16TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALSELASTIC WAVE EMISSION DURING DELAMINATIONGROWTH OF CARBON/EPOXY MONITORED WITH
FIBER-OPTIC DEFEW STRAIN RATE SENSOR
Kazuro Kageyama*, Hideaki Murayama*, Sakura Atsuta*,
Isamu Ohsawa* and Makoto Kanai** Department of Environmental and Ocean Engineering, The University of Tokyo
Keywords: CFRP, Delamination, DCB Test, Acoustic Emission, Fiber-optic Sensor

Abstract
A fiber-optic sensor with very large bandwidthand extremely high sensitivity has been developed
based on Doppler effect in flexible and expandable
light waveguide (DEFEW). Fiber-optic DEFEW
sensor is applied to detect elastic waveforms emitted
during the delamination growth in DCB specimen.
Principle, sensitivity of circular loop sensor
configuration and setup of measuring system arebriefly explained.
Unidirectional carbon/epoxy
systems of T800H/3631 (base line) and T800H/3900-2 (toughened) are used for the DCB tests.
DCB tests are carried out, and AE waveforms aredetected with a pair of upside and downside sensors.
The fiber-optic sensors are flexible and they detect
AE events inside the gauge length. The test results
indicate that signs of the first arrival peak detectedwith upside and downside sensors have related to
the failure mode of breakage of bridging fibers andmatrix cracking,1 Introduction
Delamination is one of the most critical
fractures of composite materials. It is necessary to
examine and improve the interlaminar fracture
toughness for highly reliable use of composite
materials, especially in case of being employed toprimary structure.
Mode I interlaminar fracture toughness, G IC, ismeasured by simulating the delamination processwith the double cantilever beam test. Microscopic
behavior during delamination is very complicatedand more difficult to evaluate.
Monitoring of Acoustic emission (AE) is very
appropriate tool to monitor microscopic failure
events. AE is elastic waves caused by microscopic

failure. Generally PZT sensors are applied to AE
monitoring [1]. They cannot be in use on curved
surfaces so it is difficult to put them near AE sourcesin DCB specimens. Meanwhile a fiber-optic sensor
has flexibility and it is possible to detect AE without
damping of high-frequency component.
Authors have been developed a new fiber-optic
strain rate sensor, which is based on Doppler effectin flexible and expandable light-waveguide(DEFEW) [2]. Fiber-optic DEFEW sensor has been
applied to health monitoring of composite andconcrete structures [3-5]. Fiber-optic DEFEW sensor
has extremely high resolution less than nano-strain(10-10) in the very wide frequency range from 1 kHz
to 1 MHz. In this study, transient strain rate signals
in the DCB test were detected by the fiber-optic
DEFEW strain rate sensors and analyzed in time andfrequency domains to examine microscopic behavior
during delamination.

2 Doppler effect in flexible and expandable
light-waveguide (DEFEW)
2.1 Principle
Consider the light wave transmission in amedia with refractive index, n. The light wave withfrequency f0 emitted from a moving point A (lightsource) is detected at other moving point B(observer). The distance between points A and B
changes from L to L+dL during infinitesimal timeinterval dt. Doppler frequency shift, fD , is observed
at B by the observer,
n dL(1)
fD = − ⋅λ0 dt,where λ0 is the light wavelength in a vacuum, and
λ0/ n is the light wavelength in the media.

KAZURO KAGEYAMA, H. Murayama, S. Atsuta, I. Ohsawa and M. Kanai

2.2 Circular loop sensor
In the case of a circular loop sensor as shownin Fig. 2, the sensitivity of strain rate can beevaluated by integrating Eq. (4) on the strain rate
field. For simplicity, the uniform strain rate field,
ε& x , ε& y and γ& xy is assumed on the integration path.

Fig. 1 Flexible and expandable light-waveguide.

