For a special issue of Journal of Optics A: Pure and Applied Optics, Photon06:
submitted July 2006.
Graphite anisotropy measurements using laser-generated
ultrasound
B. Dutton and R. J. Dewhurst
University of Manchester, P.O. Box 88, Manchester, M60 1QD, UK.
b.dutton@postgrad.manchester.ac.uk
Abstract. One of the contributing factors for graphite degradation is oxidation. In order to
detect this type of degradation, samples with different levels of oxidisation have been studied.
Longitudinal velocity measurements were carried out on these samples, using laser ultrasound
techniques. These techniques have the advantage that they are non-contact, and therefore did
not perturb the sample under investigation. From these measurements, the Young’s modulus
was estimated, based on a typical Poisson’s ratio of 0.20. This was performed for several
cylindrical samples; some samples had parallel microstructure orientation in the through
thickness measurement direction, while others had perpendicular orientation. Both orientations
had very different longitudinal velocities; with corresponding variations in Young’s modulus.
Degradation was measured for both cases of parallel and perpendicular oriented samples.
Furthermore, in these perpendicular oriented samples, velocities were dependent on sample
rotation about the cylindrical axis. Additional measurements were taken at 90o orientations to
confirm this feature. This may be the first time that acoustic birefringence has been observed in
graphite.
1. Introduction
Graphite has been used for many applications. One of them has been the nuclear plant industry, where
graphite is used as a neutron moderator. The literature indicates that there are two main factors
involved in the degradation of graphite in nuclear reactors, which in turn degrades its Young’s
modulus. One factor is the effect of the bombardment of neutrons [1, 2] and the other the oxidation
due to the coolant, CO2 [1], which reduces the Young’s modulus exponentially. Young’s modulus can
be reduced dramatically with either or both of these processes. Each graphite core must perform its
duty as a structure that allows unrestricted passage of fuel elements and control rods maintaining
appropriate cooling flows at all times during the operating life of the station. Over the life of the
graphite it suffers dimensional changes and loss of strength due to the neutron radiation and oxidation
caused by the coolant. The graphite used in some nuclear plants is called Pile Grade A (PGA) [1].
The degradation of PGA graphite needs to be monitored. There are several methods for testing the
structural strength of graphite, and one of them involves the measurement of Young’s modulus.
Preliminary tests were conducted on graphite that had not undergone neutron radiation. Using laser
ultrasound techniques to generate ultrasound and an out-of-plane (OP) electromagnetic acoustic
transducer (EMAT) for detection, longitudinal velocities were calculated using first arrival times.
From these measurements, Young’s modulus was estimated, based on a typical Poisson’s ratio of 0.20
for graphite. This was performed for several cylindrical samples; some samples had parallel
microstructure orientation to the through thickness measurement direction, while others had a
perpendicular orientation. Some of the graphite samples were degraded by oxidisation while others
were virgin. Additional measurements were taken at 90o rotations about the cylindrical axis to examine
any polarisation of longitudinal velocity components.
2. Measurement technique
Laser/EMAT systems for ultrasound inspection have been examined by our group in recent years due
to the robustness and physical properties of EMATs [3-7]. Signal interpretation, sometimes using Bscan images to study mode conversion, has led to a number of new insights into material
characterisation. Laser/EMAT ultrasound systems have a number of advantages that become attractive
when considering measurement on small specimens. Conventional ultrasonic transducers require
physical contact (or immersion) with the sample. This leads to damping of surface motion and
alteration of ultrasonic waveforms. Our technique is non-contact, using dry sample surfaces so
couplant problems are eliminated. Repeatable, broadband high frequency point excitation source is
feasible with MHz bandwidths. An EMAT detector with MHz frequency response bandwidths is also
achievable [8]. There is more information content from which to characterise the material under
investigation. For time-of-flight (TOF) measurements, the ability to generate a broadband, spatiallyresolved ultrasound source reduces uncertainty in the initial ultrasound arrival time (t0), dead-time,
and path length, and therefore is effective in elastic modulus determinations.
The laser used was a Q-switched Nd:YAG, which can offer at least 30 mJ in 8 ns at a wavelength
of 1.06 µm, with a repetition rate of up to 20 Hz and 2% stability in energy. In the present experiments
the repetition rate was limited to 5 Hz and the radiation was coupled through a multimode optical fibre
to the sample under test. The laser was also operated at underrated flashlamp voltages to limit the
output from the optical fibre to about 3.5 mJ per pulse. This radiation level was directed onto the end
surface of a sample with a spot size of about 1.0 mm, see figure 1. The short pulse of laser light was
absorbed at the surface causing rapid heating of a small surface area. For the energy levels applied, a
rapid expansion of the surface occurs (thermoelastic source) [9]. In this way the laser power density
was lower than that required for plasma formation in either air, or on the sample surface. The rapid
expansion set up a pattern of acoustic waves with longitudinal and shear components propagating into
the material, and Rayleigh waves travelling across the surface.
