Aramid reinforced aluminium laminates: Arall: adhesion problems and environmental effects. Volume A: Adhesion and delamination

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ARAMID REINFORCED ALUMINIUM LAMINATES: ARALL ADHESION PROBLEMS AND ENVIRONMENTAL EFFECTS

ADHESION PROBLEMS AND ENVIRONMENTAL EFFECTS volume A: adhesion and delamination

volume B: environmental effects

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus, prof.dr.ir. J.M. Dirken,

in het openbaar te verdedigen ten overstaan van een commissie aangewezen door het College van Dekanen

op dinsdag 3 februari 1987 te 16.00 uur

door

MARCEL LEONA CLEMENTINE EDUARD VERBRUGGEN

Vliegtuigbouwkundig ingenieur geboren te Merksem

TR diss^ 1524

1) Het is ongebruikelijk dat een hybride materiaal.voornamelijk de voordelen van zijn componenten bezit doordat de nadelen van elke afzonderlijke component worden opgevangen door een andere component. ARALL is een ongebruikelijk

materiaal.

2) De inspanning, geleverd om het scheursluitingsmechanisme in ARALL uit te vinden, zal klein blijken te zijn ten opzichte van de inspanning om de lucht-waardigheidsinstanties te overtuigen dat het mechanisme werkt (certificatie-problematiek van het vliegen met "schade").

3) Uit het oogpunt van "damage tolerance" zijn perfecte materialen gevaarlijk.

4) Aramide vezels nemen meer en sneller vocht op dan de gangbare harssystemen, maar het verschil is een orde kleiner dan gesuggereerd wordt in de literatuur.

R.E. Allred The room temperature moisture kinetics R.M. Lindrose of Kevlar 49 fabric/epoxy laminates.

SAND 78-0412, 1978

5) Het gebruik van een constante diffusiecoëfficient (per grensvlak) voor het

berekenen van het vochtopnameprofiel van composieten is niet correct. I

6) De bruikbaarheid van de "wedge-edge" proef voor een snelle evaluatie van de breuktaaiheid van lijm en/of prepregsystemen wordt in de literatuur sterk onderschat.

7) Het beoordelen van het delaminatiegedrag van een lijm- of een prepreg- . systeem toegepast in vliegtuigconstructies aan de hand van resultaten van ! statische proeven is onbetrouwbaar. De drempelwaarde van de spanningstoestand,

die voor delaminatie onder een wisselende belasting wordt vereist, ligt een factor 2 tot 3 lager dan de statische drempelwaarde.

8) De aanwezigheid van vocht aan de grens van een delaminatiegebied is, zowel voor een statische als een wisselende belasting, zeer nadelig voor de dela­

minatie -uitbreiding.In het geval van een wisselende belasting kan de delaminatie-groeisnelheid 5 tot 10 maal hoger zijn dan bij de omstandigheden in het labora­ torium.

9) De veronderstelling dat de breuktaaiheid van een vezel-harssysteem onaf­ hankelijk is van het vezeltype gaat alleen op zolang het breukvlak in de matrix ligt. Aan deze voorwaarde wordt niet voldaan bij een aantal vezel-harssystemen.

A.G. Miller Tougness testing of composite materials P.E. Hertsberg 12th Nat. SAMPE Techn. Conf.

V.W. Rantala Okt. 7-9, 1980

10) Stoeckel, Blasius en Crist beweren dat het karakter van de sterkte en de stijfheid van polyamide en polyester vezels verklaard kan worden met een

"tie-molecule" model. Het hiervoor aangevoerde bewijs, gebaseerd op het aantal belastingsdragende verbindingsmoleculen, is niet juist.

J.M. Stoeckel Chain Rupture an Tensile Deformation J. Blasius of Polymers.

11) Het voortbestaan van racisme toont aan dat de evolutie van de menselijke intelligentie veel langzamer verloopt dan algemeen wordt gedacht.

12) Gezien de snel voortschrijdende automatisering, is het voor de vakbonden zinvoller te streven naar een eerlijke samenleving in een toekomstige, bijna-werkloze maatschappij dan naar achterhaalde begrippen als volledige tewerk­ stelling.

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ARAMID ALUMINIUM LAMINATES: ARALL

Adhesion Problems and Environmental Effects

Part A: Adhesion and delamination

In a separate volume: Part B: Environmental effects on delamination

Abstract

Fatigue c r a c k growth i n t h e new hybrid m a t e r i a l ARALL i s accompagnied by some "controlled" delamination in the wake of t h e c r a c k in t h e t h i n aluminium s h e e t s . The aramid f i b r e s a r e b r i d g i n g t h e c r a c k , which h i g h l y reduces t h e s t r e s s i n t e n s i t y a t t h e c r a c k t i p and t h u s leads t o a very low c r a c k growth r a t e or c r a c k a r r e s t . The d e l a m i n a t i o n in t h e t h i n i n t e r m e d i a t e l a y e r s , c o n s i s t i n g of aramid f i b r e s and metal a d h e s i v e , i s t h e main t o p i c of the present t h e s i s . The i n v e s t i g a t i o n has revealed new i n s i g h t s i n t o several questions r e l a t e d to (1) p h e n o m e n o l o g i c a l d e t a i l s of t h e d e l a m i n a t i o n mechanisms i n ARALL, (2) e n v i r o n m e n t a l e f f e c t s on delamination, (3) u s e f u l n e s s and l i m i t a t i o n s of d i f f e r e n t types of specimens to study delamination under mode I , mode I I and combined modes of l o a d i n g , (4) t h e a p p l i c a t i o n of f r a c t u r e mechanics t o u n d e r s t a n d t h e delamination b e h a v i o u r , and (5) t h e a p p l i c a t i o n of d i f f u s i o n models t o a n a l y s e m o i s t u r e a b s o r p t i o n by ARALL. Topics ( 1 ) , (3) and (4) a r e covered i n P a r t A, t o p i c s (2) and (5) in Part B.

Acknowledgement: The present i n v e s t i g a t i o n was c a r r i e d out i n t h e M a t e r i a l s L a b o r a t o r y of t h e F a c u l t y of Aerospace E n g i n e e r i n g , D e l f t U n i v e r s i t y of Technology, as p a r t of a research c o n t r a c t granted by the " S t i c h t i n g Technische Wetenschappen (STW)".

Contents

pp.

Prologue *

Chapter j_: Introduction 3

Chapter _2: ARALL, a new family of aircraft materials

2.1 : History 5

2.2: ARALL concept 5

2.3: ARALL properties 7 2.4: ARALL applications in aircraft 9

structures

Chapter _3_: Characterisation of the delamination behaviour of ARALL

3.1: Introduction 10 3.2: Literature survey

3.2.1: Electrostatic and chemical inter- 11 molecular bonds

3.2.2: Wettability properties 20 3.2.3: Surface roughness 2 7

3.3: Influence of fibre/matrix adhesion on the mode I and mode II fracture behaviour of intermediate layers

3.3.1: Requirements for standardized test 30 methods

3.3,2:' Standard materials used in the test 31 series

3.3.3: Definition of possible delamination 32 paths for an adhesive and a prepreg

3.3.4: Characterization of fibre/matrix 33 adhesion by means of the Bell-peel

test

3.3.5: Characterization of fibre/matrix 38 adhesion by means of the WTDCB-test

3.3.6: Characterization of fibre/matrix 51 adhesion by means of the interlaminar

shear test

3.3.7: Characterization of fibre/matrix 54 adhesion by means of the thick

3.3.8: Characterization of fibre/matrix 59 adhesion by means of the

edge-notched flexure specimen

3.3.9: Characterization of fibre/matrix 63 adhesion by means of the central

notched delamination specimen

3.3.10: Characterization of fibre/matrix 67 adhesion by means of the fibre bundle

pull-out test

3.3.11: Evaluation of the experimental test 7 3 series

3.4: Influence of the fibre/matrix adhesion on the ARALL fracture behaviour

3.4.1: Direction of delamination extension 78 3.4.2: Influence of fibre/matrix adhesion 119

on the fracture toughness of ARALL

3.4.3: Influence of fibre/matrix adhesion 124 on the fatigue behaviour of ARALL

References 12? Tables 136 Figures 152 Appendix A: WTDCB-specimen, theoretical background 209

Appendix B: Crack growth rate for a WTDCB-specimen 2 14 Appendix C: Fracture behaviour of polymers 2 19 Appendix D: Validity of the WTDCB-test method 2 24

Appendix E: Stress distribution in an interlaminar 226 shear specimen

Appendix F: Influence of specimen geometry on the 23 1 fracture behaviour of an ILS-specimen

Appendix G: Shear lag calculation for a thick ad- 235 herend specimen (adhesive layer)

Appendix H: Shear lag calculation for a thick ad- 240 adherend specimen (prepreg intermediate

layer)

Appendix I: Sensitivity of the critical shear stress 246 for the thick adherend specimen geometry

Appendix J: The strain energy release rate for an 2 49 end - notched flexure (ENF-) specimen

Appendix K: Shear lag analysis of the fibre bundle 255 pull-out test

Prologue

A man's life is the result of a concatenation of making decisions: if he is lucky, his own decisions.

