COMPOSITES
SCIENCE AND
TECHNOLOGY
Composites Science and Technology 67 (2007) 1892–1899
www.elsevier.com/locate/compscitech
Effect of cure cycle heat transfer rates on the physical and
mechanical properties of an epoxy matrix composite
L.W. Davies
a
a,*
, R.J. Day a, D. Bond b, A. Nesbitt a, J. Ellis c, E. Gardon
c
Northwest Composites Centre, University of Manchester, Sackville St., Manchester M60 1QD, UK
b
Department of Mechanical Engineering, University of Manchester M60 1QD, UK
c
Hexcel Composites Ltd., Duxford, Cambridge CB2 4QD, UK
Received 11 August 2006; received in revised form 14 October 2006; accepted 14 October 2006
Available online 1 December 2006
Abstract
Although the autoclave technique produces composite parts of high-quality, the process is time consuming and has intrinsically highcapital and operating costs. QuickstepTM is a novel polymer composite manufacturing technique designed for the out-of-autoclave processing of high-quality, low-cost components with a reduction in cure cycle times. This paper assesses the use of the Quickstep method for
the processing of an epoxy/carbon fibre aerospace material and compares this to equivalent composites produced using an autoclave
process. Higher process ramp rates, achievable using Quickstep, have been shown to reduce resin viscosity thus facilitating void removal.
Manipulation of the Quickstep cure cycle, while the resin is at low-viscosity, has significant effects on the mechanical properties of the
product whilst simultaneously reducing the cure cycle time. Using Quickstep curing, samples were produced exhibiting comparable interlaminar properties but lower flexural strength as compared to those produced using the autoclave. However, normalisation of the data to
a common fibre volume fraction showed that better interlaminar shear strengths could be obtained using Quickstep. This improvement in
specific interlaminar shear strength was postulated to be due to the lowering of the resin viscosity over the duration of the cure, resulting
in better wet through of fibres by resin and improved interfacial adhesion between fibre and matrix. This study identifies key parameters
associated with the Quickstep process, providing a basis for further optimisation.
2006 Elsevier Ltd. All rights reserved.
Keywords: Quickstep; A. Polymer matrix composites; B. Curing; B. Mechanical properties
1. Introduction
Advanced fibre-reinforced polymer composite materials
have not been as widely adopted in industrial applications
as would be predicted from their structural performance
characteristics. The main reasons for their limited industrial use being the complexity of processing techniques
and the associated high-production costs [1]. The use of
advanced composites has, therefore, tended to be restricted
to industries such as the aerospace and high-performance
*
Corresponding author. Tel.: +44 161 306 2291.
E-mail address: leon.davies@postgrad.manchester.ac.uk (L.W. Davies).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compscitech.2006.10.014
automobile industries accustomed to high-production costs
and low-volume production.
Currently, high-performance composite components are
predominantly produced by either hand or machine lay-up
of prepreg laminates followed by treatment in an autoclave
at elevated temperature and pressure using a programmed
cure cycle. The high-pressure environment of the autoclave
facilitates the dissolution and removal of voids present in
the part, allowing the product to satisfy the stringent
mechanical performance standards required by the aerospace and other high-performance industries. The removal
of voids is of paramount importance since their presence
can have detrimental effects on the mechanical properties
of finished composite parts, manifested by a reduction in
L.W. Davies et al. / Composites Science and Technology 67 (2007) 1892–1899
strength [2–5] and fracture toughness [6]. Additionally, the
high-consolidation pressures involved in autoclave processing produces parts with high-fibre volume fractions which
in turn leads to further improvement in mechanical properties [7]. Although the autoclave technique produces parts of
high-quality, the process has intrinsically high-capital and
operating costs. Additionally, to avoid resin exotherm runaway, the cure cycles employed for autoclave processing
require relatively slow heat-up rates (1–4 C min1) which,
in addition to high-pressure generation (5–16 bar (500–
1600 kPa)), result in time consuming procedures.
