Understanding Acoustic Emission Testing- Reading 1 Part B-A

In 1965, however, Robinson used more sensitive equipment to show that
acoustic emission occurred at much lower load levels than had been reported
earlier, and hence, could be used to monitor earlier microcracking (such as
that involved in the growth of bond cracks in the interfacial region between
cement and aggregate).
In 1970, Wells built a still more sensitive apparatus, with which he could
monitor acoustic emissions in the frequency range from about 2 to 20 kHz.
However, he was unable to obtain truly reproducible records for the various
specimen types that he tested, probably due to the difficulties in eliminating
external noise from the testing machine. Also in 1970, Green reported a much
more extensive series of tests, recording acoustic emission frequencies up to
100 kHz. Green was the first to show clearly that acoustic emissions from
concrete are related to failure processes within the material; using source
location techniques, he was also able to determine the locations of defects. It
was this work that indicated that acoustic emissions could be used as an
early warning of failure. Green also noted the Kaiser effect, which suggested
to him that acoustic emission techniques could be used to indicate the
previous maximum stress to which the concrete had been subjected. As we
will see below, however, a true Kaiser effect appears not to exist for concrete.
Charlie Chong/ Fion Zhang