Multistable morphing structures have gained considerable attention over the past three decades due to their ability to efficiently work in various operating conditions. Unsymmetrical laminates, which yields two stable equilibrium shapes due to the residual thermal stresses, are widely investigated to produce multistable structures. Even though cured shapes of unsymmetrical laminates are promising candidates for the design of morphing structural components, they may not fulfil all the requirements of a continuous morphing structure requiring more than two stable states during the morphing action. As a solution to achieve more than two stable configurations, series-connected unsymmetrical laminates are proposed in the present analysis. Connecting different bistable laminates results in multiple stable configurations at the end of the curing process, which is often desired in continuous shape-changing applications. The snap-through process involving shape transition between the generated stable shapes is highly nonlinear in nature. Since morphing aerospace structures are often subjected to severe dynamic excitation, large-amplitude nonlinear vibrations are inevitable during the snap-through transition. This work aims to explore the dynamic characteristic of multistable continuous composite plates generated by connecting bistable laminates without any external fixing aids. The proposed numerical analysis is carried out within a commercially available finite element package, ABAQUS. The effect of aspect ratios, laminate layups and actuation loads on the dynamic characteristic (natural frequencies and mode shapes) are reported from systematic parametric studies. The proposed model is further extended to predict the nonlinear characteristics of single well vibration and cross well vibration.

Morphing airfoils present an effective approach to managing the different requirements in each segment of a mission profile (e.g., takeoff/landing, cruise, and active maneuvering). In this work, an approach to morphing airfoil design that couples aerodynamic performance and internal structural configuration is detailed. The internal structural topology is formulated using a Lindenmayer System (L-System) coupled with a graph-based interpreter known as Spatial Interpretation for Development of Reconfigurable Structures (SPIDRS). The L-System encodes design variables that are interpreted via SPIDRS graphical operations and governs the development of the internal configuration (composed of elastic structural members and actuators). The global optimization uses a weakly coupled fluid-structure interaction (FSI) scheme for a first-order estimation of the aeroelastic loads that are critical for airfoil aerodynamic performance and structural integrity. Each airfoil is evaluated in two states: a standard non-actuated state to determine performance in standard operating conditions (e.g., cruise) and a high lift state, where internal shape memory alloy actuators are deformed to create a high lift configuration for the airfoil (e.g., takeoff/landing). Evaluating the aerodynamic performance of airfoils in these two states results in a series of potential solutions that best manage the tradeoff between aerodynamic metrics for both evaluated cases.

Dispersion relations describe the frequency-dependent nature of elastic waves propagating in structures. Experimental determination of dispersion relations of structural components, such as the floor of a building, can be a tedious task, due to material inhomogeneity, complex boundary conditions, and the physical dimensions of the structure under test. In this work, data-driven modeling techniques are utilized to reconstruct dispersion relations over a predetermined frequency range. The feasibility of this approach is demonstrated on a one-dimensional beam where an exact solution of the dispersion relations is attainable. Frequency response functions of the beam are obtained numerically over the frequency range of 0–50kHz. Data-driven dynamical model, constructed by the vector fitting approach, is then deployed to develop a state-space model based on the simulated frequency response functions at 16 locations along the beam. This model is then utilized to construct dispersion relations of the structure through a series of numerical simulations. The techniques discussed in this paper are especially beneficial to such scenarios where it is neither possible to find analytical solutions to wave equations, nor it is feasible to measure dispersion curves experimentally. In the present work, actual experimental data is left for future work, but the complete framework is presented here.

This paper presents a motor driven wrist brace that can adjust its stiffness by changing its mesoscale geometry. The design involves a plate structure that folds from a flexible flat shape to a stiff corrugated shape by means of a motor driven tendon. The structure is built using a laminate of rigid and flexible layers, with embedded flexural hinges that allow it to fold. The paper proposes a simplified analytical model to predict stiffness, and physical three-point bending tests indicate that the brace can increase its stiffness up to fifty times by folding.

