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Journal ArticleDOI

A multi-pattern compensation method to ensure even temperature in composite materials during microwave curing process

01 Apr 2018-Composites Part A-applied Science and Manufacturing (Elsevier)-Vol. 107, pp 10-20

AbstractMicrowave 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.

Topics: Curing (chemistry) (53%)

Summary (3 min read)

1. Introduction

  • Fiber reinforced polymer composites with strong mechanical properties are increasingly used in aerospace products [1, 2].
  • The technology has a number of problems which restrict further improvement of product quality and manufacturing efficiency [4].
  • 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 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.
  • 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].

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 , 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.

HP (1)

  • Where mnP is the microwave power at a certain point on the composite surface.
  • The MCSS represents a couple of parameters regarding the resonant applicator, the microwave input and the composite part.
  • The position and number of microwave inputs were used as the control strategy of the MCSS, and can be expressed mathematically as a vector.
  • 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.
  • This is the idea of the multi-pattern compensation method.

3. Implementation of the multi-pattern compensation method

  • A process control system is developed to implement the multi-pattern compensation method.
  • As shown in Fig.3, the structure of the system can be divided into two parts.
  • One is aimed at improving the temperature uniformity of the composite part, and the other is to keep the average temperature following the setting temperature.
  • When the curing process is started, the temperature distribution of the part is monitored and analyzed in real time.
  • Once the HP is selected, the computer will rapidly adjust the switches of the magnetrons of the oven according to the related control strategy.

3.1 Strategy of HP database construction

  • Since the microwave power distribution on the composite surface is unmeasurable without disturbing the original microwave field, another physical quantity that is directly proportional to the microwave power has to be used to solve this problem.
  • As for the exothermic heat of cure reaction, it will influence the temperature distribution of the composite during microwave curing, but the temperature distribution is monitored in real-time by temperature sensors, and then a normal compensation process will be carried out.
  • As shown in Fig.4, if only one spot is considered during the compensation process, n HPs are needed to construct the HP database, making this spot, for example hot spot or cold spot, of these HPs spread over the whole surface of the composite part.
  • If not, it is better to make the control strategy programmed so that the computer can continuously execute multiple times in a single cycle for time saving.

3.2 Strategy of uneven temperature compensation

  • During the microwave curing process, the uneven temperature of the composite part is compensated based on the established HP database.
  • Only the hottest spot or the coldest spot was taken into account during one compensation step, considering the compensation accuracy and efficiency at the same time.
  • If max minT-T T-T³ (T , maxT and minT are the average, maximum and minimum of the measured temperature), the hottest spot on the part will be compensated preferentially.
  • Specifically, the computer will search the HP database very quickly for an appropriate HP which has the lowest heating rate at the corresponding position.
  • These monitoring and compensating steps are repeated until the composite part is completely cured.

3.3 Strategy of power control

  • The purpose of the power controller is to keep the measured average temperature tracking the setting temperature.
  • When the total microwave power is determined, the power variation will be equally distributed to the current microwave inputs.
  • When the power of these inputs is increased by pD simultaneously, the heating rate of each area will increase by C , but the HP of the composite (relationship between the heating rates in these areas) will not be changed.

4. The experiment carried out

  • A 2.45GHz, 20KW microwave curing system was designed and manufactured by the research team, as shown in Fig.7 (a).
  • The composite plate was first preheated by various control strategies to construct its HP database.
  • The preset threshold for compensation was set as 6°C, and the operating time of each HP is 8s.
  • The second experiment randomly changed the microwave power of all magnetrons of the oven to generate relative movement between the electromagnetic field and the composite plate, like the commonly used mode agitator or turntable.

5. Results and discussions

  • Based on the authors’ experimental findings, this paper presents a multi-pattern compensation method to realize the uniform in-plane temperature distribution of composite parts during microwave curing.
  • Then, the effectiveness of the proposed multi-pattern compensating method is investigated in detail.

5.1 Theoretical analyses of the experimental findings

  • Since the HP of a composite part is directly affected by the electromagnetic fields inside a microwave oven, the influence of MCSSs on the electromagnetic fields inside the microwave oven is systematically discussed in this section.
  • As shown in Fig.8 (e) and (f), the resonant frequency of the microwave cavity after perturbation can be computed as follows.
  • The 10lTE mode induced by each microwave input in the rectangular oven of where 10lA is an amplitude constant, b is the propagation constant of microwave.
  • Thus, the position and number of microwave inputs, as well as the power ratio between them have significant influence on the distribution of electromagnetic fields in the cavity.
  • This further confirms the authors’ experimental findings of the relationship between composite HPs and MCSSs.

5.2 Validation of the proposed multi-pattern compensating method

  • The temperature distribution on the composite surface under the traditional single pattern heating, random field variation heating and multi-pattern compensation heating was compared by the shown maximum, average and minimum temperature profiles (see Fig.9).
  • Sometimes good results can be obtained, and sometimes the situation is just the opposite.
  • The maximum temperature difference was only 11.2°C, which brings a reduction of about 67% and 58% compared with the traditional single pattern heating and random field variation heating.
  • As a consequence, the temperature distribution during the random field variation heating is more homogeneous than that in the single pattern heating.
  • Then, there was a small overshoot along with the rise of the composite temperature, but it was gradually adjusted back by the system.

6. Conclusions

  • Based on the authors’ experimental findings that there is a one-to-one correspondence between composite HPs and MCSSs, a multi-pattern compensation method was proposed to realize a homogeneous microwave curing process for advanced composite materials.
  • In order to ensure the feasibility of the multi-pattern compensation method, the theoretical analysis of the findings was investigated; the principle of this method was discussed; and the control strategy of this method is designed.
  • It was demonstrated to be a feasible plan through the comparison of resulting temperature difference between this and other two traditional microwave curing process.
  • Under the situation of only considering the hottest spot or the coldest spot during one compensation step, the maximum temperature difference of a short carbon fiber/epoxy composite plate was reduced by 67% and 58% compared with the traditional single pattern heating and random field variation heating process.
  • This technology can be potentially used in other microwave heating processes as well where a high temperature uniformity is required.

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1
!
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).

2
!
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

3
!
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

4
!
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

5
!
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.

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TL;DR: An alternative method for improving the interfacial and tensile properties of carbon fiber composites by controlling the fiber-matrix interface was developed and such multiscale reinforced composites show great potential with their improved mechanical performance to be likely applied in the aerospace and automotive industries.
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