The buckling characteristics of thin functionally graded (FG) nano-plates subjected to both thermal loads and biaxial linearly varying forces is investigated. Eringen’s nonlocal elasticity theory is employed to account for the nano-scale phenomena in the plates. Hamilton’s principle and the constitutive relations are used to derive the partial differential governing equations of motion for the thin plates that are modeled using Kirchhoff’s plate theory. The mechanical properties of the FG nano-plates are assumed to vary smoothly across the thickness of the plate following a power law. Three types of thermal loads are presented and the spectral collocation method is utilized to solve for the critical buckling loads. The accuracy of the numerical solution of the proposed model is verified by comparing the results with those available in the literature. A comprehensive parametric study is carried out, and the effects of the nonlocal scale parameter, power law index, aspect ratio, slopes of the axial loads, boundary conditions, assumed temperature distributions, and the difference between the ceramic-rich and metal-rich surfaces on the nonlocal critical buckling loads of the nano-plates are examined. The results reveal that these parameters have significant influence on the stability behavior of the FG nano-plates.

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.

This paper presents an initial step towards a new class of soft robotics materials, where localized, geometric patterning of smart materials can exhibit discrete levels of stiffness through the combinations of smart materials used. This work is inspired by a variety of biological systems where actuation is accomplished by modulating the local stiffness in conjunction with muscle contractions. Whereas most biological systems use hydrostatic mechanisms to achieve stiffness variability, and many robotic systems have mimicked this mechanism, this work aims to use smart materials to achieve this stiffness variability. Here we present the compositing of the low melting point Field’s metal, shape memory alloy Nitinol, and a low melting point thermoplastic Polycaprolactone (PCL), composited in simple beam structure within silicone rubber. The comparison in bending stiffnesses at different temperatures, which reside between the activation temperatures of the composited smart materials demonstrates the ability to achieve discrete levels of stiffnesses within the soft robotic tissue.

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.

Heat is often lost unused in industry, commerce and households and is considered a waste product — while there is a lot of energy potential in waste heat. As part of the project “Theasmart”, scientists and companies are exploring just these potentials to find out how the waste product heat can be used for further purposes through the use of an innovative smart materials technology. The goal of the project is the further qualification of shape memory alloys with special focus on thin hysteresis applications for energy harvesting. In certain applications, these metals can be used as a thermal drive, for example for thermal valves or as thermal air flow regulators. Energy efficiency in processes in industrial companies or households could be improved by their use of waste heat. By 2020, the development of thermally driven generators, so-called “energy harvesters”, and the identification of other areas of application is planned. This publication focuses on first steps towards a process tool which can be actuated by waste heat of a thermal annealing sub-process directly, or used as mechanical energy charging device combined with a releasing mechanism.

In this paper, a high performance micro piezoelectric energy harvester (PEH) fabricated on stainless substrates is presented. A PZT piezoelectric active layer with a thickness of about 10 μm was deposited on a stainless steel substrate by the aerosol deposition method. The cantilever beam-shaped PEH was then fabricated by metal-MEMS processing of the PZT/stainless steel composite structure. The size of the cantilever PEH transducer developed was about 1 cm 2 and a proof mass was attached to tune its resonant frequency to around 120 Hz for harvesting mechanical vibrations from direct drive AC motors. The PEH transducer showed an output voltage and an output power of 8.9 V p-p and 107.8 μW, respectively, when connected with optimal load and excited under 0.5 g acceleration level. In order to realize the fatigue behavior and reliability of the PEH in field applications, the PEH transducer was driven at its own resonant frequency and tested under 1.0 g acceleration level for millions of cycles and the vibration modes were measured with a laser scanning vibrometer. The PEH transducer had an operating lifetime of about 1.8 million cycles at 1.0 g cyclic loading based on the shift of its resonant frequencies and the decrease in electrical output. The experimental results show the resonant frequencies of the first, second and third modes were all shifted to lower frequencies with increasing operation cycle number due to the development of microcracks in the ceramic PZT active layer. However, the same PEH transducer could survive millions of cycles (in the high millions) at 0.5 g cyclic loading without any significant changes in the resonant frequencies and electrical output. The results confirm the operating limits of the PEH transducer and suggest further protection and reinforcement are required for the transducer to operate at high acceleration loadings.

