Analysis for propagation characteristics of longitudinal mode and torsional mode guided waves in rod
Regarding the application of guided waves in one-dimensional structure, the propagation characteristics of the longitudinal guided wave and torsional guided wave are investigated. The dispersion equations of guided wave in the longitudinal and torsional modes in one-dimensional component are derived. Through programming numerical calculation in MATLAB, the frequency dispersion curves of the two modes are obtained, which imply that the engineering application of guided waves in detection of rod needs to choose an appropriate frequency and the corresponding propagation characteristics of the waves should be combined to get reliable results. The dispersion characteristics analysis results in different radius of rods show that it is feasible for detecting the defects in rod using L(0,1) mode of ultrasonic guided waves.
Wire breaking localization of parallel steel wire bundle using acoustic emission tests and finite element analysis
The precise localization of cable damage is a major issue in bridge engineering. The acoustic emission (AE) method has been applied to detect defects for cable structures in bridges. In this paper, an AE localization method is proposed to localize both lateral and longitudinal positions of defects in a parallel steel wire bundle composed of seven steel wires. The proposed method is verified numerically and experimentally. Tension failure test of the parallel steel wire bundle composed of seven wires was carried out, and the precise localization of defects was realized by through modal AE techniques and the analysis of the characteristics of AE signal. A Multiphysics model based on COMSOL was established to facilitate the visualization of stimulated AE propagated in the parallel wire bundle. The propagation mechanism of AE signal was studied, and the feasibility of localization method was verified.
Identification of presumed shallow underwater chemical explosions using land-based regional arrays
Abstract Seismic events located in the Gulf of Bothnia, the Baltic Sea, and the North Sea, recorded at the regional seismic arrays NORESS and FINESA, have characteristics of underwater blasts observed hydroacoustically. Spectral analysis of regional phases associated with the events reveals strong time-independent spectral scalloping indicating that the waveforms are made up of correlated “pulse-echo” type signals. Cepstral analysis gives consistent delay times between the pulse-echo pairs with the most common delays between 400 and 600 msec. Some cepstra also have negative peaks at quefrencies between 150 and 350 msec. The cepstral peaks are observed in all phases associated with the event, and events recorded at both arrays have the same delay times. The higher time delays are consistent with bubble-pulse delays commonly observed in underwater blasts recorded hydroacoustically. The lower-quefrency negative peaks are caused by echoes with reversed polarities relative to the primary pulse and are consistent with reflections from the top of the water column. Generally, these events have large Pn/Lg amplitude ratios similar to those observed from quarry blasts. Somewhat smaller high-frequency Pn/Lg ratios are observed for Baltic Sea events compared with the North Sea events, which may be due to differences in the water-bottom geologies of the two regions. Simple source models of underwater blasts explain many of the features of the observed cepstra and indicate that the shot depths had to be shallow (around 40 to 80 m) for shot sizes between 75 and 150 kg. This study shows that the seismic sensor network, called for by the Comprehensive Test Ban Treaty (CTBT), can augment the smaller network of hydroacoustic stations for the detection and identification of clandestine under-water nuclear-explosion tests in the near offshore environment.