Damage Tolerance and Durability of Adhesively Bonded … · 2006. 6. 27. · Damage Tolerance and...

The Joint Advanced Materials and Structures Center of Excellence

Damage Tolerance and Durability of Damage Tolerance and Durability of Adhesively Bonded Composite Adhesively Bonded Composite

StructuresStructuresHyonny Kim, Assistant Professor, School of Aeronautics & AstronaHyonny Kim, Assistant Professor, School of Aeronautics & Astronauticsutics

C.T. Sun, Professor, School of Aeronautics & AstronauticsC.T. Sun, Professor, School of Aeronautics & AstronauticsThomas Thomas SiegmundSiegmund, Associate Professor, School of Mechanical Engineering, Associate Professor, School of Mechanical Engineering

2Purdue University – Joint Advanced Materials and Structures Center of Excellence

Damage Tolerance and Durability of Adhesively Bonded Composite Structures

• Motivation and Key Issues– failure prediction of composite adhesive joints remains a difficult problem

• multiple failure modes and complex failure processes• damage initiation and growth influenced by geometry, loading, and environmental

factors such as moisture, temperature, etc.– damage in joints is difficult to detect – must design structures to be tolerant to

reasonably-sized flaws• accurate models are needed to predict failure and assess damage tolerance

• Objectives– investigate physical phenomena and processes leading to failure in adhesively

bonded joints– account for bondline thickness and environmental conditions– develop models describing these phenomena

• Approach: combined experimental/analytical investigations supporting development of models


FAA Sponsored Project Information

• Principle Investigators & Researchers– Hyonny Kim– C. T. Sun– Thomas Siegmund

– Post-Doc: Steffen Brinkmann– Graduate Students: Haiyang Qian, Jungmin Lee,

Richard Khoo, Hee Seok Roh, Jibin Han (grad. 12/05)

• FAA Technical Monitor– Peter Shyprykevich


Focus Areas Towards Achieving Objectives:

– Adhesive constitutive behavior for use in bonded joint analyses

– Effect of adhesive thickness on mixed mode fracture of joints

– Effect of bondline thickness on strength of adhesively bonded joints – CTOA approach

– Influence of moisture and bondline thickness on joint fracture


Project I

Adhesive Constitutive Behavior in Bonded Joints

Hyonny Kim, Assistant [email protected]

Jungmin Lee and Hee Seok Roh, Graduate Students


Background and Objectives

Background• nonlinear adhesive constitutive behavior is needed to conduct modeling/analysis – e.g., FEA

– choice of constitutive curve is not clear• adhesive τ vs γ measured by ASTM D5656:

– exhibits bond thickness dependency– criticized as being inconsistent at ASTM

Symposium on Joining and Repair of Composites (March 2003), and at FAA Adhesive Joints Workshop (June 2004)

•• material propertymaterial property should be geometry independent

Objectives:• understand why ASTM D5656 behavior is

bondline-thickness dependent• establish more direct and simple test method

for determining constitutive behavior: tensile dogbone, t.b.d. method

• resolve differences observed between tensile dogbone test & ASTM D5656

Shear Stress vs. Shear Strain Relationship for PTM&W ES6292 Measured by ASTM D5656

Test Method


Modified ASTM D5656 Joint Tests

• Average shear strain =

• Average shear stress =

relative displacement adhesive thickness

applied load area of test section

Relative Displacements Measured by Laser Extensometer

ApplyCorrection

Test Specimen Grip Fixture


D5656 Test Results

• D5656 test data show strong bondline thickness dependency

• global rotation of joint is minimal (< 0.5°)


0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.160

1000

2000

3000

4000

5000

6000

7000

εx

σx (p

si)

displacement rate: 0.01 in./min displacement rate: 0.05 in./min

Bulk Adhesive Constitutive Behavior – Tensile Dogbone Tests

Cytec FM 73 film adhesive

0.000 0.005 0.010 0.015 0.020 0.0250

1000

2000

3000

4000

5000

6000

Stre

ss, p

si

Strain

0.3 in/min

0.05 in/min

1.5 in/min

PTM&W ES6292 epoxy paste adhesive

Constitutive Behavior is Strain Rate Dependent


Finite Element Modeling

0.00 0.05 0.10 0.15 0.20

0

1000

2000

3000

4000

5000

Ave

rage

She

ar S

tress

, τ (p

si)

Average Shear Strain

20mil 30mil 40mil 20mil_Test 30mil_Test 40mil_Test

• bulk tensile coupon constitutive data used to model D5656 test

• FEA models not showing bondline thickness dependency– need to account for strain rate

dependency and damage evolution


P

P

Failed Adhesive Material

Discussion

• strain rate dependency:– adhesive exhibits strain rate

dependency– strain rate in joint ~ 10-1 s-1

– strain rate in bulk tensile coupon less than ~10-3 s-1

– must model adhesive using viscoplasticmaterial (Zgoul M. and Crocombe 2004)

