Optimal Sensor Placement for Damage Detection in Complex Structures

An optimal sensor placement methodology is proposed based on detection theory framework to maximize the detection rate and minimize the false alarm rate. Minimizing the false alarm rate for a given detection rate plays an important role in improving the efficiency of a Structural Health Monitoring (SHM) system as it reduces the number of false alarms. The placement technique is such that the sensor features are as directly correlated and as sensitive to damage as possible. The technique accounts for a number of factors, like actuation frequency and strength, minimum damage size, damage detection scheme, material damping, signal to noise ratio (SNR) and sensing radius. These factors are not independent and affect each other. Optimal sensor placement is done in two steps. First, a sensing radius, which can capture any detectable change caused by a perturbation and above a certain threshold, is calculated. This threshold value is based on Neyman-Pearson detector that maximizes the detection rate for a fixed false alarm rate. To avoid sensor redundancy, a criterion to minimize sensing region overlaps of neighboring sensors is defined. Based on the sensing region and the minimum overlap concept, number of sensors needed on a structural component is calculated. In the second step, a damage distribution pattern, known as probability of failure distribute, is calculated for a structural component using finite element analysis. This failure distribution helps in selecting the most sensitive sensors, thereby removing those making remote contributions to the overall detection scheme.

Membrane Modular Structures With Inflatable Tubes and Connective Cables for Future Space Applications

Spirally folded hexagonal membrane structures with inflatable tubes and connective cable networks are presented aiming to establish possible construction scenarios of future large space structure systems over hundreds meters scale and corresponding structures based on the hierarchical modular structure concept using deployable membrane modules. Laboratory scale hand-made conceptual models are manufactured, and their deployment experiments are carried out to show their applicability to the hierarchical modular structures systems.

Modeling IPMC Material With Dynamic Surface Characteristics

This paper presents the Finite Element Analysis (FEA) of an ionic polymer-metal composite (IPMC) material. The IPMC materials are known to bend when electric field is applied on the electrodes. The material also produces potential difference on the electrodes when is bent. Several authors have used the FEA to describe that fenomenon and rather precise basic Finite Element (FE) models already exist. Therefore the current study is mainly focused on the modeling of the electrodes of IPMC. The first goal of this research is to model the electric currents in the electrodes. The basis of the electric current calculations is the Ramo-Shockley theorem, which has been used in the other areas of physics to describe the currents in a circuit due to a charge movement in a media. We have used the theorem to calculate the current density in the continuous electrodes of IPMC due to the ion migration in the backbone polymer. Along the current densities we are able to calculate voltage on the electrode at a given time moment. The model is demonstrated to give some physically reasonable results. However, the model is rather complex and as the solution times are quite large, some possible optimizations have been considered as well. The second goal of this study is to include the dynamic resistance and capacitance of the electrodes in our model. Lot of research has been done to develop a physically reasonable capacitor-resistor model of an IPMC and the results have been promising. Furthermore, some authors have managed to develop partial differential equations (PDE) to describe the model. We try to include some simplified versions of those equations into our physical model. As the FE model for IPMC is nonlinear and gets complicated very fast when additional equations are added, the final sections of this paper briefly considers some novel optimization ideas in regard to modeling IPMC with FE method.

Developing a Piezoelectric Active Sensor SHM System for Satellites

Structural Health Monitoring (SHM) is a valuable tool for in-service assessment of structural condition. Despite a broad use in many engineering fields, SHM has seen limited application to space systems. The paper explores specifics of SHM applied to space systems and satellites in particular. It is suggested that SHM may be considered for aiding rapid assembly of satellite components, monitoring system dynamics during launch and assessing in-service variation of structural properties suitable for model updating. In this paper, we present a discussion of factors affecting realization of the SHM system for satellites and provide recommendations for the system configuration and its practical use. The SHM system design based on a network of piezoelectric active sensors is considered. Piezoelectric sensors were selected due to availability of both active and passive operation modes. The passive SHM mode may find applications related to spaceship launch process and on-orbit structural monitoring. It is anticipated that the active structural assessment may be exercised during satellite pre-launch qualification and possible on-orbit characterization. Hence, the present contribution focuses on SHM of improperly tightened bolts as one of major satellite integrity concerns and embedded material characterization techniques. The developed SHM method utilizes the acousto-elastic effect manifested through the elastic wave phase shift caused by stress-induced localized changes in the sound speed. Experiments aimed at improving fundamental understanding of this technique are discussed and applicability of the technique to realistic structures is investigated. The methodology for in-situ material characterization is tested on structural elements of simple geometry and extension to complex structural systems is suggested. Synergistic use of the same hardware for acoustoelastic and material characterization methods is recommended and further system integration options are proposed.

Wind-Tunnel Testing of Shape Memory Alloys Actuators as Morphing Wing Driving Systems

A morphing wing, composed of flexible extrados, rigid intrados and a Shape Memory Alloys (SMA) actuator group located inside the wing box, is used to adapt an airfoil profile to variable flight conditions. The SMA actuator group developed for the morphing wing prototype consists of three main subsystems: the SMA active element, the transmission system, and the passive bias element. The functional requirements for the actuator group were determined using a coupled fluid-structure model of the flexible extrados. An original design approach was applied to determine the geometry and assembly conditions of the SMA active elements. For validation purposes, the morphing wing powered by SMA actuators was tested in a wind tunnel under subsonic flight conditions (Mach = 0.2 to 0.3 and α = −1 to 2°). The ability of the actuator group to move the flexible extrados up to 8 mm of vertical displacement and to bring it back to the initial profile has been successfully proven for all of the wind tunnel testing conditions. During the repetitive actuation, the force, displacement and temperature of the SMA active elements were measured and the results obtained in the force-displacement-temperature space were used to validate the SMA performances predicted during the design phase.

