“…Compared with signal classification methods based on single features detected via threshold-based acquisition, pattern recognition techniques are computationally intense. However, they are significantly more adequate for processes investigated and, fortunately, can now be implemented in a variety of efficient and elegant mathematical ways [14]. (4) The most important finding, which was observed in all tests at both temperatures, is that, as final rupture approached, the AE energy per cycle increased, while the spectral density function progressively shifted to the low-frequency domain, as reflected in the median frequency reduction.…”