Modelling the interfacial degradation in adhesively bonded joints

298 Pages·2013·46.56 MB·English

Modelling the interfacial degradation in
adhesively bonded joints
Carl David Mortimer Liljedahl
SupelVisors:
Prof. A.D. Crocombe
Dr. M.A. Wahab
Materials, Surfaces and Structural Systems
School ofE ngineering
University ofS urrey
Guildford
SU"ey
United Kingdom
Submitted for a degree of Doctor of Philosophy in February 2006
Abstract
The aim of the research was to develop predictive models for the interfacial degradation
of adhesively bonded joints when exposed to aggressive environmental conditions. Four
different joint configurations using the same adhesive system were exposed to a variety
of conditions including immersion at 50°C, 96%RH at 50°C and 80%RH at 70°C. In
addition data from joints for other adhesive systems were also incorporated into the
investigation.
Moisture has a degrading effect on the strength of adhesively bonded joints. Therefore
the diffusion into the bulk material was determined by gravimetric experiments. However,
the mobility of the water molecules at the interface between the adhesive and the
substrate may be higher than in the bulk material. In order to assess this, the spatial
moisture distribution in bonded epoxy laminates was detennined by a nuclear reaction
analysis (NRA) technique. The moisture profile found experimentally and the modelling
undertaken of the interfacial diffusion indicated that the ingress in the interfacial region
was a few times faster than in the bulk material for the adhesive system investigated.
Both hygroscopic (swelling) and thermal residual strains may affect joint durability. The
thermal expansion was determined by means of a bi-material beam and the hygroscopic
expansion was determined by measuring the expansion of bulk samples at various
moisture levels. Creep properties for the adhesives studied were determined to investigate
the relaxation of residual stresses during the aging process. The coefficients of thermal
expansion and hygroscopic expansion were of the same order of magnitude for the
adhesives investigated. Creep was seen to be enhanced in the presence of moisture. The
AVl19 adhesive was seen to creep much more than FM73 and also absorbed more
moisture. As a consequence, the residual stresses in the joints bonded with A Vl19 were
seen to relax nearly totally whilst the residual stresses in the joints bonded with FM73
relaxed to about half of their original magnitude.
Different interfacial fracture tests were carried out in order to assess which was most
appropriate. Notched coating adhesion tests (NCA) were carried out initially. However, it
was very difficult to produce a repeatable notch and the adhesive often cracked before the
coating debonded. Good results were obtained then these samples were immersed in
water. Another test investigated was a split beam specimen. However this test was of
limited use as the secondary bond was weaker than the aged interface of interest. Finally,
a mixed mode flexure specimen (MMF) was selected to determine the fracture energy of
the adhesive systems in the 80%RH and 96%RH environments. The fracture energy
degraded rapidly initially with moisture content and then at a slower rate as more
moisture reached the interface. The fracture energy was found to be a function of the
amount of moisture at the interface. No further degradation was found when the joints
were held at equilibrium.
The degradation and the progressive damage were simulated with a cohesive zone model
(CZM). The model was extended from 2D to 3D. This was ~eful when predicting where
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the crack initiated in the width direction and how the initiation site changed after aging
for a L-joint configuration. When using a CZM the interfacial strength was defmed by a
traction-separation law. The parameters governing the traction-separation law were
determined using the interfacial fracture tests (NCA and MMF). The parameters were the
tripping traction and the fracture energy. It was shown to be essential to incorporate
elasto-plastic adhesive continuum behaviour in order to simulate the complete joint
response correctly. The tripping traction was determined by correlating the deviation of
the load-displacement curve with the simulated result. The fracture energy was then
determined by correlating the experimental load-crack length response with the
simulation. This gave a unique set of moisture dependent CZM parameters for various
moisture concentrations.
These parameters were then used to predict the response of other joint configurations. For
most of the joints, the residual strength was predicted closely using the moisture
dependent CZM parameters. However, in some cases other degradation mechanisms were
active. These included stress enhanced degradation and cathodic delamination. When
these mechanisms were included in the modelling, the prediction of the durability of all
joint configurations was good.
