Shape Memory Alloys (SMAs) are often used for robotic, biomedical, and aerospace applications because of their unique ability to undergo large amounts of stress and strain during thermomechanical loading compared to traditional metals. While SMAs such as NiTi have been used in wire, plate, and tubular forms, NiTi as a woven dry fabric has yet to be analyzed for use as protective materials and actuators. Applications of SMA fabric as a “passive” material include shields, seatbelts, watchbands and window screens. Applications as an “active” material include robotic actuators, wearable medical and therapy devices, and self-healing shields and screens. This paper applies a macro-mechanical model from composites analysis to NiTi plain woven fabric to determine the effective elastic constants. The fabric model is based on actual weave geometry, including the presence of open gaps and wire cross-sectional area, and with the same diameter and alloy in the warp and weft. A woven NiTi ribbon has been manufactured (Figure 1) using a narrow weaving machine and has been tested in uniaxial tension. Planar fabric constants were measured at a range of temperatures. The analytically and experimentally derived constants for various weave patterns and cover factor combinations are presented and compared. It was determined that in uniaxial tension the fabric behaves like a collection of unidirectional wires, but has 78% of the rigidity, on average, across all test temperatures. This result is predicted by the fabric model with a 16% error, demonstrating that the proposed analytical model offers a useful tool for design and simulation of SMA fabrics.

We present a mathematical model to guide and interpret ongoing Cell-in-Gel experiments, where isolated cardiac myocytes are embedded in a constraining viscoelastic hydrogel, to study mechano-chemo-transduction mechanisms at the single cell level. A recently developed mathematical model, based on the elastic Eshelby inclusion problem, is here extended to account for viscoelasticity of the inclusion (cell) and the matrix (gel). This provides a tool to calculate time-dependent 3D stress and strain fields of a single myocyte contracting periodically inside a viscoelastic matrix, which is used to explore the sensitivity of the cell’s mechanical response to constitutive properties and geometry. A parametric study indicates that increased gel crosslink concentration significantly alters the strain and stress fields inside the cell and creates an increased time-lag in the mechanical response of the cell during contraction. It is also found that autoregulation at the cellular level in response to afterload, potentially in the form of increased cell stiffness, has a strong influence on cell contraction.

Bone is a highly adaptive biological structure. Following Wolff’s law, bone realigns and grows to adapt to its mechanical environment. This leads to structural heterogeneity of trabecular bone and orthotropic symmetry of the elastic properties. Determining the bone alignment and material properties for living patients is difficult and involves implantation of force and displacement sensors on the bone to determine the compliance and stiffness properties. Micro-computed tomography along with finite element modeling have been limited to the vertebrae of donor cadavers to evaluate trabecular architecture, material properties, and density. Here, an adaptive structure topology optimization algorithm is presented and used to predict trabecular architecture. The algorithm predicts the optimal structure by minimizing the global compliance. The lumbar 1 (L1) vertebra is used as an example. Loads common to L1 vertebrae are applied and bone volume fraction measurements that can be taken easily from living patients through bone mineral density scans are used as the only inputs. The mathematical model is an adaptation of “99 Line Topology Optimization Code Written in Matlab” developed by Sigmund (2001). Bone is locally assumed to be isotropic with an elastic modulus of 13 GPa and the Poisson ratio of 0.3 applied to each element. The resulting structural heterogeneity results in global orthotropic relations. The model uses bone volume fraction and the loading orientation as inputs and gives the corresponding ideal bone structure geometry as an output. The trabecular structure can be predicted solely from the results of a bone mineral density scan. Finite element analysis of the optimized structure is then conducted and the global material properties are determined. While this model is for two-dimensional examples representing planes within the vertebral bone, it is extended to three-dimensional modeling to develop the cortical bone geometry and define the total volume. Matlab is then used to run the topology optimization simulation. The ideal structure is defined by optimizing for a prescribed displacement field of the system following the implementation of a gradient descent optimization method. The results are compared to published values from a combined experimental and numerical procedure. The procedure on sectioned vertebrae reported average ratios between elastic moduli of E 1 / E 2 = 5.2, E 1 / E 3 = 8.8, and E 2 / E 3 = 1.4. Results between the models and the previously published data yield similar transversely isotropic symmetry in the elastic moduli of trabecular bone. However, the elastic moduli ratios are not quite in agreement. Improving the accuracy of the boundary conditions and loading of the finite element model may improve the correlation between the optimization models and published data.

