

Side Plate (Inboard)

2½ Cylindrical Shaft

Fixed-end block

Side Plate (Outboard)

Pivot-end Base

Fixed-end Base

Joint Failure Prediction

Shear Loaded Bonded Joint

(SLBJ) Theory

non-linear constitute behavior

of adhesive

Box Beam Lap Shear Torsion

Testing

Validation

Design Guidelines & Certification



Adhesive Lap Joint Specimen

Flat JointPTM&WEA9360Loctite

Joggle JointPTM&WEA9360

Gage width ~ 0.5”Gage section ~ 17.25”



Materials

• Adhesives– PTM&W ES6292 [t = 0.05” ~ 0.20”]– EA 9360 [t = 0.10”]– Loctite (CESSNA Proprietary) [t = 0.05”]

• Adherend– NEWPORT E-Glass Fabric 7781 / NB321– NEWPORT NB321/3K70P Carbon Cloth

• Fiberglass/Carbon Layup Schedule – [04/45/-45/04]– Aluminum 2024-T3 Clad

• Phosphorus Anodized & Bond Primed[CESSNA Aircraft, Wichita, Kansas]



Maximum Shear Flow (Comparison)

Constitutive behavior for 0.20 was not available

0

500

1000

1500

2000

2500

0.00 0.05 0.10 0.15 0.20 0.25

Bondline Thickness (in)

Max

. She

ar F

low

(lbf

/in)

q-Experimentalq-SLBJ

ES6292 - Newport 7781FG



ConclusionsEnvironmental Effects• Adhesives become weak and ductile at high temperatures

and brittle at low temperatures. • Yield stress and modulus of all adhesives decrease with

increasing temperature and humidity • The plastic behavior of adhesives at elevated

temperatures caused significant shear deformation• Mechanical properties of adhesives can be substantially

degraded by the absorption of moisture • Environmental condition affects the failure mode as well

as the mechanical properties



Conclusions (Contd..)

Fatigue• ‘High stress’ fatigue life of adhesive exists in a range

below Knee point and above linear limit point• Failure modes indicate that moisture affects adhesive

bulk instead of the adhesive-adherend interface (RTW cohesive failures)

• Observation – lower void in bondline = longer fatigue life

• Film adhesive indicates better resistance to moisture (less voids?)

Stress Relaxation• Stress relaxation was increased as the stress level and

temperature was increased



Conclusions (Contd…)

Box Beam Lap Shear Torsion• Load carrying capabilities of adhesive joints decreases as

bondline thickness increases• Purdue Analysis predictions (SLBJ Theory) comparable

with box beam test results• Increasing bondline thickness affects the failure mode of

bonded joints• Accumulation of large plastic strains in thin bondlines

resulted in high adherend interlaminar strains and caused substrate (first-ply) failure

• Unstable damage development of thick bondlines (lower plastic strain development) resulted in adhesive cracking in multiple locations with a cohesive type failure and lower failure strengths

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