Interfacial Phenomena in Composite Materials '91. Proceedings of the second international conference held 17–19 September 1991 in Leuven, Belgium

277 Pages·1991·11.967 MB·English

by  Ignaas Verpoest and Frank Jones (Eds.)

Interfacial Phenomena in
Composite Materials '91
Proceedings of the second international conference
held 17-19 September 1991 in Leuven, Belgium
Edited by
Professor Ignaas Verpoest
Department of Metallurgy and Materials Engineering
Katholieke Universiteit, Leuven, Belgium
and
Dr Frank Jones
School of Materials, University of Sheffield, Sheffield, UK
Sponsored by the journal COttipOSiteS
Co-organized with the Department of Metallurgy and Materials Engineering,
Katholieke Universiteit, Leuven, Belgium,
and in co-operation with the School of Materials, University of Sheffield, UK
U T T E R W O R TH
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First published 1991
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British Library Cataloguing in Publication Data
Interracial phenomena in composite materials 91
1. Composite materials. Interfaces
I. Verpoest, I. II. Jones, F. III. Composites
620.1'18
Library of Congress Cataloguing in Publication Data
Interfacial phenomena in composite materials 91
1. Composite materials—Congresses. 2. Surface chemistry—
I. Verpoest, I. II. Jones, F. III. Composites
TA418.9.C6155:1991 620.1'1892 89-15800
ISBN 0 7506 0356 9
Printed and bound in Great Britain.
Preface
Interfaces in composites are like relationships between people: without them, society would not exist,
and each individual, living in isolation, would loose his raison d'etre.
Composite materials behave similarly: without the presence of strong interfaces, they would only be
loose conglomerates, and most of the interesting properties of the individual component materials would
become completely irrelevant.
From the early development of composite materials, the optimisation of the interface between matrix
and the reinforcing material, - be it a fibre, a particle, a platelet, -, has been of major importance :
steel bars were roughened to improve mechanical interlocking with concrete, steel wires were coated to
promote chemical interactions with rubber in radial tires, glass fibres were coated with silanes to
improve their compatibility with polyesters ... .
When in the early sixties, carbon fibres were invented, the fibre-matrix interface became suddenly even
more important, because never before in the history of technology, had materials of such widely differing
mechanical properties been combined. In order to fully exploit the excellent properties of the carbon
fibres, the load transfer between fibres and matrices had to be optimised. This not only gave rise to
intensive studies of the carbon fibre surface chemistry, but also of interface micromechanics. Indeed,
the basics for most of the micromechanical models in composites which we still actually use, were
initially developed in the fifties and sixties.
These micromechanical models proved to be very useful: materials scientists were now able to predict
reasonably accurately the strength and stiffness of most composite materials. Since the calculations
predicted promising and experimentally verifiable properties, a wave of excitement caught the
imagination of the materials world. So, in the seventies, composites really took off. Carbon-epoxies took
the lead, followed by a wide variety of polymer and metal matrix composites. Composites invaded the
worlds of aircraft, spacecraft, sports goods, etc... Attention now shifted to problems of design and
manufacturing : design because designers had to relearn how to work with anisotropic materials, (viz.
wooden structures) after more than a century, and manufacturing, because completely new processing
methods had to be introduced onto the shop floor.
At the end of the seventies, micromechanics seemed to be obsolete, or at least out of fashion. Gradually
however, new problems arose, which by the end of the eighties, had put not only micromechanics of
composites, but also and even more, especially the composite interface, in the picture again.
Firstly, composites were shown to be sensitive to gradual degradation during service (fatigue, impact,
moisture, creep, etc..) by the progressive induction of microdamage, until failure. It was soon found
that the fibre matrix interface plays a major role in the damage tolerance characteristics of any
composite material.
Secondly, new fibres were introduced: not only high strength/intermediate modulus carbon fibres, which
required an extremely efficient fibre - matrix load transfer, but also new polymeric fibres, which had a
lower tendency to interact physico-chemically with polymeric matrices. Finally, the use of ceramic fibres
in metal-matrix and ceramic matrix composites demanded more accurate control of the interfacial
reactions.
XI
Thirdly, new matrix systems were used. Metal and ceramic matrices have already been mentioned, but
also the simple transition from thermosets to thermoplastics required a totally new type of fibre-matrix
interaction, to achieve wet-out during processing as well as optimisation of mechanical properties.