Numerous d e c i s i o n s led to m y p r e s e n t situation of writing a dissertation, whereas the background for most of them has disappeared. I still wonder what made m e , as a belgian c i t i z e n , step into a dutch society consisting of, as my grandmother claimed, miserly, impertinent drug addicts (she never liked dutch people). My aeronautical study at the Delft University was not the natural consequence of being an airplane-addict (I'm still not able to recognize a DC1 0 from an airbus A310) and non of my ancestors had, except for a german bombing period during the second world war, any relation with airplanes whatsoever. Another important decision, my materials specialisation had, if my memory does not betray me, something to do with the agreeable materials college hours. By all means I believe that also the other possible specialisations were very interesting, but it was their and my fate that they were presented so early in the morning. ARALL, always considered as being a petrol trade name, turned out to be a new, promising aircraft material on which still a lot of experimental and theoretical work had to be done. The choice for a durability investigation on the new material was rather self-evident considering my former participation (being a p r o d u c t of) in a t y p i c a l , actually d a n g e r o u s , durability experiment: the marriage of my parents. And if there was one decision which made me start doctoral studies, it was certainly not my salary.

If any of the previous decisions had been different, I should have joint 99.999999% of the world population who still considers ARALL to be a petrol name (or to be nothing at a l l ) . Instead of t h a t , I joint a team (0.0000002% of the world population) which, for the last five years, focused its attention on the new material. All my former decisions must have something to do with my preference to belong to small groups.

Still, ARALL deserves all the concentrated attention it has got in the past and will get in the future. Already in the early days of aircraft history, it was recognized that weight is never a stimulans for getting of the ground (it is for getting to the ground but that's a different story). Therefore reducing the weight was and

is one of the main targets for new materials; a target which is succesfully met by ARALL which promises weight

Fatigue is considered as one of the major remaining problem areas for airplane structures and, from time to time, still causes a heavy loss of life as so painfully demonstrated in the 747-crash last year in J a p a n . ARALL, for realistic loading conditions, is indeed fatigue insensitive.

Its low weight, fatigue insensitivity, combined with other advantageous properties (e.g. damage tolerance) can make ARALL a new chain in the evolution to an even more safe, reliable and economic aircraft.

Chapter 1; Introduction

At the beginning of the present study ARALL was, thanks t o t h e work of i t s i n v e n t o r s , a l r e a d y more or l e s s standardized t o t h e m a t e r i a l d e s c r i b e d i n more d e t a i l s in c h a p t e r 2. However, some i m p o r t a n t q u e s t i o n s were s t i l l unresolved. The problems involved were r e l a t e d to (1) d e l a m i n a t i o n i n s i d e ARALL and (2) d u r a b i l i t y of ARALL i n t h e long r u n . Those a r e t h e two main problems addressed in the present study.

Delamination

ARALL i s b u i l t up as a laminated m a t e r i a l of a l t e r n a t e l y t h i n a l u m i n i u m s h e e t s and t h i n c o m p o s i t e l a y e r s c o n s i s t i n g of an a d h e s i v e m a t r i x and s t r o n g aramid f i b r e s (f ig.1 ). Delamination of ARALL i s a problem t h a t should be studied in depth in order t o e v a l u a t e possible l i m i t a t i o n s of t h i s new m a t e r i a l . I t was already known t h a t f a t i g u e c r a c k growth in ARALL goes t o g e t h e r w i t h some delamination around the fatigue crack. As a matter of f a c t some l i m i t e d and controlled delamination around an i n c i p i e n t f a t i g u e c r a c k should occur t o a c h i e v e t h e high f a t i g u e r e s i s t a n c e . Unfortunately l i t t l e was known about t h e d e l a m i n a t i o n mechanism in ARALL and r e l a t e d a s p e c t s such a s f r a c t u r e modes and t h e c o n t r o l l i n g s t r e s s s y s t e m s . I t was decided t o i n v e s t i g a t e t h e delamination phenomenon and to focus the study on:

- observations on t h e d e l a m i n a t i o n phenomenon ( f r a c t u r e p a t h )

- measurements on the delamination r e s i s t a n c e under - d i f f e r e n t t y p e s of s t r e s s s y s t e m s t o be

c h a r a c t e r i z e d by f r a c t u r e m e c h a n i c s c o n c e p t s (opening mode I and I I and c o m b i n a t i o n s of I and I D

- study of d e l a m i n a t i o n under s t a t i c and c y c l i c loading ( f a t i g u e ) .

Seven d i f f e r e n t types of specimens were adopted (fig.2) t o study delamination under d i f f e r e n t c o n d i t i o n s . Some of t h e s e specimens a r e commonly used for d e l a m i n a t i o n t e s t s on adhesive bonded sheets. Other ones were chosen t o meet s p e c i a l n e e d s of t h e p r e s e n t s t u d y . The specimen configurations had to be analysed t h e o r e t i c a l l y in o r d e r t o q u a n t i f y t h e d e l a m i n a t i o n r e s i s t a n c e i n terms of the "delamination driving force" (the f r a c t u r e mechanics t e r m i s " c r a c k d r i v i n g f o r c e " o r " s t r a i n energy r e l e a s e r a t e " ) . In t h i s r e s p e c t t h e h y b r i d n a t u r e of ARALL (aluminium, f i b r e s , a d h e s i v e ) i s a complication. The weakest l i n k w i l l t u r n o u t to be t h e i n t e r f a c e between t h e aramid f i b r e s and t h e a d h e s i v e m a t r i x .

The scope of t h e d e l a m i n a t i o n s t u d y was broadened by t e s t i n g specimens with d i f f e r e n t types of aramide f i b r e s and with carbon and g l a s s f i b r e s .

The experience gained in a fairly extensive delamination test programme and in the analysis of the results will be used to discuss the engineering behaviour of the ARALL material with respect to fatigue and fracture toughness.

Durability of ARALL

Some three years ago, the long-term behaviour of the material w a s still a blank spot. B e c a u s e of its partially metallic and partially composite character, ARALL demands a kind of combined approach which takes into account the specific durability features of both the metal and composite components. Except for both individual components, also interfacial degradation (aluminium/matrix or fibre/matrix) requires special attention. In the first part of this work, the moisture absorption behaviour of ARALL is thoroughly examined. An experimental test series is performed to obtain moisture pick-up data for a range of environments. A theoretical model for the ARALL absorption behaviour is further d e v e l o p e d u s i n g both an a n a l y t i c a l and a numerical approach. The same approaches have been introduced to examine the possible beneficial influence of protective coatings and to obtain a more reliable insight in the m o i s t u r e d i s t r i b u t i o n in a fibre reinforced material.

The second part deals with the determination of the influence of absorbed moisture on the general, and more specific, on the delamination behaviour of the material. Again, a wide range of test series has been introduced to characterize possible degradation processes in the ARALL material as the result of environmental exposure. Editorial aspects

For practical reasons the thesis is presented in two parts. Volume I gives a brief introduction to ARALL (chapter 2) followed by the first part of the study on delamination (chapter 3 ). Volume I also contains a number of appendices. Volume II includes the second part of this study on durability and environmental effects. An extensive summary of the whole study is given at the end of this volume.

Chapter 2:ARALL, a new f a m i l y of a i r c r a f t m a t e r i a l s

2 . 1 : H i s t o r y

The ARALL (Aramid Aluminium Laminate) concept, resulting in a new, fatigue insensitive structural material for aircraft applications, originated some eight years ago in the materials group of the Aeronautical Department of the Delft University. As a natural consequence of their interest and experience in both metals and composites, Vogelesang and co-workers introduced the concept of combining metal and composite properties in one single material (ref.1). In c o n t r a d i c t i o n w i t h earlier investigations based on a similar idea (e.g. ref.2 and 3 ) , Vogelesang succeeded in the optimalisation of the new material. Because of the promising properties, some industries (AKZO, ALCOA, 3M) already in an early stage financially supported the investigations, which led to patent applications for two types of ARALL. Both the American and European patents were granted in 1984. The rapidly growing interest of aircraft industries has led to a pilot production of ARALL which is now sold to industries and other institutes for evaluation. The ARALL concept, its properties and applications are described below (based on ref.4).

