Reliable operation of next generation high-speed complex structures (e.g. hypersonic air vehicles, space structures, and weapons) relies on the development of microsecond structural health monitoring (μSHM) systems. High amplitude impacts may damage or alter the structure, and therefore change the underlying system configuration and the dynamic response of these systems. While state-of-the-art structural health monitoring (SHM) systems can measure structures which change on the order of seconds to minutes, there are no real-time methods for detection and characterization of damage in the microsecond timescales. This paper presents preliminary analysis addressing the need for microsecond detection of state and parameter changes. A background of current SHM methods is presented, and the need for high rate, adaptive state estimators is illustrated. Example observers are tested on simulations of a two-degree of freedom system with a nonlinear, time-varying stiffness coupling the two masses. These results illustrate some of the challenges facing high speed damage detection.

The primary goal of the Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) is to enable the design of revolutionary applications based on shape memory alloy (SMA) technology. In order to help realize this goal and reduce the development time and required experience for the fabrication of SMA actuation systems, several modeling tools have been developed for common actuator types and are discussed herein along with case studies, which highlight the capabilities and limitations of these tools. Due to their ability to sustain high stresses and recover large deformations, SMAs have many potential applications as reliable, lightweight, solid-state actuators. Their advantage over classical actuators can also be further improved when the actuator geometry is modified to fit the specific application. In this paper, three common actuator designs are studied: wires, which are lightweight, low-profile, and easily implemented; springs, which offer actuation strokes upwards of 200% at reduced mechanical loads; and torque tubes, which can provide large actuation forces in small volumes and develop a repeatable zero-load actuation response (known as the two-way shape memory effect). The modeling frameworks, which have been implemented in the design tools, are developed for each of these frequently used SMA actuator types. In order to demonstrate the versatility and flexibility of the presented design tools, as well as validate their modeling framework, several design challenges were completed. These case studies include the design and development of an active hinge for the deployment of a solar array or foldable space structure, an adaptive solar array deployment and positioning system, a passive air temperature controller for the regulation of flow temperatures inside of a jet engine, and a redesign of the Corvette active hatch, which allows for pressure equalization of the car interior. For each of the presented case studies, a prototype or proof-of-concept was fabricated and the experimental results and lessons learned are discussed. This analysis presents a collection of CASMART collaborative best practices in order to allow readers to utilize the available design tools and understand their modeling principles. These design tools, which are based on engineering models, can provide first-order optimal designs and are a basic and efficient method for either demonstrating design feasibility or refining design parameters. Although the design and integration of an SMA-based actuation system always requires application- and environment-specific engineering considerations, common modeling tools can significantly reduce the investment required for actuation system development and provide valuable engineering insight.

EAP based actuator technologies are extensively studied to design smart/intelligent systems ranging from deployable space structures, morphing wings, to medical devices and artificial muscles. Despite the extensive research on electroactive polymers (EAP), practical implementation of this technology is slow because of low induced forces and defect-driven premature electrical breakdown. Multilayered or stacked configuration can address the low induced force issue. However, construction procedure of multilayered sample is susceptible to more defects, which can further aggravate defect-driven premature breakdown of EAP actuators. Reducing the number of defects using self-clearing concept can improve the EAP actuators’ ability to withstand high electric fields. Self-clearing refers to the partial local breakdown of dielectric medium due to the presence of impurities, which in turn results in the evaporation of some of the metalized electrodes. After this evaporation, the impurity is cleared and any current path would be safely cut off, which means the actuator continues to perform, albeit with a reduced actuation area due to electrode evaporation. In this paper we study the impact of self-clearing metalized silver electrodes on the electrical and electromechanical behavior of EAPs, more specifically P(VDF-TrFE-CTFE) terpolymer. First, we use Weibull statistics to systematically estimate the self-clearing/preconditioning field needed to clear the defects. Then electrical breakdown experiments are conducted with and without preconditioning the samples to investigate their effects on the breakdown strength of the EAP. Finally, we implement this self-clearing/preconditioning field on single and multilayered P (VDF-TrFE-CTFE) unimorph actuators and investigate the resulting electromechanical performance. Due to preconditioning of the actuators using self-clearing concept, the actuators endure higher electric fields compared to a control sample. Loss of capacitance occurs during self-clearing, which in turn affects the electromechanical performance of the actuator. For that reason, we also report on the blocked force of preconditioned and controlled actuators to evaluate and compare their electromechanical performance.

Inflatable deployable structures are practical and promising candidates for serving various aerospace missions, for instance, as solar sails, antennas, space suits, and especially Lunar and Mars habitats. These structures feature flexible composites folded at high packing efficiency, which can drastically reduce launch costs. However, they can also be damaged due to the harsh extraterrestrial operating conditions, which can propagate to cause catastrophic mission failure and endanger crew safety. Therefore, it is imperative to integrate a robust structural health monitoring (SHM) system, so that damage and faults can be detected for ensuring their safe and reliable operations. While a variety of SHM technologies have been developed for monitoring conventional, rigid, structural systems, they are faced with challenges when used for these unconventional flexible and inflatable systems. Therefore, a flexible carbon nanotube-fabric nanocomposite sensor is proposed in this study for monitoring the integrity of inflatable space structures. In particular, CNT-based thin films were fabricated by spraying and then integrated with flexible fabric to form the lightweight sensor. By coupling fabric sensors with an electrical impedance tomography (EIT) algorithm, the fabric’s distribution of spatial resistivity can be mapped using only electrical measurements obtained along the material’s boundaries. The severity and location of localized pressure and impact damage can be captured by observing changes in the EIT-calculated resistivity maps. They can be embedded in inflatable habitat structures to detect and locate abnormally high pressure regions and impact damage.