As a consequence, by the end of the 1980's, interfaces had reached a more prominent role in the
development of composite materials. So the first international conference on "Interfacial Phenomena on
Composite Materials", organised in 1989 at Sheffield University (UK) by Dr. Frank Jones, jointly with
Butterworth, came perfectly on time. Intended first to be a small, rather informal meeting, it attracted a
final attendance of about 150 people, and some 50 papers were presented.
At the end of this successful, first conference, it was decided to repeat it in two years, with a focus on the
micro-mechanics of the interface (although not forgetting the microchemical aspects), and that more
attention should be paid to metal - and ceramic matrix composites. It was a further intention too to
establish a regular series of IPCM conferences.
With all this in mind, a Scientific Committee for IPCM '91 was established, which reflected this
extension to other composites. It is appropriate to thank all the Scientific Committee members for their
generous effort. They selected out of more than 100 abstracts, the 53 papers for oral presentation, and
the 38 posters.
Together with the five keynote lecturers, they offer a complete and up to date overview of the most
relevant areas in the science of composite interfaces. Some papers are specifically devoted to one of the
three composite systems : polymer matrix, metal and ceramic matrix composites. But in some sessions
topics are covered which are relevant to all three materials : theoretical modelling, test methods,
characterisation methods.
Based on the quality of the papers, and on the diversity of the sessions, this second international
conference on Interfacial Phenomena in Composite Materials will be a success. Many people have
contributed to this. First of all, Dr. Frank Jones, who came up with the idea for the first conference in
1989, and who was for this conference the best co-chairman one could ever imagine !
The great contribution of the Scientific Committee has already been mentioned, but also the support of
the members of the International Advisory Committee is deeply acknowledged.
Thanks is also due to the staff of Butterworth-Heinemann Ltd. who are organising the conference in
conjunction with the Journal of Composites.
IPCM '91 is held at the Katholieke Universiteit Leuven. Finally I would like to thank all members of the
Local Organising Committee, of the Department of Metallurgy and Materials Engineering, and
especially our secretaries Greet Bruggemans and Ann Segers, for their great help and support. Without
them, reminding me of deadlines, organising files, mailings, meetings, IPCM '91 might not have been,....
or at least would have been very different!
Ignaas Verpoest
Leuven, 12 June 1991
Xll
MICROMECHANICS OF FIBRE-POLYMER INTERFACES
M.R. Piggott
Advanced Composites Physics & Chemistry Group, Department of
Chemical Engineering and Applied Chemistry, University of Toronto,
Toronto, Ontario M5S 1A4 Canada
The composite inteface that is receiving the most attention at the moment
appears to be that between polymers and fibres such as carbon, glass, and
Kevlar. There now seems to be very direct evidence of its influence on
shear strength, compressive strength and modulus, and strength normal to
the fibre direction. It can act as a wick for water, and can also affect
fatigue durability.
Unfortunately, when it comes to measuring interface (or interphase)
properties there are problems. Four methods are currently used, and each
method has its advocates. However, the methods produce different
results; the only published study which appears to provide comparative
values on identical systems gives values which do not correlate.
A major part of the reason for this is probably the interpretation of the
results. In every test, the possibility exists that the debonding process
involves slight frictional sliding of the part of the fibre already debonded.
Thus frictional forces are involved as well as pure debonding forces. If
this is ignored the results can be quite misleading. In addition, many
users of the pull out method produce results which seem unreasonably
high. This could be associated with the brittle nature of the bond failure in
the experiments.
Keywords: Interface, polymer, fibre
1. INTRODUCTION
The interface or interphase between fibres and polymers in a fibre composite is usually
made as strong and durable as possible since a weak interphase reduces many properties.
These include shear, off axis and compression strengths in undirectional composites and
laminates, strength and modulus in short fibre composites, and also delamination and
splitting resistance. Durability with respect both to fatigue loading and unfavourable
environments is also reduced by a poor interface.
Resistance to crack propagation across the fibres is enhanced by a weak interface,
however, so there is a trade off between shear strength and toughness for fibre
reinforced polymers [1]. Thus, some optimum interphase is required, and many attempts
have been made, using yielding and soft and even viscous interphases to increase
toughness without loss of shear strength [2] These have been only moderately
successful, and significant progress here requires a knowledge of exactly what
constitutes the optimum interphase.