There has been a recent drive to develop alternative
composite manufacturing technologies aimed at producing
materials of similar quality to aerospace grade composites,
but with shorter processing time and at a lower cost, e.g.
vacuum film infusion [8,9], vacuum bag only processing
[7,10] and electron beam curing [11]. In general, although
non-autoclave techniques benefit from shorter process
cycle times than autoclave curing, they tend to produce
materials with inferior mechanical properties due to the
absence of high-pressure.
Quickstep Technologies Pty Ltd. (Perth, Australia) has
developed a novel composite manufacturing process, which
can be utilised for the out-of-autoclave manufacture of
advanced composite materials [12,13]. The Quickstep process utilises a fluid-heated, temperature controlled, balanced pressure, floating mould which takes advantage of
the thermal conductivity of fluids. The process functions
by rapidly applying heat to an uncured laminate stack; this
is achieved by pumping a glycol based heat transfer fluid
(HTF) over the part, thus encouraging convective heat
transfer. The laminate stack is assembled on a single-sided
mould tool using conventional lay-up; it is then sealed in a
vacuum bag and transferred to a low-pressure chamber
containing the HTF (Fig. 1).
Temperature control is maintained by continuously circulating the HTF through the pressure chamber. The
mould and laminate stack are separated from the circulating HTF by two flexible silicone membranes. During processing of the composite, the HTF filled pressure
chambers are clamped together. Owing to the balanced
pressure environment, the closing of the pressure chambers
permits the laminate to be compressed without subjecting
the mould to any stress or distortion. The HTF also acts
1893
as a large thermal sink capable of removing any excess heat
generated by exothermic resin curing reactions, allowing
for the maintenance of a constant and well controlled temperature throughout the cure cycle of the laminate. The
Quickstep process was described in detail by Griffiths [12]
and Coenen [14].
Liquids generally have thermal capacities greater than
those of gases, thus the heat transfer rate between the
HTF and laminate is much greater than that achievable
in an autoclave; this leads to considerably improved heating/cooling rates and hence reduced cure schedule times.
The higher heating rate allows a lower resin viscosity to
be obtained in the laminate compared to autoclave processing thus consolidation is obtainable at lower applied
pressures (typically vacuum, plus 10 kPa externally from
the fluid). In addition, a lower viscosity improves the wetting of fibres by the resin matrix which has been demonstrated to increase fibre-resin adhesion and enhance
mechanical properties [15–17]. Recent evaluation studies
on the manufacture of a simple aircraft composite component have shown that the increased heat-up rate achievable using the Quickstep process improved its physical
and mechanical properties as compared to composites
produced using the autoclave process. Additionally, an
82% reduction in tooling and operating costs was achieved
[14,18].
The aim of the current work is to manipulate the cure
cycle used in the Quickstep process in order to modify
the physical properties of laminates, whilst attempting to
maintain mechanical properties rivalling those obtained
when using high-pressure autoclave processes. The Quickstep process has the potential benefit of versatile production facilities with reduced capital outlay and faster cure
cycles. Compared to autoclave methods, the reduced tooling and operational costs render the Quickstep process
more attractive to the general composite industry.
2. Experimental
2.1. Materials
The composite system used in this study was Hexply
6376 prepreg (supplied by Hexcel Composites), comprising
an epoxy resin/amine hardener matrix with unidirectional
T800 carbon fibre reinforcement. 6376 is a prepreg system
developed for autoclave cure and is primarily used in aerospace applications (typically close to 0% void content for
autoclave cured panels).
2.2. Vacuum bagging
Fig. 1. Schematic of Quickstep process.
All cured composites were fabricated using a conventional vacuum bag lay-up, as illustrated in Fig. 2. Glass
reinforced PTFE and Elastomax 224 nylon (supplied by
Aerovac Systems) were used as release film and bagging
film respectively. The breather fabric used was Ultraweave
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L.W. Davies et al. / Composites Science and Technology 67 (2007) 1892–1899
Bagging film
2 layers of
breather
To vacuum pump
Valve
Caul plate
Tacky
tape
(iv) Autoclave cure:
1. Heat from room temperature to 175 C at
2 C min1.
2. Isothermal dwell at 175 C for 120 min.
2.5. Conventional oven cure (v)
Tool plate
Release film
Prepreg laminate
Fig. 2. Components of vacuum bag used in this work.
