FAA Workshop Chicago 2003-09 1

Strength and Damage Tolerance ofAdhesively Bonded Joints:

Stress-Based and Fracture-Based Approaches

Hyonny KimAssistant Professor

School of Aeronautics and Astronautics,

Purdue University

presented at the

FAA Workshop on Key Characteristics for Advanced Material Control September 16-18, 2003, Chicago, IL

Funding Acknowledgements:

FAA, NASA AGATE, WSU/NIAR, NASA Langley Research Center, ASEE, Purdue Research Foundation

FAA Workshop Chicago 2003-09 2

Outline

Introduction Bonded Joint Stress Analyses

in-plane shear loading combined shear + tension loading plastic strain-based ultimate load variable bondline thickness tension loading in general unbalanced single lap joint

Damage Tolerance of Disbonded Joints – Buckling/Disbond Growth experiments models

New Directions

FAA Workshop Chicago 2003-09 3

Introduction – General Joint Loading

in-plane shear + tension produces adhesive shear stress

Nx

Nx

Ny

Ny

Nxy

Nxy

Tension Due toInternal Pressure

CompressionDue to Fuselage

Bending

Shear Due toFuselage Torsion

Segment of JointLoaded by Biaxial

Tension/Compressionand Shear

Bonded Doubler

FAA Workshop Chicago 2003-09 4

IntroductionDamage Tolerance: Buckling/Disbond Growth

compression causes buckling

subsequent disbond growth possible

FAA Workshop Chicago 2003-09 5

IntroductionDamage Tolerance: Buckling/Disbond Growth

Stringer-Stiffened Panel Wing Assembly

P

FAA Workshop Chicago 2003-09 6

Overarching Objectives

bridge knowledge gap between coupon data and full scale structural performance

provide engineering and scientific community with advancements in:

the mechanics of bonded joints

damage tolerance design and analysis philosophy

assist FAA and GA companies by providing research support

FAA Workshop Chicago 2003-09 7

In-Plane Shear Loaded Lap Joint

in-plane shear load transfer across joint produces

xz shear stress in adhesive

NxyNxy

Outer Adherend

Inner Adherend

z

x

y

-1 .0 -0.5 0.0 0.5 1.0y/c a t x = L /2

0

1

2

3

4

5

,

MP

a xza

Adhesive x-z Shearfor A lum inum Jo in t

FEA

SLBJ Theory

Kim, H. and Kedward, K.T., “Stress Analysis of Adhesive Bonded Joints Under In-Plane Shear Loading,” Journal of Adhesion, Vol. 76, No. 1, 2001, pp. 1-36.

FAA Workshop Chicago 2003-09 8

In-Plane Shear Loaded Doubler

inherently 2D problem

yz shear peaks along x = 0, a

xz shear peaks along y = 0, b (not shown)

x

yz

00

b

a

NxyNxy

Bonded Doubler- Outer Adherend

Base Structure- Inner Adherend

022 ooxy

oxy C

yt

oxy

oaxz

xt

oxy

oayz

Kim, H. and Kedward, K.T., “Stress Analysis of Adhesive Bonded Joints Under In-Plane Shear Loading,” Journal of Adhesion, Vol. 76, No. 1, 2001, pp. 1-36.

FAA Workshop Chicago 2003-09 9

Combined Loading: Shear + Tension

y

x y

0 200 400 600 800 1000 1200

|N | (lb /in )

0

200

400

600

800

|N |

(lb

/in) ta = 0 .01

Von-Mises Based Failure Envelope

NxyNxy

Outer Adherend

Inner Adherend

z

x

y

Ny

Ny

Kim, H. and Kedward, K.T., “The Design of In-Plane Shear and Tension Loaded Bonded Composite Lap Joints,” Journal of Composites Technology and Research, Vol. 24, No. 2, 2002, pp 297-307. Invited paper to special issue in honor of Don Oplinger.

FAA Workshop Chicago 2003-09 10

Plastic Strain-Based Ultimate Load

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

5

10

15

20

25

30

Shear Strain

Sh

ear

Str

ess

(MP

a)

DataFitting Curve

in-plane shear loading ductile adhesive joint carries greater

load than elastic-limit design more conservative than Hart-Smith

elastic-perfectly plastic model

adhesive test data from J. Tomblin, WSU

Kim, H. and Lee, J., “Adhesive Nonlinearity and the Prediction of Failure in Bonded Composite Lap Joints,” Joining and Repair of Composite Structures, ASTM Special Technical Publication STP1455, submitted January 2003.

FAA Workshop Chicago 2003-09 11

Failure Prediction vs. Test Data

box beam torsion lap shear coupon

experiments conducted by Wichita State University (John Tomblin)

Maximum Shear Flow

0

500

1000

1500

2000

2500

3000

0.00 0.05 0.10 0.15 0.20 0.25

Bondline Thickness (in)

Max

. Sh

ear

Flo

w (

lbf/

in)

Experimental Data

Purdue Analysis*

Constitutive behavior for 0.20 was not avail. Used 0.12 data, with reduced failure strain.

Tomblin, J., Seneviratne, W., Kim. H., and Lee, J., “Characterization of In-Plane Shear Loaded Adhesive Lap Joints: Experiments and Analysis,” FAA Final Report, DOT/FAA/AR-03/21, May 2003.