• localized damage evolution:– highly constrained bondline permits

localized failure prior to joint final failure– increased compliance – effectively

showing plastic “plateau” and large final failure strain in D5656 tests run under displacement control

– FEA models must capture this phenomenon


Summary

• D5656 thick adherend data measured for PTM&W ES 6292 adhesive– show strong bondline thickness dependency

• bulk tensile coupons tested to measure adhesive constitutive behavior directly

• FEA models of D5656 specimens using bulk-measured tensile data predicts only initial portion of specimen behavior

• issues exist:– premature failure of bulk tensile specimens – not measuring entire

constitutive behavior• improved test is needed

– to replicate D5656 data using bulk tensile coupon data, FEA modeling must account for

• strain rate dependency• localized damage evolution


Project II

Effect of Adhesive Thickness on Mixed Mode Fracture of Joints

Hyonny Kim, Assistant [email protected]

Richard Khoo, Graduate Student


Background and Objectives

Background• fracture mechanics is capable tool

for damage tolerance analysis• need mixed mode strain energy

release rate (SERR) data

Objectives• measure mixed mode SERR for

range of bondline thickness– Mixed Mode Bending

(MMB), DCB, ENF• observe processes occurring at

crack tip• use modeling/analysis to

understand bondline effect in measured data – establish fracture criteria in joints that accounts for bondline thickness dependent GICand GIIC


Experiments

• Gc measured for range of mode I and mode II mix ratios– Double Cantilever Beam (DCB) – pure mode I– Mixed Mode Bending (MMB)– End Notched Flexure (ENF) – pure mode II

• test specimen details– adherends: 2024-T4 Al alloy, 0.25 x 1.0 x 6.0 in.– adhesive: PTM&W ES6292 epoxy paste adhesive– bondline thickness range: 0.008 to 0.060 in.

• test matrix

Mode Mix (% mode II)

ta = 0.008 in.

ta = 0.020 in.

ta = 0.040 in.

ta = 0.060 in.

0 done more tests to-do done

50 done done to-do more tests

75 done done to-do more tests

100 done done to-do done


Results I – Overall GC Trend


Results II – GIC vs. ta

1.91

1.62

1.89

1.72

2.77

1.71

2.45

2.24

3.44

2.35

2.56

2.98

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 10 20 30 40 50 60 70

Thickness (mil)

GIc

(lbs

/in)


Results III – 50% Mode II GC vs. ta

3.22

3.72

4.05

5.25

4

4.404.374.36

4.69

2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

0 10 20 30 40 50 60 70

Thickness (mil)

Gto

tc (l

b/in

)


Results IV – 75% Mode II GC vs. ta

5.265.56

4.86

4.17

3.25

3.8

5.545.43

4.77

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 10 20 30 40 50 60 70

Thickness (mil)

Gto

tc (l

b/in

)


Results V – GIIC vs. ta

10.94

9.34

10.869.99

2.083.072.62

6.917.036.74

23.62

22.01

1.29

3.78

21.52

2.46

6.79

0.00

5.00

10.00

15.00

20.00

25.00

0 10 20 30 40 50 60 70

Thickness (mil)

GIIc

(in/

lbs)

Rough failure surface Smooth failure Surface


Discussion

• failure modes– all specimens exhibited cohesive failure

• data omitted if any amount of adhesion (clean interface) failure observed– stable crack growth – leaves behind rough fracture surface

• pure mode II: 20 and 60 mil bondline specimens exhibited bimodal behavior– stable growth – rough fracture surface; GIIC ~ 10 - 22 lb/in– unstable growth – smooth fracture surface ; GIIC ~ 2.5 lb/in

Specimen P100-060-01

growth along center of adhesive

Specimen P100-060-10

growth near upper adherend interface


Plastic Zone Development

• significant plastic strain developed ahead of crack tip prior to growth• confinement of plastic zone by adherends known to play key role in fracture

Microscope Field of View:

Crack Tip

Pure Mode II LoadingBondline Thickness: 0.060 in.

