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A multi-pattern compensation method to ensure even temperature
in composite materials during microwave curing process
Jing Zhou
a
, Yingguang Li*
,a
, Nanya Li
a
, Shuting Liu
a
, Libing Cheng
a
, Shaochun Sui
b
,
James Gao
c
a
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics
and Astronautics, Nanjing, 210016, China
b
Chengdu Aircraft Industrial (Group) Co., Ltd., Chengdu, 610000, China
c
Faculty of Engineering and Science, University of Greenwich, Chatham Maritime,
Kent, ME4 4TB, UK
Abstract:
Microwave curing technologies have many advantages in manufacturing fiber
reinforced polymer composite materials used in aerospace products, compared with
traditional autoclave curing technologies. However, the uneven electromagnetic field of
microwave in the cavity of the curing chamber results in uneven temperature on the
surface of composite laminates during curing, which has been a major obstacle in
industrial applications worldwide. Existing methods attempted to solve the problem by
the random superposition of uneven electromagnetic fields, but the results were still not
satisfactory to meet the high quality requirements of aerospace parts. This paper reveals
the one-to-one correspondence between heating patterns of composite parts and
microwave curing system settings, and reports a new concept to solve this problem by
continuously monitoring and compensating the uneven temperature distribution in
real-time. Experimental results from both fiber optical fluorescence sensors and infrared
thermal imagers showed significant improvement in temperature uniformity compared
with existing methods.
Key words: A. Polymer-matrix composites (PMCs); D. Process monitoring; E. Out of
autoclave processing; E. Cure.
*Corresponding author at: College of Mechanical and Electrical Engineering, Nanjing
University of Aeronautics and Astronautics, Nanjing, 210016, China.
Tel.: +86 25 84895835; fax: +86 25 84895906.
E-mail address: liyingguang@nuaa.edu.cn (Yingguang Li).
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1. Introduction
Fiber reinforced polymer composites with strong mechanical properties are
increasingly used in aerospace products [1, 2]. According to an investigation recently
carried out in collaboration with Chinese Aircraft Industrial (Group) Co., around 98%
composites used in the aerospace industry are fabricated using autoclave curing
technologies, where the material is placed in a chamber and heated by the circulating
airflow [3]. However, the technology has a number of problems which restrict further
improvement of product quality and manufacturing efficiency [4]. For aerospace
applications, the most important problem is the serious deformation of composites of
large size with varying thickness, due to the large temperature gradient in the thickness
direction. Other problems include long process cycles and high energy consumption [5].
For example, the annual output of the A350XWB will be 156 airplanes after 2018 [6],
thus 312 wings need to be manufactured. For some composite parts, only the curing
process may take up to 24 hours with a maximum energy consumption of 4070KW per
hour for an autoclave of size Ф5m×14m [7, 8], and the part deformation can be very
severe [9]. This cannot meet the increasing demands for large quantity of high
performance composites in modern aircrafts.
As an alternative to traditional autoclave curing technologies, microwave curing
technologies can reduce curing time and energy consumption, and also reduce the
temperature gradient within the composite material during curing. This is because
microwaves can heat the whole volume of the material at the same time [10], thus
greatly reducing the deformation of composite parts and improving the efficiency of the
curing process [11]. To date, a lot of research work had been conducted on microwave
curing of composites materials, including fundamental principles [12], curing kinetics
[13], fiber/matrix interfaces [14], reducing temperature gradient [15] and mechanical
properties [16, 17].
However, microwave curing technologies have not been widely applied in the
aerospace industry because of the difficulties in ensuring an even temperature on the
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surface of composite laminates during curing [18, 19]. The uneven temperature
distribution is caused by the uneven resonance of the electromagnetic field in the cavity
of microwave ovens [20]. Resonance can be considered as the effect where waves are
incident from several directions at the same time. For any arbitrary point in the cavity,
the separate wave fields incident from different directions interfere each other, i.e., they
combine constructively and destructively in an alternating pattern, and form a standing
wave during the superimposition [21]. Over time ‘hot spots’ and ‘cold spots’ (relative to
the hot spots), corresponding to antinodes and nodes of the standing wave will
inevitably appear on the surface of composite materials, leading to the uneven in-plane
temperature distribution. Because composite materials are basically laminated plate
structures, the uneven temperature distribution on their surfaces has a significant impact
on their curing performance, which can directly contribute to severe warpage and even
local ablation or under-treatment.