2.2: ARALL concept

The ARALL concept i s based on t h e i d e a t h a t an i d e a l combination of metals and composites should r e s u l t in a m a t e r i a l which combines t h e f a v o u r a b l e p r o p e r t i e s of each component w i t h o u t s h a r i n g t h e i r i n d i v i d u a l disadvantages. ARALL i s b u i l t up a s a l a m i n a t e d m a t e r i a l of a l t e r n a t e l y t h i n aluminium a l l o y sheets and composite layers (fig.1 ). The f i b r e / a d h e s i v e composite l a y e r s , i n d i c a t e d in f i g . 1 as a r a m i d p r e p r e g s , c o n s i s t of s t r o n g ( u s u a l l y ) u n i d i r e c t i o n a l a r a m i d f i b r e s , impregnated by a m e t a l epoxy a d h e s i v e . ARALL i s produced as sheet m a t e r i a l by a normal autoclave bonding cycle t o a number of t h i n A l - a l l o y s h e e t s w i t h i n t e r m e d i a t e p r e p r e g f i l m s . T h e a i m of t h e o p t i m a l i s a t i o n was to a r r i v e a t a high s t r e n g t h , f a t i g u e i n s e n s i t i v e , low d e n s i t y m a t e r i a l . B e c a u s e h i g h s t r e n g t h and low d e n s i t y were a l m o s t i n h e r e n t t o t h e concept, t h e f a t i g u e i n s e n s i t i v i t y n e e d e d f u r t h e r a t t e n t i o n . T h i s f e a t u r e , f o r t h e o p t i m i s e d ARALL c o n f i g u r a t i o n , i s o b t a i n e d a s a r e s u l t of a c r a c k opening r e s t r a i n t mechanism, which g r e a t l y reduces the s t r e s s i n t e n s i t y f a c t o r a t t h e t i p of a f a t i g u e c r a c k . The mechanism i s i l l u s t r a t e d i n f i g . 3 . Opening of an i n i t i a t e d c r a c k i s s u p p r e s s e d by f i b r e s i n t h e wake of

the crack. The keypoint of the mechanism i s the absence of f i b r e f a i l u r e n e a r t h e c r a c k t i p . Due t o t h e "bridging" f u n c t i o n of t h e f i b r e s high s h e a r s t r e s s e s w i l l o c c u r i n t h e f i b r e / a d h e s i v e l a y e r . As a consequence some d e l a m i n a t i o n i s a l m o s t u n a v o i d a b l e . Actually some " c o n t r o l l e d " d e l a m i n a t i o n i s e v e n d e s i r a b l e t o reduce t h e s t r e s s in the f i b r e s and thus to avoid f i b r e f a i l u r e . This i s a major r e a s o n why more knowledge a b o u t d e l a m i n a t i o n i n ARALL i s an e s s e n t i a l need.

The f a t i g u e i n s e n s i t i v i t y can be o p t i m i s e d by t h e following v a r i a b l e s :

1- sheet m a t e r i a l : type of a l l o y and sheet t h i c k n e s s 2- type of f i b r e s

3 - adhesive bonding system 4- r e s i d u a l s t r e s s system sheet m a t e r i a l A low t h i c k n e s s (<0.5mm) of t h e i n d i v i d u a l aluminium sheets i s e s s e n t i a l t o e n s u r e t h e high f i b r e / a l u m i n i u m r a t i o n e c e s s a r y t o o b t a i n s u f f i c i e n t c r a c k opening r e s t r a i n t . One t y p e of ARALL i s b a s e d on 7075-T6 because of i t s high y i e l d s t r e n g t h . A second t y p e i s based on 2 0 2 4 - T 3 b e c a u s e of i t s b e t t e r f r a c t u r e toughness. In both c a s e s , b a r e m a t e r i a l i s used t o avoid b o n d l i n e c o r r o s i o n . Because t h e m e t a l / a d h e s i v e i n t e r f a c e p r o p e r t i e s , e s p e c i a l l y for longer p e r i o d s , are d i r e c t l y r e l a t e d t o t h e surface q u a l i t y of the aluminium s h e e t s , t h e m e t a l s h e e t s b e f o r e bonding a r e p r e t r e a t e d according t o a p p r o p r i a t e standards.

type of f i b r e s

Aramid f i b r e s were chosen i n s t e a d of carbon f i b r e s f o r a number of r e a s o n s :

- Aramid i s e l e c t r i c a l l y n e u t r a l r e l a t i v e t o m e t a l s . T h e r e f o r e no g a l v a n i c c o r r o s i o n h a s to be expected.

- Aramid has a h i g h e r s p e c i f i c s t r e n g t h and f a i l u r e s t r a i n than carbon f i b r e s . - Aramid i s l e s s expensive. Some d i s a d v a n t a g e s of t h e f i b r e s a r e l e s s o r n o t d e t r i m e n t a l t o t h e ARALL concept: - Moisture a b s o r p t i o n i s l i m i t e d t o t h e specimen edges ( s e e c h a p t e r 4). - Poor c o m p r e s s i v e s t r e n g t h of t h e f i b r e s i s releaved by the aluminium s h e e t s .

For the two standard ARALL types, the aramid f i b r e s a r e u n i d i r e c t i o n a l l y embedded i n t h e m a t r i x . For t h e prepreg a f i b r e v o l u m e c o n t e n t of 50% h a s b e e n s t a n d a r d i z e d .

adhesive bonding system

The required matrix properties are:

- good adhesion to both the aluminium and the fibre surface

- tough fracture behaviour - low density

- good d u r a b i l i t y

A 3M polyether toughened epoxy metal adhesive (AF163-2) was found t o meet these requirements.

r e s i d u a l s t r e s s system

As a consequence of t h e i r d i f f e r e n t t h e r m a l expansion c o e f f i c i e n t s , t h e ARALL c o m p o n e n t s ( a l u m i n i u m and composite l a y e r s ) have an u n f a v o u r a b l e r e s i d u a l s t r e s s d i s t r i b u t i o n a f t e r c u r i n g ( t e n s i o n i n t h e aluminium s h e e t s , compression i n t h e aramid l a y e r s ) . The s i g n of the i n t e r n a l s t r e s s e s can be r e v e r s e d by a p r e s t r a i n process performed a f t e r curing (fig.4).

The t w o s t a n d a r d ARALL t y p e s , r e s u l t i n g from t h e optimising process, are specified below.

ARALL type Al-layers: Al-alloy thickness Prepreg layers: fibres matrix

fibre volume content fibre orientation layer thickness Prestrained 7H32 7075-T6 0.3mm 2H42 2024-T3 0 .4mm

high modulus aramid fibres (TWARON) epoxy metal adhesive (AF 163-2)

50%

unidirectional 0. 2mm

yes no

2.3:ARALL p r o p e r t i e s

Some mechanical and p h y s i c a l p r o p e r t i e s of ARALL a r e compared w i t h two commonly used m o n o l i t h i c aluminium a l l o y s (7075-T6 and 2024-T3) in t h e t a b l e below (ref.4):

tensile strength (MPa) 0.2% yield stress (MPa) Young's modulus (MPa) proportional limit in compression (MPa) elongation (%) density (kg/m3) ARALL 7H32* 2H42 7132* 2142 735 590 785 610 635 380 530 340 69000 70000 63000 64700 355 255 325 240 1.9 2.4 3.5 4.2 2360 2440 2360 2440 Aluminium 2024-T3 7075-T6 470 560 360 480 72000 72000 270 480 17.0 11.0 2800 2800

I : intermediate modulus aramid f i b r e s *: p r e s t r a i n e d version

The main a d v a n t a g e of t h e c r a c k b r i d g i n g mechanism as introduced in ARALL, i s the i n s e n s i t i v i t y of the m a t e r i a l for f a t i g u e c r a c k g r o w t h . T h i s s u p e r i o r f a t i g u e behaviour was experimentally confirmed both for constant amplitude and f l i g h t simulation t e s t s for ARALL specimens with an i n i t i a l c e n t r a l c r a c k ( f i g . 5 ) , for r i v e t e d ARALL j o i n t s ( f i g . 6 ) and for ARALL lugs ( f i g . 7 ) . F i g u r e 5 shows t h a t ARALL i n t h e a s c u r e d c o n d i t i o n a l r e a d y e x h i b i t s a strong r e t a r d a t i o n of crack growth as compared to m o n o l i t h i c 2 0 2 4 - T 3 . ARALL i n t h e p r e s t r a i n e d condition i s even more f a t i g u e i n s e n s i t i v e .