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To /., P. and G.
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Acknowledgement
The author gratefully acknowledges supervisors Prof A.D. Crocombe and Dr M.A.
Wahab for their guidance, enthusiasm, encouragement and, above all, for their time
dedicated throughout the project. They have enlightened the author with their insight and
knowledge.
The author would also like to acknowledge Prof A.S. Clough and his PhD students Ms
F.E. Gauntlett and Mr M.S. Rihawy at the Department of Physics for carrying out the ion
beam experiments. The author is also grateful to Dr C. Burt at Microstructural Studies
Unit (MSSU) for expertise advice regarding the microtorning.
Thanks are also due to Prof. John Watts at the Materials, Surfaces and Structural Systems
(MaSSS) department for his ideas on the mechanisms of joint failure and the
intezpretation of the surface analytical studies and to Mr S. Greaves and Dr. S. Hinter for
their assistance with the X-ray Photoelectron Spectroscopy (XPS) analysis. Mr A Brown
at MSSU is acknowledged for the assistance with the Scanning Electron Microscopy
(SEM).
Dr I. Hamerton at the Department of Chemistry is acknowledged for assistance with the
Dynamical Mechanical Thermal Analysis (DMTA).
The author would also like to thank M. Harper in the workshop for manufacturing of
experimental equipment and test pieces. Dr B. Le-Page and Mr P. Haynes are
acknowledged for advice on experimental methods.
The author would like to thank all research partners involved in this project: Ms Y. Hua
(University of Surrey), Dr LA. Ashcroft (Loughborough University), Mr F. Jumbo
(Loughborough University), Mr S. Millington (QinetiQ), Dr SJ. Shaw (DSTL), Mr J.F.
Sargent (BAB) and Mr T. Ackerman (MBDA).
Finally the author would like to thank Dr. S. Ogin, Prof. J. Pan and Prof. R.D. Adams for
their efforts as viva chairman, internal examiner and external examiner respectively.
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Declaration
The author declares the work presented in this thesis original and appropriate to the
subject area.
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Publications
Journal papers:
CDM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, Modelling the
environmental degradation of the interface in adhesively bonded joints using a cohesive
zone approach, Journal of Adhesion, Submitted
CDM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, Modelling the
environmental degradation of adhesively bonded aluminium and composite joints using a
CZM approach, International Journal of Adhesion and Adhesives, Submitted
C.D.M. Liljedahl, A.D. Crocombe, F.E. Gauntlett, M.S. Rihawy, A.S. Clough, Interfacial
diffusion in adhesively bonded joints, International Journal of Adhesion and Adhesives,
Submitted
CDM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, Progressive damage
modelling of adhesively bonded joints, International Journal of Fracture, In press
FE Gauntlett, MS Rihawy, AS Clough, CDM Liljedahl and AD Crocombe, Using a
scanning microbeam and a CdZnTe array for nuclear reaction measurements of water
diffusion into an adhesive resin, Nuclear Instruments and Methods B, In press
CDM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, The effect of residual
strains on the progressive damage modelling of environmentally degraded joints, Journal
of Adhesion Science and Technology, 19 (7), 2005 525-547
Conference papers:
CDM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, Modelling of interfacial
degradation in bonded joints, Adhesion 2005, September 2005, Oxford, UK
AD Crocombe, COM Liljedahl, Y Hua, MA Wahab and IA Ashcroft, Extending the
application of progressive damage durability modelling of adhesively bonded joints, 28th
Annual Meeting of the Adhesion Society, February 2005, Mobile, Alabama, USA
COM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, Interfacial diffusion in
adhesively bonded joints, 7th European Adhesion Conference, September 2004, Freiburg,
Germany
CDM Liljedahl, AD Crocombe, MA Wahab and IA Ashcroft, The effect of adhesive joint
residual stresses on the predictive modelling of environmental degradation, 7th European
Adhesion Conference, September 2004, Freiburg, Germany
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Table of Content
ABSTRACT ................................................................................................................................................... 2
ACKNOWLEDGEMENT ............................................................................................................................ S
DECLARAn ON ...........................................................................................................................................6
PUBLICAT IONS .......................................................................................................................................... 7
CONFERENCE PAPERS: ............................................................................................................................ 7
TABLE OF CONTENT ................................................................................................................................ 8
LIST OF FIGURES .................................................................................................................................... 12
LIST OF TABLES ...................................................................................................................................... 20
INTRODUCTION ....................................................................................................................................... 22
1.1 FRAMEWORK AND RESEARCH OBJECTIVE ........................................................................................... 22
1.2 METHODOLOGY AND STRUCTURE ...................................................................................................... 24
LITERATURE REVIEW ........................................................................................................................... 26
2.1 MOISTURE UPTAKE IN BULK ADHESIVES AND INTERFACES ................................................................. 26
2.1.1 Equilibrium moisture content in bulk adhesives ...................................................................... 26
2.1.1.1 The effect of relative humidity on the saturation moisture content ................................................... 26
2.1.1.2 The effect of temperature on the saturation moisture content ............................................................ 27
