ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
MECHANICAL PROPERTY AND FLOWABILITY OF
QUASI-ISOTROPIC UACS LAMINATES
Y. Fujitaa*, H. Matsutania, S. Kawamotoa, T. Takeharaa, I. Taketaa
a
Toray Industries,Inc., 1515, Tsutsui Masaki-cho, Iyogun, Ehime 791-3193, JAPAN
*[email protected]
Keywords: UACS, OHT, OHC, mechanical property
Abstract
The aim of this study is to investigate the applicability of unidirectionally arrayed chopped
strand (UACS) as a structural material for complex geometry. UACS is a sheet-like molding
material made by regularly introducing slits into a conventional prepreg. UACS achieves
both distinguished mechanical properties and excellent flowablity during molding as well as
SMC. Several static mechanical tests were carried out with UACS made of T800S/3900-2B
which includes an inter laminar toughened layer. As a result, minimal difference between
with and without slits were shown in compressive strength after impact, compressive strength
with hole and modeI/mode II out of plane toughness. Improved mechanical properties were
strength retention after drilling hole and transverse crack density in cross ply.
1. Introduction
Development of fiber reinforced plastic (FRP) has become more important as a lightweight
materials to achieve efficient energy consumption of automobile or aircraft. Especially,
carbon reinforced plastics (CFRP) are applied to many engineering fields due to its
distinguish mechanical properties such as high specific strength. In the application of CFRP,
especially to complex shaped components, component formability in other words the
flowability is important as well as mechanical properties.
Sheet molding compound (SMC) is one of the molding methods of CFRP achieving high
flowability. SMC is composed of randomly distributed chopped strands and unsaturated
thermoset resin. Complex shaped products, such as bathtubs, can be easily fabricated using
SMC. However, products made of SMC are relatively weak due to the fiber agglomeration
and orientation [1]. Therefore, its application has been limited to non-structural use for safety
concerns.
Based on these backgrounds, Unidirectionally Arrayed Chopped Strands (UACS) has been
developed which has both high flowability and high mechanical properties. As shown in
Figure 1, UACS is made by introducing slits into conventional unidirectional prepreg. The
products, components and laminates are formed by stacking UACS and curing them with hot
pressing. As a result, superior flowability is achieved by discontinuous fibers and also high
mechanical properties are achieved by unidirectionally arrayed fibers. Furthermore, as shown
in Figure 2, laminate structure is maintained even in complex structures like ribs. This implies
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
that UACS has a potential to extend FRP applications to the structural components such as
automobiles.
Fiber direction
Slit
Prepreg
Slit width
Slit interval (Fiber length)
Figure 1. Production of UACS
Figure 2. Rib structure formed by UACS
In the previous studies, initially, the relationship between slit width, slit depth to tensile
strength of unidirectional (UD) and quasi-isotropic (QI) UACS laminates were considered [2].
As the result, as the slit depth increases, the tensile strength decreased while the slit width has
minimal influence to tensile strength. An analytical model was also proposed to estimate the
tensile strength based on the energy release ratio for propagating delamination. In order to
improve the tensile strength, interlaminar toughening layer was proposed [3]. In the recent
studies, UACS with angled slit was proposed as an enhanced UACS. As the slit angle to fiber
direction decreases, the tensile strength increased [4]. Furthermore, as the slit angle decreases,
opening of the slit region is minimized due to the movement of fibers not only to fiber
direction but also transverse direction. The details of UACS flowability is discussed in
reference [5].
Before applying UACS to actual application, various mechanical properties considering not
only pure tension or compressive loading to fiber direction will be required. In this study,
several additional mechanical tests were carried out to determine distinct characteristics of
UACS.
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
2. Mechanical test
2.1 Materials
In this study, UACS were fabricated from T800S/3900-2B (prepreg of Toray industries) also
known to have an interlaminar toughened layer. Slit angle is 22 degrees and the slit length and
the interval are both 1mm. Almost all of the fibers in the prepreg are cut into 25mm strands.
Before stacking QI laminate, slits were inserted to each pre-preg layer to avoid overlapping
slits. Stacked UACS plies were set into an autoclave and cured at 180 °C for 120min.
2.2 Test methods
Mechanical properties obtained in this study are displayed in Table 1 with the corresponding
abbreviated test names and testing standards. The test specimen dimensions are shown in
Figure 3. Specimens without slits were also conducted as a control for each test.
Table 1. Test methods and size of specimens
Test name
Properties
Stacking sequence
Specimen
number
Test
method
QIT
Tensile strength of QI laminate
[45/0/-45/90]s
5
ASTM D5766
OHT
Open hole tensile strength
[45/0/-45/90]s
5
ASTM D5766
QIC
Compressive strength of QI laminate
[45/0/-45/90]2s
5
ASTM D3410
OHC
Open hole compressive strength
[45/0/-45/90]2s
5
ASTM D6484
CAI
Compressive strength after impact
[45/0/-45/90]3s
5
ASTM D7136
ASTM D7137
TCD
Transverse crack density in cross ply
[02/908/02]
3
-
DCB
Mode I out of plane toughness
[020]
5
JIS K 7086
ENF
Mode II out of plane toughness
[020]
5
JIS K 7086
OHT and OHC properties are strengths to determine the composite strength after drilling
holes (=6.35mm) for bolts and rivets. QIT and QIC were carried out to investigate the
difference of strength before and after drilled hole. It must be noted that the specimen
dimensions for OHC and QIC are different due to the testing nature. CAI is compressive test
of composite with initial damage which is provided by drop test. The drop energy was 30.5J
which is calculated in accordance with ASTM 7136. Test crosshead speeds for QIT, OHT,
QIC, OHC and CAI were 1.27mm/min.
