**9. Concluding Remarks**

Limited aspects of AE uses have been reviewed, concentrating on the period of last several years. This review will hopefully provide an overview of various AE applications to large scale structures that will be discussed in this Special Issue on Structural Health Monitoring of Large Structures using Acoustic Emission Case Histories. However, many important issues were not covered here, especially on civil engineering side. On concrete issues, see a recent book edited by Ohtsu [213] and on SHM side, there are numerous books available. Another form of construction, masonry, has attracted increasing attention. Carpinteri et al. [214] examined historic masonry structures and identified internal crack distribution with AE source location methods. More recent reviews of AE studies of masonry structures were given by De Santis et al. [215] and Verstrynge et al. [216], again demonstrating the values of AE technology.

**Author Contributions:** The author has contributed from conception to writing in its entirety.

**Funding:** This research received no funding.

**Acknowledgments:** The author is grateful to N.G. for the use of Figure 9 and to UCLA Chapter of Materials Research Soc. SAMPE Competition team, N.S., team leader, for T700 CFRP plate samples. He also acknowledges numerous valuable discussions with AE colleagues in formulating opinions presented here, although any errors or omissions are his own.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **Appendix A Limiting Frequency of Rayleigh Wave Propagation**

Wave motion of Rayleigh waves decreases rapidly as the depth increases [20,21]. When the thickness of a plate, h, becomes smaller than three to four times the wavelength, wave propagation gradually shifts to Lamb wave modes. Hamstad [217] examined by experiment and by finite element analysis the limiting frequency of the Rayleigh wave propagation. Since this is of practical importance, some of his results are reproduced in Figure A1. He used a large steel plate (25.4-mm thick) in modeling Rayleigh wave propagation from a pencil-lead break. Wavelet transform spectrogram of calculated signal is shown in Figure A1a after 381-mm travel on the same side. Corresponding spectrogram for experimentally detected signal is given in Figure A1b. Both spectrograms show the strongest peak at the arrival time of Rayleigh waves (130 μs) and signal intensity diminishes below 250 kHz, shifting to Lamb wave S0 and A0 modes. His results of the low frequency limit of Rayleigh wave propagation, *f* L, (in MHz) can be described by *f* <sup>L</sup> = 8.2/h, as shown in Figure A1c. For a given thickness, h (in mm), Rayleigh waves exist only at frequencies above *f* L. At h = 100 mm, this *f* <sup>L</sup> is 82 kHz. This was the case for Graham–Alers attenuation study discussed in Section 2.2.

**Figure A1.** (**a**) Wavelet transform of calculated signal waveform, received at 381 mm from a pencil-lead break on the same surface of a steel plate of 25.4 mm thickness; (**b**) wavelet transform of experimentally received signal waveform, received at 381 mm from a pencil-lead break on the same surface of a steel plate of 24.4 mm thickness; (**c**) the low frequency limit of Rayleigh wave propagation (in MHz) vs. steel plate thickness (in mm). These figures were rearranged from Figures 13a,b and 19 by Hamstad [217]. Reproduced with permission.
