Search

We investigate the transmission and backscattering of solitary waves in granular crystals to assess their initial strains under various levels of compression. We assemble a one-dimensional granular chain that includes a pair of heavy impurities to scatter incident solitary waves. Using a scanning laser Doppler vibrometer and an instrumented sensor particle, we experimentally show that the speed of solitary waves backscattered in the region of impurities is highly sensitive to the applied strains. Based on this pulse-echo mechanism of solitary waves, we briefly demonstrate the feasibility of localized strain sensing via multiple scatterers distributed in a granular chain. We find that numerical results obtained from a discrete element model are in excellent agreement with experimental results. This study forms a foundation for constructing a solitary wave-based sensor system to measure distributed strains in ordered granular systems. [ABSTRACT FROM AUTHOR]

We investigate the tunability of phononic crystals consisting of arrays of cylindrical elements using theoretical, numerical, and experimental approaches. We show that when these systems are excited by a continuous dynamic signal under large static precompression, they support a characteristic band structure whose cutoff frequency can be controlled by changing the alignment angles or the static precompression. Furthermore, we report the formation of an additional pass band when there exists particles' eccentricity, which is caused by the coupling mechanism between longitudinal and shear modes. [ABSTRACT FROM AUTHOR]

Osteoporosis is a well recognized problem affecting millions of individuals worldwide. The ability to diagnose problems in an effective, efficient, and affordable manner and identify individuals at risk is essential. Site-specific assessment of bone mechanical properties is necessary, not only in the process of fracture risk assessment, but may also be desirable for other applications, such as making intraoperative decisions during spine and joint replacement surgeries. The present study evaluates the use of a one-dimensional granular crystal sensor to measure the elastic properties of bone at selected locations via direct mechanical contact. The granular crystal is composed of a tightly packed chain of particles that interact according to the Hertzian contact law. Such chains represent one of the simplest systems to generate and propagate highly nonlinear acoustic signals in the form of compact solitary waves. First, we investigated the sensitivity of the sensor to known variations in bone density using a synthetic cancellous bone substitute, representing clinical bone quality ranging from healthy to osteoporotic. Once the relationship between the signal response and known bone properties was established, the sensor was used to assess the bone quality of ten human cadaveric specimens. The efficacy and accuracy of the sensor was then investigated by comparing the sensor measurements with the bone mineral density (BMD) obtained using dual-energy x-ray absorptiometry (DEXA). The results indicate that the proposed technique is capable of detecting differences in bone quality. The ability to measure site-specific properties without exposure to radiation has the potential to be further developed for clinical applications. [ABSTRACT FROM AUTHOR]

We present a mesoscopic approach to granular crystal dynamics, which comprises a three-dimensional finite-element model and a one-dimensional regularized contact model. The approach investigates the role of vibrational-energy trapping effects in the dynamic behavior of one-dimensional chains of particles in contact (i.e., granular crystals), under small to moderate impact velocities. The only inputs of the models are the geometry and the elastic material properties ofthe individual particles that form the system. We present detailed verification results and validate the model comparing its predictions with experimental data. This approach provides a physically sound, first-principles description of dissipative losses in granular systems. [ABSTRACT FROM AUTHOR]

We study the interaction of highly nonlinear solitary waves propagating in granular crystals with an adjacent linear elastic medium. We investigate the effects of interface dynamics on the reflection of incident waves and on the formation of primary and secondary reflected waves. Experimental tests are performed to correlate the linear medium geometry, materials, and mass with the formation and propagation of reflected waves. We compare the experimental results with theoretical analysis based on the long-wavelength approximation and with numerical predictions obtained from discrete particle models. Experimental results are found to be in agreement with theoretical analysis and numerical simulations. This preliminary study establishes the foundation for utilizing reflected solitary waves as novel information carriers in nondestructive evaluation of elastic material systems. [ABSTRACT FROM AUTHOR]