Initial Growth Initiation


Summary

• GC measured as function of mode mixity (modes I and II), and bondline thickness

• 8 mil bondline exhibits monotonically increasing GC for higher mode II content

• 20 and 60 mil bondlines exhibit bimodal behavior for 100% mode II– stable growth / rough fracture surface – high GIIC

– unstable growth / smooth fracture surface –low GIIC

• large plastic deformation observed to develop ahead of crack tip• FEA modeling of fracture tests is under-way to quantify plastic zone size

and confinement/interaction with adherends– validation to be achieved via comparison with image-analysis

measurements of shear strain


Project III

Effect of Bondline Thickness on Strength of Adhesively Bonded Joints

C. T. Sun, [email protected]

Hiayang Qian, Graduate Student


Objectives

To understand the mechanism that effects the thickness-dependent

joint strength behavior in adhesively bonded joints

To develop a CTOA approach for predicting crack growth in bonded

joints with the capability of accounting for the effect of bondline

thickness


T

T

l

L

t

adhesvie

Adherend: Aluminum Alloy 7075

Adhesive: PTM&W ES6292

Surface Treatment: Semco Pasa-Jell 105 (etching method)

L=3in

l=1in

T=0.125in

t=0.008in, 0.01in, 0.02in, 0.06in

Single Lap Joint Specimen Configuration for Strength Test


The joint strength decreases as the adhesive thickness increases

0

1

2

3

4

5

6

7

0 20 40 60Bond Line Thickness (mil)

Stre

ngth

of J

oint

s (k

N)

Single Lap Joint Strength vs. Adhesive Thickness


Thickness of adhesive

Adhesive: Hysol EA9394

Thickness range: 27mil-120mil

Steel Hinge

L

t

l

Wta

Total length of the specimen: 4 in

Pre-crack length: 1.5 in

Adherend: Aluminum 7075

Adherend thickness: 125mil (0.125 in)

DCB Specimen for Fracture Test


CTOA Measuring with Crack Propagation

0.2 mm

Before crack initiation Initial State

Crack Propagation


012345678

0 5 10 15 20 25

Crack Extension (mm)

CO

TA (D

egre

e)

50mil 60mil90mil 120mil31mil 27mil

( )( )7.52.4

7.574.68303.01253.001.00003.0{

234

≥≤+−+−

=xxxxxx

CTOA

CTOA Curve is Independent of Bondline Thickness


0

20

40

60

80

100

120

140

0 5 10Opening End Displacement (mm)

Load

(N)

30mil50mil130mil

Load and Displacement at the Opening End of the Specimens

Effect of Bondline Thickess on DCBFracture Load


Crack tip

Aluminum

Ll

t

taAdhesive materials

P

CTOA Calculation

DCB Model and CTOA Calculation


0

2

4

6

8

10

12

0 20 40 60 80 100 120Load (N)

Ope

ning

Ang

le (D

egre

e)

6mil20mil60mil

Effect of Bondline Thickness on CTOA


0

5

10

15

20

25

30

35

40

45

-30 -10 10 30 50 70

0.01mm

0.01

mm

6mil20mil60mil

0

5

10

15

20

25

30

35

40

45

-30 -10 10 30 50 700.01mm

0.01

mm

6mil20mil60mil

Confinement of Plastic Zone

05

1015202530354045

-30 -10 10 30 50 70

0.01mm

0.01

mm

6mil20mil60mil

Plastic Zone under 35N


6mil Boundary

6mil Boundary

20mil Boundary

20mil Boundary

6mil Boundary



Interfacial Stresses Increase asThickness Decreases

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

Applied Load (N)

Inte

rfaci

al N

orm

al S

tress

es (M

Pa) s22-6mil

s22-20mils22-60mil

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120

Applied Load (N)

Inte

rfact

ial S

hear

Stre

sses

(MPa

)

s12-6mils12-20mils12-60mil

Maximum Normal Stresses Maximum Shear Stresses

Maximum Normal Stress Maximum Shear Stress


Summary of Results

• Strength of single lap joint increases as bondline thickness increases

• CTOA for crack growth in adhesive is independent of bondline thickness

• In DCB fracture test, toughness increases as bondline thickness decreases. This result may be explained in terms of greater confinement of crack tip plastic zone in thinner bondline case

• For thinner bondlines the interfacial stresses between the adhesive and adherend are higher than those for thicker bondlines. It is possible that interfacial strength failure may precede crack extension leading to a lower failure load in joints with thinner bondlines.


Project IV

Influence of Bondline Thickness and Moisture on Joint Fracture

Thomas Siegmund, Associate [email protected]

Steffen Brinckmann, Post Doctoral Research AssociateJibin Han, (PhD 12/2005)

Eric Anderson, SURF Summer Student


Damage Tolerance and Durability of Adhesively Bonded Composite Structures

• Project goals:– Develop and employ the cohesive zone model approach to fracture to the

analysis of adhesive joint failure• Major achievements/conclusions to date:

Test procedure to determine cohesive zone model parameters undermonotonic loadingTransferability of test data between independent crack growth testsTest procedure for moisture degradationCoupled cohesive zone model for moisture/load interaction3D model implementation

