Nanofiller-modified composites have shown much potential for structural health monitoring (SHM) and nondestructive evaluation (NDE) because they exhibit self-sensing behavior via the piezoresistive-effect. To date, the piezoresistive effect has been used predominantly in isolation for cases of (quasi-)static loading. There is a comparative lack of work that investigates the piezoresistive-effect under mechanically dynamic conditions. This is important because combined piezoresistive-elastodynamic approaches could leverage the relationship between electrode geometry/topology and piezoresistive information carried in elastic waves. In other words, the question of how certain factors of electrode design, such as spacing between electrode pairs, impacts the measured piezoresistive signal has been little explored within the context of elastodynamics. Addressing this gap in the state of the art may yield insights into multiphysics approaches to SHM and NDE which seamlessly marry conductivity and vibration-based techniques. To this end, a simple prismatic carbon nanofiber (CNF)-modified epoxy rod was manufactured. Piezoresistive measurements were taken as a function of time by way of normalized resistance measurements between surface mounted electrodes. An electromagnetic shaker was employed to inject highly controlled one-dimensional stress waves into the CNF-modified epoxy rod. It was found that surface mounted piezoresistive measurements were able to accurately reconstruct the profile of propagating subsurface wave packets across the length of the rod. Transmission and propagation of the wave packets were extrinsically validated with the shaker force sensor and an external laser vibrometer (LV) system, respectively. Furthermore, artificial signal filtering was achieved by changing the distance between the electrode pairs. Lastly, the dispersion curves were constructed from piezoresistive measurements and extrinsically validated. Results from this preliminary investigation seeks to lay the foundational work for a new multiphysics SHM tool, piezoresistivity-coupled elastodynamics, that aims to provide an unparalleled level of understanding into material dynamic properties from direct electrical interrogation.
Shape memory alloys (SMAs) have long been utilized as semi-active elements for the attenuation of unnecessary vibration in engineering structures. By leveraging properties of different material phases, energy dissipation capability of smart structures integrated with SMAs can be enhanced and a certain degree of tunability subjected to temperature or stress stimuli can be achieved. In this paper, the influence of axial pre-strains on the dynamic characteristics of a pinned-pinned beam at different operating conditions was systematically and comprehensively investigated. To model the material nonlinearity of SMAs, the improved one-dimensional (1D) Brinson's model with tension-compression asymmetry is exploited. The constitutive relation was integrated into the finite element model of considered SMA beam, in which geometric nonlinearity in the von Karman sense is included as well. Free and forced vibration of the SMA beam under different levels of pre-strains as well as operation conditions were analyzed. For the free decaying of SMA beam, it was observed there exists optimal pre-strain that can achieve maximum damping performance. In the forced vibration analysis, the jump phenomena in amplitude-frequency properties of SMA beam were evaluated and compared to the equivalent elastic beam. The results imply that the pre-strains affect the vibration of SMA beam distinctly at different operating temperatures as well as frequency regions. The conducted analysis can provide guidance on fully exploiting dissipation properties of SMA lamina in the development of composites.
We illustrate the design of acoustic metasurfaces based on geometric tapers and embedded in thin-plate structures. The metasurface is an engineered discontinuity that enables anomalous refraction of guided wave modes according to the Generalized Snell’s Law. Locally-resonant geometric torus-like tapers are designed in order to achieve metasurfaces having discrete phase-shift profiles that enable a high level of control of refraction of the wavefronts. Results of numerical simulations show that anomalous refraction can be achieved on transmitted anti-symmetric modes (A0) either when using a symmetric (S0) or anti-symmetric (A0) incident wave, where the former case clearly involves mode conversion mechanisms.
We use a recently developed class of metamaterials based on geometric inhomogeneities to design acoustic lenses embedded in thin-walled structural element. The geometric inhomogeneity is based on the concept of Acoustic Black Hole (ABH) that is an exponential taper fully integrated in the supporting structure. The ABH is an element able to bend and, eventually, trap acoustic waves by creating areas with carefully engineered phase velocity gradients. Periodic lattices of ABHs are first studied in terms of their dispersion characteristics and then embedded in thin-plate structures to create lenses for ultrasonic focusing and collimation. Numerical simulations show the ability of the ABH lens to create focusing and collimation effects in an extended operating range that goes from the metamaterial to the phononic regime.
In this paper, we present an approach to the generation of steerable ultrasonic beams in structures for possible application to damage detection. The proposed approach is based on the design of embedded acoustic lenses that exploit fundamental principles of wave propagation in acoustic metamaterials. In particular, the lens design relies on the concept of acoustic drop-channel where multiple waveguides can be coupled and selectively activated by simply tuning the frequency of the excitation. Numerical analyses will show that this design allows generating highly directional excitation by using a single ultrasonic transducer. Plane Wave Expansion and Finite Difference Time Domain methods are used to evaluate the dispersion characteristics of the metamaterial lens as well as to simulate the transient response when the lens is embedded in a plate-like structure. The lens design is then experimentally validated on an aluminum plate where the lens is implemented by through the thickness notches. The overall performances are estimated by reconstructing the dynamic displacement field via Laser Vibrometry.
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