In the community of structural health monitoring (SHM) and damage detection, the majority of previous research efforts has been focused on the detection of the reduction in cross-sectional areas of structural components or connections. However, the detection of the loss of stability, which can easily result in a structural failure, has been lacking. The objective of this study is to identify the occurrence of instability in civil space structures, such as domes, shells and reticulated shell structures. For this type of structure, nodal snap-through instability occurs, where a node and connected members jump to a new equilibrium position, leading the geometric shape/pattern in a local area to change significantly. In this study, we propose to use tilting angles of members to identify the nodal snap-through instability of civil space structures. Tilting angles of members actually reflect the change in geometric shape caused by instability. The effectiveness of this approach will be validated on numerical simulations of a reticulated shell structure. This research will bridge the gap between SHM and structural stability research and will make important contributions to the science of both SHM and structural instability.

Reusable and on-orbit space structures endure harsh operating conditions and can sustain damage due to impact by micrometeoroids and orbital debris. Thus, it is necessary to develop and implement suitable sensing technologies for monitoring space structure performance, detecting damage, and preventing catastrophic failure from occurring. The challenge however is to tailor these sensors for operating in such unique environments and constraints. One such limitation is the amount of energy available for powering onboard sensing systems. Thus, the objective of this study is to design and characterize the properties of a novel self-sensing photoactive thin film. Self-sensing is encoded by designing the films to generate a photocurrent in response to illuminated light so that no electrical energy is needed for powering the sensor, and photocurrent generated varies with applied strain. First, the self-sensing thin films were fabricated using poly(3-hexylthiophene) (P3HT) and double-walled carbon nanotubes (DWNT). Two different sets of films, ones with and without DWNTs, were fabricated. Second, photocurrent generation was validated. Then, tensile tests were conducted for characterizing their strain sensing performance. Lastly, current-voltage measurements were also obtained for characterizing thin film shunt and series resistance relationships to applied strains. The results showed that photocurrent varied linearly with applied tensile strains, and this was mainly due to the effects of P3HT alignment and shunt resistance changes of the photoactive thin films.

This paper presents an investigation of predictive power and energy modeling of space structures for structural health monitoring (SHM) with piezoelectric wafer active sensors (PWAS). After a review of PWAS principles, the paper developed the multi-physics modeling of pitch-catch PWAS transfer function and discusses and the power and energy transduction between structurally guided Lamb waves and PWAS. The focus is a power and energy transduction analysis between the PWAS and a structure containing multimodal ultrasonic guided Lamb waves. The use of multimodal Lamb waves solution for power modeling is an extension of our previously presented simplified model that considered axial and flexural waves with low frequency approximation. Comparison between the axial and flexural approach and Lamb waves approach was evaluated. Frequency response functions are developed for voltage, current, complex power, active power, etc. The paper ends with summary, conclusion, and suggestion for further work.

Cylindrical shells, very commonly used in aerospace applications, are susceptible to buckling when subjected to static and dynamic or transient loads. Bucking load enhancement with minimum weight addition is an important requirement in space structures. Buckling control of space structures using piezoelectric actuators is an emerging area of research. The earlier work on enhancement of buckling load on columns reported a 3.8 times enhancement theoretically and 123% experimentally [1–2]. The enhancement was (25%) when buckling control was implemented on plates [3] using PZT actuators. Buckling control of cylindrical shells is challenging because of the uncertainties in the location of buckling and the coupling between bending and membrane action. Earlier attempt to improve the buckling load carrying capacity of the cylindrical shell did not result in a considerable increase in the buckling load [4]. This is because the buckling modes of cylindrical shell are very close to each other when compared to structures like column and plate. An optimized actuator location is hence necessary to improve the load carrying capacity of the cylindrical shells. Unlike vibration control problems where the actuators locations are optimized to minimize the structural Volume Displacement (SVD) or to maximize the energy dissipation, buckling control is aimed at controlling the critical modes of buckling and hence improving the load carrying capacity of the shells [5]. Numerical analyses are carried out, comparing different configurations used in buckling control of thin shells. Experiments are performed to support the numerical analysis as the behavior of cylindrical shells under axial compression is highly sensitive to geometric imperfections. Load – Axial shortening graphs are used to compare the performance of cylindrical shell for the various actuator configurations.