Knitted Textiles made from Nickel-Titanium (NiTi) shape memory alloy wires are a new structural element with enhanced properties for a variety of applications. Potential advantages of this structural form include enhanced bending flexibility, tailorable in-plane, and through-thickness mechanical performance, and energy absorption and damping. Inspection of the knit pattern reveals a repeating cell structure of interlocking loops. Because of this repeating structure, knits can be evaluated as cellular structures that leverage their loop-based architecture for mechanical robustness and flexibility. The flexibility and robustness of the structure can be further enhanced by manufacturing with superelastic NiTi. The stiffness of superelastic NiTi, however, makes traditional knit manufacturing techniques inadequate, so knit manufacturing in this research is aided by shape setting the superelastic wire to a predefined pattern mimicking the natural curve of a strand within a knit fabric. This predefined shape-set geometry determines the outcome of the knit’s mechanical performance and tunes the mechanical properties. In this research, the impact of the shape setting process on the material itself is explored through axial loading tests to quantify the effect that heat treatment has on a knit sample. A means of continuously shape setting and feeding the wire into traditional knitting machines is described. These processes lend themselves to mass production and build upon previous textile manufacturing technologies. This research also proposes an empirical exploration of superelastic NiTi knit mechanical performance and several new techniques for manufacturing such knits with adjustable knit parameters. Displacement-controlled axial loading tests in the vertical (wale) direction determined the recoverability of each knit sample in the research and were iteratively increased until failure resulted. Knit samples showed recoverable axial strains of 65–140%, which could be moderately altered based on knit pattern and loop parameters. Furthermore, this research demonstrates that improving the density of the knit increases the stiffness of the knit without any loss in recoverable strains. These results highlight the potential of this unique structural architecture that could be used to design fabrics with adjustable mechanical properties, expanding the design space for aerospace structures, medical devices, and consumer products.

Newts display superior attachment and climbing abilities under wet conditions due to the distinct pattern of micro- and nanoscale structures (i.e., polygonal epidermal cells with raised boundaries, and dense nanopillar arrays) on their foot pad surfaces. Inspired by the surface features of newt foot pads, a microstructured surface consisting of structural elements of round pillars surrounded by a closed hexagonal ridge is produced on the polydimethylsiloxane (PDMS) elastomer, and its wet adhesion properties are studied experimentally. A homebuilt adhesion tester is developed to carry out the pull-off experiments, in which the adhesion force of samples can be measured directly. The PDMS sample is fixed on the substrate, and a flat glass cylinder of 10 mm diameter is adopted as indenter. Different amounts of liquid are added to the area of contact by using a micropipette. Influences of preload, retraction speed, area ratio of round pillars, amount of liquid and approach-retraction cycle on wet adhesion of the patterned surface of PDMS samples are investigated. Results show that the pull-off forces of all samples increase with preload and retraction speed. However, the pull-off forces increase slowly when the preload is over 3 N for both dry and wet conditions. The area ratios of round pillars increase the pull-off forces for the dry condition. When a small amount of liquid (0.1 μl) is added, the effect of the area ratios of round pillars on the pull-off force is not consistent with that of dry condition. Effects of amount of liquid on pull-off force for different area ratios of round pillars are various. For a certain amount of liquid, it is observed that the pull-off forces in general show a relatively high value for the first approach-retraction cycle, then decrease to a lower level starting from the second cycle, and suddenly increase and maintain a relatively constant value after several times of approach-retraction cycles. Our results can give insights into the repeated sliding actions of newt foot pads when climbing in wet environments as well as the possible functions of dense nanopillar arrays on newt footpad surface.

This paper focuses on the design and development of a bio-inspired mobile robot using piezoelectric transducers as drives. The design of the device aimed to imitate the trajectory movement of a crawl-like animal. Design constraints as producing controlled movement with piezoelectric transducer, as well as the combination of multiple piezoelectric patches into one mobile robot are presented in their practical aspects. The robot uses 2 piezoelectric transducers as main drives, but also as main structural components of the device. The patches are connected with a thin light rod, and the kinematic of movement is achieved with 4 tiny wooden legs connected on each of the patches. The project investigates the possibility and effectiveness of the piezoelectric transducers for movement of the bio-inspired mobile robot. From conceptual development, to the mechanical design and control, the mobile robot is used to test different trajectories of movement. Ni RIO Evaluation kit has been incorporated as a real-time and FPGA control platform for the mobile robot while using Labview programing environment. To accomplish complex trajectories of movement the velocity of the robot was measured for straight line and rotation of the robot.

