In this work, the arising of stick-slip dissipation as well as the global mechanical response of carbon nanotube (CNT) nanocomposite films are tailored by exploiting a three-phase nanocomposite. The three phases are represented by the CNTs, a polymer coating localized on the CNTs surface and a hosting matrix. In particular, a polystyrene (PS) layer coats multi-walled carbon nanotubes (MWNTs) that are randomly dispersed in a polyimide (PI) matrix. The coating phase is strongly bonded to the CNTs outer sidewalls ensuring the effectiveness of the load transfer mechanism and reducing the material damping capacity. The coating phase can be thermally-activated to modify, and in particular, decrease the CNT-matrix interfacial shear strength (ISS) thus facilitating the stick-slip onset in the nanocomposite. The ISS decrease finds its roots in a partial degradation of the coating phase and, in particular, in the formation of voids. By weakening the CNT/polymer interfacial region, a significant enhancement in the material damping capacity is observed. An extensive experimental campaign consisting of monotonic and cyclic tensile tests proved the effectiveness of this novel multi-phase material design.

In the development of high-performance hypersonic vehicles, the use of high lift and low drag concepts demonstrates improvements in thermal loading, vehicle accelerations and load factors, and total energy dissipation. The key design limitation of hypersonic waveriders is that the optimal waverider geometry is highly dependent on Mach number and deviations from the design point may significantly degrade performance. This paper addresses this fundamental limitation of waveriders by evaluating the ability to morph the bottom surface of a waverider to provide the optimal point-design performance across a broad operational range. This paper addresses the first step required to delivering this morphing capability by addressing the number of morphing control points required to accurately match the complex surfaces of the waverider for multiple Mach number designs. Additionally, the tradeoff between number of additional control points and surface error is investigated. To achieve this, a sensitivity analysis using a Q-DEIM algorithm is performed to identify and rank the optimal set of control. This control point set is verified through analysis and used to evaluate the performance of a morphing waverider structure. Future efforts will evaluate Mach number dependent pressure distribution, aerodynamic structural loading, and thermal loading. This work simply identifies the scale of the control problem and identifies a methodology to actuate a complex surface to enable viable waverider vehicles that maintain optimum performance across a range of Mach numbers.

Macro-fiber composite (MFC) piezoelectric materials are used in a variety of applications employing the converse piezo-electric effect, ranging from bioinspired actuation to vibration control. Most of the existing literature to date considered linear material behavior for geometrically linear oscillations. However, in many applications, such as bioinspired locomotion using MFCs, material and geometric nonlinearities are pronounced and linear models fail to represent and predict the governing dynamics. The predominant types of nonlinearities manifested in resonant actuation of MFC cantilevers are piezoelectric softening, geometric hardening, inertial softening, as well as internal and external dissipative effects. In the present work, we explore nonlinear actuation of MFC cantilevers and develop a mathematical framework for modeling and analysis. An in vacuo actuation scenario is considered for a broad range of voltage actuation levels to accurately identify the sources of dissipation. Several experiments are conducted for an MFC bimorph cantilever, and model simulations are compared with nonlinear experimental frequency response functions under resonant actuation. The resulting experimentally validated framework can be used for simulating the dynamics of MFCs under resonant actuation, as well as parameter identification and structural optimization for nonlinear operation regime.

