Bannister, M.K.

Herzberg, I.

Loh, A.

Thuis, H.G.S.J.

NLR Reports Repository

Nationaal Lucht- en Ruimtevaartlaboratorium
National Aerospace Laboratory NLR
NLR-TP-98022
The influence of stitching fibre and stitching density on the failure
strains of CFRP fabrics
H.G.S.J. Thuis, I. Herszberg,*A. Loh*, M.K.Bannister**
* The Sir Lawrence Wackett centre for Aerospace Design Technology
Department of Aerospace Engineering, Royal Melbourne Institute of Technology, P.O. Box 476V,
Melbourne, VIC, 3001, Australia
**Cooperative Research Centre for Advanced Composite Structures Ltd (CRC-ACS)
506 Lorimer Street, Fishermans Bend, VIC 3207, Australia
This investigation has been carried out under a contract awarded by the Netherlands Agency for
Aerospace Programmes, contract number 01511N.
This report is based on an article to be published in European Conference on Composite Materials ECCM-
8 in Napels, June 3-6, 1998.
The contents of this report may be cited on condition that full credit is given to NLR and the author(s).
Division : Structures and Materials
Issued : January 1998
Classificattion of title : Unlimited

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NLR-TP-98022
Contents
Abstract 5
1.  Introduction 5
2.  Plate configuratiosn 6
3.  Test specimen 6
4.  Test programme 6
5.  Impact test 7
6.  Test results and discussion 7
     6.1  Compression test results 7
     6.2  Tension test results 8
7.  Conclusions 8
References 9
1 Table
3 Figures
(12 pages in total)
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This page is intentionally left blank.
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NLR-TP-98022
The influence of stitching fibre and stitching density on the failure
strains of CFRP fabrics
by
H.G.S.J. Thuis1, I. Herszberg2, A. Loh2, M.K. Bannister3
1 National Aerospace Laboratory NLR
 Voorsterweg 31, 8316 PR Marknesse, The Netherlands
2
 The Sir Lawrence Wackett centre for Aerospace Design Technology
Department of Aerospace Engineering, Royal Melbourne Institute of Technology, P.O. Box
476V, Melbourne, VIC, 3001, Australia
3
 Cooperative Research Centre for Advanced Composite Structures Ltd (CRC-ACS)
506 Lorimer Street, Fisherman’s Bend, VIC 3207, Australia
Abstract
In this paper the results are described of an experimental programme in which the influence of
stitching fibre and stitching density on the failure strains of carbon fibre fabric/epoxy laminates,
impregnated by Resin Transfer Moulding (RTM), was investigated by carrying out static
compression and tension tests. The results were compared to results of typical unidirectional
prepreg specimens made in the autoclave.
The compression tests revealed that stitching did not result in a significant improvement of the
allowable strain levels, due to the presence of circumferential roving reinforcements in the RTM
fabric. Better results are expected without such reinforcements.
The compression after impact tests showed that failure strain levels for stitched and unstitched
RTM specimens are almost equal to failure strain levels for autoclave specimens.
The tension tests revealed that high density stitching (10 stitches/cm2) results in a severe
reduction of the tension failure strains compared to unstitched values. A stitching density of 4
stitches/cm2 will  result in a small reduction of tension failure strain.
The stitching fibre type, Kevlar or Dyneema, has no influence on the failure strain levels.
1. Introduction
Impact damages can cause a severe reduction of the mechanical performance of a composite
structure, especially when the structure is loaded in compression. To deal with this problem,
designers limit the maximum strain level at Design Ultimate Load to values which can safely be
reached in a damaged condition. Due to these strain level limitations, weight savings, which in
theory could be achieved by using composites instead of metals, are seldom reached in practice.
One way to increase the maximum allowable strain level at Design Ultimate Load is to increase
the damage tolerance of the composite materials used. This can be achieved by adding "through
the thickness" fibres to the composite, resulting in a 3-D composite. The "through the thickness"
fibres prevent delaminations, caused by impact, from buckling when the structure is loaded in
compression [1,2].
To investigate the influence of stitching fibre and stitching density on the failure strains of
carbon fibre fabric / epoxy laminates, impregnated by Resin Transfer Moulding (RTM), an
experimental programme was carried out.  The results were compared to results of typical