The ultrasound that propagated through the sample was detected by an OP EMAT. This transducer
was developed at the University of Manchester. The EMAT was coupled to a Tektronix TDS640 for
signal averaging and capture, before storing data under a LabView platform. Figure 1 shows the
complete experimental system used for TOF measurements. Origin software was then used for time
measurements and presentation of ultrasound waveforms. Time of arrival was defined by the 10 %
level on the leading edge of the signal. It is important to stress that significant developments had taken
place at the university to enhance the sensitivity of this EMAT to allow such measurements to take
place [10]. Its sensitivity was less than those achieved by conventional PZT devices, but greater than
those obtained from any laser interferometer detection system.
In principle, both the longitudinal and shear wave can be detected. Thus Poisson’s ratio can be
determined and the modulus calculated without recourse to any further assumptions. Again, this is
highly desirable. However, this was not the case in graphite, where there was large forward scattering
of ultrasonic signal components. The arrival of any shear wave was confused with additional signal
arrivals from scattering and geometry effects.
Digital
Oscilloscope
8 ns pulses
from Nd:Yag laser
Amplifier
1 mm PCS
multimode
optical fibre
Computer
Graphite
sample
Ultrasound detector
(EMAT)
Longitudinal
ultrasound wave
Figure 1. Graphite sample characterization experimental setup.
2.1. Calibration
In order to validate our system, measurements were first made on an aluminium sample. The
ultrasound arrival time of flight (TOF) was measured, and the longitudinal velocity, v L, was calculated
from the known sample thickness. To obtain Young’s modulus, the density and Poisson’s ratio were
also required. The density was measured directly from the samples, while the Poisson’s ratio, σ, was
estimated to be about 0.345 for aluminium. Finally, the Young’s modulus calculation was made with
the following formula [11],

v 2L  (1  2 )(1   )
.
(1   )
Young’s modulus for aluminium was calculated to be [68.02 ± 3.67] GPa, see Table 1, top row. This
value was in agreement with a reference value of 70 GPa [12].
Sample
Density, ρ
(kg/ m3)
Al measured
Al reference
DYM PERP1
DYM PERP2
DYM PERP3
DYM PARA1
DYM PARA4
DYM PARA5
2839 ± 57
2700
1364 ± 27
1335 ± 27
1337 ± 27
1477 ± 29
1476 ± 29
1446 ± 29
Longitudinal
velocity, vL
(m/s)
6135 ± 209
6374
733 ± 24
862 ± 28
881 ± 29
1595 ± 53
1887 ± 62
1707 ± 56
Young’s
modulus, Υ
(GPa)
68.02 ± 3.67
70
0.66 ± 0.03
0.89 ± 0.04
0.93 ± 0.05
3.38 ± 0.17
4.73 ± 0.24
3.79 ± 0.19
Table 1. First of three repeated measurements of 6 thermally oxidised samples with a calibration
measurement using an aluminium sample, where highlighted velocity values were measurements from
figure 2.
2.2. Thermally oxidised graphite measurements
Measurements continued with 6 thermally oxidised graphite samples, in order to assess the accuracy
of the technique on degraded PGA graphite. To assess reproducibility, the measurement procedure
was as follows: the sample was placed in the holder, and then a measurement was taken with a signal
average of 100 before removing the sample. This procedure was then repeated three times. Figure 2 is
just a small sample of the acquired signals for 6 thermally oxidized samples. Figure 2(a) shows a
sample with the microstructure perpendicular to the axis of the cylinder, while figure 2(b) shows
another sample with microstructure parallel to the axis. A change in their longitudinal velocities was
noted, with a direct correlation to microstructure orientation. An amplifier was used to improve the
signal to noise ratio (SNR). Table 1 contains the first set of measured and calculated values for 6
samples. Young’s moduli calculations were estimated values using a constant value of 0.20 for
Poisson’s ratio. Nevertheless, variations in both longitudinal velocity and Young’s moduli were
clearly detected between samples with parallel and perpendicular microstructure. Data revealed faster
velocities and therefore higher Young’s moduli for samples with parallel microstructure compared to
samples with perpendicular microstructure, as expected.
Microstructure 
to cylinder axis
7.2 ± 0.1 mm
Microstructure ||
to cylinder axis
20.0 ± 0.1 mm
vL= 7.2/8.35 = [0.862 ± 0.028] mm/µs
7.0 ± 0.1 mm
20.0 ± 0.1 mm
vL= 7.0/3.71 = [1.887 ± 0.062] mm/µs
(b)
(a)
Figure 2. Sample measurements of thermally oxidised graphite samples: (a) with microstructure
perpendicular to the cylinder axis and (b) with microstructure parallel to the cylinder axis.