F r a c t u r e t o u g h n e s s p r o p e r t i e s a r e summarized in f i g s . 8 and 9 for t h e two ARALL types r e s p e c t i v e l y . The r e s i d u a l s t r e n g t h of ARALL i s depending on whether the f i b r e s a r e cut i n the wake of the crack. As compared t o monolithic aluminium a l l o y s , a somewhat lower r e s i d u a l s t r e n g t h i s found when t h e f i b r e s a r e c u t (sawcut c o n d i t i o n ) but a considerably higher r e s i d u a l s t r e n g t h when the f i b r e s a r e i n t a c t (fatigue crack c o n d i t i o n ) . The a p p l i c a t i o n of aramid f i b r e s w i t h t h e i r r e l a t i v e l y small s t r a i n t o f a i l u r e i m p l i e s a r e l a t i v e l y n o t c h -s e n -s i t i v e b e h a v i o u r u n d e r -s t a t i c l o a d i n g ( f i g . 1 0 ) . However, i n t h e s t r e s s c o n c e n t r a t i o n range n o r m a l l y p r e s e n t i n a i r c r a f t s t r u c t u r e s , ARALL shows t o be s u p e r i o r compared t o the monolithic aluminium a l l o y s . An i m p o r t a n t a s p e c t of ARALL i s t h e b e h a v i o u r under machining and forming p r o c e s s e s . C u t t i n g , d r i l l i n g ,

ARALL can be performed using standard aluminium work shop procedures. Cold p l a s t i c forming of ARALL s t i f f e n e r s can e a s i l y be done by means of a folding process (fig.12). Because of a r e l a t i v e l y poor a d h e s i o n between aramid f i b r e s and epoxy a d h e s i v e in t h e p r e p r e g l a y e r , ARALL showed t o be r a t h e r s e n s i t i v e f o r p e e l (mode I) l o a d i n g c a s e s . This feature w i l l be thoroughly discussed in the f i r s t p a r t of t h e p r e s e n t s t u d y , where t h e f i b r e / m a t r i x adhesion i s c h a r a c t e r i z e d by m e a n s of a s e t of standardized experiments. 2 . 4 : ARALL a p p l i c a t i o n s in a i r c r a f t s t r u c t u r e s The ARALL p r o p e r t i e s i n d i c a t e a p o s s i b l e a p p l i c a t i o n of t h e m a t e r i a l for t e n s i o n dominated, f a t i g u e s e n s i t i v e a i r c r a f t s t r u c t u r e s such as t h e lower wing s k i n and t h e c y l i n d r i c a l p a r t of t h e f u s e l a g e ( p r e s s u r e c a b i n ) . In comparison w i t h m o n o l i t h i c a l u m i n i u m a l l o y s , t h e i n t r o d u c t i o n of ARALL may r e s u l t i n w e i g h t s a v i n g s t o over 3 0%. If one c o n s i d e r s t h e a c t u a l s e r v i c e l i f e of a c i v i l a i r c r a f t (10-20years), i t i s obvious t h a t t h e d u r a b i l i t y (long-term b e h a v i o u r ) of t h e m a t e r i a l i s of m a j o r importance. An i n v e s t i g a t i o n of t h i s problem area i s the second p a r t of t h i s study.

Chapter 3: C h a r a c t e r i z a t i o n of t h e d e l a m i n a t i o n behaviour of ARALL

3 . 1 : I n t r o d u c t i o n

As pointed out before (chapter 2) f a t i g u e crack growth in ARALL o c c u r s e x t r e m e l y s l o w l y as a r e s u l t of c r a c k bridging by t h e aramid f i b r e s . I t was a l s o s a i d t h a t crack b r i d g i n g i n s t e a d of f i b r e f a i l u r e r e q u i r e s some limited delamination in the f i b r e / a d h e s i v e prepreg l a y e r . Observations have shown t h a t d e l a m i n a t i o n o c c u r s a t t h e f i b r e / a d h e s i v e i n t e r f a c e . I n t h i s c h a p t e r t h e delamination behaviour w i l l be studied by carrying out a large v a r i e t y of e x p e r i m e n t s t o o b s e r v e and a n a l y s e t h e delamination phenomenon, and to r e l a t e t h e b e h a v i o u r t o r e l e v a n t delamination d r i v i n g f i e l d parameters ( s e c t i o n 3.3). A survey of t h e e x p e r i m e n t a l programme i s given i n t a b l e 1 .

In the section preceding t h e delamination e v a l u a t i o n , a brief l i t e r a t u r e survey i s presented concerning adhesion and i n t e r f a c e s t r e n g t h . In the survey (section 3.2) some general aspects (e.g. review of p o s s i b l e chemical bonds, w e t t a b i l i t y ) as w e l l a s a v a i l a b l e l i t e r a t u r e d a t a a r e t r e a t e d .

In s e c t i o n 3.4, a c a l c u l a t i o n model i s p r e s e n t e d t o p r e d i c t t h e d i r e c t i o n and o n s e t of d e l a m i n a t i o n extension. In t h e same s e c t i o n , t h e new i n s i g h t s on delamination i n i t i a t i o n and p r o p a g a t i o n a r e a p p l i e d t o the s t a t i c and dynamic f r a c t u r e behaviour of ARALL.

3.2: Literature survey

The structure of ARALL implies that a literature survey on adhesion and interface strength has to deal with two different interfaces:

- aluminium/adhesive interface

- fibre/matrix interface: In the test series performed in our laboratory, aramid prepregs are examined in comparison with glass and carbon prepregs. Therefore these three fibre types are included in the survey although carbon and glass fibres are not used in the standard ARALL material.

The survey itself is subdivided in three main topics: - electrostatic and c h e m i c a l i n t e r m o l e c u l a r bonds

(3.2.1)

- wettability properties (3.2.2) - surface roughness (3.2.3)

For the three topics, the same order has been followed: first a general survey is presented followed by detailed literature information on the typical ARALL interfaces. 3.2.1: Electrostatic and chemical intermolecular bonds 3.2.1.1:general review

The survey on a subject extensively treated in numerous publications will be relatively brief here. Much more detailed information is available in textbooks and papers.

It is noteworthy that on a chemical and physical level, there is no principal difference between cohesive and interface b o n d s . Both types of bonds c o n s i s t of a number of electrostatic and/or covalent bonds. In both cases (adhesion or cohesion) the strength of the bond is directly related to:

- type of bond(s) - number of bonds

In general three types of bonds sould be considered (ref.1):

- electrostatic bonds: These bonds are related to the interaction of atoms of an electronic structure which

is essentially independent of the proximity of other atoms around it.

- covalent bonds: For covalent bonds, the electrons between the a t o m s are shared and the e l e c t r o n i c structure of the atoms depends on the proximity of interacting atoms.

- metallic bonds: An ideal metal crystal consists of a regular array of ion cores, with the valence electrons (conduction electrons) nearly free to move throughout the w h o l e m a s s . In one c o n c e p t , the c o n d u c t i o n electrons are considered as a kind of electron gas (ref. 5,6,7). Pauling (ref.2) describes the electrons as resonating between bonding orbits (there are more bonding orbits than electron pairs). Because the electrons are free to m o v e , they can respond to polarizing fields (e.g. caused by external ions or dipoles). The ion cores can themselves interact in two ways:

1: repulsion: the result of the net positive charge of each ion core. This force drops as the square of the distance and is influenced by the screening of the conduction electrons (ref.6). 2: attraction: the result of the dispersion forces

of the electrons in the ion cores. This force decreases as the seventh power of the distance. Hydrogen bonds (e.g. ref. 2,3,4), often encountered when interfaces a r e c o n s i d e r e d , h a v e a p a r t i a l l y electrostatic character (dipoles formed by chemical bonds of hydrogen to more electronegative polyvalent atoms such as N, F and 0) and a partially covalent character. In general one may state:

electrostatic < hydrogen bond < covalent bond

bond strength strength strength

Both the e l e c t r o s t a t i c and c o v a l e n t bonds can be subdivided in different groups. Here only the more relevant ones will be briefly discussed. The bonds are presented in order of decreasing bond strength.

a) electrostatic bonds

- ionic bond: a positive and a negative ion attract each other, each ion acting as a nucleus surrounded by a rigid spherical distribution of electrons.

I n t e r a c t i o n energy: U + 4i7rer t

, + ■

where q ; q : charge of the ions

r : d i s t a n c e between t h e ions E Q : d i e l e c t r i c c o n s t a n t

- d i p o l e - d i p o l e bonds: t h e d i p o l e - d i p o l e f o r c e can be described by the e l e c t r i c dipole moment :

P = q.1

The origin of the dipole-dipole force is shown in the figure below:

U = yly2

Interaction energy:

^j—. (2 cose cos9_ - sine.. sine» cos(<|> _{♦ 2)>} (fixed dipole orientation)

U = - 2 U = - =r PiP _1^2 r 2 1 2 3' kTr6 (fixed dipole orientation; head to tail configuration) (free r o t a t i o n dipoles)

where y^: electric dipole moments

r: distance between the two dipoles kT: average thermal energy

- dipole-induced-dipole bonds: in an electric field, an originally neutral atom or symmetric molecule can be polarized because of an attraction in opposite direction of the electron cloud and nucleus. The electric field necessary to induce the dipole can be produced by a neighbouring dipole.

I n t e r a c t i o n energy: 2

yl a2

U = g— (dipolar + nonpolar molecule; ref.27) r

yl a2 + v2 al

U = ( 2 permanent dipoles) where y^: d i p o l e moments

a^: p o l a r i z a b i l i t i e s

- London d i s p e r s i o n bonds: any i n s t a n t in t i m e , one can find a p a r t i c u l a r configuration of nuclei, and e l e c t r o n s t h a t p r e s e n t s an i n s t a n t a n e o u s d i p o l e " m o m e n t . This produces an e l e c t r i c f i e l d which i n t e r a c t s w i t h t h e p o l a r i z a b i l i t y of an a d j a c e n t molecule and r e s u l t s in an instantaneously induced d i p o l e and mutual i n t e r a c t i o n to provide t h e d i s p e r s i o n f o r c e . The London d i s p e r s i o n force i s t h a n c a l l e d t h e i n s t a n t a n e o u s f o r c e of a t t r a c t i o n a v e r a g e d o v e r a l l i n s t a n t a n e o u s configurations of t h e e l e c t r o n s in t h e m o l e c u l e . I t i s noteworthy t h a t t h e London bond i s t h e o n l y bond p o s s i b l e between two non-polar atoms ( r e f . 2 8 ) .