The velocity vector and unit normal vector are
given by Eqs. (5) and (6), respectively, where polar
coordinate system is employed as shown in Fig. 2.
yNext, consider an arbitrary light path (lightwaveguide), Γ , as shown in Fig. 1. The path isflexible and expandable. It has finite overall length,
L, and two ends, denoted as points A and B,
respectively. An incident light from the one end A(light source) is transmitted through the waveguideand detected at the other end B (observer). When
the waveguide moves or vibrates, the same equationas Eq. (1) is applicable,
neq dL(2)⋅
fD = −
λ0 dt, where neq is the equivalent refractive index of the
waveguide and λ0/ neq is the equivalent length oflight wave in the waveguide.
From a geometrical consideration, dL/dt is given by
Eq. (3),dLB(3)= [v • t ]A + ∫ k ⋅ v • ndsdtΓ
, where κ , v and n are the curvature, the velocity
vector and the unit normal vector of the infinitesimal
segment, ds, respectively, and t is the unit direction
vector defined at the end points A and B. (See Fig.
1.) The operation • indicates inner product of two
vectors. From Eqs. (2) and (3), we obtain thefollowing equation;
neq
n
(4)
[v • t ]BA − eq ∫ k ⋅ v • nds
fD = −λ0
λ0 ΓThis equation implies “Doppler effect in flexible andexpandable light-waveguide”. Displacement ratenormal to the small segment of bent optical fiber
effects on the frequency shift of light transmitted
through the optical fiber and that the intensity of thefrequency shift is proportional to the curvature of thebent fiber. We can detect the local displacement rate
at the bent region in the optical fiber. As a feature of
sensors, output signal shall return to steady-statevalue when the external excitation is removed. It
requests that the waveguide is elastic, ordeformation is reversible.

V
n
R
θ
0

x

Fig. 2 Circular loop sensor

1⎫⎧&⎪ε x cosθ + 2 (γ& xy + ω& xy )sin θ ⎪v = R⎨⎬
1⎪ (γ& xy − ω& xy )cosθ + ε& y sin θ ⎪⎭⎩2⎧cos θ ⎫n=⎨⎬⎩ sin θ ⎭

(5)

(6)

, where ω& xy and R are rotation term of distortion andradius of the loop, respectively. Substituting Eqs.
(5) and (6) into Eq. (4), the theoretical frequency
shift, fDth, is obtained.
πRneq(ε&x + ε& y ) = − πRneq (ε&1 + ε&2 ) (7)f Dth = −λ0

λ0

The sum of the axial strain rates or the sum of
the principal strain rates is converted into the
Doppler frequency shift by applying circular loop
sensor. Shear strain and rotation have no effect on
the frequency shift. In the case of N turns of loop,
as shown in Fig. 2 (left), the frequency shift
becomes N times larger.
NπRav neq
(ε&x + ε& y ) = − NπRav neq (ε&1 + ε&2 ) (8)f Dth = −

λ0
λ0
R
=
(R
+R
)/2
is
the
average radius of, where av
max
min
the loop. The circular loop sensor has no directional
sensitivity.
We can control the sensitivity of the sensor bychanging the radius and number of turns. The sizeof the loop is equivalent to a gauge length of thesensor, and it is recommended that the gauge length
should be sufficiently smaller than the wavelength ofthe elastic wave measured.

ELASTIC WAVE EMISSION DURING DELAMINATION GROWTH OF CARBON/EPOXYMONITORED WITH FIBER-OPTIC DEFEW STRAIN RATE SENSOR

2.3 Setup of measurement system
A Laser Doppler velocimeter (LDV) as shownin Fig. 3 is used to detect the frequency shift.
Detection electronics is FM discriminator. Lightsource is He-Ne laser (output power; 1mW,
wavelength. λ0: 632.8 nm), and heterodyne
interference technique is applied to the measurementin the present paper. An acousto-optical modulator(AOM) changes the frequency of the reference lightsource from f0 to f0+fM (fM = 80 MHz) in order toproduce beating signals with frequency of fD+fM.

by two fiber-optic sensors (first, set A and next, setB) and a PZT sensor at the same time during thetest.

Fig.4 DCB specimen

3.2 Test conditions

3 Test Methods
3.1 Specimen and Sensors
Unidirectional carbon/epoxy systems of
T800H/3631 (baseline) and T800H/3900-2(toughened) are used for the DCB tests (JISK7086-1993/ISO15024-2001). The specimen isshown in Fig.4. The width, B, is 25mm, the overall
length, L, is 175mm, and normal thickness, 2H, is3mm. The initial crack was introduced bypolyimide film with thickness of 13μm. The film
was removed and a precrack was induced before
the test.
Four fiber-optic sensors (average diameter,
20mm; number of turns of upside sensors, 20/downside sensors, 20) and a PZT sensor wereattached to the specimen. AE signals were detected

