FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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bands are initiated right after the first yield drop. High acoustic emission
activity occurs in the Lüders strain region of the stress-strain curve. Each
peak on the RMS curve corresponds to a stress drop on the stress-strain
curve. This indicates that the major source of acoustic emission in this
steel is due to inhomogeneous deformation. No systematic difference in
bahaviour was observed between longitudinal and transverse samples. Thus
acoustic emission activity is detected mostly within ehe first 3% plastic
strain during tensile testing.
In fracture testing, three-point bend specimens, 12.7 mm thick with a notch
in the TL and LT directions were obtained from the as-received A53B pipeline
steel. Fig. 4 shows a typical RMS voltage of the acoustic signal during a
test as a function of load point displacement. The acoustic emission
activity remains high throughout the displacement range investigated which
represents the development of the plastic zone ahead of ehe crack tip without
extending the crack length. Most of the emission were found to occur during
the plastification of the ligament ahead crack tip and no significant
emission accompany the stable crack growth process itseLî. This is in
agreement with Blanchette, Bassim and Dickson(8) for A516 grade 70 steel.
3.2 Leak and attenuation characterization
The leak tests were performed on a segment of pipe, 400 cm long with inside
and outside diameters of 160 and 170 mm respectively. The pressure and flow
rate of the air through the pipe were measured accurately. Two series of
tests, one using an accelometer with a range of up to 25 KHz and the other
with an acoustic transducer with a resonant frequency of 1140 KHz were
conducted. An artificial leak was made by drilling a hole directly in the
middle of the pipe and the signal was analysed for its frequency content.
Fig. 5 represents a plot of the RMS from different leak sizes as a function
of pressure while Fig. 6 shows the effect of hole diameter on the leak
frequency obtained at different air pressures through the pipe. It is
noticed that for a given pressure, as the hole diameter decreases, the leak
frequency also decreases. On the other hand, the value of the R.M.S. which
represents the relative amplitude of the signal increases with the increase
in hole diameter as well as increase of the pressure. Finally, measurement
of the signal amplitude at different points on the pipe shows no significant
attenuation for this relatively short segment of pipe. Turbulence due to end
effects appears to increase the signal amplitude to some extent.
In the attenuation tests, the aim is to determine the attenuation coefficient
aj which is defined for a plane wave as
P = Po e~ ad (1)
where PQ is the initial pressure and P is the sound pressure at distance d.
In this study the attenuation coefficient a, expressed as dB/m, was