I n t e r a c t i o n energy: IT = - 2 ^ 2 ,C1C2 " " 4 r6 • 2C1 +C2 where ex: p o l a r i z a b i l i t y r : d i s t a n c e between two d i p o l e s C=I: i o n i z a t i o n energy b) covalent bonds

The d i f f e r e n t c o v a l e n t bonds ( t r i p l e , double* s i n g l e ) describe t h e number of i n v o l v e d e l e c t r o n s ( s i x , f o u r , two r e s p e c t i v e l y ) . C o v a l e n t bonds a r e formed by t h e sharing of u n p a i r e d o u t e r o r b i t a l e l e c t r o n s between atoms ( p a i r s of e l e c t r o n s w i t h opposing, s p i n s which occupy the same o r b i t s a r e not capable of bonding).

Two t y p e s of m o l e c u l a r o r b i t s r e s u l t from covalent.-. . i n t e r a c t i o n of simple s , p o r b i t s or t h e i r sp- h y b r i d s :

- sigma b o n d s : a r e o b t a i n e d by m o l e c u l a r .bonding i n t e r a c t i o n s of s - o r b i t s w i t h p - o r b i t s or , s p - . _ . hybrid o r b i t s . I n t h i s c a s e t h e b o n d i n g • e l e c t r o n s a r e centered between two n u c l e i .

- pi-bonds: a r e t h e r e s u l t of m o l e c u l a r i n t e r a c t i o n s of p - t y p e o r b i t s . The i n t e r a c t i o n may o c c u r t h r o u g h l a t e r a l o v e r l a p p i n g of t h e i n d i v i d u a l l o b e s o r i e n t e d i n t o an e q u i v a l e n t

plane of r o t a t i o n .

A s i n g l e c o v a l e n t bond (e.g. C-C) i s formed by a sigma bond.

A double covalent bond (e.g. C=C) is formed by a sigma and a pi bond.

A triple covalent bond (e.g. C=C) is formed by a sigma and two pi bonds.

Resonance e f f e c t s ( e . g . b e n z e n e CgHg) can c a u s e i n t e r m e d i a t e p r o p e r t i e s b e t w e e n s i n g l e and d o u b l e covalent bonds.

3.2.1 . 2 : Intermolecular forces at the aluminium/adhesive i n t e r f a c e

Before d i s c u s s i n g p o s s i b l e b o n d s a t t h e aluminium/adhesive i n t e r f a c e i t i s n e c e s s a r y t o recognize t h e d i f f i c u l t y of d e f i n i n g a s t a n d a r d aluminium i n t e r f a c e . Aluminium, j u s t l i k e most metals

(except g o l d ) i s , i n c o n t a c t w i t h o x y g e n , thermodynamically u n s t a b l e and w i l l form o x i d e s u r f a c e l a y e r s . Because of i t s high energy surface, water w i l l spread on t h e o x i d e l a y e r s and form s t r o n g hydrogen bonds. Further, not only the surface elements but a l s o t h e i r s p a c i n g and a c t i v i t y w i l l l a r g e l y i n f l u e n c e p o s s i b l e i n t e r a c t i o n s . I t i s t h e r e f o r e d e s i r a b l e t o d e s c r i b e f i r s t t h e b u i l t - u p of oxide l a y e r s ( n a t u r a l l a y e r s or l a y e r s r e s u l t i n g from p r e t r e a t m e n t s ) b e f o r e p o s s i b l e i n t e r a c t i o n s with polymers can be d i s c u s s e d . The n a t u r a l growth of an oxide l a y e r on an aluminium surface i s shown in fig.1 3. I t i s c l e a r t h a t t h e m e t a l elements i t s e l f do not c o n t r i b u t e t o t h e o x i d e / p o l y m e r i n t e r f a c e s t r e n g t h a l t h o u g h t h e i r o r i e n t a t i o n may have an i n f l u e n c e on t h e o r i e n t a t i o n of t h e growing o x i d e l a y e r . For ARALL, t h i s n a t u r a l o x i d e l a y e r i s r e p l a c e d by a p r o c e s s - c o n t r o l l e d oxide l a y e r d u r i n g a s u r f a c e p r e t r e a t m e n t c o n s i s t i n g of d e g r e a s i n g , p i c k l i n g and chromic acid anodising. The oxide l a y e r , r e s u l t i n g from an a n o d i s i n g p r o c e s s , c o n s i s t s p r i m a r i l y of boehmite (Al^O-j.HoO) a n d / o r p s e u d o b o e hm i t e (AI2O3.2H9O) ( r e f . 1 1 , T 2 , 1 3 ) . The e x i s t a n c e of e l e c t r o s t a t i c (secundary) bonds across the oxide/adhesive i n t e r f a c e i s s e l f e v i d e n t s i n c e d i s p e r s i o n bonds w i l l occur between any c o m b i n a t i o n of p o l a r a n d / o r n o n - p o l a r m o l e c u l e s . Depending on t h e n a t u r e of t h e a d h e s i v e ( p o l a r or non-polar) and t h e oxide l a y e r , the d i s p e r s i o n f o r c e s can c o n t r i b u t e dominantly (e.g. ref.14,15) or only p a r t i a l l y t o the i n t e r f a c e s t r e n g t h .

In recent l i t e r a t u r e (e.g. ref.1 6,17,18,19) i t i s s t a t e d t h a t i n the case of epoxies and p h e n o l i c s , a l s o primary (covalent or ionic) bonds do e x i s t a t the oxide/adhesive i n t e r f a c e a l t h o u g h e x p e r i m e n t a l p r o o f s e e m s t o be

absent. In r e f . 2 0 , Comyn p r o p o s e s a model for s t r o n g ionic bonds between an epoxy a d h e s i v e and an o x i d e surface:

1: an epoxy r i n g opens because of t h e p r e s e n c e of an

amine group: _

0

2: ion exchange sites on the aluminium surface are present as the result of an acid treatment:

/ A l — OH + HCl-*- XA1+C1~ + H„0

3: at the aluminium interface, ion exchange may occur:

o~

_ NH „ — CH _ — CH — + Al+X —► NAl+0~— CH— CH 0— NHtx~ 2 2 / / I 2 | 2

For the sake of completeness i t should be mentioned t h a t also between a n o n - o x i d i s e d , n o n - h y d r a t e d aluminium surface and a p o l y m e r , o t h e r i n t e r m o l e c u l a r bonds besides d i s p e r i o n forces may e x i s t . This type of bonds i s r e l a t e d t o m e t a l l i c bonds and i s c a l l e d d i p o l e -mirror-image bonds ( e . g . r e f . 9 , 1 0 ) .

3 . 2 . 1 . 3 : I n t e r m o l e c u l a r f o r c e s a t t h e f i b r e / a d h e s i v e i n t e r f a c e

Good a d h e s i o n o b v i o u s l y r e q u i r e s m o l e c u l a r groups in both a d j a c e n t s t r u c t u r e s which can r e a c t w i t h each other. B e c a u s e b o t h f i b r e s and m a t r i x c o n s i s t of macromolecules b u i l t up from long c h a i n s w i t h r e a c t i v e end g r o u p s , t h e s e end groups can r e a c t w i t h each o t h e r during t h e c u r i n g p r o c e s s . These c h e m i c a l bonds w i l l not r e s u l t i n a s t r o n g chemical a d h e s i o n s i n c e t h e number of bonds i s very l i m i t e d . Mostly by e t c h i n g (carbon f i b r e s ) , t h e number of a c t i v e groups on t h e fibre s u r f a c e can be i n c r e a s e d . If t h e s e groups a r e unable t o r e a c t c h e m i c a l l y with t h e m a t r i x g r o u p s , coupling a g e n t s can be used with a d e q u a t e r e a c t i v e groups t o both s i d e s of t h e i n t e r f a c e . As mentioned before, t h e s t r o n g e r t h e i n t e r a c t i o n energy of t h e chemical bond, t h e b e t t e r t h e adhesion (if a s u f f i c i e n t number of bonds i s i n v o l v e d ) . I t i s t h e r e f o r e n o t s u r p r i s i n g t h a t , considering t h e order of p o s s i b l e bonds in c h a p t e r 3.2.1 . 1 , f o r a good a d h e s i o n , c o v a l e n t ,

hydrogen and dipole bonds are aimed at. The three examined fibre/matrix combinations will now be discussed briefly.