Based on a recently developed shakedown theory for non-smooth nonlinear materials, we derive a criterion for high-cycle fatigue in shape memory alloys (SMAs). The fatigue criterion takes into account phase transformation as well as reorientation of martensite variants as the source of fatigue damage. The mathematical derivation of the criterion is based on the requirement of elastic shakedown for a given structure to achieve unlimited fatigue endurance. Elastic shakedown is defined as an asymptotic state in which damage due to time-varying load becomes confined at the mesoscopic scale, or the scale of the grain, with no discernable inelasticity at the macroscopic scale. From an energy standpoint, elastic shakedown corresponds to a situation where energy dissipation becomes bounded and the response elastic after a certain number of loading cycles. A sufficient condition to achieve this state was established by Melan (1936) [1] and Koiter (1960) [2] for elastoplastic materials and later generalized to hardening plasticity by Nguyen (2003) and to non-smooth non-linear materials by Peigney (2014). The latter formulation is applicable to SMAs obeying the ZM constitutive model (Zaki & Moumni, 2007) and is shown here to allow the derivation of a high-cycle fatigue criterion analogous to the one proposed by Dang Van (1973) for elastoplastic materials. The criterion allows establishing a safe domain in stress deviator space at the mesoscopic scale consisting of a hypercylinder with axis parallel to the direction of martensite orientation. The hypercylinder is delimited along its axis by two transverse hyperplanes representing bounds on admissible stress states consistent with the loading conditions for phase transformation. Safety with regard to high-cycle fatigue, upon elastic shakedown, is conditioned by the persistence of the macroscopic stress path, as the load varies and at every material point, strictly within the hypercylinder. The size of the hypercylinder is shown to strongly depend on the relative amount of martensite present in the SMA.

Fiber reinforced polymer (FRP) composites have been increasingly used in engineering applications due to their lightweights, high strength, and high corrosion resistance. However, the conventional FRPs exhibits brittle failure, low toughness, limited fatigue strength, and relatively low ultimate tensile strains. Shape memory alloys (SMAs) are a class of metallic alloys that can recover large strains upon load removal with minimal residual deformations. Besides their ability to recover large deformations, SMAs possess excellent corrosion resistance, good energy dissipation capacity, and high fatigue properties. This study explores the use of superelastic SMA fibers to reinforce a thermoset polymer matrix to produce a polymer composite with enhanced mechanical properties. Nickel-Titanium wires with a diameter of 495 micrometer are used as fibers. SMA coupons with different reinforcement ratios are fabricated using a special-made mold and following a modified hand lay-up technique. The uniaxial tensile tests are conducted under cyclic loading protocols. The results of the tests are assessed in terms of ultimate strength, ultimate strain, residual strain, and failure modes of the composites.

Structural health monitoring can enhance reliability, increase safety, and decrease maintenance costs by detecting damage at an early stage. By taking advantage of the electromechanical coupling, piezoelectric materials have the potential to harvest energy from ambient vibration sources to provide low-power electricity for self-powered electronic devices. In comparison with other piezoelectric transducers, zinc oxide (ZnO) nanowires carry the added advantages of structural flexibility, lower cost, compactness, and lighter weight. In this study, the energy harvesting capabilities of nanoscale ZnO piezoelectric nanowires (NW) grown on the surface of glass fiber fabrics are investigated experimentally. A series of cantilevered carbon fiber beams containing a controlled amount of ZnO nanowires is evaluated. The absolute electrical energy dissipation is quantified by measuring the output power over a broad spectrum of known vibratory loads and frequencies. The maximum amount of power extracted is obtained by employing resistive impedance matching. Here, a maximum peak of ∼6.7 mV was generated when the beam containing ZnO nanowires was excited at 2.90g and connected to a 10 MΩ load. At that excitation level, a maximum of 20.0 pW was generated when an optimal resistor of 1 MΩ is connected. A tip mass of ∼0.6 gram added to the sample with ZnO NWs increased the peak-voltage by 2.21 mV and increased the peak-power by 13.3 pW. A series of DC voltage applied to the ZnO sample suggests the equivalence of poling treatment, where the dipole alignment of the ZnO NWs are disrupted. Here, a maximum peak-power of 45 pW is reported, showing promising potential of scaling-up to harvest ambient energy for low-powered electronics.