FAA Workshop Chicago 2003-09 12

Variable Bondline Thickness (VBT)

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VBT: Increased Stress Due to Bondline Thinning

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

t1 / tu n if

0

1

2

3

4

5

6

7

8

M in. f

M ax. f

f = 1

Kim, H. “The Influence of Adhesive Bondline Thickness Imperfections on Stresses in Composite Joints,” Journal of Adhesion, Vol. 79, No. 7, 2003, pp 621-642.

FAA Workshop Chicago 2003-09 14

Tension Loading of Unbalanced Single Lap

closed-form stress analysis of generalized unbalanced joint

Joint Specs: - glass/epoxy, - overlap 2c = 25.4 mm, - to = 2.49 mm, ti = 2to, ta = 0.33 mm

FAA Workshop Chicago 2003-09 15

Damage Tolerance of Disbonded Joints

damage tolerance is safety concern thin flanges

typically 1 mm (0.04 in.) susceptible to buckling + disbond growth

disbond not easily detectable adhesive properties affect growth critically

processing of joint very important

FAA Workshop Chicago 2003-09 16

Buckling/Debonding Experiments

Construction: glass/epoxy &

carbon/epoxy face sheets 6.3 mm foam core two halves secondarily

bonded with PTM&W ES6292 paste adhesive

FAA Workshop Chicago 2003-09 17

Experimental Measurements

Kim, H., Kwon, H., and Keune, J., “Buckling Initiation and Disbond Growth in Adhesively Bonded Composite Flanges,” 44th AIAA / ASME / ASCE / AHS / ASC Structures, Structural Dynamics, and Materials (SDM) Conference, April 7-10, 2003, Norfolk, VA

FAA Workshop Chicago 2003-09 18

Disbond Growth Model

one-edge free flanges

fracture-mechanics based compute strain energy

release rate assumed post-buckled mode

shape

33

2212

1 )by(c)by(c)by(c[)y,x(w

a

xmcos])by(c...)by(c n

n

214

4

m = 1:

m = 2:

FAA Workshop Chicago 2003-09 19

Comparison of Model and Experiments

0 50 100 150 200 250

D isb o n d L en g th , a (m m )

0.000

0.001

0.002

0.003

0.004

0.005

App

lied

Far

field

Str

ain

, o

Buckling - SS

Buckling - C

D isbonding - SS

D isbonding - C

Experim ent - Buckling

Experim ent - D isbond

G c = 578 J/m 2

Glass/Epoxy [0]4; b* = 25.4 mm – 0.8 mm Flanges

FAA Workshop Chicago 2003-09 20

Non-Uniform Disbond Growth Front

FAA Workshop Chicago 2003-09 21

Strain Energy Release Rate Profile

detailed view of G along disbond front

predicts corner disbond initiation

comparison with VCCT/FEA as programmed by R. Kreuger

0 0.2 0.4 0.6 0.8 1y/b *

0

100

200

300

400

500

G (

J/m

2)

FEA

M odel

o = 2,450

E xtrapo la ted G peak

FAA Workshop Chicago 2003-09 22

New Directions:Intrinsic Material Properties vs. Joint Behavior*

understand relationship between intrinsic material properties vs. “properties” inferred from structural (joint) behavior intrinsic material properties should

be independent of joint configuration, e.g., bondline thickness, mode of loading

resolve differences observed from different test methods: tensile test dogbone napkin ring Krieger Gage / ASTM 5656

use nonlinear analyses and correlation with tests to extract TRUE, consistent, description of intrinsic material behavior

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

5

10

15

20

25

30

Shear Strain

Shea

r Stre

ss (M

Pa) t

a= 2.0 8 mm

ta= 1.0 7 mm

ta= 0.3 3 mm

* this topic was raised at the ASTM

Symposium on Joining and Repair of

Composite Structures, March 2003,

Kansas City, MO

* Adhesive test data from J. Tomblin and W. Seneviratne, WSU

FAA Workshop Chicago 2003-09 23

New Directions: Bonded Joint Impact

(a)

(b)

(c)

Damage on 38.1 mm Overlap Joint Due to 40 J Impact;

(a) Impact Side; (b) Back Side, (c) C-Scan Showing Disbond Area

characterize damage modes

identify damage formation mechanisms

determine governing parameters

advanced: model

predictions

basic guidance studies needed:

FAA Workshop Chicago 2003-09 24

New Directions: Shear Buckling and Disbonding

investigate in-plane shear buckling and subsequent disbond growth

leads into combined compression + shear loading

Disbond in JointBetween FuselageHalves

N xy

o

A

B

FAA Workshop Chicago 2003-09 25

New Directions: Health Monitoring of Bonded Joints

real time (or time interval based) monitoring of joint health

existing: ultrasonic scanning common method for discrete

interval inspection real-time implementation

difficult electrical resistance external or embedded sensors /

crack gages / fiber optics acoustic waves

useful for internal, non-accessible locations correlate measurements with damage shape and location

input for damage tolerance prediction models

FAA Workshop Chicago 2003-09 26

New Directions: Full-Scale Behavior Prediction

Disbond in JointBetween FuselageHalves

N xyo

A

B

PDisbond

SandwichConstruction- Halves Bonded Along Top and Bottom C-L

Vertical Load

Side Load at25% Chord

Path to get from Coupons & Elements to Full Scale Component Level

UnderstandExperimental Data

Refined Models:Adhesive Plasticity, Peel StressFailure Under Multiaxial Stress,

Fracture Mechanics

???Other Factors

develop predictive models:maintain balance between simplicity and complexity