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.

Shape memory alloys (SMAs) have tremendous potential use as actuators in mechanical systems due to their high specific energy density. Large recovery stresses can be generated when Nickel Titanium (NiTi), the most widely used SMA, undergoes constrained recovery where it is held in a deformed geometry and heated from a detwinned martensite phase to austenite phase. Recent experimental results have found that residual stresses can also be generated in NiTi after returning to a low temperature geometrically constrained state. This paper presents experimental results performed on NiTi wire samples where wire was: 1) deformed from a low temperature twinned martensite state to produce a strain that would be recoverable in an unloaded state 2) held at that strain state and heated above the austenite finish transition temperature and then cooled back below the martensite transition finish temperature while recording the forces generated. It was found that a residual load was produced in the low temperature state. Results from further testing beyond this point showed repeatability with application of small and large strains. Post constrained recovery stresses have the potential to be used to generate residual stresses in structures in a low energy, un-actuated state with a remaining potential for thermal actuation.

Cellular contact-aided compliant mechanisms (C3M) are cellular structures with integrated self-contact mechanisms, i.e. the segments can come into contact with each other during deformation. The contact changes the load path and can influence on the mechanism’s performance. Cellular contact-aided compliant mechanisms can be tailored for a specific structural application, such as energy absorption. Nickel Titanium compliant mechanisms can exploit the superelastic effect to improve performance and increase energy absorption. The potential for compliant mechanisms designed specifically for metal additive manufacturing opens the possibility of functional grading and tailoring the material properties locally for achieving overall performance. The combined effort of the geometry and the nonlinear material property increases the local compliance of the unit cell, resulting in higher energy absorption. A functionally graded 3D energy absorbing contact-aided compliant mechanisms cell with curved walls is analyzed. Functionally graded zones of higher flexibility are explored with different superelastic material properties. Introducing different moduli of elasticity as a function of the critical transformation stress results in different energy absorption. This approach can be used for tailoring the overall performance based on the application.

Functionally graded compliant mechanisms can be fabricated with additive manufacturing technology by engineering the microstructural and compositional gradients at selected locations resulting in compositionally graded zones of higher and lower flexibility. The local compliance depends on the geometry of the structure as well as the material property in the selected region. As Nitinol (NiTi) is well suited for applications requiring compliance, the critical transformation stress and the superelastic modulus of elasticity are crucial parameters for defining the local compliance. To understand the behavior at the interface between two different material compositions, three models of gradient change between the alloys are analyzed: step change, linear and polynomial gradients. In addition to localize the deformation in the interface, three different flexure designs in the interface are analyzed. This paper will address a methodology for modeling and parametrization of material properties and transition at the interface, for different flexure designs. The combined effort in the interface of the functional grading and the geometry will be used for the design of monolithic self-deployable structures, initially folded in compact shape. The design motivation comes from the self-deploying mechanisms inspired by insects’ wings.

This paper presents the design, fabrication, and operation of a chemo-mechatronic system that changes its geometry and electrical functionality in the presence of specific chemical signals. To accomplish this, we integrated a protein hydrogel with an aluminum substrate and flexible circuit in a low-profile laminate. To demonstrate the concept, we have built and tested a sensor that lights an LED when actuated in the presence of polyethylene glycol (PEG).

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.

Early research on a new concept for a morphing system based on unit structures or cells containing pressurized fluid is presented in this article. The motivation stems from the desire to achieve 3D smooth variations with multiple degrees of freedom and variations in surface area. Such a cell is composed of a hybrid between elastomeric material and stiffening material, creating an orthotropic system. When connected in a network of cells, the morphing system is highly integrated in terms of the components of the skin, substructure and actuation means. Numerical predictions of small simple prismatic cells show a force and axial strain capability of above 200 N and 14% respectively for typical elastomeric and stiffening materials at 8 bar pressure. PolyJet™ additively-manufactured models of wings show how such actuators can be integrated into aircraft structures, including when 3D geometry is highly challenging. These additively-manufactured models were operated at low pressures in the order of 0.5 bar, and a number of open questions on the design, manufacture and operation of such structures are discussed along with intended future work towards higher grade materials and working pressures.