3
0.5 1.0
Embedded length (mm)
Microcompression Fragmentation
Fig. 2. Debonding forces vs embedded length
Fig. 1. Four methods currently used for for glass in polyester resin. Pull out method
measuring interface/interphase properties. used.
The first step in establishing this is to measure the properties of interphases produced by
current methods of making composites. There are four methods of doing this, which are
in use at present, see fig. 1. These methods all measure the shear strength of the
interphase,and each has its merits and weaknesses. This paper will concentrate on pull
out and fragmentation since they can be interpreted without needing numerical analysis.
2. SINGLE FIBRE PULL OUT
The pull out method has several advantages. (1) It can be used to estimate debonding
forces as a function of bonding area while retaining geometric symmetry. (2) It gives
independent information about the friction that follows debonding. (3) It may, in
favourable circumstances, be used to estimate interfacial pressures. (4) When the
pressure is known the friction coefficient can also be estimated.
Its disadvantages include experimental difficulty, which may be alleviated by the use of
special fibre embedding equipment [3], and high fibre Poisson's shrinkage stresses,
which reduce the interfacial pressure below that observed in a normal composite. Also,
the analysis of the results appears to be far from straightforward.
The simplest analysis assumes that the interface yields at a stress Ti, so that the
y
debonding force, FA, is a linear function of embedded length L for a fibre of diameter 2r:
FA = 27irx. L (1)
A ly
Such a linear relationship is seldom observed, except at very small embedded lengths.
For longer lengths a maximum stress criterion is used. The interface is assumed to fail
when the maximum stress reaches a failure stress, Td> and the shear lag analysis gives
F = (27cr2t/n) tanh(nL/r) (2)
A d
n is a constant, dependent mainly on fibre and matrix Young's moduli, Ef and E [4]:
m
4
1/2
[EJE(lw )ln(R/r)]
n = f m (3)
Here v is the Poisson's ratio of the polymer matrix and 2R is the diameter of the
m
polymer cylinder in which the fibre is embedded.
The analysis has two problems: (1) the results cannot always be fitted to equation 2, and
(ii) the values of T(j needed for even moderately good fits are often too large.
Energy criteria instead of a stress criterion have been used [5], with some success in
curve fitting. However,the analysis due to Penn and Lee [6] produces the result
F = 2TH-2 jEfift tan(nL/r) (4)
A
which is indistinguishable in its variation with L from equation 2. Here Gi is the work of
fracture of the interphase. This neatly sidesteps the problem with Td, although Gi values
calculated from this equation are sometimes rather small. (Note that Gi and Td are related;
Gi = TdVEm2. Thus they are mutually interchangeable.)
However, the curve fitting problem requires a different approach. Equations 2 and 4
have an asymptote FA = constant which is independent of L. Although Penn and Lee and
Desarmot and Sanchez [7] have results which can be fitted, our results seldom can; see
figs 2 and 3 These results can sometimes be fitted if we include friction during
debonding [4]. Fig. 4 shows the process envisaged. Debonding starts from close to the
point where the fibre emerges from the polymer. It proceeds towards the remote end,
and involves a displacement of the debonded part of the fibre, resisted by friction. The
curve fits shown in fig. 2 were obtained in this way.
Unfortunately, this approach also has problems: (i) not all results are suitable for this
analysis (e.g. fig. 3) and (ii) the frictional data before and after debonding are in conflict.
—(E "^r r E 0
1
r—o
T
0.1 0.2 0.3 0.4
Kmhedded length (mm)
Fig. 3. Debonding force vs embedded length Fig. 4. Debonding processes envisaged ir
the pull out tests. Top; thermoset, bottom
for carbon (AS4; in epoxy resin. Pull out
thermoplastic.
method used.
5
It would seem, therefore, that no one failure process can be used to describe all pull out
results. For glass-polyester (fig. 2) it seems reasonable to assume friction during
debonding and a maximum shear stress criterion, indicating a relatively weak interphase,
although the discrepancy in pre and post debonding interfacial pressures (P) is
unexplained. For the carbon-epoxy as shown in fig. 3, for small embedded lengths (<
0.2 mm) a yielding process appears to be involved, since FA is approximately
proportional to L. This also applied to Pitkethly and Doble's results [8]. In both cases,
the shear yield strength of the interphase is extremely high, and in the former, about
twice that of the neat resin. At longer lengths, the failure mode appears to change to
brittle fracture. Desarmot and Sanchez and Penn and Lee's results with carbon-epoxy
appear to be wholly explicable on the basis of equations 2 or 4, indicating that friction
during debonding was negligible.
