Study on Monitoring of Stress and Strain during Curing Process of
Fiber Metal Laminates
Xuhui Kang
1
, Lihua Zhan
1, 2 *
, Xintong Wu
1
, Tengfei Chang
2
and Jiayang He
2
1
School of Mechanical and Electrical Engineering, Central South University, Changsha, China
2
Institute of Light Alloy, Central South University, Changsha, China
Keywords: Fiber Metal Laminates; Stress and Strain Monitoring; FBG and Thermocouples; Strain Definition Method.
Abstract: Traditional metal materials and fiber reinforced composites are unable to meet the requirements of
lightweight and impact resistance of aerospace components and the advent of fiber metal laminates has
solved this problem. Most of the existing researches on FMLs focus on the improvement of material system
and impact properties, while little attention has been paid to the changes of stress and strain during the
forming process. Moreover, most of the existing monitoring methods are not suitable for non-destructive
monitoring of strain during the whole forming process. In this paper, a strain monitoring method based on
Fiber Bragg Grating (FBG) and Thermocouple (TC) is proposed, which can be applied to the forming
process of autoclave without destroying the original stress distributions and component shapes of the parts.
The strain variation process during the curing of FMLs was monitored and analyzed by using FBG+TC, so
as to establish theoretical model of forming process, propose strain monitoring method based on Strain
Definition (SD) and analyze comprehensively the characteristics and applicability of the three methods.
1 INTRODUCTION
Aerospace components are gradually developing
towards lightweight and impact resistance, which
cannot be met by traditional aerospace composites
and single metals comprehensively. Fiber metal
laminates have attracted more and more attention all
over the world due to their primely comprehensive
properties. Fiber Metal Laminates (FMLs) were first
introduced and developed by Delft University of
Technology in the Netherlands (Vlot and Gunnink,
2001; Vlot, 2001). It is a new type of super-hybrid
composite material which is cured by alternating
layers of metal sheets and fiber reinforced
composites at specific temperatures and pressures
(Sinmazçelik et al., 2011; Asundi and Choi, 1997),
as is shown in Figure 1. FMLs, which not only
possess the highly stiffness and strength of
composites and the toughness and machinability of
metals, but also derive specifically excellent damage
tolerance and fatigue properties and impact
resistance of FMLs, combine the properties of metal
and fiber reinforced composites to overcome the
shortcomings of single components (Vogelesang and
Vlot, 2000; Aniket et al., 2016). Because of the
above advantages, FMLs have been well applied to
aviation. There are many theoretical and
experimental studies on FMLs all over the world,
such as the material system improvement (Sexton et
al., 2012; Vo et al., 2013; Pan and Yapici, 2015),
performance prediction like basic mechanical
properties (Moussavi-Torshizi et al., 2010), low-
speed impact performance (Vlot, 1996;
Rubiogonzález et al., 2016; Tsartsaris et al., 2011.)
and bird impact (Zhu et al., 2014), and molding
methods like shot peening (Miao et al., 2010).
However, the research on the stress and strain of
FMLs is relatively rare, especially that of the
evolution of stress and strain and the formation of
residual stress in the curing process. The residual
stress of FMLs is mainly formed during the curing
process. The residual stress mainly results from the
curing shrinkage caused by the curing reaction of the
matrix resin and the thermal mismatch stress
generated during the temperature history change due
to the difference in the thermal expansion coefficient
of the component materials. In addition, insufficient
or excessive defects and pores of the resin after
curing also contribute to the formation of residual
stress. The excessively residual stress of FMLs not
only makes for the local delamination, warping,
interfacial stripping and other damage by the
Kang, X., Zhan, L., Wu, X., Chang, T. and He, J.
Study on Monitoring of Stress and Strain during Curing Process of Fiber Metal Laminates.
DOI: 10.5220/0008186301030109
In The Second International Conference on Materials Chemistry and Environmental Protection (MEEP 2018), pages 103-109
ISBN: 978-989-758-360-5
Copyright
c
2019 by SCITEPRESS – Science and Technology Publications, Lda. All rights reser ved
103
interfacial stress concentration, especially for the
parts of complex structure, but also situates the fiber
in an unfavorable compression state, degrades the
performance of the composite, and even breaks the
fiber or cracks matrix.
