Admittance Signatures Research Articles

The electro-mechanical impedance method for damage identification in circular plates

Among structural acoustics methods, the electromechanical (E/M) impedance method is a new method that is easy to implement on complex structures with minimum instrumentation and straightforward results. This new acoustic method benefits from the advent of commercially available low-cost piezoceramics that have opened new opportunities for structural identification. The structural dynamics is identified by obtaining the E/M impedance or admittance signatures of piezoelectric active sensors that are permanently attached to the structure. In the presented study, the E/M impedance method was used to identify the structural dynamics and the presence of damage in circular plates. An analytical expression, which accounts for both axial and flexural vibrations of a plate, was derived and validated through a set of experiments. The experimental results show good matching with the theory. Additional acoustic experiments were conducted to assess the presence of damage in circular plates for a particular set of boundary conditions. Some damage metrics were suggested to analyze the frequency spectrum obtained. The results show the changes in the E/M impedance spectrum due to damage presence. Future research will focus on the application of the E/M impedance to the acoustical diagnostics of aging aircraft structures.

Electro-Mechanical Impedance Method for Crack Detection in Thin Plates

This paper describes the utilization of Electro-Mechanical (E/M) impedance method for structural health monitoring of thin plates. The method allows the direct identification of structural dynamics by obtaining its E/M impedance or admittance signatures. The analytical model for two-dimensional structure was developed and verified with experiments. Good matching of experimental results and calculated spectra was obtained for axial and flexural components. The ability of the method to identify the presence of damage was investigated by performing an experiment where the damage in the form of crack was simulated with an EDM slit placed at various distances from the sensor. It was found that the crack presence dramatically modifies the E/M impedance spectrum and this modification decreases as the distance between the sensor and the crack increases. Several overall-statistics damage metrics, which may be used for on-line structural heath monitoring, were investigated. Among these candidate damage metrics, the αth power of the correlation coefficient deviation, CCDα,3< α <7, used in the high frequency band 300-450 kHz, was found to be most successful. Careful selection of the high frequency band and proper choice of the appropriate damage metric were found to be essential for successful damage detection and structural health monitoring.

Origin of Corona-Dominated Topographic Rises on Venus

Both large-scale mantle upwellings, comparable to terrestrial hotspots on Earth, and smaller scale mantle upwellings, known as coronae, occur on Venus. Corona-dominated rises have many of the characteristics of large-scale mantle upwellings, or hotspots, such as broad topographic rises greater than 1000 km in diameter and large positive gravity anomalies. Due to the presence of clusters of three to eight coronae, three large volcanic rises (or hotspots) on Venus have been classified as corona-dominated rises (CDRs): Themis, Eastern Eistla, and Central Eistla Regiones. CDRs have been interpreted to result from the break-up of a large-scale plume. Comparison of the topographic morphology for individual coronae at Themis and Eastern Eistla Region to a model of corona evolution indicate that they are in varying stages of evolution. At Eastern Eistla Regio all the coronae have essentially the same topographic morphology, consistent with a late stage of evolution and the presence of a depleted mantle layer at depth. The complex deformation sequences and stratigraphic relationships both between coronae and with respect to the regional plains observed at all three rises indicate a prolonged origin. This observation, as well as the varying stages of evolution, rule out the previously proposed interpretation of corona-dominated rises as a manifestation of the break-up of a large-scale mantle upwelling, which requires essentially simultaneous formation of the coronae. Instead we suggest that other large topographic rises are the manifestation of deep mantle plumes, likely to originate at the core–mantle boundary, and that CDRs are clusters of coronae that originate at a shallower interface, perhaps at an upper–lower mantle boundary. Using top- and bottom-loading flexural models to fit the gravity/topography admittance spectrum for each of the three CDRs yields elastic thickness estimates that are 10–15 km greater for bottom-loading at longer wavelengths than top-loading at shorter wavelengths. Estimates of elastic thickness assuming top-loading are 10, 12, and 22 km and 20, 25, and 35 km from bottom-loading for Eastern Eistla, Central Eistla, and Themis Regiones, respectively. We believe that the bottom-loading elastic thickness estimates are more reliable because using a top-loading model when both types of loading are present yields an unrealistically low elastic thickness estimate. As there is no obvious source of surface loading at either Themis or at Eastern Eistla, we interpret the top-loading admittance signature to be a result of delamination of the lower lithosphere depressing the surface, which is consistent with the observed coronae morphologies.

Admittance signatures of rifted and transform margins: examples from eastern Canada

Examination of the gravity and bathymetry data across the rifted Scotian margin and the sheared Grand Banks margin, off the Canadian east coast, using a spectral analysis or admittance approach. yielded different results. The admittance for the Scotian margin best fitted a regional compensation model; in contrast, a local compensation mechanism reproduced the transfer function for the Grand Banks margin. The results for the longer wavelengths were verified using satellite altimetry data. The effect of a hidden or subsurface load was investigated for both margins and this was also found to differ. The differences in the compensation mechanism between rifted and sheared margins was attributed to their formation mechanisms. Rifted margins are formed by upper crustal faulting and lower crustal flow parallel to the spreading centre. These characteristics are compatible with thin plate flexure compensation. In contrast, transform margins are formed perpendicular to the spreading ridge and the isotherms are only slightly raised as the spreading centre migrates along the margin producing a weak suture which accommodates local compensation.