Current acoustoelastic-based stress measurement techniques operate at the high-frequency, weakly-dispersive portions of the dispersion curves. The weak dispersive effects at such high frequencies allow the utilization of time-of-flight measurements to quantify the effects of stress on wave speed. However, this comes at the cost of lower sensitivity to the state-of-stress of the structure, and hence calibration at a known stress state is required to compensate for material and geometric uncertainties in the structure under test. In this work, the strongly-dispersive, highly stress-sensitive, low-frequency flexural waves are utilized for stress measurement in structural components. A new model-based technique is developed for this purpose, where the acoustoelastic theory is integrated into a numerical optimization algorithm to analyze dispersive waves propagating along the structure under test. The developed technique is found to be robust against material and geometric uncertainties. In the absence of calibration experiments, the robustness of this technique is inversely proportional to the excitation frequency. The capabilities of the developed technique are experimentally demonstrated on a long rectangular beam, where reference-free, un-calibrated stress measurements are successfully conducted.

Light-weight, compact actuators capable of delivering large linear stroke based compliant structures offer interesting potential for adaptive structures and robotics given their capability to carry a load and to be geometrically scalable. Furthermore, the utilization of compliant structures replacing mechanisms and moving parts offers the possibility for reducing losses due to friction. Solid state actuators have been proposed as a means to attain such capabilities with particular success for smart systems based on piezoelectric, shape memory, and electroactive polymer materials. Despite this success, smart material systems do not concurrently offer large strokes, high blocking force and wide response bandwidth. Recently, a new class of multi-stable structures that generate large linear strokes from twisting have been introduced. Furthermore, the snap-through action characterizing the changes between stable states of multi-stable systems has been shown to be a viable mechanism for inducing controlled actuation at high rates. In this paper, the novel design of a linear actuator adapted from twisting multi-stable structures is tailored to have geometric instability that is exploited to achieve large axial strokes under the action of small deformations from a single structural component. Finite element modeling is used for analysis of the structure, where parameter studies of composite layup and structure geometry are conducted to adjust equilibrium positions and stroke length of the actuator design. Different smart actuator topologies demonstrating the ability for compliant multi-stable systems are coupled with smart materials to produce large linear deformations and wide actuation bandwidth. The herein presented unconventional design serves as a useful linear actuator, as well as a load carrying component. The introduced multi-functional light-weight load-carrying actuators relevant for aerospace and robotics applications.

Traditionally, sensors to be integrated into a structural component are attached to or mounted on the component after the component has been fabricated. This tends to result in unsecured sensor attachment and/or serious offset between the sensor reading and the actual status of the structure, leading to performance degradation of the host structure. This paper describes a novel extrusion-based additive manufacturing process that has been developed to enable embedment of sensors in ceramic components during the part fabrication. In this process, an aqueous paste of ceramic particles with a very low amount of binder content (< 1 vol%) is extruded through a moving nozzle to build the part layer-by-layer. In the case of sensor embedment, the fabrication process is halted after a certain number of layers have been deposited. The sensors are placed in their predetermined locations, and the remaining layers are deposited until the part fabrication is completed. Because the sensors are embedded during the fabrication process, they are fully integrated with the part and the aforementioned problems of traditional sensor embedment can be eliminated. The sensors used in this study were made of sapphire optical fibers of 125 and 250 micro-meters diameter and can withstand temperatures up to 1600 °C. After the parts were built, two different drying processes (freeze drying and humid drying) were investigated to dry the parts. The dried parts were then sintered to achieve near theoretical density. Scanning electron microscopy was used to observe the embedded sensors and to detect any possible flaws in the part or embedded sensor. Attenuation of the sensors was measured in near-infrared region (1500–1600 nm wavelength) with a tunable laser source. Raman spectroscopy was performed on the samples to measure the residual stresses caused by shrinkage of the part and its slippage on the fibers during sintering and mismatch between the coefficients of thermal expansion of the fiber and host material. Standard test methods were employed to examine the effect of embedded fibers on the strength and hardness of the parts. The result indicated that the sapphire fiber sensors with diameters smaller than 250 micrometers are able to endure the freeform extrusion fabrication process and also the post-processing without compromising the part properties.

Structural supercapacitors are very interesting multifunctional devices combining the properties of an electrical energy storage device and a structural component simultaneously. These types of supercapacitors are mostly equipped with solid state electrolytes, instead of traditional liquid electrolytes, avoiding leakage and safety problems and supporting the mechanical performance of the composite materials. In the present study, the Lithium-ion based solid ceramic electrolyte Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 was successfully synthesized by sol-gel method. Its electrical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Results show that Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 possesses a conductivity of 2.94×10 −4 S/cm at room temperature and a specific capacity of 55.57 μF/g. The as-prepared samples were embedded into fiber composite material using the aviation approved resin RTM6 with an injection process making the composite structure flexible. Subsequently, the specific capacity and conductivity were tested getting values of 53.44μF/g and 2.00×10 −4 S/cm respectively. The reason for electrical properties loss was investigated by computerized tomography (CT) and EIS tests and the results provide reference for the future research.

