FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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determined experimentally for pipeline steels through the thickness of plate
as well as in the pipe segment described earlier. The procedure for
estimation of a through plate thickness using the pulse reflection method was
described by Krautkramer(9), and will not be repeated here. For X70 pipeline
steel, the attenuation coefficient was found to be equal to 1.09 dB/m while
for X42 steel, the attenuation was much higher with a = 2.125 dB/m.
In the tests on the pipe segment, a wave generator was used to produce
frequencies swept between 01 to 1 MHz. An exciter transducer was connected
with the wave generator. A receiving transducer was placed at specific
distances from the exciter and the signal, after amplification, pass-as
through an R.M.S. voltmeter and is plotted as a function of frequency. The
value of a determined in this test corresponded to 0.89dB/m. This value is
affected by the reflection of the elastic waves in the free ends of the
pipe.
3.3 Theoretical modelling
Significant effort in the theoretical modelling of the acoustic emission
system and its functioning was made. This ranges from modelling the sources
of burst emission in terms of lattice perturbation and dislocation
interaction (Bassim and Wassef(lO)), to the prediction of the stresses and
displacement in a plate due to a propagating crack (Bassim, Wassef and
Tangri(ll)) and finally the prediction of the type and frequency spectre! of
the noise due to the flow of a gas (or fluid) in a circular hole or
rectangular slit (Wassef et al(12)).
The above studies have for general conclusions that acoustic emissiori is
caused by the perturbation of an otherwise perfect lattice to cause phonon
emission. Involved quantum mechanics approach shows that the interaction of
two or more dislocations is capable to producing a phonon emission
corresponding to about 200 KHz which is the range where acoustic emission is
most observed.
Once acoustic emission is produced, its propagation will depend on the
position of the source with respect to the structure and on the relative
dimension of the emitting crack and the rest of the structure. Modelling,
involving spectral representation, Green's functions as well as solutions to
Fredholm integral equations are used to predict the stresses and displacement
fields in a body containing a crack and subjected to a loading. These
predictions give an accurate indication of the type of transducer to be used
in conjunction with a given geometry as well as the most appropriate
transducer configuration to use in a structure.
4. CONCLUSIONS
A summary of a research program which was carried out in conjunction with
development of a new acoustic emission system is presented. The specific
application is the continuous monitoring of pipelines. The program involves
materials studies on pipeline steels, model tests to measure signals