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|Author(s):||Marvin A. Hamstad;|
|Title:||Some Observations on Rayleigh Waves and Acoustic Emission in Thick Steel Plates|
|Published:||December 31, 2009|
|Abstract:||Rayleigh waves in acoustic emission (AE) applications were examined for nominal 25 mm thick steel plates. Pencil lead breaks (PLBs) were introduced on the top and bottom surfaces as well as on the plate edge. The plate had large transverse dimensions to minimize edge reflections arriving during the arrival of the direct waves. An AE data sensor was placed on the top surface at both 254 mm and 381 mm from the PLB point or the epicenter of the PLB point. Also a trigger sensor was placed close to the PLB point. The signals were analyzed in the time domain and the frequency/time domain (with a wavelet transform). For most of the experiments, the two data sensors had a small aperture (about 3.5 mm) and a high resonant frequency (about 500 kHz). These sensors effectively emphasized a Rayleigh wave relative to Lamb modes. In addition finite element modeling (FEM) was used to examine the presence or non-presence of surface Rayleigh waves generated by dipole point sources buried at different depths below the plate top surface. The resulting out-of-plane displacement signals were analyzed in a fashion similar to the experimental signals for propagation distances of up to 1016 mm for out-of-plane dipole sources. Surface Rayleigh waves were generated in the experiments for all three locations of PLBs. In the case of the bottom surface PLBs, the Rayleigh wave propagated to the plate edge, up the edge and then along the plate top surface to the sensors. Due to the time delay from the propagation up the edge to the plate surface, surface Rayleigh waves from edge PLBs resulted in a strong signal that interfered with a straightforward analysis of the intense frequency/time regions of the Lamb modes from these source positions. The FEM results for the out-of-plane dipoles showed the surface Rayleigh wave out-of-plane displacement amplitude decayed relative to the Lamb mode amplitudes as the depth of the source below the surface pseudo sensors increased. A surface Rayleigh wave was|
|Pages:||pp. 114 - 136|
|Keywords:||Acoustic emission, pencil lead break, Rayleigh waves, source depth, thick plate, wavelet transform|
|PDF version:||Click here to retrieve PDF version of paper (4MB)|