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NLR-TP-98022
unidirectional prepreg specimens made in the autoclave. The programme was carried out by
NLR in collaboration with the Australian Research Centre for Advanced Composite Structures
(CRC-ACS), under a contract awarded by the Netherlands Agency for Aerospace Programmes
(NIVR). The CRC-ACS produced the stitched preforms that were needed for the fabrication of
flat plates and performed open hole fatigue Cooperative tests. The NLR fabricated the flat
plates, machined the flat plates to specimens and carried out the static test programme.
The results of the open hole fatigue tests are reported in [3]. This paper presents the results of
the static test program. The test programme revealed that the stiffness of the RTM and autoclave
specimens were nearly the same. This means that the influence of the stitching density and
stitching fibre on the failure strain can also be interpreted as the influence of the stitching
density and stitching fibre on the failure stress.
2. Plate configurations
A number of flat plates were fabricated, eight by RTM and two by autoclave processing. After
curing, these plates were machined to the dimensions of the test specimens. The plates differed
from each other by stitching density (0 stitches/cm2, 4 stitches/cm2 and 10 stitches/cm2),
materials used (unidirectional carbon fabric / epoxy for the RTM plates and unidirectional
carbon tape/epoxy prepreg for the autoclave plates), stitching fibre (Kevlar and Dyneema) and
by laminate lay-up (“soft” laminates with a high percentage of 450 layers and "hard" 00 layer,
dominated laminates (see table 1)).
The thickness of one layer RTM fabric did not correspond to the thickness of one layer
unidirectional prepreg tape. To make the RTM plates as compatible as possible to the autoclave
plates, the number of prepreg layers for the autoclave specimens was chosen in a way that the
same amount of fibres was used, as close as possible, in the RTM plates as in the autoclave
plates while maintaining laminate symmetry. Therefore, one prepreg layer was added to
autoclave laminate no. 4 to make it compatible with RTM laminate no. 2 and two prepreg layers
were added to autoclave laminate no. 3 to make it compatible to RTM laminate  no. 1 (only one
layer was needed for compatibility  of the amount of fibres, however, because of symmetry, two
layers had to be added). After curing all plates were inspected by C-scan.
3. Test specimens
The cured flat plates were machined to the dimensions of the following specimens for the static
test program (see fig. 1): tension specimens, short block compression specimens, compression
after impact specimens, open hole compression specimens and open hole tension specimens.
Three specimens were made for each test configuration.
4. Test programme
A test programme was composed to obtain a global characterization of the mechanical
properties of the materials used. The test programme can be divided into two areas: compression
and tension tests.
The compression tests were subdivided into short block compression tests, open hole
compression and compression after impact tests. The short block compression tests were used as
a reference to determine the reduction in failure strain caused by an impact or an open hole. The
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NLR-TP-98022
compression after impact and open hole compression tests were used to evaluate the damage
tolerance behaviour of the materials used.
The tension tests were subdivided into standard tension and open hole tension tests. The
standard tension tests were used as a reference to determine the reduction in tension failure
strain caused by an open hole (which is considered as the most severe type of damage in tension
loaded structures).
All tests were carried out at ambient conditions. The tests were performed on a mechanically
driven static test machine. All tests were carried out under displacement control with a velocity
of 1.0 mm\min.
To avoid buckling of the open hole compression and compression after impact specimens, an
anti-buckling guide (with a window of 125 mm x 60 mm) was used during the tests of these
specimens. During the tests, failure loads and strains were recorded. To check the validity of the
compression after impact and open hole compression tests, out of plane deformations of the
specimens were recorded also during these tests.
5. Impact tests
Before being tested, the compression after impact specimens were impacted. To apply these
impacts an instrumented impact tower was used. During impact the specimens were clamped in