- carbon/epoxy adhesion

The epoxy group can be presented as a very reactive three membered ring consisting of two carbon an one oxygen atom: Q Q

The epoxy polymer mostly used in adhesives, has the following structure for its repeating unit:

CH. C — CH. — OCH — CHCH„0 I | 2 OH The a v a i l a b l e u n r e a c t e d s e c o n d a r y h y d r o x y l g r o u p s (0H-) s e r v e a s r e a c t i v e c e n t r e s f o r c r o s s - l i n k i n g o r a d h e s i o n . The p o l y m e r i s f o r m e d f r o m e p y c h l o o r h y d r i n e a n d b i s p h e n o l - A : b i s p h e n o l - A e p y c h l o o r h y d r i n e CH„ HO C -I CH. — OH C H _ — C H — CH„C Z / 2. \ 0 The b a s i c m a t e r i a l f o r t h e p r o d u c t i o n o f c a r b o n f i b r e s i s m o s t l y PAN ( p o l y a c r y l o n i t r i l e ) : CN CN CN 1 X ' CH CH CH

\_„/ V../ V . /

\

After some heating stages, the two-dimensional network of the carbon fibre is obtained. The process is mostly followed by an oxidation process of the fibre surface to improve the adhesion properties:

.N <? CH / \ C CH, II C CH

S

The surface reactivity of carbon fibres is low because of the parallel orientation of the graphite planes with the fibre s u r f a c e . The C-atoms in the six-ring structure are strongly coupled by covalent bonds; only the boundary C-atoms can be reactive and functional groups can be added to these boundary atoms.

In general, three methods exist to improve the carbon fibre/matrix adhesion:

1: whiskerisation of the fibre surface: It is possible to create m o n o - c r y s t a l l i n e w h i s k e r s on the fibre surface, growing perpendicular to the surface. This method improves the mechanical joining and results in an improved interfacial strength.

2: oxidation of the fibre surface: Functional groups of the matrix system can be coupled with the O-atoms.

3: use of a coupling agent: A polymer can be added to the fibre surface with both coupling possibilities towards the fibre and the matrix. Mostly a dipole bond is created between the polymer and the fibre. To enable this bond, the carbon structure is supplied with a suitable reactive group (e.g. carboxyl-group).

Because the addition of functional groups to the fibre structure is well possible, very strong interfacial bonds can be created with the reactive functional groups of the epoxy matrix.

- glass/epoxy adhesion

The chemical structure of glass is based on that of silica (SiÜ2) with addition of oxides, boron, calcium, sodium, iron and other elements. The structure of glass shows no longe range ordening at a molecular level but consists of a three dimensional array of atoms.

Glass fibres have the following reactive groups at the fibre surface:

- Si-end atoms which have OH-groups - alkali metals which have OH-groups

Although interfacial bonding of the end atoms with the OH-groups of the epoxy is well possible, coupling agents are mostly used to further improve the adhesion. Silane coupling is a well-known method which consists of:

The X - g r o u p i s h y d r o l y s e d and i n w a t e r c o n t a i n i n g s o l v e n t s one g e t s :

Y - R - Si - Xn ^ ^ Y - R - Si - (0H)n + nHX

The OH-group is bonded with the glass surface.

The Y-group can be chosen in relation to the matrix structure and can form strong covalent bonds with the epoxy matrix.

The silane coupling agent has also a water repulsive function (in the presence of water, the OH-groups can form c h e m i c a l bonds with H 2 O ) . As for the carbon fibres, the presence of reactive functional groups

(added or not) at the fibre surface results in a very strong interfacial bond strength.

- aramid/epoxy adhesion

The aromatic polyamides (aramid) are manufactured in a wet spinning process and consists of sequential segments of phenylene and amide (located at the para-carbons):

-O-

T'-phenylene amide

This segmental arrangement results in rigid chains like poly-p-phenylene terephtalamide (PpPTA) which is the chain structure actually used in the production of both KEVLAR and TWARON fibres.

The formation of the chain is governed by competitive intramolecular i n t e r a c t i o n s b e t w e e n the c o n j u g a t e d groups in the chain. Regularly p o s i t i o n e d a m i d e segments allow for medium strong intermolecular hydrogen

(0--H) bonds; adjacent hydrogen-bonded planes interact predominantly by means of van der Waals forces (dipole-induced-dipole, d i p o l e - d i p o l e or London d i s p e r s i o n forces) with some pi-bond overlap of the phenylene segments (ref.21).

The one-dimensional structure of the chain and the already weak bonds between adjacent chains and planes point to the absence of reactive functional groups in the chain structure. The addition of functional groups to the fibre surface or the use of coupling agents is extremely difficult and did, up to now, not result in

any significant improvements (ref.67). One method to improve interfacial adhesion is introduced by Allred and consists of getting amine groups (R-NH2) on the fibre surface by a t r e a t m e n t w i t h a m i n e c o n t a i n i n g low temperature plasmas. The amine groups provide reactive sites for covalent bonds with the epoxy system (ref.22). Although this m e t h o d proved to be s u c c e s f u l , the accompanying costs are so excessive that it is not yet used on an industrial scale.

3.2.2: Wettability properties 3.2.2.1: Interfacial adhesion

Before discussing wettability of liquids on solids, it is necessary to introduce the concept of interfacial tension. This concept only deals with electrostatic forces, m o s t l y s u b d i v i d e d in n o n - p o l a r , p o l a r and hydrogen bond groups (no covalent bonds). Interfacial tension is acting on a molecular level which, depending on the molecular distance between the adjacent phases and the type of attraction energy, may consist of two to three m o l e c u l a r m o n o l a y e r s on each side of the interface.

The first studies performed on interfacial tension were executed on a non-polar liquid/liquid interface which limited the interfacial reactions to dispersion forces. The adjacent layers of dissimilar molecules at the interface are in a different force field than the bulk liquid: The non-polar liquid molecules are attracted towards the bulk liquid by intermolecular forces which result in the surface tension of the liquid (Y-|). This internal attraction force is opposed by the attraction of liquid 2 due to dispersion forces. The counteracting by the dispersion forces is predicted by the geometric mean of the dispersion force components of the surface tension of the non-polar liquid and the second liquid as defined by Berthelot: ,

(Y Y o ) <M

bulk l i q u i d 1

bulk liquid 2

The total interfacial tension results in (for a non-polar liquid/liquid interface):

, - , d d.k ,, . Y12 = Yl + Y2 " 2 ( Y1 -Y2 > ( 1 ) where Y-|2: i n t e r f a c i a l t e n s i o n Y-| , 2: s u r f a c e t e n s i o n of l i q u i d 1 and 2 r e s p e c t i v e l y s u p e r s c r i p t d: d i s p e r s i o n component

When p o l a r a n d / o r hydrogen bonds a r e i n v o l v e d , eq.(1) has t o be extended t o :

Y12 = Y l + Y2 " 2(Y1 d.Y2 d)J 5 - 2 (Y ] L P / h. Y2 P /V (2) where s u p e r s c r i p t p : polar bonds

h: hydrogen bonds

This r e l a t i o n i s a l s o v a l i d f o r a s o l i d / l i q u i d i n t e r f a c e . The u s e f u l n e s s of t h e g e o m e t r i c mean of B e r t h e l o t i s e x t e n s i v e l y d i s c u s s e d i n t h e l i t e r a t u r e s i n c e i t can o n l y be used c o r r e c t l y when t h e i n t e r f a c i a l molecules have about t h e same r a d i u s ( r e f . 25). Good

(ref .26) p r o p o s e d t h e i n t r o d u c t i o n of a f a c t o r <> which j i s a function of the molecular dimensions and p r o p e r t i e s of t h e two i n t e r f a c i a l m a t e r i a l s . Y12 = Y, + Y2 " 2*<Y1.Y2>* ( 3 ) where * = * v • ^ (4) ^V = 4(v!v 2 ) 1 / 3 / (vi ^ + v2 ^ ) 2 *A = Al2/ (A11A22) h

V^: molar volumes of the molecules

A^ a t t r a c t i o n c o n s t a n t s in an r " ° p o t e n t i a l energy function as proposed by Lennard-Jones*

* The L e n n a r d - J o n e s p o t e n t i a l i s a m o d e l f o r t h e i n t e r m o l e c u l a r p o t e n t i a l energy of molecules: e = - ( a / r6 - b / r1 2) where e : p o t e n t i a l energy a / r6: a t t r a c t i o n p o t e n t i a l (due t o cohesive f o r c e s b e t w e e n molecules) b / r ' : r e p u l s i o n p o t e n t i a l

In ref. 28, a more extended survey is presented in which also other adjustments of the geometric mean of Berthelot are discussed (e.g. the theory of Huntsberger, ref.29 and Wu, ref.30).

3.2.2.2: Contact angles and wettability

In 1805, Young performed the first attempt to use contact angle studies for the description of interfacial forces (ref.31). He proposed to treat the contact angle of a liquid on a solid surface as the result of the mechanical equilibrium under the action of three surface tensions:

YL V: surface tension at the liquid/vapor interface

YSL: surface tension at the solid/liquid interface

Ys v: surface tension at the solid/vapor interface

(5)

7SL

. o o A liquid is called nonspreading if Of 0 . If 8 =0 , the liquid completely wets the solid and spreads freely over the surface at a rate depending on the liquid viscosity and the solid surface roughness.