Ghasemi A R et al. (Ghasemi et al., 2016.)
utilized incremental hole drilling (IHD) method to
measure the residual stress distribution of FMLs
along the thickness direction. The residual stress was
calculated in combination with the coefficient
matrix. The theoretical value of the residual stress is
calculated based on the classical laminate theory. By
comparing the experimental and theoretical values, it
was found that the two are well matched. The IHD
method can be used to evaluate the non-uniform
residual stress of the FMLs along the thickness
direction. However, the IHD method will destroy the
part due to drilling at the point to be measured, and
because the borehole releases part of the residual
stress, changing the original stress distribution. IHD
method makes the shape of the part change.
Abouhamzeh M et al. (Abouhamzeh et al., 2015)
based on the classical laminate theory, established a
theoretical expression for the FMLs curing residual
stress and the warp deformation of the asymmetric
layup structure by adding resin shrinkage and other
corrections. Compared with the finite element
simulation results, the accuracy of the model is
verified.
Figure 1: Fiber Metal Laminates structure.
The research on the residual stress of existing
FMLs rarely centers around the evolution law of the
stress of the part during the curing process, and the
damage detection method has done harm to the part
and changed its original stress distribution greatly.
On the ground of the advantages and disadvantages
of existing research, a method suitable for measuring
the strain evolution of FMLs laminate curing process
is proposed, namely "Fiber Bragg Grating +
thermocouple (FBG + TC)" strain measurement
method. This method does not damage the part and
the original stress distribution of the part. The strain
evolution law can be monitored throughout the
curing process and even in the service phase, and
reliable measurement results can be obtained. In this
paper, the curing process of fiber metal laminates is
firstly analyzed, and a simplified theoretical model
is established. Then the FBG+TC method is used to
measure the strain evolution process of FMLs during
the curing process, and compared with the
theoretical calculation results, the accuracy of the
measurement method is confirmed. This study will
lay a solid foundation for those on the stress
evolution and formation of FMLs.
2 METHOD
In this paper, the theoretical solution is obtained by
establishing a process theory model associated with
temperature and stress. And the experimental value
is measured by FBG+TC method and definition
method. The stress-strain evolution process of FMLs
was explored by comprehensive comparative
analysis.
2.1 Materials and Instruments
The FMLs used in this experiment are made of 0.3
mm thick 2024 aluminum alloy sheet and 0.188 mm
thick carbon fiber reinforced epoxy resin based
T800/X850 prepreg (Cytec) alternately laminated.
The size of the product is 150 mm * 50 mm. The
hot-pressing process is used to cure the FMLs. The
autoclave used in this experiment is an experimental
YT-13-03 autoclave (Dalian Yintian) whose size is
650mm1000mm. The FBG sensor and the K-type
thermocouple used for monitoring separately
measure the wavelength and temperature data, and
the strain data are obtained by the temperature
compensation method. The FBG wavelength data
are collected in real time by the FBG demodulator
(ZenOptics960, Shanghai), the thermocouple
temperature data are collected in real time using a
data recorder (TP700, Shenzhen TOPRIE). The
instrument used is shown in Figure 2.
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104
(A) Autoclave YT-13-03 (B) FBG (C) K-type thermocouple
(D) FBG Demodulator (E) Data recorder
Figure 2: Instruments for monitoring the FMLs curing
process.
The instrument employed to measure the length,
which is used for calculating the strain based on the
definition of strain is 3D scanner (ATOS) shown in
Figure 3.
Figure 3: ATOS 3D scanner.
2.2 Method
2.2.1 Theoretical Model of Forming Process
Since the standard layups of FMLs are
symmetrically layered, despite of the influence of
the interface between the metal sheets and the fiber
reinforced composites, the FMLs are considered to
be a combination of two layers of heterogeneous
materials without internal moments, as is shown in
Figure 4. There are three assumptions (Oken and
June, 1895):
a. The residual stress is a plane stress;
b. Symmetrical layup of the laminate, the
resultant force of residual stress is zero;
c. The interface is firmly bonded, and the metal
layer and the composite layer are uniformly
deformed during the cooling process.
Figure 4: Simplified structure of FMLs.
Considering the same deformation, there are:
(1)
E, , , are Young's modulus, Poisson's ratio,
thermal expansion coefficient and residual stress,
respectively. 1 and 2 represent metals and
composites, L is the composite along the fiber
direction, and T is the composite material along the
direction perpendicular to the fiber.
Considering that the resultant force of residual
stress is zero, there are:
(2)
Where t1 is the total thickness of the metal layer
and t2 is the total thickness of the composite layer.