Durable Biomolecular Assemblies for Protein-Powered Device Concepts

Physically encapsulated droplet-interface bilayers are formed by confining aqueous droplets surrounded by lipid mono-layers in connected compartments within a solid substrate. The droplets reside within each compartment and are positioned on fixed electrodes built into the solid substrate. Full encapsulation of the network is achieved with a solid cap that inserts into the substrate to form a closed volume. Encapsulated networks provide increased portability over unencapsulated networks by limiting droplet movement and by integrating the electrodes into the supporting fixture. The formation of encapsulated droplet-interface bilayers is confirmed with electrical impedance spectroscopy and cyclic voltammetry is also used to measure the effect of alamethicin proteins incorporated into the resulting lipid bilayers. The durability of the networks is quantified using a mechanical shaker to oscillate the bilayer in a direction transverse to the plane of the membrane and the results show that single droplet-interface bilayers can withstand several g’s of acceleration. Observed failure modes include both droplet separation and bilayer rupturing, where the geometry of the supporting substrate and the presence of electrodes are key contributors. Physically encapsulated DIBs can be shaken, moved, and inverted without bilayer failure, enabling the creation of portable, protein-powered devices.

Enhanced Electromechanical Properties in Poly(Vinylidene Fluoride) (PVDF)-Based Nanocomposites

Efforts to enhance the electromechanical properties of Poly(vinylidene fluoride) (PVDF) and its copolymers have been directed at optimizing the molecular chemistry, stretching and poling parameters. This study investigates an alternative approach to enhancing the properties via adding nanoinclusions in PVDF. We investigate the enhanced electrostrictive response in PVDF by adding Single Walled Carbon Nanotubes (SWNTs). We also show the change in the non-polar morphology of “as-is” PVDF to the polar γ phase by adding SWNTs and eventually to the piezoelectric β phase by stretching the nanocomposites.

Preliminary In Vivo Experimental Validation of SMA Based Bowel Extender for Short Bowel Syndrome

Short Bowel Syndrome is a serious medical condition caused by insufficient small bowel length resulting in significantly high rates of morbidity and mortality. The limited success of current therapies has prompted the investigation of a new treatment approach based on mechanotransduction — the process through which mechanical tensile loading on the bowel induces longitudinal growth. To enable clinically relevant mechanotransduction growth studies in large animals, such as pigs, a fully implantable and instrumented bowel extender device based on a Shape Memory Alloy (SMA) ratchet was developed and validated in benchtop and ex vivo tests. These devices, however, must also be validated against the unique in vivo environment which presents challenges such as sealing, battery life, surgical implantation, signal attenuation from tissue, and isolating the measurement of tensile loading on the bowel wall. This paper extends the earlier development work to in vivo validation experiments within live pigs. A brief summary of the bowel extender architecture and operation is provided along with earlier ex vivo results that established device limits for in vivo testing. The wireless communication rate was updated to extend battery life and new surgical implantation procedures and lengthening schemes were developed. Two bowel extenders were tested in in vivo experiments ranging from 2.5 to 4.5 days with data collected to validate the wireless communication, SMA ratcheting and load/displacement measurements, confirming that the bowel extender successfully operates in vivo. More importantly, the bowel extenders successfully induced significant growth, which is promising for future studies comparing different lengthening schemes for optimal growth and the development of a clinical device for treating short bowel syndrome in humans.

Development of an Active Squeeze Film Journal Bearing Using High Power Ultrasonic Transducers

A novel active squeeze film journal bearing actuated by high power piezoelectric transducers is developed aiming for non-contact suspension of axial rotating member with active error compensation and active axis positioning. A mathematical model based on acoustic radiation pressure theory is developed to predict the levitation force of the proposed bearing system. The levitation force model is then integrated into the model of the electro-mechanical system to describe the total dynamic behavior of the bearing system. Experimental results are carried out using a prototype system, which show good agreement with the calculation.

Dispersion of Carbon Nanotubes in Cement-Based Composites and Its Influence on the Piezoresistivities of Composites

Sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (NaDDBS) are used as surfactants to improve the dispersion of multi-walled carbon nanotubes (MWNTs) in cement mortar and fabricate piezoresistive carbon-nanotube/cement mortar composite. The piezoresistivity of carbon-nanotube/cement mortar composite with different content levels of MWNTs and different surfactants were explored under repeated loading and impulsive loading. Experimental results indicate that NaDDBS has higher efficiency than SDS for the dispersion of MWNTs in cement mortar. The response of the electrical resistance of carbon-nanotube/cement mortar composite with NaDDBS to external force is more stable and sensitive than that of carbon-nanotube/cement mortar composite with SDS. These findings indicate that the use of NaDDBS is an effective way for improving the dispersion of MWNTs in cement-based composite and fabricating MWNTs filled cement-based composite with stable and strong piezoresistive response.