Menglong Liu

A coatable, light-weight, fast-response nanocomposite sensor for the in situ acquisition of dynamic elastic disturbance: from structural vibration to ultrasonic waves

Inspired by an innovative sensing philosophy, a light-weight nanocomposite sensor made of a hybrid of carbon black (CB)/polyvinylidene fluoride (PVDF) has been developed. The nanoscalar architecture and percolation characteristics of the hybrid were optimized in order to fulfil the in situ acquisition of dynamic elastic disturbance from low-frequency vibration to high-frequency ultrasonic waves. Dynamic particulate motion induced by elastic disturbance modulates the infrastructure of the CB conductive network in the sensor, with the introduction of the tunneling effect, leading to dynamic alteration in the piezoresistivity measured by the sensor. Electrical analysis, morphological characterization, and static/dynamic electromechanical response interrogation were implemented to advance our insight into the sensing mechanism of the sensor, and meanwhile facilitate understanding of the optimal percolation threshold. At the optimal threshold (~6.5 wt%), the sensor exhibits high fidelity, a fast response, and high sensitivity to ultrafast elastic disturbance (in an ultrasonic regime up to 400 kHz), yet with an ultralow magnitude (on the order of micrometers). The performance of the sensor was evaluated against a conventional strain gauge and piezoelectric transducer, showing excellent coincidence, yet a much greater gauge factor and frequency-independent piezoresistive behavior. Coatable on a structure and deployable in a large quantity to form a dense sensor network, this nanocomposite sensor has blazed a trail for implementing in situ sensing for vibration- or ultrasonic-wave-based structural health monitoring, by striking a compromise between sensing cost and sensing effectiveness.

Characterizing Hypervelocity Impact (HVI)-Induced Pitting Damage Using Active Guided Ultrasonic Waves: From Linear to Nonlinear

Hypervelocity impact (HVI), ubiquitous in low Earth orbit with an impacting velocity in excess of 1 km/s, poses an immense threat to the safety of orbiting spacecraft. Upon penetration of the outer shielding layer of a typical two-layer shielding system, the shattered projectile, together with the jetted materials of the outer shielding material, subsequently impinge the inner shielding layer, to which pitting damage is introduced. The pitting damage includes numerous craters and cracks disorderedly scattered over a wide region. Targeting the quantitative evaluation of this sort of damage (multitudinous damage within a singular inspection region), a characterization strategy, associating linear with nonlinear features of guided ultrasonic waves, is developed. Linear-wise, changes in the signal features in the time domain (e.g., time-of-flight and energy dissipation) are extracted, for detecting gross damage whose characteristic dimensions are comparable to the wavelength of the probing wave; nonlinear-wise, changes in the signal features in the frequency domain (e.g., second harmonic generation), which are proven to be more sensitive than their linear counterparts to small-scale damage, are explored to characterize HVI-induced pitting damage scattered in the inner layer. A numerical simulation, supplemented with experimental validation, quantitatively reveals the accumulation of nonlinearity of the guided waves when the waves traverse the pitting damage, based on which linear and nonlinear damage indices are proposed. A path-based rapid imaging algorithm, in conjunction with the use of the developed linear and nonlinear indices, is developed, whereby the HVI-induced pitting damage is characterized in images in terms of the probability of occurrence.

Applications of a nanocomposite-inspired in-situ broadband ultrasonic sensor to acousto-ultrasonics-based passive and active structural health monitoring

A novel nanocomposite-inspired in-situ broadband ultrasonic sensor previously developed, with carbon black as the nanofiller and polyvinylidene fluoride as the matrix, was networked for acousto-ultrasonic wave-based passive and active structural health monitoring (SHM). Being lightweight and small, this kind of sensor was proven to be capable of perceiving strain perturbation in virtue of the tunneling effect in the formed nanofiller conductive network when acousto-ultrasonic waves traverse the sensor. Proof-of-concept validation was implemented, to examine the sensor performance in responding to acousto-ultrasonic waves in a broad frequency regime: from acoustic emission (AE) of lower frequencies to guided ultrasonic waves (GUWs) of higher frequencies. Results have demonstrated the high fidelity, ultrafast response and high sensitivity of the sensor to acousto-ultrasonic waves up to 400kHz yet with an ultra-low magnitude (of the order of micro-strain). The sensor is proven to possess sensitivity and accuracy comparable with commercial piezoelectric ultrasonic transducers, whereas with greater flexibility in accommodating curved structural surfaces. Application paradigms of using the sensor for damage evaluation have spotlighted the capability of the sensor in compromising sensing cost with sensing effectiveness for passive AE- or active GUW-based SHM.