It is of interest to exploit the insight from the lateral line system of fish for flow sensing applications. The lateral line consists of arrays of flow sensors, known as neuromasts, with hair cells encased within a gel-like structure called cupula. There are two types of neuromasts, superficial neuromasts, which reside on the surface, and canal neuromasts, which are recessed within a channel with its ends open at the body’s surface. In this work we investigate the modeling of a canal-type artificial lateral line system. The canal is filled with viscous fluid to emulate its biological counterpart. The artificial neuromast consists of an ionic polymer-metal composite (IPMC) sensor embedded within a soft molded cupula structure. The displacement of the cupula structure and the resulting short-circuit current of the IPMC sensor under an oscillatory flow are modeled and solved with finite-element methods. The Poisson-Nernst-Planck (PNP) model is used to describe the fundamental physics within the IPMC, where the bending stimulus due to the cupula displacement is coupled to the PNP model through the cation convective flux term. Comparison of the numerically computed cupula displacement with an analytical approximation is conducted. The effects of material stiffness and and device size on the device sensitivity are further explored.

In the field of Additive Manufacturing (AM), one of the major applications of laser-based 3D metal printing is the creation of custom implants for medical purposes. However, a significant challenge in the manufacturing of implants using Selective Laser Melting (SLM) is the formation of partially melted particles on the surface of medical implants. These particles result in a multitude of issues including plurality of structurally weak points on the designed implants, obstruction of important design features, and possibility of dislodgement over the service life span, thereby posing a threat to the recipient. To address the above challenges, it is imperative to develop a simple but effective surface cleaning method to remove partially melted particles from the surface without damage to the designed medical implants. In this work, a comparative study was conducted to investigate the effect of both chemical and electro-plasma based cleaning processes on the removal of partially melted particles from the surfaces of 3D printed Ti-6Al-4V medical screw implants. These techniques include chemically polishing implants with HF-HNO 3 acid solutions and using an electro-plasma based cleaning process. With the field of additive manufacturing rapidly expanding, this work offers valuable insight on proper post-process treatment of 3D printed parts for future medical purposes in biomedical fields.

Strain sensors are one of the most widely used transducers for structural health monitoring, since strain can provide rich information regarding structural integrity. Recently, it has been shown that thin film sensors that incorporate nanomaterials can be engineered to possess unique properties, such as flexibility, high sensitivity, and distributed sensing capabilities, to name a few. To date, a plethora of different nanomaterials have been explored for fabricating strain sensors, such as by using conductive polymers, metal nanowires, and carbon nanotubes, among others. The aim of this work is to leverage the unique properties of graphene to fabricate next-generation thin film strain sensors. While graphene exhibits impressive mechanical and electrical properties, it remains challenging to harness these properties for sensing, primarily because of difficulties associated with high-quality synthesis and to incorporate them in a scalable fashion. In this study, few-layered graphene nano-sheets (GNS) were first synthesized using a low-cost, liquid-phase exfoliation technique. Second, GNS was dispersed in an aqueous solution with a low-concentration polymer acting as the dispersing agent. Third, the dispersion was printed onto flexible polymer substrates to form complex geometrical patterns, such as strain rosettes. Then, the electrical and electromechanical properties of the printed thin film sensors were characterized. It was found that the strain rosettes could resolve multi-axial strains applied during coupon tests. Overall, the GNS-based strain sensors showed excellent signal-to-noise ratio, stable sensing performance, high strain sensitivity, and remarkable reproducibility.