Since the differences in thermal expansion of the
composite material along the fiber direction and the
metal layer are much larger than the direction of the
vertical fiber, the stress distribution in the direction
of the vertical fiber is ignored, and the expression of
residual stress is:
(3)
Study on Monitoring of Stress and Strain during Curing Process of Fiber Metal Laminates
105
2.2.2 FBG+TC Method
FBG is widely used in temperature, strain, and
corrosion and curing of composite materials (Qiu et
al., 2013; Huang, 1998; Lo and Xiao, 1998) due to
its anti-electromagnetic interference, light weight,
small size, and corrosion resistance (Majumder et
al., 2008). However, FBG has strain-temperature
cross-sensitivity problem (Dewynter-Marty et al.,
1998; Farahi et al., 1990). (Majumder et al., 2008)
gives a variety of temperature compensation
methods to solve the cross-sensitivity scheme.
Considering the advantages and disadvantages and
the actual situation, this paper uses thermocouple to
realize temperature compensation.
The relationship between the wavelength
measured by FBG and the temperature measured by
the K-type thermocouple is shown in equation (4):
(4)
In the formula, , T and are wavelength,
temperature and strain respectively. The foot mark 0
represents the initial value, the foot mark i stands for
the real-time value at time i, and K
T
and K
are the
temperature sensitivity coefficient and the strain
sensitivity coefficient respectively. It is considered
that the initial value of the strain is 0, and the strain
calculation formula (5) can be derived from the
equation (4).
(5)
2.2.3 Strain Definitions Method
The strain definition method is based on the
definition of strain, and the surface of the part before
and after the curing process are scanned with an
ATOS three-dimensional scanner, moreover the
original length L and the length after curing L
1
are
measured from scan results, and the residual strain
value is calculated by the formula (6).
(6)
2.3 Experimental Procedure
The experimental procedure shown in Figure 5 was
designed to incorporate the FBG+TC method and
the definition method. The surface anodizing
treatment process is intended for changing the
surface micromorphology of sheet metal so that the
metal and the composite material are more firmly
bonded, which is a necessary step towards the
commercial GLARE laminate (Sinmazçelik et al.,
2011; Botelho et al., 2006). Table 1 shows the
stacking order of the parts. Figure 6 shows the
experimental layout. Table 1 shows the layup
information of the parts. The experiment is
repeatedly conducted three times.
Figure 5: Experimental procedure.
Table 1: Layup information of the parts.
Sample No.
Lay-up
1
2024-T3
2
2024-T3/CFRP 0°/CFRP 0°
/2024-T3
Figure 6: The experimental layout.
3 RESULTS AND ANALYSIS
The theoretical model method, FBG+TC method and
definition method are used to solve the strain in the
FMLs molding process. By comparing the results of
the three methods, it is found that the FBG+TC
method and the definition method perform better in
monitoring the strain. The results are shown below.
Regarding the solution of the theoretical model,
since the thermal property parameters of the material
T800/X850 cannot be directly detected from the
literature, it is obtained by the approximate solution
method. A method for solving the equivalent
thermal expansion coefficient and equivalent
Young's modulus of fiber composites based on
volume fraction is given by (Schapery, 1968.). By
consulting the literature, the parameters contain
elastic modulus (E), coefficient of thermal expansion
(CTE) and volume fraction (VF) of T800 fiber and
X850 resin are shown in Table 2.
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106
Table 2: Material properties of CFRP.
Material
E
11
(GPa)
CTE
11
(m/)
VF
T800
294
-0.56
0.65
X850
2(**)
60(***)
0.35
(** Moussavi-Torshizi et al., 2010; ***Zhang et al., 2006)
The physical properties of the composite
materials T800/X850 and aluminum alloys 2024-T3
obtained by literature and calculation are shown in
Table 3:
Table 3: Material properties of CFRP and 2024-T3.
Material
E
11
(GPa)
CTE
11
(m/)
t (mm)
T800/X850
191.8
-0.339
0.376
2024-T3
72.4
23.2
0.6
The process temperature of T800/X850 is 180
°C. In order to ensure the uniformity with the
experiment, the cooling temperature is 24 °C.
According to the equations (3), the average stress of
the aluminum alloy layer is
al
= 165.917MPa, and
the average stress of the composite layer is
cf
= -
264.762 MPa. This indicates that for the internal
stress of Al-CFRP laminate, the residual tensile
stress in the aluminum alloy layer is 165.917 MPa;
the composite layer subjected to residual
compressive stress and the size is 264.762 MPa.
