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IMECE2015-52706

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Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition
IMECE2015
November 13-19, 2015, Houston, Texas
IMECE2015-52706
INVESTIGATION OF NON-DESTRUCTIVE EXAMINATION FOR MECHANICAL
DAMAGE OF FRP
Keisuke ONO
Kyoto Institute of Technology
Kyoto-shi, Kyoto, Japan
Yoshimichi FUJII
Kyoto Institute of Technology
Kyoto-shi, Kyoto, Japan
Akihiro WADA
Kobe City College of Technology
Kobe-shi, Hyogo, Japan
The possibility of dividing of Lamb wave modes by reducing
the thickness of samples was confirmed and the variance of
distribution of frequency of S0 mode wave by micro fracture
in GFRP.
ABSTRACT
Nowadays, fiber reinforced plastic (FRP) has been widely
used in many areas such as auto mobile, airplane and marine
vessel due to its high specific strength, good corrosion
resistance, relatively low cost and so on. However, it still
remains unknown that what kind of damage will happen in the
internal structure when an automobile, which is made from
FRP, has a slight impact with something such as a wall. Then
the following road safety of the automobile cannot be
guaranteed because certain parts may be exposed to damage in
what seems even like a slight impact. In addition, it is well
known that initial fracture can bring damage and great effect to
the mechanical properties of the FRP material. The novelty of
this paper is that the object of this research is micro crack such
as transverse crack. While, almost previous report is aimed at
delamination. Actually, before the delamination happens, micro
crack has already occurred. The mechanical property of FRP is
beginning to decrease by delamination.
However, when the
delamination occurred in the FRP is examined, it is already too
late because the delamination can bring great influence to the
safety of the FRP products. Therefore, it is important to
investigate and detect the presence of micro crack with
ultrasonic wave. In this way, some accidents might be avoided.
While, because of the variety of the constraints in the fracture
mechanism, the damage behavior is very difficult to evaluate
and there are rarely researches and data on it. Although damage
assessment by visual observation and the durable service life on
FRP has become a general tendency in these recent years, the
appropriate way of non-destructive examination has not been
confirmed yet.
The purpose of this study is to investigate the possibility of
non-destructive examination with ultrasonic wave testing for
mechanical damage of glass fiber reinforced plastics (GFRP).
INTRODUCTION
The ultrasonic wave testing, one of the non-destructive
examinations, is commonly used for homogeneous material
such as metal or plastics. In the test, a wave of high frequency
will be introduced to the internal part of the material, and then
the fracture situation or presence of the flaws can be detected
and analyzed by the reflected wave. This method is very
suitable and easy for damage detection for homogenous
material. [1-4] However, the situation becomes complicate
when applying this method to FRP because the composite
consists of more than one component, that is, the matrix resin
as well as the reinforcement such as glass fiber. If the ultrasonic
wave testing can be applied to detect the damage situation of
FRP, it becomes possible for service life prediction and damage
assessment of FRP material after unlocking the initial fracture
behavior.
Thus, great effort has been made to carry out research
about non-destructive examination with ultrasonic wave testing
in this paper, the purpose of which is to figure out the solution
to non-destructive examination for FRP. The possibility of
assessment of mechanical damage of FRP with the ultrasonic
wave testing is investigated. The damage has been detected by
the reflected ultrasonic wave because the distance decay of
reflected wave or its frequency distribution will be changed if
the material has subjected to damage. In general ultrasonic
wave testing of micro fracture of FRP case, the object is
delamination by impact from outer surface. However, in this
research, the object is micro fracture such as transverse crack or
1
Copyright © 2015 by ASME
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its delamination. The schematic of oblique incident ultrasonic
wave testing is shown in Fig. 1.
=
MATERIAL AND EXPERIMENT
Preparation of test specimen
The materials tested were composed of a combination of
glass roving cloth (Nitto Boseki co., ltd., RS110QL-0520) and
vinyl ester resin (Showa Denko K.K., R806). The GFRP plates
were made by hand lay-up method at room temperature, the
thickness of 4 plies laminated plate is 3.2 mm and 1ply is
0.8mm. Those plates were cut into specimens with size of 250
mm × 25 mm.


,
2
2


 = √ 2 −  2 ,  = √
Fig. 1
Schematic of oblique incident ultrasonic test.
Tensile direction
tan( ⁄2)
tan(  ⁄2)
=−
( 2 − 2 )
5mm
5mm
(f)
(e)
(d)
5mm
(g)
Fig. 2 Fracture level of specimen of (a) Blank, (b) 20%, (c)
30%, (d) 40%, (e) 50%, (f) 60%, (g) 70%.
2
4 2  
4 2  
(c)
(b)
(a)
5mm
( 2 − 2 )
5mm
5mm
5mm
Propagation of Lamb wave
Lamb wave is one of the guided wave which propagates in
flat plate, property of which depends on material and thickness
of the flat plate as well as the frequency of ultrasonic wave. [5]
Lamb wave has two modes, S mode (Symmetric) and A mode
(Anti-symmetric). Both S mode and A mode have different
propagation forms including high and low order modes which
are propagating at same time. The schematic of each mode
form is illustrated in Fig. 3 (a), (b). If the acoustic velocity of
material is known, the dispersion curve of phase velocity of
Lamb wave can be calculated by Rayleigh- Lamb equation (1),
(2).