In the framework of Clean Sky 2 Airgreen 2 (REG-IADP) European research project, a novel multifunctional morphing flap technology was investigated to improve the aerodynamic performances of the next Turboprop regional aircraft (90 passengers) along its flight mission. The proposed true-scale device (5 meters span with a mean chord of 0.6 meters) is conceived to replace and enhance conventional Fowler flap with new functionalities. Three different functions were enabled: overall airfoil camber morphing up to +30° (mode 1), +10°/−10° (upwards/downwards) deflections of the flap tip segment (mode 2), flap tip “segmented” twist of ±5° along the outer flap span (mode 3). Morphing mode 1 is supposed to be activated during take-off and landing only to enhance aircraft high-lift performances and steeper initial climb and descent. Thanks to this function, more airfoil shapes are available at each flap setting and therefore a dramatic simplification of the flap deployment system may be implemented. Morphing modes 2 and 3 are enabled in cruise and off-design flight conditions to improve wing aerodynamic efficiency. The novel structural concept of the three-modal morphing Fowler flap (3MMF) was designed according to the challenges posed by real wing installation issues. The proposed concept consists of a multi-box arrangement activated by segmented ribs with embedded inner mechanisms to realize the transition from the baseline configuration to different target aero-shapes while withstanding the aerodynamic loads. Lightweight and compact actuating leverages driven by electromechanical motors were properly synthesized to comply with stringent requirements for real aircraft implementation: minimum actuating torque, minimum number of motors, reduced weight, and available design space. The methodology for the kinematic design of the inner mechanisms is based on a building block approach where the instant center analysis tool is used to preliminary select the locations of the hinges’ leverages. The final geometry of the inner mechanisms is optimized to maximize the mechanical advantage as well as to provide the kinematic performances required by the three different morphing modes. The load-path was evaluated, and the cross-sectional size of leverages was subsequently optimized. Finally, actuating torques predicted by instant center analysis were compared to the calculated values from finite element analysis. The structural sizing process of the multi-box arrangement was carried out considering elementary methods, and results were compared with finite element simulations.

Industrial robots are commonly designed to be very fast and stiff in order to achieve extremely precise position control capabilities. Nonetheless, high speeds and power do not allow for a safe physical interaction between robots and humans. With the exception of the latest generation lightweight arms, purposely design for human-robot collaborative tasks, safety devices shall be employed when workers enter the robots workspace, in order to reduce the chances of injuries. In this context, Variable Stiffness Actuators (VSA) potentially represent an effective solution for increasing robot safety. In light of this consideration, the present paper describes the design optimization of a VSA architecture previously proposed by the authors. In this novel embodiment, the VSA can achieve stiffness modulation via the use of a pair of compliant mechanisms with distributed compliance, which act as nonlinear springs with proper torque-deflection characteristic. Such elastic elements are composed of slender beams whose neutral axis is described by a spline curve with non-trivial shape. The beam geometry is determined by leveraging on a CAD/CAE framework allowing for the shape optimization of complex flexures. The design method makes use of the modeling and simulation capabilities of a parametric CAD software seamlessly connected to a FEM tool (i.e. Ansys Workbench). For validation purposes, proof-concept 3D printed prototypes of both non-linear elastic element and overall VSA are finally produced and tested. Experimental results fully confirm that the compliant mechanism behaves as expected.