4 Detected AE waves
As shown in Fig.5, AE events started at the
beginning of the crack growth. The total number ofdetected AE events during about 20 minutes’ tests
was around 2000. G IC during crack propagation was
nearly constant in the both case of T800H/3631 andT800H/3900-2.
100
4.5
Load P Crack Growth
4
80
3.5
3
60
2.5
2
40
1.5
1
AE total events
20
0.5
0
0
0
250 500 750 1000 1250
Time[s]Crack Growth a [mm],AE total events/20

The sensitivity of the optical circular loop
sensor (average diameter: 20 mm, number of turn:10) is calculated theoretically. By applying
commercially available performance data of theLDV (Melectro, V1002) [6], the resolution anddynamic range were obtained and extremely highresolution less than nano-strain (10-10) is expected in
the very wide frequency range from 1 kHz to 1 MHz.
Low frequency vibration of 0.1 Hz is detectable with
sufficient sensitivity. The resolution of the newly
developed sensor is extremely superior to the otherfiber optic sensors, such as FBG. The developed
fiber-optic sensor covers the measurement range
from conventional strain gauge to AE sensor.

The cross head speed was controlled at
0.5mm/min during the DCB tests. Monotonic load
was applied in order to detect AE signals
continuously. AE signals were detected with thesampling rate 5MHz and the number of words of5000. 10Hz high pass filter was used for signals
derived from fiber-optic sensors. Crack length wasmeasured using a traveling microscope.

Load[kgf]

Fig. 3 Setup of measurement system.

Fig.5 Relation of load, crack growth and AE events
Fig.6 shows an example of sets of AE signals
detected by upside and downside fiber-optic
DEFEW sensors at the same time. The waves show
3

KAZURO KAGEYAMA, H. Murayama, S. Atsuta, I. Ohsawa and M. Kanai

clear inverse correlation. The AE source might be arelease of dipole of forces, which is antisymmetricwith respect to crack surface, though symmetric
Mode I load is applied to the specimen..
3

Output[V]

1.5
0
-1.5

5 Discussions
5.1 Signs of first arrival peaks
Model specimens are prepared for breakage ofbridging fiber (see Fig. 8) and for matrix cracking inadhesive layer between a pair of composite
laminates. DCB tests of the model specimens arecarried out, and the signs of first arrival peakdetected with upside and downside sensors are
examined.

-3
0

0.002
0.004
Time [μs]0.006(a) Detected AE wave at upside surface
Fig. 8 Model specimen of bridging fiber
1.2
0.8
0.4
0
-0.4
-0.8
-1.2

UpsideDownside

Output(V)Output[V]

0.7

0

-0.7
0

0.002
0.004
Time [μs]

0

0.006

(b) Detected AE wave at downside surface
Fig.6 Example of comparison of AE signals
between upside and downside fiber-optic sensors(a) 20 kHz peak(b) 570 kHz peakFig.7 Results of time-frequency analysis

0.01
Time(ms)

0.015

0.02

Fig. 9 Time history of elastic wave of
breakage of bridging fiber
As shown in Fig. 9, signs of first arrival peakof breakage of bridging fiber are positive/positive(expansion/expansion), and it well explains therelease of dipole forces normal to the crack surface.
On the other hand, most of signs of first arrival peakat matrix cracking are positive/negative or
negative/positive, and the AE sources might be arelease of dipole of forces which is antisymmetricwith respect to crack surface, or shear fracture.
Signs of first arrival peak of DCB tests of
T800H/3631(baseline)
andT800H/3900-2(toughened) specimens are listed in Table 1
compared with the results of the model specimens.
Table 1 Percentage of sign of first arrival peaks
detected with upside and downside sensor
Sign

In this experiment, several signals withdifferent characteristics were detected. Two
examples of the results of time-frequency analysis
are given in Fig.7 (a) and (b) which show different
frequency peak of 20kHz and 570kHz, respectively.
These differences are considered of a result ofdifferent failures during delamination.

0.005

+/+
-/+/-/+

Baseline
T800H/3631

Toughened
T800H/3900-2

Matrixcracking

Fiber
break

15
17

10
19

5
0.