1032 nylon (Tygavac Ltd). All prepared laminates consisted of 8 unidirectional prepreg plies 130 mm in width ·
80 mm in length, with the fibres aligned along the length
axis. Prior to curing the prepreg, the laminates were
de-bulked for 20 min in order to remove bulk trapped air
from lay-up and to consolidate the laminates further. The
maximum external consolidation pressure achieved, as a
consequence of using the vacuum system, was approximately 1 bar (100 kPa).
Panels were manufactured using a conventional thermal
oven to the manufacturer’s recommended cure cycle as
described above in Section 2.4. The conventional oven
was employed in order to simulate the autoclave cure using
vacuum alone with the absence of any applied consolidation over-pressure. Full vacuum was applied throughout
the cure process.
2.6. Mechanical testing
2.6.1. Flexural strength
Flexural testing was conducted on an Instron 4505 test
apparatus using a 3-point bend jig according to British
Standard [19]. The flexural strength rfs (MPa) of the composite samples were determined according to:
2.3. Quickstep cure
rfs ¼
Three different cure cycles (i–iii) were chosen in order to
investigate the effect of the cure cycle on the properties of
composite panels and are described below. The Quickstep
plant was found to be capable of producing average ramp
rates of approximately 10 C min1. Full vacuum was
applied throughout the cure process.
(i) Spike cure designated QSspike:
1. Heat from room temperature to 175 C at
10 C min1.
2. Cool from 175 C to 130 C at 8 C min1.
3. Isothermal dwell at 130 C for 20 min.
4. Heat from 130 C to 175 C at 10 C min1.
5. Isothermal dwell at 175 C for 120 min.
(ii) Dwell cure designated QSdwell:
1. Heat from room temperature to 130 C at
10 C min1.
2. Isothermal dwell at 130 C for 20 min.
3. Heat from 130 C to 175 C at 10 C min1.
4. Isothermal dwell at 175 C for 120 min.
(iii) Straight cure designated QSstraight:
1. Heat from room temperature to 175 C at
10 C min1.
2. Isothermal dwell at 175 C for 120 min.
2.4. Autoclave cure
The manufacturer’s recommended cure cycle was
employed for autoclave curing of the prepreg panels, as
described below. A consolidation pressure of 8 bar
(800 kPa) was employed throughout the cure process with
full vacuum applied.
3FL
2bh2
ð1Þ
where F(N) is the maximum applied load on the force–displacement curve, L is the span (mm), h is the sample thickness (mm) and b is the sample width (mm). Four specimens
were tested from each panel.
2.6.2. Interlaminar shear strength
Interlaminar shear testing was conducted on an Instron
4505 according to ASTM Standards [20]. The interlaminar
shear strength ILSS (MPa) was calculated according to:
ILSS ¼ 0:75
F max
bh
ð2Þ
where Fmax is the maximum load (N) on the load versus
deflection graph, b is the sample width (mm) and h is the
sample thickness (mm). Six specimens were tested from
each panel.
2.7. Rheology
The viscosity of neat 6376 resin was determined as a
function of time and temperature using a Bohlin Instruments Parallel Plate Rheometric Scientific Analyser. Heating rates of 2 C min1, 5 C min1, 10 C min1 and
15 C min1 were employed. The resin temperature was
raised at the given heating rate to 175 C where the temperature was held until gelation of the resin occurred. Additionally, the change in viscosity of neat resin over heating
profiles equivalent to the Quickstep cure cycles was
determined.
L.W. Davies et al. / Composites Science and Technology 67 (2007) 1892–1899
2.8. Void content and fibre volume fraction
The percentage void content /v and percentage fibre
volume fraction /f of the composite panels was determined
using hot acid digestion according to British Standard [21].
Five specimens from each panel were tested.