Embedded fiber Bragg grating (FBG) sensors are attractive for in-situ structural monitoring, especially in fiber reinforced composites. Their implementation in metallic structures is hindered by the thermal limit of the protective coating, typically a polymer material. The purpose of this study is to demonstrate the embedding of FBG sensors into metals with the ultimate objective of using FBG sensors for structural health monitoring of metallic structures. To that end, ultrasonic additive manufacturing (UAM) is utilized. UAM is a solid-state manufacturing process based on ultrasonic metal welding that allows for layered addition of metallic foils without melting. Embedding FBGs through UAM is shown to result in total cross-sectional encapsulation of the sensors within the metal matrix, which encourages uniform strain transfer. Since the UAM process takes place at essentially room temperature, the industry standard acrylate protective coating can be used rather than requiring a new coating applied to the FBGs prior to embedment. Measurements presented in this paper show that UAM-embedded FBG sensors accurately track strain at temperatures higher than 400 °C. The data reveals the conditions under which detrimental wavelength hopping takes place due to non-uniformity of the load transferred to the FBG. Further, optical cross-sectioning of the test specimens shows inhibition of the thermal degradation of the protective coating. It is hypothesized that the lack of an atmosphere around the fully-encapsulated FBGs makes it possible to operate the sensors at temperatures well above what has been possible until now. Embedded FBGs were shown to retain their coatings when subjected to a thermal loading that would result in over 50 percent degradation (by volume and mass) in atmospherically exposed fiber.

Equivalent electromechanical circuit analysis models provide efficient tools for analysis of power flow across multiple physical domains. In this work, we use this tool to develop a model of the power flow through mechanical, magnetic and electrical domains for analysis of magnetostrictive material-based devices. The magnetostrictive unimorph system in this study consists of a magnetostrictive galfenol (Fe-Ga alloy) layer bonded to a non-magnetic flexible metal layer, a pickup coil wound around the bimetallic strip and an electrical load. Permanent magnets are used to set a magnetic bias field and to provide a tip mass load at the free end of the cantilevered unimorph. The electrical load is connected to the pickup coil, such that vibration in the magnetostrictive alloy layer caused by the vibrating structure generates electrical energy that is dissipated by the electrical load, thereby damping the vibrations in the structure. The pickup coil output voltage varies with fluctuation in the magnetic flux density due to vibration of the beam. The electrical load discussed in this paper includes an inductance and a resistor (work that also includes capacitance is on-going). An electromechanical circuit model of the electrically loaded magnetostrictive unimorph system is used to study system dynamics. For this purpose the circuit description is simplified by a transformation of the electrical and magnetic elements into the mechanical domain. The network model of this system and simulations of its dynamic behavior are presented.

Damage nucleation and growth can be complex in hybrid structures composed of layers of metal and laminated composites. Presently there are limited reliable damage growth analytical and empirical methods to evaluate the bond integrity of such structures and to quantify the state of bonding in such joints. Depending on the geometry and accessibility of hybrid joints, ultrasonic nondestructive testing (NDT) techniques are available for inspection of these structures. However there are some limitations for the usage of typical bulk or guided waves to quantify the integrity of bondline in hybrid structures. This work suggests the use of specific forms of ultrasonic guided waves that propagate along the bondline of these hybrid structures. This study is dedicated to modeling of interface guided waves for the purpose of disbond crack damage assessment. The nature of interface waves is discussed and the numerical simulation based on the material properties and geometries of hybrid interfaces as well as composite stacking sequence is verified. A finite element model of a hybrid structure with isotropic and anisotropic multilayer composites is constructed. The behavior of interface guided waves influenced by disbond cracks at free edges of hybrid bonded joints is numerically studied. The propagation characteristics of interface waves is shown to be sensitive to the size of disbond cracks. The velocity of interface waves is shown to have an inverse relation to the disbond damage size. Results show the speed is also a function of the interfacing ply orientation at the bondline. These results suggest that interface waves can be used to monitor the condition of bonded joints in hybrid structures.