Stator insulation quality assessment is an important issue for pulsewidth modulation inverter-fed machines, since the stator insulation is exposed to increased thermal/electrical stresses. In this paper, a new approach for monitoring the condition of the stator insulation for inverter-fed machines is proposed. The main concept of the proposed approach is to apply various types of test voltages to the motor stator insulation using the inverter, to perform standard offline insulation tests whenever the motor is not operating. Two methods for testing the insulation with modifications to the inverter circuit are proposed and verified experimentally. The results show that the proposed technique provides a simple low cost solution for detecting stator insulation degradation at an early stage for reliable operation of inverter-fed machine systems, since the insulation condition can be monitored frequently. This not only helps prevent forced outages and safety risks due to insulation breakdown, but also helps perform maintenance in an efficient manner since individual motor maintenance can be prioritized and scheduled based on its present condition. [ABSTRACT FROM AUTHOR]

An online technique for monitoring the condition of the groundwall (GW) and phase-to-phase (PP) insulation for three-phase ac-machine stator windings is proposed in this paper. Insulation-condition indicators such as capacitance, dissipation factor (tan δ), and ac insulation resistance are calculated online based on the differential leakage-current measurements for each phase. A model for a three-phase ac-machine insulation system is derived and analyzed for both offline and online testing. Guidelines for interpreting the measured indicators for determining the: 1) overall condition and 2) significant degradation in the individual GW and PP insulation for each phase of the stator are given based on the analysis. Experimental results on a 15-hp induction motor under intentional GW and PP insulation degradation conditions show that incipient insulation degradation can be detected and classified with high sensitivity. [ABSTRACT FROM AUTHOR]

We present a resistive pressure sensor based on the thin film printed via melt electrospinning of polyether block amide (PEBA). This thin film is created by arranging the electrospun cylindrical fibers periodically into a narrow vertical wall. We coat this film using poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and place it between top and bottom counter electrodes for the pressure sensor. With the application of pressure, the variation of the contact area between the electrode and the cylindrical surface of the film produces a sensitive current response for detecting a wide range of pressure with tunable sensitivities. We present the optimized process conditions of the melt electrospinning for the construction of the micro structured thin film. We also develop theoretical model based on the classical contact theory to characterize and predict the sensor responses. Finally, a multi-touch interface for a mobile device is demonstrated based on the described methods. The reliable, cost effective, and scalable characteristics of the presented method offer great promise for mobile or wearable applications. [ABSTRACT FROM AUTHOR]

We experimentally investigate a diagnostic technique for identifying a weak bond in composites using highly nonlinear solitary waves (HNSWs). We set up a one-dimensional chain of granular crystals, consisting of spherical particles with nonlinear interactions, to generate HNSWs. These solitary wave packets are transmitted into an inspection area of composites by making a direct contact with the chain. We demonstrate that a strong type of solitary waves injected to the weak bond area can break the weak bond of laminates, thereby causing delamination. Then, to identify the creation of the delamination, we transmit a weak type of solitary waves by employing the same apparatus, and measure the solitary waves reflected from the specimens. By analyzing these reflected solitary waves, we differentiate the weak bond samples with the pristine bond ones in an efficient and fast manner. The diagnostic results based on the proposed method are compared with the strength and energy release rate at bond interfaces, which are measured via standard testing methods such as three point bending and end notched flexure tests. This study shows the potential of solitary wave-based detection of weak bonds for hot spot monitoring of composite-based structures. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the feasibility of visualizing stress waves propagating in plates using air-coupled acoustic emission sensors. Specifically, we employ a device that embeds arrays of microphones around an optical lens in a helical pattern. By implementing a beamforming technique, this remote sensing system allows us to record wave propagation events in situ via a single-shot and full-field measurement. This is a significant improvement over the conventional wave propagation tracking approaches based on laser doppler vibrometry or digital image correlation techniques. In this paper, we focus on demonstrating the feasibility and efficacy of this air-coupled acoustic emission technique by using large metallic plates exposed to external impacts. The visualization results of stress wave propagation will be shown under various impact scenarios. The proposed technique can be used to characterize and localize damage by detecting the attenuation, reflection, and scattering of stress waves that occurs at damage locations. This can ultimately lead to the development of new structural health monitoring and nondestructive evaluation methods for identifying hidden cracks or delaminations in metallic or composite plate structures, simultaneously negating the need for mounted contact sensors. [ABSTRACT FROM AUTHOR]