2.9. Optical microscopy
Optical micrographs of the composite specimens were
taken at 20· and 50· magnification. The specimens were
mounted in a polyester resin, ground, and then polished.
3. Results and discussion
Fig. 3 shows the temperature profiles achieved throughout the Quickstep and autoclave cure cycles. Owing to the
increased ramp rates the cure cycle time has been significantly reduced using the Quickstep process. The shortest
recorded cure cycle time was that of the QSstraight cure
at approximately 160 min. The longest cure cycle times
were those of the autoclave and oven cured samples at
approximately 275 min.
Table 1 shows the average values of flexural strength,
interlaminar shear strength, void content, fibre fraction
and panel thickness obtained for the manufactured composite panels. Table 1 shows the effect of changing the cure
cycle on the void content and, as a consequence, the
mechanical properties of the panels; generally, increased
void content resulted in reduced flexural strength and inter200
180
temperature (˚ C)
160
140
120
100
80
60
QSdwell
QSspike
QSstraight
autoclave/oven
40
20
0
0
30
60
90
120
150
180
210
240
270
time (mins)
Fig. 3. Measured temperature profiles for Quickstep, autoclave and oven
processing of composite panels.
1895
laminar shear strength. The QSspike panel however yielded
comparable ILSS values to the autoclave panel despite having a higher void content. The superior flexural strength of
the autoclave panel is due primarily to a higher fibre volume fraction as exemplified by comparing Figs. 4–6, which
show typical optical micrographs taken at 20· magnification for the autoclave, thermal oven and QSspike panels
respectively. Since the autoclave and oven cured panels
were produced using identical thermal cure profiles but differing pressure profiles, comparison of these processes
highlights the effect of using a high-pressure process compared to a vacuum (no overpressure) only process on the
fibre fraction; the autoclave panel exhibiting higher fibre
fraction. The presence of resin rich areas observed throughout the vacuum only composite samples (Figs. 5 and 6) was
responsible for the decreased fibre volume fraction. As
shown in Table 1, a lowering of the fibre volume fraction
correlated to an increase in the average thickness of the
manufactured panels.
The presence of resin rich areas was observed in all of
the vacuum only manufactured composites as illustrated
in Figs. 7 and 8. Further examination of Figs. 7 and 8 suggests that resin rich areas are regions previously occupied
by voids (dark areas on micrographs), the voids being
removed during the cure cycle of the composite and
replaced by flowing lowered viscosity resin. By observation, comparison of all the optical micrographs show typically that the frequency and size of resin rich areas found in
autoclave samples are negligible compared to those found
in the Quickstep samples. This indicates that a different
mechanism for void removal may exist between high-pressure consolidation processes as used in the autoclave process and vacuum consolidation as used in both the
thermal oven and Quickstep processes. In high-pressure
processing voids are encouraged to dissolve and disperse
within the matrix [22,23] resulting in low-void content
and high-fibre volume fraction.
In vacuum processing it is thought that throughout the
cure cycle the resin viscosity plays an important role in the
ultimate quality of the panel produced. It is suggested that
a lower viscosity over the duration of the cure cycle
accounts for a lower void content. Fig. 9 shows the viscosity profiles of neat 6376 resin as a function of the heating
rate and Table 2 shows the value of minimum viscosity gmin
obtained. It can be seen that the minimum viscosity of the
resin decreased significantly as the ramp rate was changed
from 2 C min1 to 15 C min1. This result is significant in
Table 1
Mechanical and physical properties of 6376 composite panels
Flexural strength rfs (MPa)
ILSS (MPa)
Void content /v (%)
Fibre fraction /f (%)
Average panel thickness (mm)
(i) QSspike
(ii) QSdwell
(iii) QSstraight
(iv) Autoclave
(v) Oven
1755 ± 47
115 ± 7
1.8
60.2
2.00
1477 ± 99
84 ± 5
8.1
55
2.12
1322 ± 65
71 ± 5
12.3
49
2.25
1923 ± 39
111 ± 2
0.6
64.1
1.97
1505 ± 75
84 ± 4
8.9
54.2
2.10
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L.W. Davies et al. / Composites Science and Technology 67 (2007) 1892–1899
Fig. 4. Optical micrograph of autoclave specimen taken at 20·
magnification.