2.1.1.3 The effect of stress during exposure on the equilibrium moisture content. ....................................... 29
2.1.1.4 The effect of water type on the moisture uptake ................................................................................. 30
2.1.2 Rate ofu ptake in bulk adhesive samples ................................................................................. 30
2.1.2.1 The effect of the temperature on the diffusion coefficient ................................................................. 31
2.1.2.2 Two stage models .................................................................................................................................. 32
2.1.2.3 Stress dependent diffusion .................................................................................................................... 33
2.1.3 Interfacial diffusion ................................................................................................................. 36
2.2 MECHANICAL PROPERTIES OF BULK ADHESIVES ................................................................................ 37
2.2.1 The effect ofm oisture on the mechanical properties ............................................................... 38
2.2.1.1 The effect of moisture on the elastic properties and ultimate strength .............................................. 38
2.2.1.2 The effect of moisture on the adhesive viscoelastic properties .......................................................... 39
2.2.1.3 The effect of moisture on the glass transition temperature ................................................................. 39
2.2.2 The effect oft emperature on the adhesive mechanical properties ........................................... 40
2.2.2.1 The effect of temperature on the elastic properties and ultimate strength ........................................ .40
2.2.2.2 The effect of temperature on the viscoelastic properties .................................................................... 41
2.3 THERMAL EXPANSION ....................................................................................................................... 41
2.4 HYGROSCOPIC SWELLING ................................................................................................................... 42
2.5 FAILURE AND RUPTIJRE OF ADHESIVES AND BONDED JOINTS ............................................................. 43
2.5.1 Peak strains (or stresses) ......................................................................................................... 43
2.5.2 Fracture mechanics ................................................................................................................. 43
2.5.3 Progressive damage modeJ/ing. ............................................................................................... 45
2.5.3.1 Crack band model ................................................................................................................................. 46
2.5.3.2 Cohesive zone modelling ...................................................................................................................... 47
2.6 JOINT DURABILITY ............................................................................................................................. 60
2.6.1 Mechanisms of the degradation oft he adhesion ...................................................................... 60
2.6.2 Durability ofa dhesively bonded joints .................................................................................... 61
2.6.2.1 The effect of temperature on the rate of strength degradation ........................................................... 63
2.6.2.2 The effect of corrosive environments on the durability ...................................................................... 63
2.6.2.3 The effect of pre-treatment on joint durability .................................................................................... 64
2.6.2.4 Reversible effects .................................................................................................................................. 66
2.6.2.5 The effect of applied stress during the ageing ..................................................................................... 67
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2.6.2.6 The effect of residual strains .................. '" ........................................................................................... 68
2.7 CONCLUSIONS .................................................................................................................................... 69
BULK ADHESIVE AND INTERFACIAL FRACTURE PROPERTIES .............................................. 71
3.1 AV 119 (WITH STEEL) ...................................................................................................................... 72
3.1.1 Moisture ingress in bulk adhesive films ................................................................................... 72
3.1.2 Moisture and temperature dependent mechanical properties ................................................. 74
3.1.3 Glass transition temperature ................................................................................................... 74
3.1.3.1 Test configuration ................................................................................................................................. 75
3.1.3.2 Experimental results .............................................................................................................................. 76
3.1.4 lime dependent material properties ........................................................................................ 77
3.1.4.1 Creep tests .............................................................................................................................................. 77
3.1.4.2 Stress relaxation .................................................................................................................................... 78
3.1.5 Thermal and hygroscopic expansion ....................................................................................... 79
3.1.6 Moisture dependent inteifacialfracture test ............................................................................ 80
3.2 EA932 1 AND FM73 (WITH ALUMINIUM AND IM7/8552) ............................................................. 82
3.2.1 Bulk adhesive and composite manufacturing .......................................................................... 82