Steady-state traveling waves in structures have been previously investigated for a variety of purposes including propulsion of objects and agitation of a surrounding medium. In the field of additive manufacturing, powder bed fusion (PBF) is a commonly used process that uses heat to fuse regions of metallic or polymer powders within a loose bed. PBF processes require post-process removal of loose powder, which can be difficult when blind holes or complex internal geometry are present in the fabricated part. Here, a preliminary investigation of a simple part is conducted examining the use of traveling waves for post-process de-powdering of additively manufactured specimens. The generation of steady-state traveling waves in a structure is accomplished through excitation at a frequency between two adjacent resonant frequencies of the structure, resulting in two-mode excitation. This excitation can be generated by bonded piezoceramic elements actuated by a sinusoidal voltage signal. The response of the structure is affected by the parameters of the excitation, such as the particular frequency of the voltage signal, the placement of the piezoceramic actuators, and the phase difference in the signals applied to different actuators. Careful selection of these parameters allows adjustment of the quality, wavelength, and wave speed of the resulting traveling waves. In this work, open-top rectangular box specimens composed of sintered nylon powder and coated with fine sand are used to represent freshly fabricated parts yet-to-be cleaned of un-sintered powder. Steady-state traveling waves are excited in the specimens while variations in the frequency content and phase differences between actuation points of the excitation are used to affect the characteristics of the dynamic response. The effectiveness of several response types for the purpose of moving un-sintered nylon powder within the specimens is investigated.

The aerodynamic foil bearing is a special type of air bearing in which the flexible foil structure between rotor and rigid housing supports the rotor bearing system with a greater robustness against thermal distortion and production misalignments. In such bearings, the generation of an aerodynamic pressure in the lubricating film after reaching the lift-off speed prevents the solid contact between rotor and foil structure. Since many static and dynamic properties of air foil bearings strongly depend on the inner contour of the bearing, the idea of an adaptive air foil bearing (AAFB) is developed to optimize the bearing’s performance at different operating points. This paper concentrates on a semi-analytical model based on plate theory using Ritz method for simulating the static shape control of piezoelectrically actuatable supporting segments for an AAFB under different loading conditions. The elastic suspension of the supporting segments and symmetries of the bearing are considered in the modeling. After validation by means of FEM analyses and experimental tests the influence of geometry and material is examined in a parametric study. Later on, the model is used for parameter optimization in order to achieve the most effective shape morphing.

Vibration-based energy harvesting has been widely investigated to as a means to generate low levels of electrical energy for applications such as wireless sensor networks. However, due to the fact that vibration from the environment is typically random and varies with different magnitudes and frequencies, it is a challenge to implement frequency matching in order to maximize the power output of the energy harvester with a wider frequency bandwidth for applications where there is a time-dependent, varying source frequency. Possible solutions of frequency matching include widening the bandwidth of the energy harvesters themselves in order to implement frequency matching and to perform resonance-based tuning approach, the latter of which shows the most promise to implement a frequency matching design. Here three tuning strategies are discussed. First a two-dimensional resonant frequency tuning technique for the cantilever-geometry energy harvesting device which extended previous 1D tuning approaches was developed. This 2D approach could be used in applications where space constraints impact the available design space of the energy harvester. In addition, two novel resonant frequency tuning approaches (tuning via mechanical stretch and tuning via applied bias voltage, respectively) for electroactive polymer (EAP) membrane-based geometry energy harvesters was proposed, such that the resulting changes in membrane tension were used to tune the device for applications targeting variable ambient frequency environments.

Self-healing material structures with the inherent capability to mend damage will lead to a paradigm shift in design as fracture may no longer constitute a failure. Generally, there are two techniques of self-healing that operate at different scales, require different approaches and often are dealt with separately; geometric restoration and crack filling/bonding. Geometric restoration uses shape memory materials that can mechanically close fractures after they occur. Crack filling and bonding fills and chemically bonds fractured parts in place. Materials capable of recovering from complete fractures, that have propagated across the entire component, have typically taken a sparse fiber composite form with a structural matrix encapsulating shape memory fibers. This form of self-healing material has demonstrated the ability recover original bulk geometry. However, lacking bonding, the healed structures have not had the ability to resist subsequent externally applied loads without re-opening the crack. A new approach of pre-straining the shape memory fibers before curing them in a matrix in the pre-strained state is presented in this paper with basic theory and experimental results. Pre-straining the shape memory fibers before casting them in the matrix causes them to undergo constrained recovery upon activation. Thus, the samples create closing loads across the crack which are capable of withstanding external loads without re-opening.

