In the present work a plate of steel (St37) has been used with
dimensions of 1 m*1 m and 3 mm thickness. The mechanical properties
and chemical composition of plate is shown in Table 1. In
order to facilitate sensor placing and pencil lead breaking, the
surface of plate has been divided to 10 cm intervals. As can be seen
in Fig. 2 four sensors were installed on the plate in a rectangular
array. The sample was laid on a foam pad on the table and a
0.5 mm pencil lead was broken as artificial defect. In order to generate
homogeneous artificial defects, it is necessary to break leads
in a certain angle and length with similar forces. A 16 channel
acquisition system was used to analyze the waveform and record
data. Four PAC preamplifiers (model 1220A) with 40 dB gain were
utilized for signal conditioning. Four general purpose sensors
(model PAC 15a) were also applied with operational frequency
range of 0.1–1 MHz to detect all AE signals propagated through
the material. In order to provide suitable acoustic coupling
between specimen and sensors, high vacuum silicone grease was
used to cover the surface of the sensors. Acoustic signals were
acquired at 2 MHz sampling rate and therefore frequencies up to
1 MHz were considered. AE Win 2006 of physical Acoustic Corporation
was used as acoustic emission software for acquisition
processing.