3.2.1.1 FM73 ...................................................................................................................................................... 82
3.2.1.2EA9321 .................................................................................................................................................. 83
3.2.1.3 IM7/8552 ............................................................................................................................................... 84
3.2.2 ArtifiCial aging environments .................................................................................................. 85
3.2.3 Moisture uptake in bulk specimens .......................................................................................... 85
3.2.3.1 Equilibrium moisture uptake ................................................................................................................ 86
3.2.3.2 Fickian diffusion ................................................................................................................................... 87
3.2.4 Swelling due to moisture uptake .............................................................................................. 89
3.2.5 Thermal expansion .................................................................................................................. 90
3.2.6Mechanicalproperties ............................................................................................................. 92
3.2.6.1 Moisture dependent mechanical properties for the adhesives ............................................................ 93
3.2.6.2 Creep properties for FM73 at various moisture levels ........................................................................ 94
3.2.6.3 Mechanical properties for the aluminium ............................................................................................ 95
3.2.6.4 Moisture dependent mechanical properties for the composite ........................................................... 96
3.2.7 Glass transition temperaturefor EA9321, FM73 and IM7J8552 ............................................ 98
3.2.8 Moisture dependent inteifacial fracture tests .......................................................................... 99
3.2.8.1 Acid etch ................................................................................................................................................ 99
3.2.8.2 Assessment of secondary bond adhesive ............................................................................................. 99
3.2.8.3 Notch coating adhesion test ................................................................................................................ 100
3.2.8.4 Split beam ............................................................................................................................................ 102
3.2.8.5 Mixed mode flexure ............................................................................................................................ 10 3
3.3 CONCLUSIONS .................................................................................................................................. 108
JOINT TESTS ........................................................................................................................................... 109
4.1 JOINT TEST CONFIGURATIONS AND TEST RESULTS ............................................................................ 11 0
4.1.1 Configurations ....................................................................................................................... 110
4.1.2 Test resu/ts ............................................................................................................................. 111
4.2 CHARACTERISATION OF FAILURE MECHANISMS ............................................................................... 114
4.2.1 Characterisation oft he joint constituents suifaces ................................................................ 115
4.2.1.1 Aluminium substrates ......................................................................................................................... 115
4.2.1.2 Adhesive FM73 ................................................................................................................................... 116
4.2.2 Degradation oft he bondedjoints .......................................................................................... 116
4.2.2.1 MMF (PM73) ...................................................................................................................................... 117
4.2.2.2 NCA ..................................................................................................................................................... 118
4.2.2.3 Composite SLJ .................................................................................................................................... 120
4.2.2.4 Aluminium SLJ ................................................................................................................................... 121
4.2.2.5 DLJ ....................................................................................................................................... '" ............. 128
4.2.2.6 L·joint. .................................................................................................................................................. 131
4.3 CONCLUSIONS .................................................................................................................................. 134
DEVELOPMENT OF FE MODELLING TECHNIQUES ................................................................... 135
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5.1 COHESIVE ZONE MODELS ...•.....•.•••..••.................•..•................•......................................•............•..•.. 135
5.1.1 Benchmark models ................................................................................................................. 13 6
5.1.2 The effect oft he CZM parameters ......................................................................................... 139
5.1.2.1 Mesh and configurations ..................................................................................................................... 139
5.1.2.2 The effect of the CZM parameters on the failure load ...................................................................... 140