In order to solve the uneven temperature problem, different ways had been
attempted in the past which can be classified into four categories. The first one is to
focus on the design of the shape and size of the microwave cavity [22]. For example, the
uniformity of the microwave field can be improved by increasing the size of the cavity.
This is because the number of resonant modes (the distribution state of the
electromagnetic field) within a microwave applicator increases rapidly as the size of the
cavity increases, and sometimes the different resonant modes within the applicator are
possible to have complementary effects. The second way is to use multiple microwave
sources within the cavity since the resonant modes associated with different sources are
able to overlap, which may further enhance the heating uniformity [23]. The third way
is to generate a relative movement between the material and the electromagnetic field
[24]. An example can be found in a microwave oven at home which is often equipped
with a turning table that rotates the plate with food during operation. The purpose of the
turning table is to reduce the effect of multiple hot spots by moving the object being
heated through areas of high and low power fields alternately, so as to achieve
uniformity in temperature of the food. The fourth way is to adopt variable-frequency
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microwave systems for materials processing, which can generate many different
resonant modes by repeatedly applying different microwave frequencies thus achieving
uniformity of power within the microwave cavity [25, 26].
The above existing methods have, to different extents, improved the uniformity of
microwave heating by random superposition of the uneven electromagnetic field within
the microwave cavity. However, these methods cannot solve the problem from the
scientific point of view, and the uneven temperature problem during microwave curing
remains as a major challenge in the manufacturing of advanced composite materials [18,
19]. This paper reveals the relationship between heating patterns of composite parts and
microwave curing system settings. On this basis, a multi-pattern compensation method
is proposed to achieve better uniformity of temperature on the surface of composite
laminates during microwave curing. This method, through monitoring the uneven
temperature distribution and applying appropriate compensating HPs in real-time, can
significantly improve the homogeneity of the temperature field of composite parts
during curing.
2. Idea of the multi-pattern compensation method
Through extensive experimental research, the authors found that there is a
one-to-one correspondence between heating patterns (HPs) of composite parts and
microwave curing system settings (MCSSs), as illustrated in Fig.1. Corresponding
theoretical analysis is presented in Section 5.1. Here, HP are defined as the distribution
law of the microwave power on the composite surface, which can be mathematically
expressed as a matrix which contains the information of the microwave power and
position.
11 12 1
21 22 2
12
n
n
mm mn
PP P
PP P
PP P
éù
êú
êú
=
êú
êú
ëû
HP
(1)
where
mn
P
is the microwave power at a certain point on the composite surface. The
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MCSS represents a couple of parameters regarding the resonant applicator, the
microwave input and the composite part. Among them, parameters of the composite part
(material, ply, shape, size and position) and the microwave equipment (shape, size,
filling medium of the applicator, and frequency of the microwave input) can be regarded
as constants and are difficult to be adjusted during curing, when a certain composite part
and a certain industrial microwave oven (often with multiple microwave magnetrons)
are selected. Fortunately, the position and the number of microwave inputs, as well as
the power ratio between them can be easily controlled by adjusting the switches of
various magnetrons of the oven as an electronic process. In this paper, the position and
number of microwave inputs were used as the control strategy of the MCSS, and can be
expressed mathematically as a vector.
[ ]
12
,,
l
dd d
=MCSS
(2)
where
l
d
is the switch state of the lth microwave input, which is a binary number and
can be valued at 0 or 1. As mentioned above, the heating pattern of the composite can be
controlled by adjusting the control strategy of the MCSS.
( )
f =MCSS HP
(3)
According to the above analysis, the HP of the object being heated will not change
as long as the related MCSS remains constant. Hence, when a part (or a new one of the
same) is heated for a new run, the HPs collected beforehand can be used as a useful
database to adjust its uneven temperature distribution. More specifically, when a certain
temperature distribution is monitored, a HP with a complementary heating preference
would be most beneficial to realize a uniform in-plane temperature distribution,
especially when the high/low power sections of the HP are cold/hot spots for the current
temperature distribution (see Fig.2). This is the idea of the multi-pattern compensation
method. It overcomes the limitation of random superposition principle in traditional
method, and use complementary HPs to ensure even curing temperature during the
whole curing process.