Fig. 7. Optical micrograph of QSdwell specimen taken at 50·
magnification.
Fig. 5. Optical micrograph of oven specimen taken at 20· magnification.
Fig. 8. Optical micrograph of QSstraight specimen taken at 50·
magnification.
10000
2˚C/min
5˚C/min
10˚C/min
15˚C/min
Viscosity (Pas)
1000
100
10
1
0
Fig. 6. Optical micrograph of QSspike specimen taken at 20·
magnification.
10
20
30
40
50
60
70
time (mins)
Fig. 9. Viscosity of neat 6376 resin as a function of heating rate.
80
L.W. Davies et al. / Composites Science and Technology 67 (2007) 1892–1899
Table 2
Effect of heating rate on the value of minimum resin viscosity
Heating rate (C min1)
gmin (Pa s)
2
5
10
15
4.4
2.2
1.8
1.6
the present work since processing of composites using the
autoclave method was conducted at a ramp rate of
2 C min1 and Quickstep conditions use significantly
higher ramp rates of approximately 10 C min1. The initial ramp region (Fig. 9), before the preset dwell temperature of 175 C, illustrates the balance between the rate of
decrease of the viscosity with increasing temperature and
the rate of crosslinking which is causing the viscosity to
rise. Since the temperature dependent component of viscosity does not change at the dwell temperature, the region
beyond the dwell temperature of 175 C shows the effect
of crosslinking on the viscosity. Once at 175 C the resin
viscosity rose rapidly due to the increased rate of crosslinking at this temperature.
During the processing of the composites, a resin pregelation processing window was generated in which the
removal of voids from the composite matrix was facilitated
by a lower resin viscosity over a period of time. Once the
resin was sufficiently crosslinked, resin flow became negligible and voids were trapped in the composite structure. This
processing window was extended by manipulating the
Quickstep cure cycle to incorporate a spike cure (QSspike).
Fig. 10 shows the variation in resin viscosity during the
cure cycles used in this work, illustrating the concept of
the processing window. Fig. 10 shows that the time averaged viscosity over approximately 60 min is lowest for
the QSspike cure and highest for both the autoclave and
oven cure.
When producing a composite using the QSspike profile, upon reaching 175 C during the initial ramp period,
the temperature is rapidly reduced to 130 C thus maintaining a plateau period of relatively low-viscosity. This
lower temperature has the effect of preventing significant
crosslinking before voids are displaced by flowing resin.
10000
QSstraight
QSdwell
QSspike
Autoclave/Oven
Viscosity (Pas)
1000
100
1897
The average viscosity of the QSdwell cycle does not
appear to differ too greatly from the QSspike cure with
the exception that gmin occurs at a later time in the cycle.
However, the results in Table 1 show that void content is
lower for the QSspike cure, suggesting that the time at
which gmin occurs plays an important role in the strengthening and consolidation mechanisms involved during
manufacture. Fig. 10 illustrates how the processing window, i.e. the section of the cure cycle during which the
resin viscosity was relatively low, could be extended by
manipulation of the Quickstep cure cycle e.g. the processing window was extended from the QSstraight cure by
utilising the QSspike cure. The incorporation of a spiked
temperature profile followed by an isothermal dwell period allowed the viscosity to remain relatively low over a
longer period of time before the viscosity rose significantly. The relatively poor mechanical performance of
the QSstraight panel reflects the inefficient removal of
voids due to a significantly shortened processing window.
In addition, the QSspike panel showed significant
improvement in physical and mechanical properties to
that of the oven cured panel. Since the thermal oven process was employed as a model for the autoclave process
in the absence of consolidation over-pressure, comparison
of the data (Table 1) obtained for the QSspike cure to
those of both the autoclave and oven cure highlights
the significance of a lowered viscosity over a suitable period of time and the effect of applied pressure in
approaching a void free laminate.