a frame with a window of 75 mm x 125 mm. The impacts were applied by a steel spherical tub
with a diameter of 0.5" at a velocity of 3.96 m/s.
The Barely Visible Impact Damage (BVID) was considered as the most severe impact damage
for a structure, since a structure with a BVID still has to be able to withstand Design Ultimate
Load (DUL). To determine the BVID energy level, a number of impact tests were conducted on
spare specimens of configuration no. 2.3. These impact tests showed that the BVID energy level
for these plates was 18 Joule. Therefore it was decided to impact all CAI specimens with this 18
Joule energy level, resulting in indentations between 0.25 mm to 0.55 mm.
6. Test results and discussion
6.1 Compression test results
Figure 2 shows the average compression, open hole compression and compression after impact
test results, based on the results of three specimens of the same configuration. From this figure
the following observations can be made:
1) The short block compression failure strains of the RTM specimens (configuration 1 and
2) are much lower than the short block compression failure strains of the autoclave
specimens (configuration 3 and 4). This can be explained partly by the fact that the
RTM specimens were made of fabric, whereas the autoclave specimens were made of
unidirectional tape. However, the main reason for the limited failure strains of the RTM
specimens must be accredited to the type of fabric that was used. In the  fabric used, a
number of rovings are configured with a circumferential reinforcement. Because of this
reinforcement, these rovings will not be compacted when the RTM mould is closed and
therefore the permeability of the fabric will be enhanced. However, it will also
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NLR-TP-98022
introduce resin pockets in the cured laminate. These resin pockets lead to instability
when the laminate is loaded in compression and will cause a severe reduction in
compression failure strain [4].
2) The autoclave specimens (configuration 3 and 4) show a tremendous reduction in
failure strain level due to an open hole or when damaged by impact.
3) The RTM specimens (configuration 1 and 2) show a much smaller reduction in failure
strain level due to an open hole or when damaged by impact.
4) RTM specimens with high density stitching (configuration 1.3 and 2.3) show higher
compression after impact failure strains than RTM specimens without stitches
(configuration 1.1 and 2.1) and medium density stitching (configuration 1.2 and 2.2).
5) Compression after impact failure strain levels of the RTM specimens are nearly the
same as the compression after impact failure strain levels of the autoclave specimens,
when comparing soft laminates 1 and 3, and hard laminates 2 and 4.
6) The compression after impact failure strain level of the unstitched RTM specimens
(configuration 1.1 and 2.1) is almost equal to the compression after impact failure strain
level of the stitched RTM specimens (configuration 1.2, 1.3 and 1.5 and configuration
2.2, 2.3 and 2.5).
Although the motivation to use a stitching fibre to improve damage tolerance was not
clearly substantiated, stitching may still be a viable concept to reduce fabrication costs.
A stitching fibre facilitates handling of a preform since the stitching of fibres stabilizes
the preform. Tooling costs will be reduced because the stitching of fibres compact the
preform and therefore the loads required to close the mould will be limited. The
assembly costs will be reduced, because the amount of fasteners needed for assembly
will be reduced.
7) The stitching fibre type has no effect on the failure strain (compare configuration 1.2
with 1.5 and configuration 2.2 with 2.5).
6.2 Tension test results
Figure 3 shows the average tension and open hole tension test results, based on the results of
three specimens of the same configuration. From this figure the following observations can be
made:
1) The failure strains of the tension specimens made by RTM (configurations 1 and 2
except configuration 2.3 with 10 stitches/cm2) are close to the failure strains of the
tension specimens made in the autoclave (specimens 3 and 4).
2) The RTM specimens (configurations 1 and 2) show similar open hole failure strain
levels as the autoclave specimens (configurations 3 and 4).
3) Specimens with laminate configuration 2.3 (Kevlar stitching fibre and 10 stitches/cm2)
show a severe decrease in tension failure strain, due to the high stitching density in these
specimens.
4) The stitching fibre type (Kevlar or Dyneema) has no effect on both the tension and the