More than hundred years after Young, Sumner (ref.32) proved that equation (5) can also be found using a thermodynamic approach. After some initial work of Thompson (ref.33,34), Dupre (ref.35) defined the reversible work of adhesion of a liquid on a solid as:

Wa = YSV + YLV ~ YS L (6)

The work of adhesion i s d i f f i c u l t to obtain by means of experiments because of:

1 : YSL and Ysv can h a r d l y be measured

2: necessary e q u i l i b r i u m c o n d i t i o n s can h a r d l y be obtained

3: adsorption of v a p o r on t h e s o l i d s u r f a c e i s h a r d l y avoidable

Especially the last point is extensively treated in the literature. It has lead to an extended Dupré relation

(e.g. ref.36,37,38):

wa = (YS° " YSV°> + YLV°-(1 + c o s 6> <7>

where Ys°: free energy at the solid/vacuum

interface

Ys vo: free energy at the solid/saturated

vapor interface

YL vo: free energy at the liquid/saturated

vapor interface

On its turn, the first term (Ygo - Y sv-o ) defined as the

equilibrium pressure of the adsorbed vapor of the liquid on the solid (*e) is also extensively treated in the

literature (e.g. ref.39 to 43). A survey of the literature is presented in ref.28.

In ref.29, Huntsberger derived an experimentally useful relation between the contact angle and the free energy of the interfaces:

v , 2 (rsod)^(yLV.d)^ 2 (T8.P)^(Tl|V.P)^

1 + COS 6 = + > + (8)

YLV° YLV° YLV°

Wetting of a liquid on a solid is mostly subdivided in different groups according whether the liquid and solid have a high or low surface free energy. Solids which can only interact by means of dispersion forces have a low energy surface; when also polar and/or hydrogen bonds may interact, a solid will have a high energy surface. The same definition can be used for liquids. In general, low energy liquids will spread on high energy surfaces because the liquid decreases the surface free energy of the system. However adsorption of a film by the solid can transfer it into a low energy surface and in that case nonspreading can occur (low energy liquids do not have to spread on low energy solids).

For the wetting of low energy liquids on low energy solids, Zisman (ref.39) proposed a powerful method for examining wettability using a critical surface tension concept ( YC) :

For low energy liquids and solids, eq.(8) reduces to: 1 + cos 9 = 2(Ysod)V(YL Vod)^

U s i n g s o m e w e l l - d e f i n e d l i q u i d s ( y L V " l i n e c a n b e p l o t t e d i n a c o s e -s t r a i g h t o k n o w n ) . . , c o s Ö - ( YT "* f i g u r e w i t h an o r i g i n (Y_./-1)and a s l o p e of 2 ( YSO 1

'W?V

^l_yo (inverse square root scale)

f o r l i q u i d s w i t h yLy0 ^ YC: s p r e a d i n g w i l l o c c u r YLyO > Yc: n o n s p r e a d i n g w i l l o c c u r A s i m i l a r a p p r o a c h c a n b e u s e d t o o b t a i n t h e s u r f a c e t e n s i o n of a n unknown s o l i d . W e t t i n g o f l o w e n e r g y l i q u i d s on h i g h e n e r g y s o l i d s i s , i n t h e a b s e n c e of a d s o r p t i o n , a l w a y s p o s s i b l e . U s i n g a r e t r a c t i o n m e t h o d i n s t e a d o f s p r e a d i n g , a s o r t o f c r i t i c a l s u r f a c e t e n s i o n a p p r o a c h c a n b e m a d e ( r e f . 4 4 , 4 5 ) . W e t t i n g w i t h h i g h e n e r g y l i q u i d s d e m a n d s a s p e c i a l t e c h n i q u e t o o b t a i n t h e s u r f a c e t e n s i o n c o m p o n e n t s ( n o n p o l a r a n d p o l a r ) . By c h o s i n g s p e c i a l n o n p o l a r s o l i d s , t h e d i s p e r s i o n c o m p o n e n t c a n b e o b t a i n e d . On t h e d e s i r e d s o l i d s u r f a c e t h e t o t a l s u r f a c e t e n s i o n c a n be o b t a i n e d a n d t h e r e f o r e a l s o t h e p o l a r o r h y d r o g e n p a r t of i t : YLV° = Y dLV° + YPLV° + y hLV° A f a i r l y c o m p l e t e s u r v e y of e x p e r i m e n t a l t e c h n i q u e s t o m e a s u r e t h e s u r f a c e t e n s i o n of l o w a n d h i g h e n e r g y l i q u i d s on l o w a n d h i g h e n e r g y s o l i d s i s p r e s e n t e d b y Good a n d S t r o m b e r g ( r e f . 4 6 ) . 3 . 2 . 2 . 3 : S p r e a d i n g o f a d h e s i v e s on a l u m i n i u m a l l o y a n d f i b r e s u r f a c e s I t was m e n t i o n e d b e f o r e t h a t a l l low e n e r g y l i q u i d s w i l l wet a n d s p r e a d o n h i g h e n e r g y s o l i d s . T h e r e f o r e i t i s d e s i r a b l e t h a t t h e s u r f a c e t e n s i o n o f t h e s o l i d ( l a r g e l y ) e x c e e d s t h e s u r f a c e t e n s i o n of t h e l i q u i d . I n t h e p a s t 30 y e a r s , s e v e r a l e x p e r i m e n t s w e r e p e r f o r m e d t o

obtain the surface tension data on aluminium oxides, fibres and adhesives.

From nucleation experiments, the interfacial tension (Ygr ) between pure aluminium and its melt was found to be 93 mJ/m2 (ref.47). As mentioned before, oxidation of the aluminium surface is inevitable and in most cases even desirable. It is therefore not surprising that, in connection with adhesion and adhesives, the wetting properties of a l u m i n i u m oxides (AI2O3) got more attention. Using a heats of solution concept, Fricke and Blaschke (1940) published the first data concerning the surface tension of A I 2 O 3 . It was found to be 560mJ/m2 (ref.48). Later studies performed on AI2O3 showed the distinction between the dispersion and polar component of the AI2O3 surface tension (ref.49,50):

Y d s = 100 mJ/m2

y p s = 538 mJ/m2 Y s = 638 mJ/m2

An interesting study was performed by Smith and Kaelble (ref.51). They examined the influence of pretreatment processes on 2024-T3: FPL-etch Plasma Vapor de-greased

A 2

Although the results are rather interesting (FPL-etching results in the highest surface tension) it is not understood by the author why the observed data are almost a factor 10 lower than the well-known data for AI2O3. For the three other solids, used in our test series

(carbon, glass and aramid fibres), the data are far less numerous and, what is worse, mutually disagreeing. However, this is not surprising considering the number of d i f f e r e n t adhesion p r o m o t e r s or o t h e r sizings introduced in the past.

In ref.20, the surface tension of carbon fibres was found to be:

Yd s = 25.9 1.5 mJ/m2 Yp s = 25.7 3.3 mJ/m2

Ys = 51.6 mJ/m2

In ref.10, the dispersion part of the surface tension of graphite was determined with three different methods:

Y c= 119 mJ/m'1 (free energy of adsorption concept) 108 mJ/m2 (heats of adsorption concept)

107 mJ/m2 (heats of immersion concept)

More recent investigations, performed with a flotation method, showed the following data for carbon and glass fibres:

critical surface tension ( Yc) :

carbon fibre (Thornell 5 0 ) : Yc ~ 24 mJ/m2 (ref.52) polyethylene coated glass: Yc - 30 mJ/m (ref.54) silicane coated glass: Y<-, - 22 mJ/m^ (ref.54)

surface tension ( Ys) :

nitric acid oxidized Thornell 50: Ys ~ 25 mJ/m2 (ref.53) ozone-oxidized Thornell 50: YS - 53 mJ/mz

(ref.53) Not only the low values obtained in these references are remarkable, but also the minor difference between the silicane and polyethylene coating on glass (from the latter coating very poor adhesion might be expected). Contact angle measurements on silane coated glass proved that a silane coating does not improve the wetting properties of epoxies (ref.55).

Information on the surface tension of aramid fibres is almost completely absent. To the author, no data on this subject are known. In ref.56, it is stated that the surface energetics of untreated PpPTA (aramid) are similar to that of a liquid epoxy resin. This statement is confirmed in ref.57 where the wetting of different liquids on aramid fibres is compared with a solid epoxy surface. H o w e v e r in both r e f e r e n c e s , no data are available concerning the interfacial tension between

aramid fibres and an epoxy adhesive.

The data on different epoxy adhesives are more common and fit rather well. All the results show that an epoxy adhesive is slightly polar:

For an amine-cured epoxy (DGEBA) (ref.58): Yd = 41 .2 ±2. 4 mJ/m2 Yp = 5.0 ±0.8 mJ/m2 Y = 46.2 mJ/m2 F o r an e p o x y p h e n o l i c ( r e f . 5 9 ) : Yd = 3 0 . 3 ± 4 . 5 mJ/m2 Yp = 1 2 . 0 ± 2 . 6 mJ/m2 Y = 42.3±2.5 mJ/m2

In general, an organic slightly polar epoxy with a surface tension of 40 to 50 mJ/m will wet and spread on an oxide layer of aluminium (Ys = 638 m J / m2) as long as the high energy surface is not c o n t a m i n a t e d . The wetting of an epoxy on a carbon, glass and aramid fibre

surface is more debatable since the surface tension of the fibres lies in the same region of the epoxy surface tension.

