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The acoustic emission sensors, model AE-
SS1 (Holroyd Instruments, UK) which were made
of piezoceramic elements, were mounted on the
circumference along the surface of the cylinder
(Fig. 2). The acoustic emission sensors of 100 kHz
resonance frequency responded well with the material
degradation and microscopic crack initiation and
growth. The captured acoustic emission signals were
then conditioned and amplified with 60 dB-gain signal
conditioning unit. The PCI-1714UL was used for
A/D conversion and data logger. In the experiments,
a normal operating cylinder was tested with four AE
sensors by means of mechanical coupling (Fig. 2).
The cylinder used in this experiment had a 60 litre
capacity. It was made of a steel sheet with a diameter
of 300 mm, length of 990 mm and thickness of 8
mm.
To perform hydrostatic test, the cylinder was
placed in a chamber which could facilitate a high
pressure testing (Fig. 3). For an abnormal operating
condition cylinder, a simulated surface crack was
performed on a good cylinder. The simulated
(artificial) crack (Fig. 4) was made using a grinder to
generate a surface defect with length of 30 mm (along
the length of cylinder), width of 3 mm and depth of
2 mm.
To eliminate background noise, the threshold of
AE signal was set to 39 dB AE . Evidently, the threshold
level affects the value of the AE parameters. A typical
Figure 3. Installation of CNG cylinder in a
high-pressure hydrostatic test chamber.
example is the event duration. By definition, it is the
time that the AE event is above the threshold. A step
wise increase in hydrostatic pressure was performed
up to 400 bars with the testing cylinder. During the test,
a pressure sensor was attached with the hydro-testing
machine as for load reference for data acquisition.
EXPERIMENTAL RESULTS AND
DISCUSSION
To verify the proposed approach, the experiment on
the normal operating cylinder was performed using
hydrostatic test. A hydrostatic test was applied on the
testing cylinder with increased pressure up to 300
bars. The AE signals captured from all four sensors
Figure 4. Artificial surface crack generated
at the side wall of the cylinder.
Figure 5. AE signals versus time (data point)
from a normal CNG cylinder.
Jostt vol 6.indd 55 7/22/10 10:09:43 PM
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Journal of Science and Technology in the Tropics (2010) 6: 53-57