100
0

30
38

68

40
31

71

67
28

95

0
0

0

ELASTIC WAVE EMISSION DURING DELAMINATION GROWTH OF CARBON/EPOXYMONITORED WITH FIBER-OPTIC DEFEW STRAIN RATE SENSOR

14
12
10
8
6
4
2
0

Positive・
Positive
Negative・
Negative

8
4
0

50

Positive・Negative
Negative・Positive

15
10
5
0
0

30

30

2

0.6

Positive・Positive
Negative・Negative

10
0
0

20
10
0
-2

0.1
0.2
0.3
0.4
0.5
T800H/3631:Maximum amplitude(V)Fig. 11 Histograms of maximum amplitudeof AE events of T800H/3631

Positive・
Negative
Negative・
Positive

40

0.6

AE events-1.5 -1 -0.5 0
0.5
1
1.5
T800H/3631:Difference of arrival times (⎠ s)

0.1
0.2
0.3
0.4
0.5
T800H/3631:Maximum amplitude(V)

20

20

-2

AE events

Negative・Negative0-1.5 -1 -0.5
0
0.5
1
1.5
2
T800H/3631:Difference of arrival times (⎠ s)

Fig. 10 Time difference between first arrival peaks
detected with upside and downside sensor
Histograms of maximum amplitude of AE
events of T800H/3631 (baseline) and T800H/3900-2(toughened) are shown in Figs. 11 and 12,
respectively.
Average of peak amplitude of
T800H/3631 is larger than that of T800H/3900-2 inpositive/negative or negative/positive sign datagroup, which relates to matrix cracking. Histogramsof positive/positive sign data group show small
difference between T800H/3631 and T800H/3900-2,
because they are reinforced with same reinforcement
fiber.

AE events

AE events

AE parameters, such as time difference
between first arriving peaks, maximum amplitudeand half cycle of the first arriving peak, arecategorized as signs of first arriving peak.
Histograms of time difference between firstarriving peaks detected with upside and downside
sensors are shown in Fig. 10. AE events withpositive/positive signs have longer time difference
between upside and downside sensors than thosewith opposite signs. AE events of fiber breakoccur at upper and lower ends of a bridging fiber,
which result in difference of arrival time.

Positive・Positive
12

AE events

5.2 Histograms of AE parameters16

AE events

Around 70% of first arrival peaks of baseline
and toughened carbon/epoxy systems show opposite
signs, which suggest major failure mode is matrix
cracking in the DCB test.

40
35
30
25
20
15
10
5
0

0.1
0.2
0.3
0.4
0.5
0.6
T800H/3900-2:Maximum amplitude(V)Positive・Negative
Negative・Positive

0

0.1
0.2
0.3
0.4
0.5
0.6
T800H/3900-2:Maximum amplitude(V)Fig. 12 Histograms of maximum amplitudeof AE events of T800H/3900-2
Half cycle of first arrival peak might relate to
duration of failure process. Histograms of the halfcycle, Tf, of opposite sign AE event group of
T800H/3631 and T800H/3900-2 are shown in Fig.
13. Average time of T800H/3900-2 is longer thanthat of T800H/3631, and it suggests that failure
process of T800H/3900-2 is more ductile than thatof T800H/3631. Toughness of matrix system has
5

KAZURO KAGEYAMA, H. Murayama, S. Atsuta, I. Ohsawa and M. Kanai

AE events

some effects on AE parameters detected with fiberoptic DEFEW sensor.
40
35
30
25
20
15
10
5
0

Positive・Negative
Negative・Positive

AE events

0

0.5

1
1.5
2
T800H/3631:Tf（μs)

40
35
30
25
20
15
10
5
0

2.5

3

Positive・Negative
Negative・Positive

0

0.5

1
1.5
2
T800H/3900-2:Tf（μs)

2.5

3Fig. 13 Histograms of half cycle of first arrival peak

6 Conclusions
AE signals, which have physical meaning ofstrain rate, were detected by the fiber-optic DEFEW
sensors during Mode I delamination tests. Fiberoptic DEFEW sensor has successfully applied tomicroscopic evaluation of failure process ofcomposite materials.
The results suggest that failure of matrix resin
occurs in shear mode under Mode I loading. First
arrival peaks of AE events are analyzed and they are
categorized four modes, and three of them might berelated to breakage of bridging fibers and matrix
cracking. Several different characteristics in timefrequency domain were also observed.

7 Acknowledgements
The authors thank Mr. Yuich Machijima,
LAZOC, Inc. for technical supports to the newfiber-optic sensing system.

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