The same measurement procedure was repeated for a second time, but this time a velocity variation
was measured when the sample was rotated through the cylinder axis by about 90º. Results are shown
on Table 2. Figure 3 shows a sample with perpendicular microstructure at 0o rotation (a) and 90o
rotation (b), and a different sample with parallel microstructure at 0o rotation (c) and 90o rotation (d). It
can be seen that the variation from a 90o rotation is more pronounced when the sample had
microstructure perpendicular to the cylinder axis. Repeat measurements confirmed these results.
Sample
Density, ρ
(kg/ m3)
DYM PERP1
DYM PERP2
DYM PERP3
DYM PARA1
DYM PARA4
DYM PARA5
1364 ± 27
1335 ± 27
1337 ± 27
1477 ± 29
1476 ± 29
1446 ± 29
0o Rotation
Longitudinal
Young’s
velocity, vL
modulus, Υ
(GPa)
(m/s)
951 ± 31
1.11 ± 0.06
822 ± 27
0.81 ± 0.04
894 ± 29
0.96 ± 0.05
1797 ± 59
4.29 ± 0.22
1847 ± 61
4.53 ± 0.23
1660 ± 55
3.59 ± 0.18
90o Rotation
Longitudinal
Young’s
velocity, vL
modulus, Υ
(GPa)
(m/s)
736 ± 24
0.66 ± 0.03
736 ± 24
0.66 ± 0.03
768 ± 25
0.71 ± 0.04
1757 ± 58
4.10 ± 0.21
1950 ± 64
5.05 ± 0.26
1797 ± 59
4.20 ± 0.21
Table 2. Measurements from 6 thermally oxidised samples after 0 o and 90o sample rotations, where
highlighted velocity values were measurements from figure 3.
vL = 7.2/7.57 = 0.951 ± 0.031 mm/µs
(a)
vL = 7.2/9.78 = 0.736 ± 0.024 mm/µs
(b)
vL = 7.1/4.04 = 1.757 ± 0.058 mm/µs
vL = 7.1/3.95 = 1.797 ± 0.059 mm/µs
(c)
(d)
Figure 3. Measurements of a sample with perpendicular microstructure (a) at 0 o rotation and (b) at 90o
rotation, and a sample with parallel microstructure (c) at 0 o degree rotation and (d) at 90o rotation.
2.3. Virgin graphite measurements
The last measurements were made on 4 long (20 and 40 mm) virgin PGA cylinders. Results are shown
on Table 3. These include short and long samples; with microstructure parallel and perpendicular to
cylinder axis; and with 0o and 90o rotations. Variations of longitudinal velocities and therefore
Young’s moduli were detected for samples with microstructure parallel and perpendicular to the
cylindrical axis. Again, velocity variation on rotation about 90o was more noticeable for samples with
microstructure perpendicular to axis.
Sample
Density, ρ
(kg/m3)
BRW PR 2A PERP
BRW PR 4A PERP
BRW PL 2A PARA
BRW PL 4A PARA
1718 ± 34
1762 ± 35
1760 ± 35
1720 ± 34
0o Rotation
Longitudinal
Young’s
velocity, vL
modulus, Υ
(GPa)
(m/s)
1969 ± 65
5.99 ± 0.30
1976 ± 65
6.19 ± 0.32
2703 ± 89
11.57 ± 0.59
2554 ± 84
10.10 ± 0.51
90o Rotation
Longitudinal
Young’s
velocity, vL
modulus, Υ
(GPa)
(m/s)
2088 ± 69
6.74 ± 0.34
2041 ± 67
6.61 ± 0.34
2658 ± 88
11.19 ± 0.57
2554 ± 84
10.10 ± 0.51
Table 3. Measurements from 4 virgin PGA cylindrical samples with 0 o and 90o sample rotations.
3. Conclusions
For porous and virgin graphite samples, measurements have shown that it was possible to measure
Young’s modulus using longitudinal velocity as the experimental measureand. Estimated values for
Young’s modulus values agree with previous PGA graphite samples values [2]. Young’s modulus for
parallel and perpendicular microstructure for virgin samples were [11.13 ± 0.60] GPa and [6.42 ±
0.35] GPa respectively, while previous work has reported Young’s modulus for parallel and
perpendicular microstructure 11.7 GPa and 5.4 GPa respectively, but without uncertainty estimates
[2]. More interestingly, we have detected velocity variations when samples have been rotated about
their cylindrical axis. The data positively shows polarisation of longitudinal velocity components.
This report may be the first time that acoustic birefringence has been observed in graphite.
Acknowledgments
We would like to thank Dr. Paul Mummery and Mark Turski from University of Manchester,
Materials Science Centre, for initiating this work and Dr. Siridech Boonsang for his support on
experimental arrangements. One of the authors (B. Dutton) would like to acknowledge scholarships
from the Mexican government CONACYT No. 178491, PROMEP Oficio/103.5/03/950 and the
University of Yucatan.
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