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.

Several aerospace, space, and naval platforms process a core structural system assembled wholly or partially by aluminum alloy elementary structures, such as beams, plates, and cylinders, among others. Aluminum alloys are preferred due to light weight and corrosion resistance. The formed complex aluminum alloy structure is characterized by a number of joints where different structural elements are joined together. Areas with joints are prone to damage. Some joints are made by bolts for assembly and reassembly purposes. A loose bolt can be considered as a man-induced damage in the whole complex structure. This damage can result in a catastrophic failure. The vibration and elastic wave propagation-based monitoring of a critical area bearing bolt joints can offer a reliable monitoring. In this work, we present a basic experimental research effort. We consider an aluminum alloy beam composed of two sunbeams joined by means of two bolts. The bolts are healthy in the sense that they are tight to the level of design recommended force. We use three mono-axial sensors in three dimensional arrangement and mine vibration-wave data in the form of collocated signals. These signals are analyzed by means of the proper orthogonal decomposition transform. The local tri-axial arrangement of mono-axial sensors is used to collect collocated acceleration signals in two local areas. One of the local areas contains the bolted joints. The other local area contains the free end of the structure. This is not considered as a critical area but a boundary accessible area. The POD modal structures of collocated vibration signals are quite different. The behavior is compared to that of structure without bolted joints.

Fatigue cracks often occur in structural components due to dynamic loadings acting on them, such as wind loads and ground motion. If the static deflection due to dead loads is smaller than the vibration amplitude caused by dynamic loadings, these fatigue cracks alternately open and close with time, exhibiting a breathing-like behavior. This type of crack leads to a smaller change in structural dynamic characteristics than open cracks, and thus it is more difficult to be detected. If undetected timely, these fatigue cracks may lead to a catastrophic failure of the overall structure. Considering that breathing cracks introduce bilinearity into the structure, the present authors first developed a simple and efficient system identification method for bilinear systems by separating the global responses into two parts and performing Fourier transform on each set of separated data [1]. By applying this method, the natural frequency of each stiffness region can be identified. Then, breathing fatigue cracks can be detected by looking for the difference in the identified natural frequency between stiffness regions [1]. That approach is only applicable to the cases where the intact structure is linear. This study is to extend the approach in [1] to the cases when the intact structure is nonlinear, e.g., a structure with large displacements (geometrical nonlinearity). Once breathing cracks occur, there will exist both bilinearity (caused by breathing cracks) and cubic nonlinearity (caused by large displacements). To detect fatigue cracks in this case, Hilbert transform is proposed to be employed to process the separated data, instead of employing Fourier transform as in [1]. This approach has been successfully validated by numerical simulations.

Shape memory alloys (SMAs) have attracted a great deal of attention as a smart material that can be used in various civil engineering applications due to their favorable mechanical properties such as ability to undergo large deformations, high corrosion and fatigue resistance, good energy dissipating capacity, and excellent re-centering ability. In contrast to the use of SMAs in the biomedical, mechanical and aerospace applications, which requires mostly small diameter of material, the larger size bars are usually needed in a civil engineering application. It is well known that properties of large-section SMA bars are generally poorer than those of wires due to difficulties in material processing. Furthermore, large diameter SMA bars are more expensive than thin SMA wires. Shape memory alloy cables have been recently developed as an alternative and new structural element. They leverage the superior mechanical characteristics of small diameter SMAs into large-size structural tension elements and possess several advantages over SMA bars. This study explores the performance of NiTi SMA cables and their potential use in civil engineering. The SMA cable, which has a diameter of 8 mm, is composed of 7 strands and each strand has 7 wires with a diameter of 0.885 mm. The uniaxial tensile tests are conducted at various loading rates and strain amplitudes to characterize the superelastic properties of the SMA cable and study the rate-dependent mechanical response of the SMA cable under dynamic loads. An optical digital image correlation measurement system and an infrared thermal imaging camera are employed to obtain the full-field strain and temperature fields. Potential applications of SMA cables in civil infrastructure applications are discussed and illustrated.