As the required power for wireless, low-power sensor systems continues to decrease, the feasibility of a fully self-sustaining, onboard power supply, has increased interest in the field of vibration energy harvesting — where ambient kinetic energy is scavenged from the surrounding environment. Current literature has produced a number of harvesting techniques and transduction methods; however, they are all fundamentally similar in that, the harmonic excitation frequency must fall within the resonant bandwidth frequency of the harvesting mechanism to maintain acceptable energy output. The purpose of this research is to investigate the potential for natural frequency tuning by means of passive electrical components, that is, using an imposed electrical inductance to adjust the equivalent stiffness, and resulting resonant frequency of a vibration energy harvester. In past literature, it was concluded that an “active” frequency tuning mechanism would be infeasible, as the power required by an equivalent “stiffening transducer” would require more power to maintain the system at resonance, than the system would actually produce as a result of maintaining resonance, i.e., a net energy loss (Roundy and Zhang 2005). It is believed that the model used in this conclusion can be improved by directly modeling changes in system stiffness as an equivalent mechanical spring, instead of an external inertial loading. Due to the conservative nature of the harmonic spring, the compliance of a harvesting mechanism can be theoretically altered without energy losses, whether the actuation is applied using “active” or “passive” means. This revised model departs from the traditional, base excitation model in most vibration energy harvesting systems, and includes additional stiffness, and damping elements, representative of induced mechanical spring, and related losses. We can validate the feasibility of this technique, if it can be shown that the natural frequency of an energy harvester can be altered, and still maintain energy output similar to its “untuned” natural frequency. If feasible, this tuning method would provide a viable alternative to other bandwidth-increasing techniques in literature, e.g., wideband harvesting, bandwidth normalizing, high-damping, etc. In this research, a change in natural frequency of the experimental energy harvesting system of 0.5 Hz was demonstrated, indicating that adjusting the natural frequency of a vibration energy harvesting system is possible; however, there are many new challenges associated with the revised energy harvesting model, related to the new introduced losses to the system, as well as impedance matching between the mechanical and electrical domains. Further research is required to better quantitatively characterize the relationship between natural frequency shift, and imposed electrical inductance.

This study presents mechanical energy dissipation with a proof-of-concept prototype magnetostrictive (Fe-Ga alloy, galfenol) based shunt circuit using passive electrical components. Magneto strictive material can harvest electricity out of the structural vibrations based on the Villari effect using permanent magnet and pickup coil configuration. The device in this study consists of a polycrystalline galfenol strip bonded to a brass cantilever beam. Two brass pieces, each containing a permanent magnet, are used to mass load at the end of the beam and to provide a magnetic bias field through the galfenol strip. The voltage induced in an induction coil closely wound around the cantilever beam captures the time rate of change of magnetic flux within the galfenol strip as the beam vibrates. To dissipate the electrical voltage output from the pickup coil and/or to change the phase of eddy current from the magnetic flux density fluctuation, a shunt circuit is attached. The effective mechanical impedance for the magnetostrictive shunt circuit is derived in a model. The effectiveness of a series L-R and L-C shunt circuit is demonstrated theoretically and experimentally. The non-linear model parameters, which include the mechanical-magnetic coupling factors, α and α T , and the permeability of galfenol, β , are extracted from experimental measurement. The shunted magnetostrictive damping model of both resistive and capacitance shunt cases compare well with the experimental results.

This paper presents a finite element based numerical study on controlling the postbuckling behavior of thin-walled cylindrical shells under axial compression. With the increasing interest of various disciplines for harnessing elastic instabilities in materials and mechanical systems, the postbuckling behavior of thin-walled cylindrical shells may have a new role to design materials and structures at multiple scales with switchable functionalities, morphogenesis, etc. In the design optimization approach presented herein, the mode shapes and their amplitudes are linearly combined to generate initial geometrical designs with predefined imperfections. A nonlinear postbuckling finite element analysis evaluates the design objective function, i.e., the desired postbuckling force-displacement path. Single and multi-objective optimization problems are formulated with design variables consisting of shape parameters that scale base eigenvalue shapes. A gradient-based algorithm and numerical sensitivity evaluations are used. Results suggest that an optimized shape for a cylindrical shell can achieve a targeted response in the elastic postbuckling regime with multiple mode transitions and energy dissipation characteristics. The optimization process and the obtained geometry can be potentially used for energy harvesting and other sensing and actuation applications.