The development of variable-stiffness systems is key to the advance of compact engineering solutions in a number of fields. Rigidizable structures exhibit variable-stiffness based on external stimuli. This function is necessary for deployable structures, such as inflatable space antennas, where the deployed structure is semi-permanent. Rigidization is also useful for a wide range of applications, such as prosthetics and exoskeletons, to help support external loads. In general, variable-stiffness designs suffer from a tradeoff between the magnitude of stiffness change and the ability of the structure to resist mechanical failure at any stiffness state. This paper presents the design, analysis, and fabrication of a rigidizable structure based on inflatable octet-truss cells. An octet-truss is a lattice-like configuration of elements, traditionally beams, arranged in a geometry reminiscent of that of the FCC lattice found in many metals; namely, the truss elements are arranged to form a single interior octahedral cell surrounded by eight tetrahedral cells. The interior octahedral cell is the core of the octet-truss unit cell, and is used as the main structure for examining the mechanics of the unit as a whole. In this work, the elements of the inflatable octet truss are pneumatic air muscles, also called McKibben actuators. Generalized McKibben actuators are a type of tubular pneumatic actuator that possess the ability to either contract or expand axially due to an applied pressure. Their unique kinematics are achieved by using a fiber wrap around an isotropic elastomeric shell. Under normal conditions, pressurizing the isotropic shell causes expansion in all directions, like a balloon. The fiber wrap constrains the ability of the shell to freely expand, due to the fiber stiffness. The wrap geometry thus guides the extensile/contractile motion of the actuator by controlling its kinematics. It is their ability to contract under pressure that makes McKibben actuators unique, and consequently they are of great interest presently to the robotics community due to their similitude to organic muscles. Kinematic analysis from constrained maximization of the shell volume during pressurization is used to obtain relations between the input work due to applied pressure and the resulting shape change due to strain energy. Analytical results are presented to describe the truss stiffness as a function of the McKibben geometry at varying pressures.

Morphing is a technology with high potential to reduce emissions in aviation by adapting the shape of the wings to varying external operating conditions. This paper is presenting results from the EU FP7 funded CHANGE project, where different concepts to adapt a UAV wing airfoil to different demands were investigated. The paper is concentrating on the design and experimental testing of a droop nose, which transforms the leading edge part of the 60 cm chord airfoil from a NACA 6510 shape for loiter and low speed to a NACA 2510 shape for a high speed mission. This paper is presenting the use of an especially soft skin, which reduces the needed force for morphing. That way the requirements for the servos driving the droop nose could be reduced significantly. This paper is showing the implications of such a soft design on the accuracy of the shape generated. For such a skin design, the driving mechanism of the system is designed as a compliant mechanism, which was generated by topology optimization, taking into account aerodynamic loads. For easy manufacturing reasons, thermoplastic polylactic acid (PLA) with zero warp property was used for the manufacturing of this compliant mechanism. Finally deformation measurements of the morphing skin were carried out in a series of lab tests. The match between measured and numerically derived section is quite good, especially in the root region of the wing. Finally an example of an alternative concept to the soft approach is presented. It is the metal based compliant mechanism with a rather stiff GFRP skin. A discussion on the use of different materials and the way forward towards 3D skin optimization is wrapping up the paper.

In this paper, we describe the fabrication and testing of a tunably-compliant tendon-driven finger implemented through the geometric design of a skeleton made of the low-melting point Field’s metal encased in a silicone rubber. The initial prototype consists of a skeleton comprised of two rods of the metal, with heating elements in thermal contact with the metal at various points along its length, embedded in an elastomer. The inputs to the systems are both the force exerted on the tendon to bend the finger and the heat introduced to liquefy the metal locally or globally along the length of the finger. Selective localized heating allows multiple joints to be created along the length of the finger. Fabrication was accomplished via a multiple step process of elastomer casting and liquid metal casting. Heating elements such as power resistors or Ni-Cr wire with electric connections were added as an intermediate step before the final elastomer casting. The addition of a tradition tendon actuation was inserted after all casting steps had been completed. While preliminary, this combination of selective heating and engineered geometry of the low-melting point skeletal structure will allow for further investigation into the skeletal geometry and its effects on local and global changes in device stiffness.

Single or array FBGs in fiber optics are embedded inside materials through various processes where sometimes integrity of the fibers may not be guaranteed. This paper reports on investigations of various aspects securing integrity of the measurements received from these sensors under various conditions of strains, temperature and geometry of sensors placement while embedding at materials subsurface. FBGs are very sensitive and delicate in the manipulation but offer great comprehensive measurement depending on the applications. These are required for the manual and automated embedding process of the fibers inside metals and hence the tensile, bending and temperature tests. The results have shown that if no adequate conditions of embedding are used, the sensors can be damaged and hence measurement readings can be severely affected. These conditions will constitute the parameters in the process control to be defined and monitored while embedding the sensors.