The subject of origami design is garnering increased attention from the science, mathematics, and engineering communities. However, relatively little research exists on understanding the behavioral aspects of the material system undergoing the folding operations. This work considers the design and analysis of a novel concept for a self-folding structure. It consists of an active, self-morphing laminate that includes thermally actuated shape memory alloy (SMA) layers and a compliant passive layer. Multiple layers allow folds in both the positive and negative directions relative to the laminate normal. The layers are configured to allow continuously variable folding operations based only on which regions are heated. For the purposes of demonstration, an example problem is considered whereby a thin structure is designed that can be stored in a flat sheet configuration and then morph using sets of folds toward two distinct shapes. We examine the effects of fold width, layer thicknesses, and activation power history on the geometric configurations that can be obtained. The design efforts are supported by a comprehensive and accurate three-dimensional constitutive model for SMAs implemented into a finite element analysis (FEA) framework. Shell elements and laminate theory are used to increase the computational efficiency of the analysis. Discussion of the complex effects of active folding in an SMA laminate sheet with in-plane homogeneity, including transient effects, are discussed.

This paper presents an investigation of predictive modeling of space structures for structural health monitoring (SHM) with piezoelectric wafer active sensors (PWAS) transducers. The development of a suitable SHM system for complex space structure is not trivial; creating a robust SHM capability requires at least: (a) flexible accommodation of numerous configurations; (b) detection of damage in complex multifunctional structures; (c) identification if mechanical interfaces are properly connected. To realize this, we propose a predictive modeling approach using both analytical tools and finite element method (FEM) to study the health status of the structure, the power and energy transduction between the structure and the PWAS. After a review of PWAS principles, the paper discusses the modeling and the power and energy transduction between structurally guided waves and PWAS. The use of guided wave (GW) and the capability of embedded PWAS to perform in situ nondestructive evaluation (NDE) are explored. FEM codes are used to simulate GW of 2D and 3D space structure using the commercials software ABAQUS. PWAS transducers placement at different location on a flat plate and on an isogrid panel was simulated. The signal scattered by a crack emerging from the hole is simulated. Predictive modeling of power and energy transduction is discussed using an analytical approach. This model of 2-D power and energy transduction of PWAS attached to structure allows examination of power and energy flow for a circular crested wave pattern. Wave propagation method for an infinite boundary plate, electromechanical energy transformation of PWAS and structure, and wave propagation energy spread out in 2-D plate are considered. The parametric study of PWAS size, impedance match gives the PWAS design guideline for PWAS sensing and power harvesting applications.

The use of elastic wave based Structural Health Monitoring has shown its usefulness in both characterizing and diagnosing composite structures. Techniques using elastic wave SHM are being developed to allow for improved efficiency and assurance in all stages of space structure development and deployment. These techniques utilize precise understanding of wave propagation characteristics to extract meaningful information regarding the health and validity of a component, assembly, or structure. However, many of these techniques focus on the diagnostic of traditional, isotropic materials, and questions remain as to the effect of the orthotropic properties of resin matrix composite material on the propagation of elastic waves. As the demands and expectations placed upon composite structures continue to expand in the space community, these questions must be addressed to allow the development of elastic wave based SHM techniques that will enable advancements in areas such as automated build validation and qualification, and in-situ characterization and evaluation of increasingly complex space structures. This study attempts to aid this development by examines the effect of cross ply, off-axis fiber orientation on the propagation characteristics of lamb waves. This is achieved by observing the result of symmetric and anti-symmetric wave propagation across materials in cases containing both off-axis and axially-aligned elements. In both cases the surface plies of the test specimen are axially aligned with the wave propagation direction. Using these results, the relative effect of core ply orientation on lamb wave propagation, and lamb wave sensitivity to bulk properties, or alternatively, the dominance of surface properties on propagation characteristics, can be seen, and this information can be used to aid in future research and application of lamb waves for interrogation of advanced, high-strain composite space structures. It was found that the core orientation caused significant variation in the S 0 wave velocity, while yielding little influence on the A 0 wave velocity.

Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive enablers for a large class of structural health monitoring (SHM) applications. This paper presents and discusses the challenges and opportunities related to the use of PWAS in the structures specific to space applications. The challenges posed by space structures are often different from those encountered in conventional structures. After a review of PWAS principles, the paper discusses the multi-physics power and energy transduction between structurally guided waves and PWAS; predictive modeling results using a simplified analytical approach are presented. Experimental results on space-like specimen structures are presented. Survivability of PWAS transducers under cryogenic space-like conditions are experimentally verified. The paper ends with conclusions and suggestions for further work.

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.

Dimensional stability of the space structures, such as large telescope mirrors or metering substructures, is very important because even extremely small deformations of these structures might degrade the optical performances. Therefore, precise deformation data of the space structures according to environment change are required to design these structures correctly. Also, real-time deformation monitoring of these structures in space environment is demanded to verify whether these structures are properly designed or manufactured. FBG (fiber Bragg grating) sensors are applicable to real time monitoring of the space structure because they can be embedded onto the structures with minimal weight penalty. In this research, therefore, thermal deformation measurement system for the space structures, composed of FBG sensors for real time strain measurement and DMI (displacement measuring interferometers) for accurate specimen expansion data acquisition, is developed. Thermal strains measured by distributed FBG sensors are evaluated by the comparison with the strains obtained by highly accurate DMI.