The value of the interlaminar shear strength determined for the QSspike panel yields an interesting result.
Despite exhibiting a higher void content, the QSspike
composite had comparable interlaminar shear strengths
to those of the autoclave panel implying a form of
mechanical enhancement to the QSspike composite structure. This can be seen clearly in Table 3, where the QSspike panel exhibited a higher average interlaminar shear
strength to that of the autoclave when the mechanical
data was normalised to an equivalent fibre volume fraction of 60.2%.
The mechanical enhancement associated with the QSspike cure is postulated to be due to the lowering of the resin
viscosity over the duration of the cure cycle improving both
wet through of the reinforcement and surface wetting of
fibres by the resin matrix. Evidence has suggested that
improved wet through enhances both resin wetting contact
angles and wetting tension resulting in better physical
Table 3
Average values of ILSS normalised to 60.2% fibre volume fraction
10
Normalised ILSS (MPa)
1
0
10
20
30
40
50
60
70
time (mins)
Fig. 10. Viscosity of neat 6376 resin during cure cycles.
80
(i) QSspike
(ii) QSdwell
(iii) QSstraight
(iv) Autoclave
(v) Oven
115
92
87
104
93
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L.W. Davies et al. / Composites Science and Technology 67 (2007) 1892–1899
adhesion of resin to the fibre surface [17,24], in turn leading
to improved mechanical properties. This improved interfacial adhesion has been reported previously for glass fibre/
epoxy composites by using single fibre pull-out tests [25]
and in carbon fibre/epoxy composites using Raman spectroscopy [26].
light the potential for further optimisation of the Quickstep cure cycle conditions in a bid to reduce void
content further and improve mechanical properties. However, due to the inability of vacuum only processes to generate high laminate consolidation pressures, Quickstep
cured panels may not be able to match the fibre volume
fraction and thickness tolerances obtained by using autoclave processing.
4. Conclusions
The moulding of an aerospace prepreg has been performed using the Quickstep process and the physical
properties of cured laminates compared to those produced using both autoclave and thermal oven processing.
Owing to the increased rates of heat-up and cool-down
during the applied cure cycles, the overall fabrication
cure time of equivalent prepreg panels could be reduced
by up to 90 min using Quickstep. The higher ramp rates
associated with the process resulted in a significant
decrease in the resin minimum viscosity as compared to
those observed using the autoclave process. Using Quickstep, the cure cycle of the laminate was manipulated in
order to maintain resin viscosity at a low-level for as
long as possible thus generating an extended processing
window. The processing window provided an opportunity to exploit the lowered resin viscosity whilst wet
through occurred before significant crosslinking caused
a rapid rise in the viscosity. As demonstrated using the
QSspike cure, maintaining a lowered resin viscosity for
a longer period improves consolidation of the laminate
by lowering the void content of the system. The lowering
of the void content, and the subsequent increase in fibre
volume fraction, correlated with an improvement in the
mechanical properties of the Quickstep cured panels as
manifested by an increase in both the flexural and interlaminar shear strengths.
The QSspike cured panel showed a significant improvement in mechanical properties compared to the other
Quickstep cure cycles. The flexural strength of the QSspike
cured panel was found to be around 10% lower than an
equivalent panel produced using the autoclave whereas
the maximum ILLS was found to be comparable to that
of the autoclave panel despite a higher void content. However, the ILSS for the QSspike cured panel showed an
improvement over the autoclave cured panel when the data
was normalised to an equivalent fibre volume fraction providing evidence for an improvement in interfacial resin/
fibre adhesion. The interfacial enhancement was postulated
to be due to improved wetting of the fibres by resin as a
result of lower viscosity and improved mechanical interlocking of fibre and resin.
Since the oven process simulated the autoclave process
without any applied consolidation pressure, the improvement in the physical and mechanical properties of the
QSspike cured panel over the oven cured panel shows
the significance of a lowered resin viscosity against the
applied consolidation pressure. The results presented high-
Acknowledgement
The authors thank the Engineering and Physical Sciences Research Council (EPSRC) for providing financial
support to undertake this work.
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