The paper discusses recent attempts to support the development of nervous materials based on structural health monitoring, augmented with corrective actions using actuators against any external effect or impending failure within the structure. This configuration features embedded sensors inside materials of structural importance e.g. metal and composites that could include comprehensive monitoring in terms of coverage area of the structure, variety of parameters to be measured, types of signals received and with fast information processing capabilities. Fast processing of information would allow the smart material to respond quickly and effectively to any external stimuli e.g. force, pressure or temperature. A review is provided to establish grounds to work on novel methods to embed off-the-shelf sensors for the development of smart material/structures. Actuation options have not been considered in this current review. Existing embedding technologies are reviewed for the sake of refining direction of research and possibilities on improvement of plausible methods. It is envisaged throughout this review to establish a clear understanding of the existing methods and develop improved and alternatives for performance improvement when developing smart and nervous materials.

Shape memory alloys (SMAs), are a class of metals that possess the capability to recover substantial deformations resulting from applied mechanical loads through a solid-solid phase transformation. Typical deployment systems for solar arrays on microsats only allow for one-way deployment. However, by using an SMA actuator in place of a conventional deployment system, repeatable deployment and retraction can be achieved. Relative to conventional actuators, SMA-based solid state actuators offer a reduction in the weight, volume, and overall complexity of the system. In this study, a design of an SMA-based solar panel deployment mechanism for a typical microsat is presented. In this design, a conventional actuation system is replaced with a system of SMA torsional actuators, which allows for a deployed and stowed phase to be reached independent of environmental conditions. This design study illustrates that an SMA-based solar array deployment system can provide a viable replacement for a conventional deployment system while significantly reducing the deployment system weight, volume, and complexity.

This paper has explored and analyzed new routes to design new concepts of materials capable of feeling external effects and feed information back to a monitor and/or react through embedded actuators to resist any deformation. The material with its new artificial sensing property can be easily scaled-up to govern a whole structure at macro scale. The research has investigated a variety of manufacturing routes to build prototypes to be tested for the sake of characterization and performance assessment as well as cost analysis to assess effectiveness. This has included ultrasonic fiber optics embedding in thin Metals e.g. Aluminum which has shown some challenges to be discussed. The host materials included mainly layered manufacturing based materials e.g. powder based materials (Alloy Al6061) and additive process e.g. 3D printing with ABS material. This work has considered samples with concepts having embedded fiber optics in 1D, 2D and 3D. The integrity of the fiber optics and the host materials as well as the sensors performance has been investigated under several conditions of pressure, temperature and geometric placement of the fiber optics. A parametric compromise between materials standard performance and integrity of the sensors is to be found.

This paper describes a new three-dimensional (3D) additive manufacturing (AM) technique in which electroactive polymer filament material is used to build soft active 3D structures, layer by layer. The proposed manufacturing process is well-suited for creating electroactive soft complex structures and devices, whereby the entire system can be manufactured from an electroactive polymer material. For the first time, the unique actuation and sensing properties of ionic polymer-metal composite (IPMC) is exploited and directly incorporated into the structural design to create sub-millimeter scale cilia-like actuators and sensors to macro-scale soft robotic systems. Because ionic polymers such as Nafion are not melt-processable, in the first step a precursor material (non-acid Nafion precursor resin) is extruded into a thermoplastic filament for 3D printing. The filament is then used by a custom-designed 3D printer to manufacture the desired soft polymer structures, layer by layer. Since, at this stage the 3D-printed samples are not yet electroactive, a chemical functionalization process follows, consisting in hydrolyzing the precursor resin in an aqueous solution of sodium hydroxide (NaOH) and dimethyl sulfoxide (DMSO, C 2 H 6 OS). Upon functionalization, metal electrodes are applied on the samples through an electroless plating process, which enables selected areas of the 3D-printed electroactive structures to be controlled by voltage signals for actuation, while other parts can function as sensors. This innovative AM process is described in detail and experimental results are presented to demonstrate the potential and feasibility of creating 3D-printed IPMC actuator samples.