This work is based on the original concept of coupling two resonant vibration modes to reproduce insect wing kinematics and generate lift. The key issue is designing the geometry and the elastic properties of the artificial wings to achieve quadrature coupling of the bending and twisting motions using only one actuator. Qualitatively, this implies bringing the frequency of the two resonant modes closer. In the light of this challenge, an optimal wing configuration was determined for a micromachined polymer prototype three centimeters wide and validated through experimental modal analyses to illustrate the proximity of the frequencies of the bending and twisting modes. Then, a dedicated lift force measurement bench was developed and used to demonstrate a lift force equivalent to 110% of the prototype weight. For the first time, high-speed camera measurements of the wing motion confirmed that maximum lift was obtained as expected for bending and twisting motions in phase quadrature with a fully resonant motion of the wings using a single actuator.

Materials capable of exhibiting inherent morphing are rare and typically reliant on nanometric chemical properties. The resulting shape adaptability is thus slow and limited to specific environmental conditions. In contrast, natural composites, such as those found in carnivorous plants, have evolved hierarchical architectures displaying remarkably fast adaptation upon environmental stimuli. These biological materials have inspired the fabrication of snapping composite shells through the careful design of the internal microstructure of synthetic materials by magnetic alignment of reinforcements. The ability to accurately model such programmable materials using finite element analysis (FEA) is necessary to facilitate the design optimization of resulting structures. Using similar material parameters as explored in previous experimental studies, we employ nonlinear FEA to investigate the effects of introducing curvilinear spatially distributed micro-reinforcements on the deformation of a shell with bioinspired geometry. The FEA model is subject to a preliminary experimental validation. Comparison to a conventional [0/90] composite layup and simplified models demonstrates the advantages of magnetically aligned reinforcements to achieve complex, snapping morphing structures with tailored characteristics.

This manuscript investigates energy harvesting from arterial blood pressure via the piezoelectric effect for the purpose of powering embedded micro-sensors in the human brain. One of the major hurdles in recording and measuring electrical data in the human nervous system is the lack of implantable and long term interfaces that record neural activity for extended periods of time. Recently, some authors have proposed micro sensors implanted deep in the brain that measure local electrical and physiological data which is then communicated to an external interrogator. This paper proposes a way of powering such interfaces. The geometry of the proposed harvester consists of a piezoelectric, circular, curved bimorph that fits into the blood vessel (specifically, the Carotid artery) and undergoes bending motion because of blood pressure variation. In addition, the harvester thickness is constrained such that it does not modify arterial wall dynamics. This transforms the problem into a known strain problem and the integral form of Gauss’s law is used to obtain an equation relating arterial wall motion to the induced voltage. The theoretical model is validated by means of a Multiphysics 3D-FEA simulation comparing the harvested power at different load resistances. The peak harvested power achieved for the Carotid artery (proximal to Brain), with PZT-5H, was 11.7 μ W. The peak power for the Aorta was 203.4 μ W. Further, the variation of harvested power with variation in harvester width and thickness, arterial contractility and the pulse rate is investigated. Moreover, potential application of the harvester as a chronic, implantable and real-time Blood pressure sensor is considered. Energy harvested via this mechanism will also have applications in long-term, implantable Brain Micro-stimulation.

Smart materials are unique in their ability to change properties in response to an environmental stimulus. These materials provide promising opportunities for adaptable aerospace structures, where they can be altered to suit their need. In this research, Honeycomb Polymer Composites (HPCs) were investigated as potential materials for this need. HPCs are new materials that consist of a polymer embedded in a honeycomb structure, and exhibit a significantly higher stiffness than the polymer or honeycomb alone. This stiffness amplification is due to the nearly incompressible polymer resisting the volume change within the honeycomb cells. HPC samples were fabricated using an aramid honeycomb, with either silicone or urethane rubber as the matrix materials to fill the honeycomb. Varying polymer stiffness, honeycomb geometry, and testing temperature were all tested to observe the effects on the material properties. The results indicated that the HPCs could be effectively tailored and modeled to suit the need for different effective moduli. This research provides important insight and results in the development of programmable honeycomb polymer composites (PHPCs), which rely on shape memory polymers (SMP) as the internal working polymer.