The length dimensions of the parts before and
after curing are scanned using an ATOS scanner.
The internal stress of the aluminum alloy layer of the
FMLs after compensation is obtained, which is
shown in Table 4.
Table 4: Result of strain definition method.
L1 (mm)
L2 (mm)
L3 (mm)
Uncured
2024
149.8597
149.8627
149.8627
FMLs_1
149.7815
150.0544
150.0509
FMLs_2
149.774
150.054
150.059
FMLs_3
150.181
150.344
150.411
Cured
2024
149.8597
149.8627
149.8627
FMLs_1
149.7815
150.0544
150.0509
FMLs_2
149.774
150.054
150.059
FMLs_3
150.181
150.344
150.411
Stress (MPa)
2024
FMLs_1
FMLs_2
FMLs_3
50.55011
169.0378
176.3602
177.2773
The monitoring results of Group 1 using
FBG+TC method is displayed in Figure 7.
The part temperature follows the air temperature
in the tank, and the strain changes with temperature.
Compared with the strain history of 2024 aluminum
alloy, the strain reduction rate of FMLs in the
cooling stage is significantly lower than that of 2024
aluminum alloy. This is because the thermal
expansion coefficient of the T800/X850 composite
after curing is negative. The temperature drop causes
the composite to expand. The composite material
transforms this expansion into tensile stress applied
to the aluminum alloy layer through the interface, so
that the rate of strain reduction of the 2024
aluminum alloy in the cooling stage is slowed down.
There is a sharp rise in the stress-strain data near the
end point. Because the load from the mold and the
vacuum bag no longer applied to the part. The
tensile stress of metal layer suddenly increased. The
monitoring results of three groups is shown in
Figure 8 and Figure 9.
Figure 7: The monitoring results of Group 1 using
FBG+TC method.
The internal stress of the metal layer at the end of
the cooling of the FMLs is σ
fbg
=173.307MPa.
The stress results of the FMLs curing process
obtained by the three methods are numerically close
overall. If the calculation result of the process model
is true, the relative errors of the values obtained by
the FBG+TC method and the strain definition
method are 4.616% and 5.170%, respectively, and
the error of the FBG+TC method relative to the
"definition method" is 0.5295%. It is evident that the
results of the two experimental values are closer.
This is because the strain measured of FMLs is
determined by many factors, while the theoretical
model only considers the most important factors of
thermal expansion coefficient mismatch. In another
experiment, the asymmetrically laminated parts were
solidified and then reheated to a process temperature
of 180 °C. It was found that the asymmetrical plates
warped at normal temperature were substantially
flattened, and the warpage substantially disappeared.
The literature (Oken and June, 1895) also shows that
the thermal expansion coefficient mismatch is the
Study on Monitoring of Stress and Strain during Curing Process of Fiber Metal Laminates
107
main factor contributing to the curing stress of
FMLs, so this paper does not consider the effect of
curing shrinkage on the curing strain.
Figure 8: The 3 groups results of FBG+TC method.
Figure 9: The monitoring results of three methods.
4 CONCLUSIONS
Based on FBG and thermocouple, a strain
measurement method suitable for complex process
environment is proposed and applied to the
monitoring of FMLs autoclave forming process. The
strain and temperature of the part are obviously
positively correlated. During the cooling process, the
metal layer is subject to tensile stress, while the
composite layer is to compressive stress. The tensile
stress of the metal layer is imposed by the composite
layer, and the compressive stress of the composite
layer is by the metal layer. Strain measurement
methods based on strain definition were proposed,
and experiments were carried out. The monitoring
results are close to the FBG+TC monitoring results.
Since the theoretical model only considers the main
factors of thermal expansion coefficient mismatch,
but ignores other secondary factors, the model
calculation results deviate significantly from the
FBG+TC detection results. In general, the FBG+TC
method is suitable for the monitoring of the strain
during the curing process of FMLs composite parts
because it can be used normally in the autoclave
process environment and its damage to the measured
parts is very small. The definition method is suitable
for measuring the strain value at a certain moment
and the process model also needs to take more
influencing factors into consideration.
ACKNOWLEDGEMENTS
The authors are grateful to the Central South
University Graduate Research Innovation Project
(1053320171669) and The National Natural Science
Foundation of China (No.51675538) for supporting
this work.
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