JEC COMPOSITES MAGAZINE - Issue #84 - October 2013 - 45

Materials and manufacturing
TeXtreme® spread tow tapes and TeXtreme® spread tow plain
weave fabric (100 gsm) consisting of Pyrofil TR50S carbon
fibres were used to produce the specimens for this study. UD
composite specimens were manufactured by winding the UD
tapes on a plate followed by vacuum infusion of LY556/HY917/
DY070 epoxy resin. TeXtreme® spread tow plain weave laminates
were manufactured by placing 22 layers of fabric on a flat mould
and vacuum infusing the same epoxy resin. The required number
of cut plies was stacked on top of each other, each one positioned
in such a way (half tape width shift) as to avoid having the crossing points of the fibre bundles placed at the same horizontal and
vertical location. In both cases, the resin was mixed and preheated to 50°C. Full vacuum was achieved after 15 minutes of
degassing. The temperature was kept at 50°C while the resin infused. After infusion, the temperature was raised to 80°C and the
laminates were cured at this temperature for 16-20 hours. After
demoulding, the laminates were post-cured at 140°C for 4 hours.
The average thickness of the plates was approximately 2 mm.
To study impact response and damage formation in impacted
laminates, plates were made from 44 plies of 100-gsm TeXtreme®
weave, with the weave pattern equally positioned in all plies (no
shift). To achieve proper wetting of the plate through the thickness, pressure was increased to 4 bars using an RTM system.
The average thickness of the plates used for the impact tests was
approximately 4 mm.

Mechanical properties
and damage formation under static loading
The standard properties of UD composite specimens made from
TeXtreme® spread tow tapes were tested. The results of these
tests can be used to facilitate things like numerical modelling
at the meso level, which requires properties and characteristics
of the fibre tapes. In the tests, strain was measured with strain
gauges, and the elastic properties (longitudinal modulus, E1,
and Poissons's ratio, ν12) were determined in the region between
0.15% and 0.7% of axial strain. Transverse modulus was due to
the low failure strain measured between 0.1% and 0.2%. The
elastic properties and strength data obtained during the test
campaign, which also comprise results from tests on the plain
weave laminate [1], are summarized in Table 1.
The 25 MPa transverse tensile strength value is lower than
expected. This may be due to the quality of the materials used
to manufacture the plate. Some voids/defects were observed on
micrographs of polished specimens after testing. Additionally,
some specimens showed uneven fibre packing, with a layer-like
structure where very densely packed regions are clearly seen locally (See figure 3).
When testing the plain weave laminates, one of the objectives
was to study the possible formation of damage in thin-ply spread
tow fabric reinforced composites. Therefore, when loaded in tension, not all specimens were loaded to catastrophic failure. For
two specimens, the tests were deliberately stopped at strain levels

Tab.1: Mechanical properties measured for the TeXtreme® UD composite
and TeXtreme® plain weave laminate

TeXtreme® TeXtreme® plain
UD composite weave laminate

Property
Longitudinal modulus (GPa)

138

Transverse modulus (GPa)

8.1

In-plane shear modulus (GPa)

5.5

Poisson's ratio

0.25

0.033

Longitudinal tensile strength (MPa)

2390

1260

Transverse tensile strength (MPa)

(25)

Longitudinal compressive strength (MPa)

1170

709

In-plane shear strength (MPa)

102

of approximately 1.1% and 1.3% in order to assess possible initial
damage. The third specimen was loaded to catastrophic failure at
1.8% strain. After testing, the specimens were cut parallel to the
loading direction and polished in order to study the cracks with
an optical microscope. Two pieces were cut from each of the
two specimens that were unloaded when the strain level reached
1.1% and 1.3%, respectively, in order to study the cracks both at
the centre of the specimen and on the surface of the specimen
edge. It was expected that matrix cracks would be apparent at
strain levels of 1.1% and 1.3%. However, no matrix cracks were
detected when using the microscope's brightest light settings for
the specimen encased in fluorescence epoxy. There are a number
of possible explanations for this. The main one is that the study
was carried out after the specimens had been fully unloaded, so
that micro-cracks could have formed during the tensile loading stage. However, this would not become apparent because
the cracks close upon unloading. Another is that it is uncertain
whether the fluorescence epoxy could penetrate any cracks, due
to its high viscosity. To eliminate this potential source of error,
the specimens were also inspected with a low-viscosity penetrant
fluid. The dye penetrant inspection is governed by capillary action, since the surface tension of the penetrant fluid is low; the
fluid penetrates into the surface discontinuities of the specimen.

Fig. 3: Defects in the material due to manufacturing resulting in voids and
nonuniform mesostructure in some parts of the UD composite specimens
lead to low transverse tensile strength

No84 October 2013 /

jec composites magazine 45

JEC COMPOSITES MAGAZINE - Issue #84 - October 2013

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