For the TCD, the number of transverse cracks was counted while tensile load was paused at a
constant strain interval. The UACS specimen for TCD was composed of 0 degree layer
without slit and 90 degree layer with slits. DCB/ENF tests were carried out to investigate the
out of plane toughness of UACS laminate. Both specimens were formed including initial
crack in the interlaminar. The crosshead speed for DCB and ENF were 1.0mm/min and
2.0mm/min respectively.
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
OHT
QIT
GFRP tab
25mm
65
250mm
152.4
GFRP tab
10
304.8
65
OHC
Impact damage
Strain guage
101.6
38.1
12.7
(a) OHT, QIT, OHC
Hole is for OHT and OHC
(b) QIC
(c) CAI
(d) TCD
Pre crack
Initial crack
Stainless block
Initial crack
25.4
101.6
(e) ENF
25
(d) DCB
(unit : mm)
Figure 3. Size of specimen
3 Results of tests
3.1 QIT, OHT, QIC and OHC
The tensile strengths obtained from QIT/OHT test and QIC/OHC test are both shown in
Figure 4 and Figure 5 respectively. Figure 4(b) and Figure 5(b) show the strength retention
after drilling the hole. Both QIT and OHT strength for UACS are lower than the control;
however strength retention is higher than the control. As shown in Figure 6, both specimens
after OHT were broken around hole which suggests OHT strength of both specimens were
dominated by stress concentrations around hole.
Although accurate comparison is impossible for QIC and OHC because of the difference in
specimen dimensions, the retention of strength in compressive strength resembles the results
of OHT. This result implies fiber buckling around hole is a dominant factor for compressive
strength in UACS as well as the controlled specimen.
From these results, it is uncovered that UACS have lower notch sensitivity than controlled
specimen.
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
Figure 4. Result of OHT and QIT
Figure 5. Result of QIC and OHC
Figure 6. Damaged specimen after OHT and QIT
3.2 CAI
The relationship between the initially damaged area and CAI strength are both shown in
Figure 7. From this result, there are minimal difference in both the damaged area and CAI
strength. Similar to the result of OHC, this result implies fiber buckling in the stress
concentration around the damaged area which is dominant for CAI strength.
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
Figure 7. Result of CAI
3.3 TDC
The relationship between applied strain and transverse crack density is shown in Figure 8. As
a result, the initiation of the crack delays in UACS. One possible mechanism of this is the
reduction of residual thermal stresses in UACS due to resin region around slits shown in
Figure 9. Confirmation of the fracture mechanism must be further determined by observations
and numerical analysis in future studies.
Figure 8. Result of TCD
Figure 9. Transverse clack in UACS TCD specimen
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
3.4 DCB
The relationship between crack propagation length and energy release rate is shown in Figure
10. The initial energy release rate of UACS is equivalent to the controlled specimen, however
as the crack propagates difference in energy release rate is obvious. The reason to this
occurrence is the distinguishable fiber bridging in the controlled specimen. The short fiber in
the UACS is not possible to generate fiber bridging. However, application with small
deformation, this difference of energy release rate in the long crack length region will not be
critical.
Figure 10. Result of DCB
3.5 ENF
The mode II out of plane toughness is shown in Figure 11. There was minimal difference
since the crack is originally in interlaminar layer region where slits do not affected the
mechanical property.
Figure 11. Result of ENF
4 Conclusions
In this study, various mechanical tests have been carried out to obtain distinguished UACS
characteristics. As the conclusions, following results were obtained:
(1) Strength retention after drilling hole is higher than controlled specimen.
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ECCM16 - 16TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014
(2) Strain for initial transverse crack occurrence was higher than controlled specimen.
(3) Minimal differences in out of plane toughness compared with the control was obtained.
Figure 12 shows the retention of mechanical properties of UACS compared with the
controlled specimen. In the graph, CAI is compressive strength, TCD is the strain when
transverse crack density is 0.1 and DCB is the energy release rate at a crack length of 15mm.
Apart from the pure tension and compression properties, there are no remarkable differences
in mechanical properties for UACS. These results imply UACS will maintain high mechanical
properties after forming to complex shapes. Further investigation will optimize slit
dimensions and patterns to approach continuous fiber prepreg in the aspect of QIT, OHT and
QIC.
Figure 12. Retention of mechanical properties of UACS
References
[1] Marissen R, Linsen J. Variability of the flexural strength of sheet moulding compounds.
Composites Science and Technology, volume (59): 2093-2100, 1999.
[2] I. Taketa, T. Okabe and A. Kitano. A new compression-molding approach using
unidirectionally arrayed chopped strands, Composites : Part A, volume (39):1884-1890,
2008.
[3] I. Taketa, N. Sato, A. Kitano, M. Nishikawa, T. Okabe. Enhancement of strength and
uniformity in unidirectionally arrayed chopped strands with angled slits, Composites :
Part A, volume (41) : 1639-1646, 2010.
[4] I. Taketa, T. Okabe, A. Kitano. Strength improvement in unidirectional arrayed chopped
strands with interlaminar toughening, Composites : Part A, volume (40) : 1174-1178,
2009.
[5] I. Taketa, T. Okabe, H. Matsutani, A. Kitano. Flowability of unidirectionally arrayed
chopped strands in compression molding, Composites : Part A, volume (42) : 1764-1769,
2011.
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