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Introduction
Conclusion
This report describes the findings of a study performed by
GARTEUR AG20, aimed at examining the fractographic aspects of fatigue failure
in polymer composites. An investigation into the mechanisms of fatigue failure
in unidirectional materials was performed to establish the
macroscopical/microscopical features associated with fatigue fracture. The
micromechanisms by which different fractographic features form, under different
loading conditions, as well as the effect of material on their appearance, was
studied. Investigations to establish whether there was a relationship between
the fractographic features and crack growth rate and direction were also
performed.
Fractographic investigations were undertaken by exchanging
fatigue specimens between members within three round robin exercises. Members
findings were reported at bi-annual meetings, six of which were held over the
three year life of the project.
The results of this study identified a number of
fractographic features unique to fatigue failure including; striations within
the fibre imprints and matrix rollers, both of which were observed within mode
II (shear) dominated failures. Striations were also observed in the matrix
between the fibres in some materials, but these were only visible using
electron microscopes with very high resolutions. Macroscopically the fatigue
fracture surfaces were noticeably smoother that their static equivalents,
which, in the mode I specimens, was attributed to a reduced level of fibre
bridging. The single phase resin systems gave rise to rollers and striations,
with the fibre/matrix bond appearing to be the most important factor
controlling striation formation within the fibre imprints. The main difference
observed between materials was most apparent in the two phase system, where in
mode II, rubbing out of the toughening thermoplastic particles was mainly
observed.
Studies of the matrix rollers showed they were not useful
for determining crack growth directions. Matrix striations could be used to
indicate the local directions of crack propagation, but the striations within
the fibre imprints were the most useful for determining global directions of
fracture. By observing which side of the fracture surface was fibre rich or
imprint rich and the appearance of the striations (bright or dark), the crack
growth direction could be ascertained. Due to the significant variation in the
inter-striation spacing observed, efforts to correlate striation spacing to
crack growth rates proved inconclusive.
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In all stages of the life of a
composite structure the systematic study of microstructure and fracture
surfaces provides essential feedback into material developments, design and
ultimately certification. As such, fractographic analysis is increasingly being
recognised as fundamental to the application of composite materials in
aerospace structures.
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Prior to
this fractographic programme, the GARTEUR Structures and Materials Action Group
AG14 (Fractography of Composites) [1] undertook a study aimed at establishing a
common terminology and methodology for the assessment of fracture surfaces.
Whilst conducting this work it became apparent that the examination of fatigue
fracture surfaces presented particular difficulties to fractographers, both in
terms of their identification and their interpretation. To rectify this lack of
understanding, the AG14 final report recommended that a detailed study of
fatigue should be pursued within GARTEUR.
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It is the
general perception that current fibre reinforced composites are not susceptible
to fatigue. Part of this perception has originated due to the use of
conservative design strains which have resulted in composite components being
subjected to only modest loads insufficient to promote fatigue failure.
However, with improved understanding and the development of improved fibres and
matrix materials, it is likely that design strains will increase, which could
lead to potential fatigue problems. It is essential therefore that the ability
to recognise andunderstand
fatigue failures in composites structures is developed before they arise;
either during the development of a component, or when it is in service.
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The
approach taken in this study involved the generation of a series of specimens,
tested under controlled conditions, which were then examined by members of the
group. Three round robin exercises, involving the exchange of mode I, mode II
and mixed-mode (I+II) interlaminar fracture specimens, were conducted on a
range of unidirectional materials. The fracture surfaces were examined by one
or more participants and the results reported, discussed and evaluated at
Action Group meetings. In addition, relevant data generated in-house was
presented as appropriate by individual members to aid the interpretation of the
fracture features.
The proposed programme of work sought to address the five
objectives listed below:
1) To
establish the macroscopical and microscopical features which indicated that
fatigue played a role during failure.
2) To
establish the microscopical mechanisms by which the features identified (in 1)
occur under different loading modes and stress
intensities.
3) To
establish the material dependency of the fractographic features associated with
fatigue failure (If appropriate unreinforced resins would be
examined).
4) To
establish the relationship between crack growth direction and the appearance of
the fatigue features.
5) To
establish the possibility of relating the crack growth rate to the
inter-striation distances on the fracture surface.
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A review of
the general literature has shown that much of the published work relating to
fatigue fracture in composites describes only the fracture processes occurring
in fatigue superficially. Often important information relating to the
examination conditions, or specimen test conditions, have been omitted. The
review has therefore highlighted the importance of including detailed
information of this kind when describing the findings of fractographic
research.
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In this
study a number of microscopic fractographic features, unique to fatigue
failure, were observed. These features included; matrix rollers, matrix striations and
striations within the fibre imprints. The mode of loading was important in the
development of these fatigue fracture features such
that:
1) Matrix rollers require
high shear stresses for their formation.
2) Striations in the fibre
imprints also appear to require the presence of shear stresses for their
formation.
Striations in the matrix were observed in both mode I and mode II loaded
specimens. The former was attributed to the presence of local shear stresses at
the fibre/matrix interface, caused by fibre bridging.
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Although the presence of such
features appears to confirm that fatigue played a role in failure, their
absence does not necessarily mean that cyclic loading has not played a part in
failure. The fractographic features observed on the mating surfaces of
specimens subject to bending were quite different, with one side often
containing many fibres, whilst the other contained mainly imprints.
Macroscopically the fatigue fractures were smoother than their static
counterparts.
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Fractographic features in the
material containing single phase resins were broadly similar. The PES present
in the two phase 914 resin masked many of the fatigue fracture features or
significantly modified their appearance.
A relationship between crack
growth direction and the appearance of the striations was only partly proven.
The striations have to have only limited value for the determination of crack
growth rates.
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The work
presented in this report provides increased understanding of fatigue damage
development, which will be invaluable if the mode and sequence of failure in
composite components, in use on both civil and military vehicles, are to be
identified successfully.
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