John Tomblin Waruna Seneviratne Paulo Escobar Yoon-Khian Yap Pierre Harter National Institute for...

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 by

FAA

Partnered with

Purdue 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)

Ap

par

ent

Sh

ear

Str

eng

th (

psi

)

ASTM D1002

ASTM D3165

ASTM 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 Joint characterization

– 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 ES6292

Bondline 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

Sh

ear

Str

ess

t = 0.013 in

t = 0.043 in

t = 0.083 in

t = 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 0.8

Shear Strain

She

ar S

tres

s (p

si)

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

Film Adhesive

She

ar S

treng

th (k

si)

RTD

ETD

ETW

0

1

2

3

4

5

6

7

Paste Adhesive

Sh

ear S

treng

th (

ksi)

RTD

ETD

ETW

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

Film Adhesive

She

ar M

odul

us (

Msi

)

RTD

ETD

ETW

0.00

0.03

0.05

0.08

0.10

0.13

0.15

0.18

0.20

Paste Adhesive

She

ar M

odul

us (M

si)

RTD

ETD

ETW

Film Adhesive

Paste Adhesive

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



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% LL

10000Cy SL2≈72% UL ≈202% LL

1000Cy 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 i

2 117 p s i

19 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


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


Lin e a r Lim it Lo a d2 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 U

ltim

ate

RTD

RTW

CTD

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 U

ltim

ate

RTD

RTW

CTD

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 U

ltim

ate

RTD

RTW

CTD

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

Ge ’

G1 ’

G2 2

G1



Modified ALCOA Stressing Fixture

tt

Calibration for each environmental condition

Test Results Format

P

L o a d C e l l

P

L o a d C e l l

t)

t)

t)

t)

Time

Time



Stress Level DeterminationShear

Str

ess

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)

Str

ess

(psi

)

10% YS

15% YS

25% YS

0

50

100

150

200

250

0 10000 20000 30000 40000 50000

Time (sec)

Str

ess

(psi

)

25% YS

10% YS

15% YS

0

20

40

60

80

100

120

0 10000 20000 30000 40000 50000

Time (sec)

Str

ess

(psi

)10% YS

15% YS

25% YS

150 °F

180 °F

210 °F



Box Beam Lap Shear Torsion Test

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)

Ma

x. S

hea

r F

low

(lb

f/in

)

q-Experimental

q-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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