We experimentally and numerically investigate a diagnostic method for detecting hidden delamination in composite panels, using highly nonlinear solitary waves. Solitary waves are a type of nonlinear waves with strong energy intensity and non-distortive nature, which can be controllably generated in one-dimensional granular crystals. In this study, we use granular crystals as a combined sensor and actuator to detect hidden delamination in carbon fiber reinforced polymer (CFRP) composite panels. Specifically, we locally excite a CFRP composite specimen using the granular crystal as an actuator and measure the reflected waves that carry the specimen’s diagnostic information using the same device as a sensor. We first investigate the effect of the panel’s boundary conditions on the response of the reflected solitary waves. We then investigate the interactions of a solitary wave with delamination hidden in the CFRP composite specimen. Lastly, we define a damage index based on the solitary waves’ responses to identify the location of the hidden delamination in the CFRP composite panel. The solitary wave-based diagnostic method can provide unique merits, such as portable and fast sensing of composites’ hidden damage, thereby with the potential of being used for hot spot monitoring of composite-based structures. [ABSTRACT FROM AUTHOR]

A new diagnostic method to identify the location and magnitude of external impact on a sandwich structure using granular crystal sensors was studied. The granular crystal sensors are composed of one-dimensional chains of spherical particles that are inserted in a thick core of the sandwich structure. Given an external impact, the embedded sensors generate compact-supported, highly nonlinear solitary waves resulting from the dispersive and nonlinear characteristics of granular crystals. In this study, the propagating mechanism of highly nonlinear solitary waves in relation to various impact conditions was investigated. Particularly, it was reported that the flight time and magnitude of solitary waves are highly sensitive to the location and amplitude of impact. By analyzing measured solitary waves, the striker's impact location and drop height was successfully predicted nondestructively. It was found that the diagnostic results are in agreement with the numerical simulations obtained from a combined spectral element and discrete element model. The findings in this study imply that the granular crystal sensors can form a new type of impact monitoring system for a sandwich structure to achieve enhanced diagnostic accuracy and potentially to improve sensors' survivability under harsh mechanical and thermal environments. [ABSTRACT FROM AUTHOR]

We present a gradient-index crystal that offers extreme tunability in terms of manipulating the propagation of elastic waves. For small-amplitude excitations, we achieve control over wave transmission depth into the crystal. We numerically and experimentally demonstrate a boomeranglike motion of a wave packet injected into the crystal. For large-amplitude excitations on the same crystal, we invoke nonlinear effects. We numerically and experimentally demonstrate asymmetric wave transmission from two opposite ends of the crystal. Such tunable systems can thus inspire a novel class of designed materials to control linear and nonlinear elastic wave propagation in multiscales. [ABSTRACT FROM AUTHOR]

In this study, we investigate the enhancement of mechanical properties that shallow biangle, thin-ply laminates bring to fiber reinforced polymer composites. Coupon- and structural-level tests are conducted along with numerical simulations. According to the coupon tests, we find that shallow biangle fibers and thin plies can increase the axial stiffness of laminates significantly at the cost of a relatively small decrease in their transverse and shear moduli. For the structural tests, we fabricate composite wing structures using an out-of-autoclave vacuum-assisted resin transfer molding process. By conducting static bending tests, we show superior structural performances of the wing structure that employs shallow biangle fibers and thin-ply fabric, compared to those that use conventional fiber-angle and thick plies. Shallow biangle and thin-ply technologies can open new routes to designing composite structures with improved stiffness and strength with fast and cost-effective fabrication processes. [ABSTRACT FROM AUTHOR]

We study scattering of waves by impurities in strongly precompressed granular chains. We explore the linear scattering of plane waves and identify a closed-form expression for the reflection and transmission coefficients for the scattering of the waves from both a single impurity and a double impurity. For single-impurity chains, we show that, within the transmission band of the host granular chain, high-frequency waves are strongly attenuated (such that the transmission coefficient vanishes as the wavenumber k→±π), whereas low-frequency waves are well-transmitted through the impurity. For double-impurity chains, we identify a resonance--enabling full transmission at a particular frequency--in a manner that is analogous to the Ramsauer-Townsend (RT) resonance from quantum physics. We also demonstrate that one can tune the frequency of the RT resonance to any value in the pass band of the host chain. We corroborate our theoretical predictions both numerically and experimentally, and we directly observe almost complete transmission for frequencies close to the RT resonance frequency. Finally, we show how this RT resonance can lead to the existence of reflectionless modes in granular chains (including disordered ones) with multiple double impurities. [ABSTRACT FROM AUTHOR]