Traditional composite materials invented to be used in structures with the purpose of high load-bearing with excellent in-plane properties. Continuous fiber reinforced composites are one of the mostly used categories of advanced composites. This class of composites has gained a lot of attention due to their light-weight and decent mechanical properties. However, additional material design is required to tune both mechanical and structural properties of these composites. Since the load transfer between reinforcement phase and polymer matrix happens at the interfacial region, a better interphase might result in a composite with higher vibration damping. In this study, a gradient interphase between carbon fiber and polymer matrix has been created by using ZnO nanowires to engineer the damping loss factor of the carbon fiber composites. For the growth of ZnO nanowires on the carbon fabric, low temperature hydrothermal reaction has been used. Then the carbon fabrics with ZnO nanowires were infiltrated with a low viscosity epoxy using vacuum assisted resin transfer molding technique. The stiffness and structural damping of the composite were examined using dynamic mechanical analysis. The results show that the damping properties of hybrid composites using ZnO nanowires are enhanced compare to the bare carbon fabric composites. Since the growth of ZnO nanowires is a tunable process, the length, diameter and aspect ratio of the nanowires and consequently the architecture of the interphase can be tailored for the desired vibration damping in the system. Thus, the hybrid composites with ZnO nanowire interphase can be used to enhance the energy dissipation in a structural system.

In this paper two innovative concepts for adjustable energy absorbing elements are presented. These absorbers can serve as an essential element in a smart crash management system e.g. for automotive applications. The adaptability is based on the basic idea of adjusting the stiffness of the absorber in relation to the actual load level in a crash event. Therefore the whole length of the absorber element can be used for energy dissipation. The adjustable absorbers are made from fiber reinforced plastics and shape memory alloy wires as actuating elements. Two possibilities for the basic design of the absorber elements are shown, the performance of the actuating SMA elements is characterized in detail and the switching behavior of the whole elements, between a stiff “on” state and a flexible “off” state, is measured.

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.

Novel semi-active vibration controllers are developed in this study for magnetorheological (MR) fluid-based vibration control systems, including: (1) a band-pass frequency shaped semi-active control algorithm, (2) a narrow-band frequency shaped semi-active control algorithm. These semi-active vibration control algorithms designed without resorting to the implementation of an active vibration control algorithms upon which is superposed the energy dissipation constraint. These new Frequency Shaped Semi-active Control (FSSC) algorithms require neither an accurate damper (or actuator) model, nor system identification of damper model parameters for determining control current input. In the design procedure for the FSSC algorithms, the semi-active MR damper is not treated as an active force producing actuator, but rather is treated in the design process as a semi-active dissipative device. The control signal from the FSSC algorithms is a control current, and not a control force as is typically done for active controllers. In this study, two FSSC algorithms are formulated and performance of each is assessed via simulation. Performance of the FSSC vibration controllers is evaluated using a single-degree-of-freedom (DOF) MR fluid-based engine mount system. To better understand the control characteristics and advantages of the two FSSC algorithms, the vibration mitigation performance of a semi-active skyhook control algorithm, which is the classical semi-active controller used in base excitation problems, is compared to the two FSSC algorithms.

As the recent structures seek lighter weight, damping becomes a more critical issue. Decreasing weight increases flexibility causing vibrations to become more prevalent. Damping treatments to reduce unwanted vibrations are usually classified into three categories: passive, semi active and active depending on the degree to which external energy and complexity is needed to achieve the required reduction in vibration. Here we examine the use of a multifunctional structure’s philosophy to introduce and research the concept of “electronic damping” offering an alternative to traditional damping solutions and a capability of providing uniform energy dissipation across a wide range of ambient frequencies and temperatures. The proposed research addresses increasing the range of effectiveness of damping by addressing the temperature and frequency dependence of material damping by using a multifunctional composite system containing an active element. Our approach is to model the mechanics using Lagrange’s formulation for multi-physics systems and to experimentally validate our models using careful experiments. We propose to examine the strength models and properties of the system and to examine the performance by constructing and testing some prototype systems. The focus is on both the electrical integration and the structural integration of the different material systems required to design a completely stand alone multifunctional composite with superior damping properties useful for suppressing vibrations.

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.