• Benefits the aviation industry: – CZ model approach well established in e.g. microelectronics, civil engineering– Aid in establishing approach to aviation industry– Establish approach to long term problems (fatigue, environmental

degradation) to reduce testing time



• The Cohesive Zone Model:– Describes local energy dissipation during fracture and fatigue– Is conveniently coupled to other fields (moisture, heat, electrical…)

F

F

Global Parameters:• Force (F) – Displacement (COD)• Environment (H2O)

COD

H2O

Δ

T

T

Local Parameters:• Traction (T) – Separation (Δ)• H2O Concentration C(H2O)

C(H2O)

Finite element model withcohesive elements & H2O transport

Adherent

Adhesive

CZ ElementsDiffusion Elements



Experimental Set-up

Crack Growth Resistance Environmental Degradation

Displacements andStrain fields

Force –Displacement

Record

Finite Element Method with

Cohesive Zone


Record

SpeckleImages

Displacements andStrain fields


Record

Finite Element Method with

Cohesive Zone


Record

SpeckleImages



Transferability of CZ Model

0

10

20

30

40

50

60

70

80

90

100

0 0.02 0.04 0.06

0.508mm1.524mm3.048mmcz law

Δn [mm]

T n [M

Pa]

0

10

20

30

40

50

60

70

80

90

100

0 0.02 0.04 0.06

0.508mm1.524mm3.048mmcz law

Δn [mm]

T n [M

Pa]

F

F

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0.0 5.0 10.0 15.0 20.0 25.0

Δa [mm]

CTO

A [d

egre

e]

Experiment (3.048mm)

Experiment (1.524mm)

Simulation_2D

Simulation_2D

Simulation_3D_on surface

From large specimens

model for smallspecimens

model for large specimens

L=50 mmb=10 mmt=3.175 mma0=25 mm

Experiment in Lab Siegmund Experiment in Lab Sun

L=125 mmb=17 mmt=3.175 mma0=38 mm

G( )( )nTCTOD∗

∂Δ =

∂



Moisture Degradation: Experiments0 hours 24 hours: Stable crack extension

a0 a0+Δa

1 mm

Unstable crack extension1 mm



Moisture Degradation: Experiments

25 μm/day=18 cm/20 years



Moisture Degradation: Simulation

As aggressive environment (moisture) enters the crack, it enhances the crack growth.

Diffusion of water:• In the crack the water concentration is 100%.• In the Crack Process Zone (CPZ) the water concentration reduces.• In the virgin material the water concentration is 0%.

The diffusion depends on the opening of the crack in the CPZ. At sites where the crack is wide open, water diffuses fast. And vice versa.




As moisture enters the crackprocess zone, the polymer ligaments loose their strength.In the current model they retain36% of their strength at fullsaturation with moisture.

Implementation: Coupled mechanical – transport solution using ABAQUS




deflection of DCB

hold deflection constant

add moisture


0

0.25

0.5

0.75

1

1E+0 1E+1 1E+2 1E+3 1E+4 1E+5

N (Cycles)0.

5 Δσ

/ σm

ax,0

σmean= 0


• A Cohesive Zone Model for Fatigue Failure

predicted S-N curve

B-737 composite stabilizer after 18 years of service (CECAM Bulletin)

σmax=σmax,0(1-D)Damage law

Goal Year #3

Previou

s Res

ults


A Look Forward

• Benefit to Aviation – in response to increasing use of adhesive bonding– supports use of more sophisticated computation-based design and

analysis tools• failure process prediction, including adhesive plasticity• CTOA criterion simple to implement• VCCT and cohesive zone (cracked & un-cracked) now available in

commercial codes• simulation tools can reduce time to conduct extensive environmental

degradation tests– addressing important issues of bondline thickness

• quantify phenomena governing why “properties” seemingly depend on bondline thickness

• definition and use of local failure criteria that are not bondline thickness dependent

– simpler test methods to obtain fracture and constitutive data• seeking to define simpler tests and remove necessity to collect data as

function of bond thickness


A Look Forward

• Future Needs– account for strain rate dependency and localized failure evolution in

constitutive modeling of adhesive – demonstrate transferability to joints of generic configuration

– quantify mixed mode fracture tests via local criterion – e.g., CTOA or CZ– experimentally characterize the interfacial strength between the

adhesive and adherends– fatigue crack growth characterization– investigate other adherend (namely composite) and adhesive types

and failure modes: interfacial (a.k.a. adhesion) and mixed interfacial/cohesive failure + composite failure

– use the developed CTOA and CZ approaches to further investigate the competing nature of interfacial strength and fracture toughness of the adhesive in determining performance of bonded joints

– theoretically study the adhesive properties and bondline thickness for optimal performance of bonded joints

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Damage Tolerance and Durability of Adhesively Bonded … · 2006. 6. 27. · Damage Tolerance and...

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