FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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orientation of these specimens with respect to the rolling plate was
investigated. In the fracture tests, parameters such as the stress intensity
factor K-£C and the J-integral, Jjc which is used to characterize ductile
fracture, were determined.
Together with the mechanical testing, acoustic emission was obtained using a
system similar to that in Fig. 1. Acoustic emission parameters, namely the
total count, count rate and root mean square of the amplitude were determined
as a function of the applied load and elongation. In the fracture tests,
acoustic emission was obtained by positioning a transducer on the side of the
specimen and following the emissions as the plastic zone ahead of the crack
is developed followed by stable crack growth.
2.2 Model tests on pipeline materials
In these tests, two aspects were considered: Firstly, the continuous
acoustic signals due to leaks were detected with transducers of different
frequencies located at different distances from leaks induced by drilling
holes in the pipe as described by Bassim and Tangri(5). Air flow at
different pressures was maintained through the pipe. The experimental set-up
is shown in Fig. 2. Secondly, the attenuation coefficient a was determined
using ultrasonic methods described in (6). The attenuation characteristics
of the pipe were thus established and calculations of the optimum spacing
corresponding to a transducer with a given resonant frequency was
calculated.
2.3 Theoretical consideration of acoustic emission sources
Theoretical modelling of the sources of emissions in terms of dislocation
motion and interaction was carried out to arrive at the optimal frequencies
corresponding to an acoustic emission signal. Also, once an emission has
occurred, modelling of the type of waves and its propagation characteristics
was carried out to determine the proper position of transducers on the pipe
and their optimum spacing. The effect of pipe (or specimen geometry) on the
destructive and constructive interference of acoustic signals, following the
analysis of Hamel, Ballon and Bassim(7) was also investigated.
3. RESULTS AND DISCUSSION
Following are representative data obtained in the course of the research
program as well as an interpretation of their physical meaning and the extent
that they can be used in the design of a dedicated acoustic emission system.
3.1 Material characterization
Flat tensile samples, from two orientations with respect to the rolling
direction, were obtained from A53B pipeline steel. Fig. 3 shows a typical
stress-strain and RMS data from a longitudinal specimen. It is observed that
the A53B deforms initially by a complex Lüders band propagation. Several