This work presents a strategy to identify the optimal localized activation and actuation for a morphing thermally activated SMP structure or structural component to obtain a targeted shape change or set of shape features, subject to design objectives such as minimal total required energy and time. This strategy combines numerical representations of the SMP structure’s thermo-mechanical behavior subject to activation and actuation with gradient-based nonlinear optimization methods to solve the morphing inverse problem that includes minimizing cost functions which address thermal and mechanical energy, morphing time, and damage. In particular, the optimization strategy utilizes the adjoint method to efficiently compute the gradient of the objective functional(s) with respect to the design parameters for this coupled thermo-mechanical problem.

Many three-dimensional constitutive models have been proposed to enhance the analysis and design of shape memory alloy (SMA) structural components. Phenomenological models are desirable for this purpose since they describe macroscopic responses using internal variables to govern the homogenized material response. Because they are computationally efficient on the scale of millimeters to meters, these models are often the only viable option when assessing the response of full-scale SMA components for engineering applications. Thus, many different 3D SMA constitutive models have been developed. However, for their intended user, the application engineer, a clear and straightforward methodology has not been established for selecting a model to use in a design process. A primary goal of the Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) modeling working group has been establishment of model selection methodology. One critical step in this process is the development of benchmark problems that clearly illustrate the capabilities and efficiencies of models. In this paper, we propose a set of benchmark problems centered on an SMA tube component. These problems have been selected to demonstrate both uniaxial and multiaxial, actuation and superelastic capabilities of 3D SMA models. We then use finite element simulations of these benchmark problems to compare and contrast both the material modeling and implementation of three unique SMA constitutive models.

To localize small damage from mode shapes, the polynomial annihilation edge detection method has been proposed and demonstrated its effectiveness on different types of structural components [7]. However, much computational effort involved in this approach lowers the damage detection speed. To alleviate this difficulty, in this paper, we improve the approach by first using the divided difference approach to identify the region(s) in which jump discontinuities are located, and then only applying the polynomial annihilation method to points in the identified region. In this way, the computational burden of this approach is significantly relieved, while the accuracy is still maintained. The improved approach has been validated by numerical simulations on a cable-stayed bridge model. This approach only requires post-damage mode shapes.

A prototype cage implant for spinal fusion surgery has been designed and developed. Spinal fusion surgery is sometimes performed to alleviate low back pain. The cage implant is a spacer that sits in between two vertebrae to allow for bone growth and fusion, all while relieving compression of the spinal cord. The cage implant is minimally invasive in nature, utilizing embedded nitinol hinges as dual purpose actuators and assembly structural elements. The cage implant utilizes elliptical shaped nitinol hinge pins as actuators to allow the cage to be in a straightened before deployment and manipulate its shape to an oblong octagon once within the disc space. A new modeling technique was developed to aid with the design of the nitinol ellipses. The model is MATLAB based and accounts for the non Mises behavior of nitinol through a correction factor for mapping the effective stress and strain. A nitinol rod and an elliptical geometry were examined experimentally and show the robustness of the developed model. These experiments were conducted to design the nitinol hinges for the cage implant. The cage implant is made of two different materials, nitinol hinge actuators and the containing titanium structural segments. The nitinol hinge actuators are completely enclosed within the medical grade titanium segments through the use of selective laser sintering.

This paper focuses on the design, fabrication, testing and analysis of a novel load-bearing element with good energy dissipation capability over a decade of variation in frequency and harmonic load amplitude. A single layer of the compact sandwiched-plate-like element is comprised of two von-Mises trusses (VMTs) between an upper and lower plate, connected to two dampers which stroke in the in-plane direction as the VMTs cycle between the two stable equilibrium states. The elements can be assembled in-plane to form a large plate-like structure or stacked with different properties in each layer for improved load-adaptability. Also introduced in the elements are pre-loaded springs (PLSs) that provide very high initial stiffness and allow the element to carry a design static load even when the VMTs lose their load carrying capability under harmonic disturbance input. Simulations of the system behavior using the Simscape environment show good overall correlation with test data. Good energy dissipation capability is observed over a frequency range from 0.1 Hz to 2 Hz. While the VMT parameters of a single layer can be optimized to a particular harmonic load amplitude, having two layers with softer and stiffer VMTs allow the system to show good energy dissipation characteristics at different harmonic load amplitude levels. The test and simulation results show that a two layer prototype can provide good energy dissipation over a decade of variation in harmonic load amplitude, while retaining the ability to carry static load on account of the PLSs. The paper discusses how system design parameter changes affect the static load capability and the hysteresis behavior.