The guided wave (GW) field excited by a wedge-shaped, anisotropic piezocomposite transducer, surface-bonded on an isotropic substrate is investigated with applications to large area structural health monitoring. This investigation supports the development of the composite long-range variable-direction emitting radar (CLoVER) transducer. The analysis is based on the three-dimensional equations of elasticity, and the solution yields expressions for the field variables that are able to capture the multimodal nature of GWs. The assumption of uncoupled dynamics between the actuator and substrate is used, and their interaction is modeled through shear tractions along the transducer's radial edges. A similar problem is modeled using three-dimensional finite element simulations to assess the spatial and transient accuracy of the solution. Experimental tests are also conducted on pristine structures to validate the accuracy of the theoretical approach. The experimental studies employ CLoVER transducers developed in-house, and their manufacturing procedure is briefly described. Frequency response experiments based on piezoelectric sensors are conducted to assess the performance of the solution in the frequency domain. These tests are complemented by laser vibrometer measurements that allow the spatial and temporal evolution of the solution to be evaluated. The numerical simulations and experimental tests show that the wave time of arrival, radial attenuation, and azimuthal distribution are well captured by the theoretical solution.
The guided wave (GW) field excited by piezoelectric wafers and piezocomposite transducers in carbon-fiber composite materials is experimentally investigated with applications to structural health monitoring. This investigation supports the characterization of the composite long-range variable-length emitting radar (CLoVER) transducer introduced by the authors. A systematic approach is followed where composite configurations with different levels of anisotropy are analyzed. In particular, unidirectional, cross-ply [0/90] 3S and quasi-isotropic [0/45/-45/90] 2S IM7-based composite plates are employed. A combination of laser vibrometry and finite element analysis is used to determine the in-plane wave speed and peak-to-peak amplitude distribution in each substrate considered. The results illustrate the effect of the material anisotropy on GW propagation through the steering effect where the wavepackets do not generally travel along the direction in which they are launched. After characterizing the effect of substrate anisotropy on the GW field, the performance of the CLoVER transducer to detect damage in various composite configurations is explored. It is found that the directionality and geometry of the device is effective in detecting the presence and identifying the location of simulated defects in different composite layups.
Structural health monitoring (SHM) is the component of damage prognosis systems responsible for interrogating a structure to detect, locate, and identify any damage present. Guided wave (GW) testing methods are attractive for this application due to the ability of GWs to travel over long distances with little attenuation and their sensitivity to different damage types. The Composite Long-range Variable-direction Emitting Radar (CLoVER) transducer is introduced as an alternative concept for efficient damage interrogation and GW excitation in GWbased SHM systems. This transducer has an overall ring geometry, but is composed of individual wedge-shaped sectors that can be individually excited to interrogate the structure in a particular direction. Each wedgeshaped sector is made with piezoelectric fibers embedded in an epoxy matrix surrounded by an interdigitated electrode pattern. The multiple advantages over alternative transducer concepts are examined. In particular, it is shown that the geometry of each sector yields actuation amplitudes much larger than those obtained for a ring configuration under similar electric inputs. The manufacture and characterization procedures of these devices are presented, and it is shown that their free strain performance is similar to that of conventional piezocomposite transducers. Experimental studies of damage detection simulating the proposed damage interrogation approach are also presented.
Convective heat transfer in aluminum metal foam sandwich panels is investigated with potential applications to actively cooled thermal protection systems in hypersonic and reentry vehicles. The size effects of the metal foam core are experimentally investigated and the effects of foam thickness on convective transfer are established. Four metal foam specimens are utilized with a relative density of 0.08 and pore density of 20 ppi in a range of thickness from 6.4 mm to 25.4 mm in increments of approximately 6 mm. An exact-shapefunction finite element model is developed that envisions the foam as randomly oriented cylinders in cross flow with an axially varying coolant temperature field. Our experimental results indicate that larger foam thicknesses produce increased heat transfer levels in metal foams. Initial FE simulations using a fully developed, turbulent velocity profile show the potential of this numerical tool to model convective heat transfer in metal foams.
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