In this paper, we describe preliminary analysis for a method of constructing homogeneous actuated electromechanical composite structures from many smaller electromechanical “active cells”. The preliminary cell design consists of a contractile shape memory alloy (SMA) elements (Nitinol) connecting two electrically conductive terminals. The composite structure is actuated by applying a gross electrical potential across the entire structure. This causes connected cells to activate through resistive heating. The material properties of the ensemble structure can be varied by changing the properties of the binding material, enabling a wide range of stiffness and damping properties. A structure made from these cells can be easily fabricated in a nearly arbitrary shape — simply by producing an appropriate mold — and would be highly redundant and robust to localized cell failures. The primary focus of this paper is to perform a static analysis of the connectivity of arrays of active cells and the associated power dissipation in the individual cells.

The nonlinear, hysteretic stress-strain characteristic of superelastic shape memory alloys (SMA) results in energy dissipation and therefore in high damping capacities. Due to the nonlinearity the damping capacity strongly depends on the amplitude of the applied excitation. In this work, a rheological non-smooth model is used to describe the principle behavior of superelastic SMA undergoing harmonic displacements. The equivalent mechanical model consists of a spring representing the elastic deformation of the superelastic SMA in austenitic and detwinned martensitic state. A friction element represents the stress plateaus for forward and backward transformation between austenitic and martensitic state. A constant force is applied to the system to generate an offset which shifts the hysteresis to positive force values. Two mechanical stops are implemented to describe the end of the stress plateaus and therefore correspond to the strain differences of the stress levels for forward and backward transformation. Thus, the system behavior is highly amplitude-dependent. A harmonic approximation of the force generated by the superelastic SMA element during one excitation period is calculated by applying the Harmonic Balance Method to the nonlinear force signal of the rheological model. In this context the Fourier coefficients are calculated by performing a piecewise integration of the force signal. The Integrals are being calculated for each steady interval. The equivalent stiffness and damping coefficients are given for this approximation as functions of excitation amplitude and the system parameters. Based on these results, the damping capacity of a superelastic shape memory element undergoing harmonic displacements is presented using an analytical expression for the damping ratio.

A thermodynamically consistent model to simulate the electromechanical response of ionic polymer-metal composite (IPMC) beams has been developed based on Euler-Bernoulli beam theory. Appropriate assumptions have been made and suitable forms for the Helmholtz free energy and the rate of dissipation have been chosen. The governing equations, describing the actuation and sensing behavior of IPMC strips in air, have been formulated using a set of kinematic assumptions, the power theorem, and the maximum rate of dissipation hypothesis, neglecting inertial effects. The model has been extended to solve for large deformations in IPMC cantilevers with certain loading conditions. The model has been shown to simulate the electromechanical responses of both Nafion and Flemion based IPMC strips. This includes the initial overshoot followed by a gradual back-relaxation observed in the tip deflection measurements of Nafion based IPMC strips under the application of a step voltage. It has been shown that a coupled convective heating term in the rate of dissipation function is crucial for simulating this overshoot and the back relaxation.

A negative capacitance shunt is a basic, analog, active circuit electrically connected to a piezoelectric transducer to control vibrations of flexural bodies. The electrical impedance of the negative capacitance shunt modifies the effective modulus of the piezoelectric element to reduce the stiffness and increase the damping which causes a decrease in amplitude of the vibrating structure to which the elements are bonded. The negative capacitance circuit is built around a single operational amplifier using passive circuit elements. To gain insight into the electromechanical coupling, the power consumption of the op-amp and the power dissipated in the resistive element are measured. The power output of the op-amp increases for increasing control gain of the negative capacitance. The power characteristics of the shunt are compared to the reactive input power analysis developed in earlier work.

Piezoelectric shunt damping is a well known technique to damp the vibrations of mechanical structures. The design of the electrical shunt aims to maximize the energy dissipation. Beside linear networks, switching circuits are a technique that can adapt to varying excitation frequencies. In this paper, the maximum damping performance of the SSDI technique is analyzed and compared to linear LR-shunts. It is shown that it can perform about 60% better.