5.1.2.3 The effect ofCZM parameters on the load-displacement curves .................................................... 141
5.1.3 The effect ofa dhesive plasticity ............................................................................................. 143
5.1.4 Extension to 3D ..................................................................................................................... 145
5.2 ApPROACH TO INCLUDE RESIDUAL STRAINS ..................................................................................... 146
5.3 DIFFUSION ....................................................................................................................................... 147
5.3.1 DiffUSion in composite joints (mass transfer) ........................................................................ 148
5.3.2 Stress or strain dependent diffusion ....................................................................................... 149
5.4 STRESS ENHANCED DEGRADATION ................................................................................................... 150
5.5 CATHODIC DELAMINATION .............................................................................................................. 151
5.6 CZM IN CONJUNCTION WITH COHESIVE FAILURE ............................................................................. 151
5.7 COMPOSITE FAILURE ........................................................................................................................ 153
5.8 CONCLUSIONS .................................................................................................................................. 154
MODELLLING OF JOINTS BONDED WITH AVI19 ........................................................................ 156
6.1 MODELLING APPROACH ................................................................................................................... 156
6.2 MODELLING WITH NO RESIDUAL STRAINS ........................................................................................ 157
6.2.1 Determination oft he CZM parameters for various moisture levels ...................................... 157
6.2.2 Failure prediction oft he NCA specimen at various moisture levels ...................................... 159
6.3 MODELLING INCLUDING RESIDUAL STRAINS .................................................................................... 161
6.3.1 Linear elastic adhesive continuum modeL ........................................................................... 161
6.3.2 TIme dependent adhesive mechanical properties when saturated in water at 50°C. ............. 163
6.3.3 Time dependent adhesive mechanical properties when saturated in 95.8roRH at 50°C ....... 164
6.3.4 Including creep properties in the CZ modelling .................................................................... 167
6.4 CONCLUSIONS .................................................................................................................................. 168
ASSESSMENT OF mE MODELLING ASSUMPTIONS FOR THE JOINTS BONDED WITH
FM73 .......................................................................................................................................................... 169
7.1 ASSESSMENT OF THE DIFFUSION ...................................................................................................... 169
7.1.1 A comparison between 1D and 2D diffusion ......................................................................... 169
7.1.2 Effect on the moisture concentration in the adhesive due to the diffusion through the
composite in the DU. ..................................................................................................................... 17J
7.2 A COMPARISON OF ADHESIVE STRESSES FROM 2D AND 3D MODELLING .......................................... 172
7.2.1 NCA ....................................................................................................................................... 173
7.2.2 Aluminium SL.!. ...................................................................................................................... 175
7.2.3 MMF ...................................................................................................................................... 177
7.2.4 Composite SU. ...................................................................................................................... 179
7.2.5 Aluminium-composite DU .................................................................................................... 181
7.2.S.1 External mechanical load .................................................................................................................... 182
7.2.5.2 Residual stresses .................................................................................................................................. 183
7.3 CONCLUSIONS .................................................................................................................................. 185
COHESIVE ZONE MODELLING OF JOINTS BONDED WITH FM73 .......................................... 187
8.1 PRELIMINARY DAMAGE MODELLING ................................................................................................ 187
8.1.1 Simplified geometry (without fillets) ...................................................................................... 187
8.1.1.1 Calibration of the CZM parameters using the SLJ... ......................................................................... 188
8.1.1.2 Prediction of the DLJ .......................................................................................................................... 191
8.1.2 Including the fillets ................................................................................................................ /93
8.1.2.1 Calibration of the CZM parameters using the SLJ.. .......................................................................... 193
8.1.2.2 Prediction ............................................................................................................................................. 196
8.2 PLASTIC ADHESIVE PROPERTIES ....................................................................................................... 196
8.2.1 Interaction ofc ontinuum plasticity and CZM ........................................................................ 197
8.2.2 Determination ofCZM parameters from the interfacialfracture tests .................................. 201
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