open hole tension failure strain (compare configuration 1.3 with 1.5 and configuration
2.2 with 2.5).
7. Conclusions
An experimental static test program was carried out to investigate the influence of stitching fibre
and stitching density on the mechanical performance of CFRP fabrics by testing specimens
statically in tension and compression. The program showed that for the RTM fabric specimens
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NLR-TP-98022
with a circumferential roving reinforcement, stitching did not result in a significant
improvement of the allowable strain levels at Design Ultimate Load. However, compression
after impact tests showed that the reduction in compression failure strain due to an impact is
much smaller for the RTM specimens than for the autoclave specimens while the compression
after impact failure strain levels for these materials are almost equal.
This means that when the compression failure strain in undamaged condition can be improved
(by using fabrics without circumferential roving reinforcements) while the reduction in
compression failure strain due to damage remains in the same order of magnitude, then the
allowable compression strain levels at Design Ultimate Load will increase. As a result, the
allowable strain levels at Design Ultimate Load for RTM composites may reach beyond those of
the traditional autoclave prepreg tape composites.
In the framework of a follow-on program, unidirectional CFRP fabric without circumferential
roving reinforcement was impregnated with epoxy resin by RTM. Compression tests on
specimens of this material revealed that the failure strain in undamaged condition was doubled
in comparison to the failure strains of the RTM specimens with circumferential roving
reinforcement [5]. In future work the damage tolerance behaviour of this fabric will be
investigated further.
References
[1] Gary, Farley; Compression response of thick layer composite laminates with through
the thickness reinforcement. U.S. Army Aerostructures Directorate, AVSCOM, NASA
Langley Research Center. Journal of Reinforced Plastics and Composites, Vol. 11, July
1992.
[2] Dexter, Benson, Funk, Joan; Impact resistance and interlaminar fracture toughness of
through the thickness reinforced graphite/epoxy, NASA Langley Research Center,
Hampton, VA 23665-5225.
[3] I. Herzberg, A.Loh, M.K. Bannister and H.G.S.J. Thuis, Open hole fatigue of stitched
and unstitched carbon/epoxy laminates, 11th International Conference on Composite
Materials, Gold Coast, Australia 1997
[4] J. Summerscale; P.R. Griffin; S.M. Grove, Quantitative microstructural examination of
RTM fabrics designed for enhanced flow, Composite Structures, Vol. 32, 1995, Special
issue, Eight International Conference on Composite Structures.
 [5] H.G.S.J. Thuis, The influence of stitching fibre, stitching density and lay-up on the
mechanical performance of CFRP fabrics, NLR CR 97208 L, 1997.
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Config: Material: Fabrication
process:
Stitch density and stitching fibre: Lay-up
no.:
1.1 1 RTM 0 1
1.2 1 RTM 4 st/cm2
Stitching fibre: 2x40 tex Kevlar
1
1.3 1 RTM 10 st/cm2
Stitching fibre: 2x40 tex Kevlar
1
1.5 1 RTM 4 st/cm2
Stitching fibre: 3x16.5 tex Dyneema
1
2.1 1 RTM 0 2
2.2 1 RTM 4 st/cm2
Stitching fibre: 2x40 tex Kevlar
2
2.3 1 RTM 10 st/cm2
Stitching fibre: 2x40 tex Kevlar
2
2.5 1 RTM 4 st/cm2
Stitching fibre: 3x16.5 tex Dyneema
2
3.1 2 Autoclave
curing
0 3
4.1 2 Autoclave
curing
0 4
Material 1: Unidirectional carbon fabric GU230-E01-100, HTA fibre, epoxy
resin LY5052 and Hardener HY5052
Material 2: Unidirectional carbon prepreg tape, HTA fibre, epoxy resin 6376
Lay-up 1:
(soft laminate)
[45,-45,45,0,-45,90,45,-45,0,-45,45,90,-45,0,45,-45,45]
E1compression = 34532 N/mm2
Lay-up 2:
(hard laminate)
[45,0,-45,0,45,90,0,-45,0,-45,0,90,45,0,-45,0,45]
E1compression = 51782 N/mm2
Lay-up 3:
(soft laminate)
[45,-45,45,0,-45,90,45,-45,45,0,45,-45,45,90,-45,0,45,-45,45]
E1compression = 34898 N/mm2
Lay-up 4:
(hard laminate)
[45.0,-45,0,45,90,0,-45,0,0,-45,0,90,45,0,-45,0,45]
E1compression = 59953 N/mm2
** The compression modulus was determined by taking the tangent of the load-strain
curves ,measured during the short block compression tests, divided by the area of the
specimens. Fibre volume fractions of the plates was 55%.
Table 1: Plate configurations and laminates used
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The influence of stitching fibre and stitching density on the failure strains of CFRP fabrics

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