=−
− 2
Where, k is wave number(−1 ), d is thickness of the flat
plate(), ω is angular frequency ( ⁄) and c is phase
velocity of lamb wave(⁄ ). In this study, aluminum which is
homogeneous material (longitudinal wave velocity  =
6420 / and transverse wave velocity = 3040 /) was
used as material of the flat plate. The dispersion curve of
aluminum is calculated by equation (1) and (2) as shown in Fig.
4.
Propagation property of ultrasonic wave in mechanical
damaged GFRP
The propagation property of ultrasonic wave in mechanical
damaged GFRP was tested by oblique incident ultrasonic wave
testing for investigation. In this study, pulsar oscillator/receiver,
digital oscilloscope, personal computer and transducer (12mm
in diameter, the frequency of 1.0 MHz) were used. The
ultrasonic echo were detected at 6 points, distance between
transducers were 15mm, 25mm, 35mm, 45mm, 55mm and 65
mm. The distance between transducers represented for the
distance of points at the intersection of specimen with center
line of transducers as shown in Fig. 1. The angle of transducers
was set up 22°by Snell’s law and the results of acoustic
velocity measurement. The electric voltage of the pulsar
oscillator/receiver was 100 V, and gain was 20 dB. On the other
hand, mechanical damaged samples were under damage test by
tensile fatigue tester at different stress amplitudes of 20 %,
30%, 40%, 50%, 60% and 70% of tensile strength (149.4 MPa).
Thus, 7 fracture level specimens including these 6 types of
mechanical damaged specimens and no damaged specimen
were used in this study. The repeated cycle number was
adopted as 10,000 and cyclic frequency as 5 Hz. Photos of 7
fracture levels specimens were displayed in Fig.2 respectively.
tan( ⁄2)
tan(  ⁄2)
2
2
(1)
(a)
(b)
(2)
Fig. 3 Schematic of (a) S mode and (b) A mode.
2
Copyright © 2015 by ASME
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Dividing of modes
When it comes to wave analysis, it will become very
complex and difficult if there is more than one mode mixed in
one receiving wave. Thus, dividing those modes and analyzing
with specific mode are necessary. In this study, the possibility
of dividing S0 mode from A0 mode by reducing the thickness
of aluminum flat plate was verified. Similar to the previous
section, the thickness of aluminum flat plates is 3 mm and 1
mm and frequency of the transducer is 1.0 MHz for oblique
incident ultrasonic wave testing. And the distance between
transducers was 50 mm.
Fig. 5 Frequency distribution of GFRP (t= 3mm).
Dividing of modes
According to the receiving wave of aluminum of which
thickness is 1 mm, S0 mode wave was different from A0
mode wave on the time axis. The fast Fourier transform was
computed. The frequency distributions of aluminum of which
thickness is 3 mm, S0 mode of aluminum of which thickness
is 1 mm, and A0 mode of aluminum of which thickness is 1
mm were shown in Fig. 6 (a)- (c). Fig. 6 (a) suggests that two
peaks were indicated at 0.7 MHz and 1.0 MHz. Although the
frequency of the oscillated wave was 1.0 MHz, the results
indicated two peaks. Furthermore, the difference of S0 mode
wave velocity and A0 mode wave velocity was small in this
condition as shown in Fig. 4. Therefore, it is assumed that S0
mode and A0 mode were mixed, two peaks were existed. In
fig. 6 (b) and (c), each graph indicates one peak at 1.0 MHz and
0.7 MHz. Hence the difference of S0 mode wave velocity and
A0 mode wave velocity was increased by reducing thickness
from 3 mm to 1 mm. It was verified that each mode had been
divided.
Fig. 4 Dispersion curve of aluminum.
Propagation property of  mode wave in mechanical
damaged GFRP
The oblique incident ultrasonic wave testing was carried out
on mechanical damaged 1 ply laminated GFRP. The possibility
of non-destructive examination for mechanical damage of
GFRP was investigated with S0 mode wave of Lamb wave. The
frequency of transducer is 1.0 MHz and the ultrasonic echo was
detected at 6 points, distance between transducers were 15mm,
25mm, 35mm, 45mm, 55mm and 65 mm. The angle of
transducers was set up 22° by Snell’s law and the results of
acoustic velocity measurement. The electric voltage of the
pulsar oscillator/receiver was 100 V and gain was 20 dB. On
the other hand, mechanical damaged samples were damaged by
tensile fatigue tester. At that time, the stress amplitudes were
20 %, 30%, 40%, 50%, 60% and 70% of tensile strength (149.4
MPa). The repeated cycle was 10,000 cycle and cyclic
frequency was 5 Hz. The 7 fracture level specimens were
prepared as same as the ultrasonic wave testing.