A more detailed look at the interfacial shear stress Ti, using numerical and finite element
results does not unfortunately, clarify the situation. Results due to Pagano [9], depend
strongly on the assumptions about the degree of support at the fibre end. Thus when
there is no displacement at the fibre end, the shear stress is a maximum near the fibre
entry point, while when the polymer is free to deform at the fibre end, it is highest at the
fibre end. This analysis does at least indicate why the meniscus is left adhering to the
fibre. This is because equilibrium conditions give %\ = 0 at the polymer surface and for
the same reason P becomes tensile there. Marmonet et al's finite element results [10], on
the other hand, suggest that %[ has two maxima, one at the surface, and one at the
embedded end. The shear lag analysis gives higher values than the Pagano results and
lower values than the Marmonet et al results.
1 1 0
x (mm)
Free length (mm)
Fig. 6. Partly debonded carbon fibre in a
Fig. 5. Fibre strength vs length: Hercules
fragmentation test with an interphasial shear
AS1 fibres.
strength of 60 MPa.
3. FIBRE FRAGMENTATION
This test has the advantage of being relatively easy to perform. It does not require the
use of optical techniques to observe the fragment lengths; instead they can be estimated
from acoustic emissions accompanying fibre breaks [11].
6
Its major disadvantage is the assumptions that need to be made in order to determine the
interface strength from the fragment lengths. It is assumed that the shear stress at the
interface is constant, and merely reverses at the centre. It is high improbable that this is
ever true. Instead, there is likely to be partial debonding, and large variations in shear
stress, as shown in fig.5. [5] The.theoretical curve here shown for the fibre stress is
confirmed by the work of Galiotis [12],
Another problem is that the true value of the fibre strength, (jfu, is needed since
^ = 3^1/41^ (5)
where Lm is the mean fragment length. Ideally, the fibre strength should be measured on
lengths close to L . Since L can be less than 1 mm, this is difficult. Instead,
m m
measurements at longer lengths (e.g. 5 and 50 mm) are made, and extrapolated using
Wiebull [13] or other [14] statistics. This may not be a good substitute. Fig. 6 shows
some recent results obtained in our laboratories with carbon fibres. (Previous
measurements only went down to lengths of 0.5 mm,[15]) The strength does not appear
to be a power law function of length. Failure of these assumptions may account for the
lack of correlation between pull out results and fragmentation [16].
Another approach, which requires very careful microscopic observation of the
fragmentation process, may be used to overcome both these problems.[17] However,
this is painstaking, and therefore not suitable for fast, comparative interface tests.
The results of these tests are often somewhat higher than the polymer shear strength, and
again, a fracture mechanism has been invoked. Works of fracture estimated from these
results tend to be somewhat lower than expected, based on the toughness of the polymer
[18]. This is also the case when G[ values are estimated from pull out tests [4].
4. MICROCOMPRESSION METHOD
This technique has two major advantages (1) equipment is commercially available for it,
and it can be carried out relatively easily, and (2) it may be applied to fibres in a real
composite. Its disadvantages are (i) it is difficult or impossible with Kevlar fibres, (ii) it
produces Poisson's expansion stresses which increase the interfacial pressure above that
present in a normal composite and (iii) it requires numerical analysis [19]. In view of the
complex behaviour of the interface or interphase observed in pull out tests, this is a major
limitation.
5. DISCUSSION AND CONCLUSIONS
Each test method has its advantages and disadvantages. Only shear strength and friction
data can be obtained. In view of the complexity of the behaviour of the interface,
correlation between the results of any particular test and composite properties data is
likely to be of doubtful value at the present time.
Clearly, a great deal of work is needed to clarify interphase failure mechanisms.
ACKNOWLEDGEMENTS
The author is grateful for financial support from the USAF Office of Scientific Research,
The Ontario Centre for Materials Research, and NSERC (Canada), and I am also
grateful to S.R. Dai and Z-N. Wang for provision of experimental results.
7
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