Analytical insight into “breathing” crack-induced acoustic nonlinearity with an application to quantitative evaluation of contact cracks

To characterize fatigue cracks, in the undersized stage in particular, preferably in a quantitative and precise manner, a two-dimensional (2D) analytical model is developed for interpreting the modulation mechanism of a breathing crack on guided ultrasonic waves (GUWs). In conjunction with a modal decomposition method and a variational principle-based algorithm, the model is capable of analytically depicting the propagating and evanescent waves induced owing to the interaction of probing GUWs with a breathing crack, and further extracting linear and nonlinear wave features (e.g., reflection, transmission, mode conversion and contact acoustic nonlinearity (CAN)). With the model, a quantitative correlation between CAN embodied in acquired GUWs and crack parameters (e.g., location and severity) is obtained, whereby a set of damage indices is proposed via which the severity of the crack can be evaluated quantitatively. The evaluation, in principle, does not entail a benchmarking process against baseline signals. As validation, the results obtained from the analytical model are compared with those from finite element simulation, showing good consistency. This has demonstrated accuracy of the developed analytical model in interpreting contact crack-induced CAN, and spotlighted its application to quantitative evaluation of fatigue damage.

Interrogation of Linear/nonlinear Features of Guided Waves for Characterizing Hypervelocity Impact-induced Pitting Damage in Shielding Structures

Hypervelocity impact (HVI) is a scenario ubiquitous in low Earth orbit, where HVI is typified by the collision between meteoroids and orbital debris and spacecraft with a relative speed greater than 10 km/s. A linear/nonlinear guided-wave-based approach for characterizing HVI-induced damage in a two-layer aluminum shielding structure (comprising inner and outer layers) is developed. After penetrating the outer layer, the generated debris cloud further impacts the inner layer, producing a unique form of damage with multitudinous small-scale pitting. In this study, aluminum spheres are discharged using a two-stage light gas gun, at an impact speed of ~6 km/s, to introduce HVI to the outer shielding layer. Both linear/nonlinear features of guided waves propagating in the inner layer including various nonlinearity sources are investigated using finite element models, corroborated by experiment. With the models, the accumulation of nonlinear second harmonics (nonlinear features) in the case of phase matching is analyzed. Based on the numerical models and experimental discovery, linear/nonlinear indices are developed, via which a detection approach is developed, able to characterize HVI-induced pitting damage. In the approach, the second harmonics (nonlinear feature) show higher sensitivity to pitting damage compared to the fundamental wave (linear feature). Combining a path-based probability imaging algorithm with defined linear/nonlinear indices, this approach can identify HVI-induced damage to the spacecraft precisely and intuitively.

Modeling on Propagation of Shock Waves induced by Hypervelocity Impact (HVI) with Application to Evaluation of HVI Damage

A hypervelocity impact (HVI) of an aluminium sphere into an aluminium plate with a speed around 4000 m/s is numerically modeled and experimentally verified. Ubiquitous in outer space and significantly different from low velocity impact (LVI), HVI features transient, localized, and extreme material deformation in an adiabatic process, under which the induced shock waves present unique yet complex features. To numerically study this normal HVI phenomenon, a dedicated hybrid modeling combining the three-dimensional smooth-particle hydrodynamics (SPH) with the finite element analysis was developed, to gain an insight into characteristics of HVI-induced shock wave propagation. The effectiveness and accuracy of the modeling and simulation was demonstrated through quantitative coincidence in results between simulation and HVI experiment. Shock wave signal features on both time and frequency domain are analyzed intensively based on the theoretical model of HVI. Upon understanding the characteristics of HVI-induced shock waves, an acoustic emission (AE) based characterization strategy, targeting HVI-committed damage, was subsequently established using an enhanced delay-and-sum-based diagnostic imaging algorithm, and this strategy was validated by locating orbital debris-induced penetration in space structures, showing precise identification results. doi: 10.12783/SHM2015/193

On propagation of shock waves generated under hypervelocity impact (HVI) and application to characterizing orbital debris-induced damage in space vehicles

The propagation characteristics of shock waves generated under hypervelocity impact (HVI) (an impact velocity leading to the case that inertial forces outweigh the material strength, usually on the order over 1 km/s) and guided by plate-like structures were interrogated. A hybrid numerical modeling approach, based on the Smoothed-Particle Hydrodynamics (SPH) and Finite Element Method, was developed, to scrutinize HVI scenarios in which a series of aluminum plates, 1.5- mm, 3-mm and 5-mm in thickness, was considered to be impacted by an aluminum sphere, 3.2-mm in diameter, at an initial velocity of 3100 m/s, 3050 m/s and 2490 m/s, respectively. The meshless nature of SPH algorithm circumvented the inefficiency and inaccuracy in simulating large structural distortion associated with HVI when traditional finite element methods used. The particle density was particularly intensified in order to acquire wave components of higher frequencies. With the developed modeling approach, shock waves generated under concerned HVI scenarios were captured at representative gauging points, and the signals were examined in both time and frequency domains. The simulation results resembled those from earlier experiment, demonstrating a capability of the developed modeling approach in canvassing shock waves under HVI. It has been concluded that in the regions near the impact point, the shock waves propagate with higher velocities than bulk waves; as propagation distance increases, the waves slow down and can be described as fundamental and higher-order symmetric and anti-symmetric plate-guided wave modes, propagating at distinct velocities in different frequency bands. The results will facilitate detection of orbital debris-induced damage in space vehicles.