400
350
Amplitude
300
250
200
150
100
50
0
0
RESULTS AND DISCUSSION
Propagation property of ultrasonic wave in mechanical
damaged GFRP
Fig. 5 shows the frequency distribution of 20% in each
distance cases between transducers. It was found that there are
a few peaks which appeared at 0.6 MHz and 1.0 MHz.
Thereby, it is assumed that some mode waves are mixed in
receiving wave. As mentioned above, if some modes are
included in the receiving wave, to analyze that wave is very
complex and difficult. Therefore, aluminum as a homogeneous
material was adopted to verify S0 mode from A0 mode in the
next section.
0.5
1
Frequency[MHz]
1.5
2
(a)
1200
Amplitude
1000
800
600
400
200
0
0
0.5
1
Frequency[MHz]
1.5
2
(b)
3
Copyright © 2015 by ASME
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the delamination between warp and weft of glass roving cloth
are black. S0 mode wave is the wave that alternate between
bloat and compress in the thickness direction. Therefore, it is
assumed that the S0 mode wave is attenuated in the hole by
delamination between warp and weft. Thus, the damage
evaluation of delamination level fracture by S0 mode wave of
Lamb wave is found. In order to evaluate the initial fracture
such as transverse crack, it is necessary to increase the
sensitivity of the receiving wave.
16
14
Amplitude
12
10
8
6
4
2
0
0
0.5
1
Frequency[MHz]
1.5
2
(c)
Fig. 6 Frequency distribution of aluminum. of (a) t= 3mm
(S0 +A0 ), (b) t= 1mm (S0 ) and (c) t= 1mm (A0 )
Propagation property of  mode wave in mechanical
damaged GFRP
In the previous section, it is found that modes of Lamb
wave are divided by reducing thickness of sample. Thus, the
oblique incident ultrasonic wave testing was run to mechanical
damaged 1 ply laminated GFRP which is inhomogeneous
material to investigate the possibility of non-destructive
examination for mechanical damage of GFRP with S0 mode
wave of Lamb wave in this section. Fig. 7 shows the frequency
distribution of Blank in each distance between transducers.
Although it was verified that S0 mode and A0 mode are
divided, Fig. 7 indicates two peaks at 0.5 MHz and 1.0 MHz
because two matters that are glass fiber and vinyl ester resin
exist in GFRP. The modulus of reinforcing fiber and resin is
dramatically different. Thus, it is assumed that velocities of
each wave that propagates in glass fiber and vinyl ester are
different. Therefore, it is suggested that the dispersion curve is
different with aluminum one, some mode waves other than S0
mode are mixed. Equation (1) and (2) was consisted as
dispersion curve of waves which propagate in isotropic
materials. So, it is found that dividing mode and calculation
velocity by different methods are necessary in aeolotropic
materials such as GFRP.
Fig. 8 Relationship between amplitude and fracture level.
CONCLUSIONS
I. In homogeneous material, the difference of S0 mode
wave velocity and A0 mode wave velocity was increased
by reducing thickness, it was verified that each mode is
divided.
II. It is found that dividing mode and calculation velocity by
different methods are necessary in aeolotropic materials
such as GFRP.
III. It is found that damage assessment of the delamination level
fracture can be achieved by analysis of S0 mode wave of
Lamb wave.
IV. It is found that if initial fracture such as transverse crack can
be evaluated, to increase the sensitivity is necessary.
REFERENCES
[1] Wada, A., Motogi, S., Yamasaki, T., 2012, “Impact Damage
Detection of FRP Laminates Based on Spectrum Analysis of
Lamb Waves”, Transactions of the Japan Society of Mechanical
Engineers, Series A, 78 (790), pp. 879-889.
[2] Wada, A., Motogi, S., 2004, “Evaluation of Matrix Cracks
in FRP Laminates Using Angular Dependence of Lamb Wave
Propagation”, Transactions of the Japan Society of Mechanical
Engineers, Series A, 70 (699), pp. 1650-1657.
[3] MAL A, K., 1991, “Ultrasonic nondestructive evaluation of
cracked composite laminates”, Composite Engineering, 1(2),
pp. 85-101.
[4] Guo,N., Cawley, P., 1993, “The interaction of Lamb waves
with delaminations in composite laminates”, Journal of the
Acoustical.
Society of America, Vol.94, No.4, pp.2240-2246.
[5] Cegla, F, B., 2006, ”ULTRASONIC WAVEGUIDE
SENSORS FOR FLUID CHARACTERISATION AND
REMOTE SENSING”, Ph. D. thesis, Imperial College London,
London.
Fig. 7 Frequency distribution of GFRP (t= 1mm).
S0 mode wave is the wave that has peak at 1.0 MHz in
previous section. So the relationship between peak value of 1.0
MHz in Fig. 7 and fracture level was considered. Fig. 8 shows
the relationship between peak value and fracture level of each
distance cases between transducers. Peak values are extremely
fall down between 30% and 40% fracture level. In Fig. 2 (d),
4
Copyright © 2015 by ASME
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