3.2.3: Surface roughness

It should be clear that the effect of surface roughness is much more important for the aluminium oxide surface, with a micromorphology characterized by micro etch pits, than it is for the surface of the fibres which are, even on a microscopical level, relatively smooth.

The influence of roughness on adhesion is twofold:

1: improved adhesion by means of mechanical interlocking 2: effect on surface area and wettability

ad 1 : The importance of mechanical interlocking on the adhesion properties of a bond has been largely discussed in the past. Already in 1947, Bikerman (ref.60) stated that surface roughness plays an important role in the bond strength. His statements were confirmed in later studies on a l u m i n i u m oxides (e.g. ref. 20,61) and polyester films (ref.62).

ad 2: It is clear that surface roughness may largely influence wettability. Adhesives with a high viscosity may cause the appearance of trapped air in the cavities and therefore induce inacceptable wetting. On the other hand, surface roughness increases largely the wetted area and consequently can result in an improved chemical and/or electrostatic strength.

Rough surfaces may also strongly influence contact angle measurements leading to largely deviating results. To account for surface roughness, some simple relations have been proposed:

cose' r = cose

where r: roughness factor 6 : true contact angle 6 ': measured contact angle

r = A/a (ref.64) where A: true area

a: projected area on a plane parallel to the apparent surface

A thorough examination of surface texture is presented in the literature by Johnson and Dettre (ref.65) and J.F. Marian (ref.66).

Chapter 3 . 3 : I n f l u e n c e of f i b r e / m a t r i x a d h e s i o n on t h e model and mode I I f r a c t u r e b e h a v i o u r of

intermediate layers

In t h i s p a r t of chapter 3, the influence of f i b r e / m a t r i x adhesion on the f r a c t u r e behaviour of i n t e r m e d i a t e l a y e r s i s examined through a s e r i e s of more or l e s s standardized experimental t e s t methods such a s :

- Bell peel t e s t (mode I f r a c t u r e )

- Width t a p e r e d double c a n t i l e v e r beam t e s t (mode I fracture)

- Interlaminar shear t e s t (dominant mode I I f r a c t u r e )

- Thick adherend t e s t (mode I I f r a c t u r e )

- End - notched flexure t e s t (mode I I f r a c t u r e ) - Central notched d e l a m i n a t i o n t e s t (combined

mode I + I I )

- Fibre p u l l - o u t t e s t (mode I I f r a c t u r e )

Most of t h e above t e s t i n g methods were developed f o r metal adhesives, p a r t i a l l y for obtaining c h a r a c t e r i s t i c m a t e r i a l p r o p e r t i e s of t h e a d h e s i v e s , and for a n o t h e r p a r t for c o m p a r a t i v e t e s t i n g and q u a l i t y c o n t r o l . For prepreg l a y e r s the s i t u a t i o n i s more complex. Due to the f i b r e s t h e p r e p r e g l a y e r i s no l o n g e r a homogeneous and i s o t r o p i c m a t e r i a l . T h a t w i l l a f f e c t t h e s t r e s s d i s t r i b u t i o n s i n t h e p r e p r e g l a y e r . Moreover, f r a c t u r e mechanisms which cannot occur i n an a d h e s i v e b o n d l i n e , a r e p o s s i b l e i n a p r e p r e g l a y e r . I n t e r f a c e s between f i b r e s and m a t r i x m a t e r i a l can be weak l i n k s . Under such c o n d i t i o n s i t i s not a t a l l obvious whether t h e s t a n d a r d t e s t i n g m e t h o d s w i l l g i v e u s e f u l a n d r e l e v a n t information. C o n s e q u e n t l y , an a n a l y t i c a l e v a l u a t i o n of the t e s t methods was c o n s i d e r e d t o be e s s e n t i a l for t h e p r e s e n t i n v e s t i g a t i o n . Such a n a l y s e s a r e p r e s e n t e d i n t h i s chapter before the t e s t programmes and t e s t r e s u l t s a r e discussed. Calculations for the a n a l y s i s are mostly presented in appendices.

In t h i s chapter the following t o p i c s w i l l be t r e a t e d : In s e c t i o n 3.3.1 some a t t e n t i o n i s p a i d t o t h e r e a s o n s f o r examining f i b r e / m a t r i x a d h e s i o n and i n t h a t r e s p e c t t o the r o l e of standardized specimens. In the next section (3.3.2) some general experimental d e t a i l s a r e presented such as t h e s t a n d a r d i z e d c o m p o s i t i o n of t h e m a t e r i a l s (both i n t e r m e d i a t e l a y e r s and t h e a d h e r e n d s ) u s e d throughout t h e t e s t s e r i e s . In s e c t i o n 3 . 3 . 3 , t h e b u i l t - u p and p o s s i b l e d e l a m i n a t i o n p l a n e s of an i n t e r m e d i a t e l a y e r a r e d e f i n e d . S e c t i o n s 3.3.4 t o 3.3.10 a r e d e a l i n g w i t h t h e above mentioned e x p e r i m e n t a l t e s t methods. F i n a l l y in section 3.3.11, a general d i s c u s s i o n i s p r e s e n t e d about t h e v a l i d i t y and u s e f u l n e s s of t h e

t e s t methods t o d e s c r i b e the f i b r e / m a t r i x adhesion.

Chapter 3.3.1 : R e q u i r e m e n t s f.p_r; s t a n d a r d i z e d t ^ s t : methods

Throughout the y e a r s , a whole range of experimental t e s t methods w i t h t h e i r f e l l o w specimen t y p e s have been introduced t o d e s c r i b e t h e d e l a m i n a t i o n b e h a v i o u r of adhesives a n d / o r p r e p r e g s i n b o t h notched and unnotched conditions. S e v e r a l t e s t m e t h o d s w i l l be d i s c u s s e d further on ( s e c t i o n s 3.3.4. t o 3.3.10.). However, i t i s necessary f i r s t t o r e c o g n i z e t h e g e n e r a l s t r e n g t h and weakness of t h e t e s t methods and t o d e f i n e t h e t a r g e t s which s h o u l d be a i m e d a t i n d e f i n i n g d i f f e r e n t experiments. S t a r t i n g w i t h t h e l a t t e r , in t h e a u t h o r ' s opinion, two f a c t o r s are of major importance to e v a l u a t e the usefulness of a t e s t method:

1: The r e s u l t s obtained with a c e r t a i n t e s t method should be i n d e p e n d e n t of specimen geometry t o a r r i v e a t widely v a l i d d a t a which d e s c r i b e t h e i n t e r f a c i a l f r a c t u r e behaviour of an adhesive or a prepreg.

2: The a p p l i c a t i o n of a f r a c t u r e parameter to a r e a l i s t i c s t r u c t u r e r e q u i r e s t h a t t h e s t r e s s ( o r s t r a i n ) d i s t r i b u t i o n in t h e s t a n d a r d specimen i s w e l l known. To f a c i l i t a t e e v a l u a t i o n s these s t r e s s d i s t r i b u t i o n s should r e s e m b l e a s much a s p o s s i b l e r e a l i s t i c s i t u a t i o n s . ad 1 : If t e s t r e s u l t s a r e a f u n c t i o n of t h e specimen geometry, i t i s a n o t h e r problem t o uncouple t h e d a t a from t h e t e s t method and make t h e r e s u l t s w i d e l y v a l i d and u s e f u l . In t h e a u t h o r ' s o p i n i o n , t h i s can only be r e a l i z e d by a c a r e f u l e x a m i n a t i o n of t h e g e o m e t r y influence on t h e f r a c t u r e b e h a v i o u r and by i n t r o d u c i n g equations which take i n t o account the specimen geometry parameters. In t h e a p p e n d i c e s a t the end of the

f i r s t volume, some a t t e m p t s a r e made t o c r e a t e such a s e t of e q u a t i o n s . I t i s c l e a r t h a t w i t h o u t knowledge of geometry e f f e c t s on f r a c t u r e d a t a , s e v e r e underestimations, or even worse, overestimations of the s t r e n g t h of a c e r t a i n p a r t can be made i f t h e t e s t r e s u l t s a r e introduced in c a l c u l a t i o n methods.

ad 2: Although p o i n t 2 seems t o be r a t h e r s e l f - e v i d e n t i t i s n o t always s o . As mentioned i n p o i n t 1, t e s t r e s u l t s o b t a i n e d w i t h some t y p e of specimen, can be highly depending on the specimen geometry. Even when a valid s e t of e q u a t i o n s i s a v a i l a b l e i n which t h e geometry e f f e c t s a r e i n c l u d e d , a d i r e c t c o r r e l a t i o n towards r e a l i s t i c s t r u c t u r e s t i l l may be dangerous (e.g. the occurrence of secondary bending in an adhesive j o i n t which w i l l never be accounted for in a t h i c k adherend t e s t a l t h o u g h one may s u b s t i t u t e t h e geometry of t h e