This work reports on the wave transmission characteristics of a hybrid one dimensional (1D) medium. The hybrid characteristic is the result of the coupling between a mechanical waveguide in the form of an elastic beam, and an electrical network. The network configuration investigated is an LC high-pass, consisting of a series of capacitors connected in series through grounded inductors. The capacitors correspond to a periodic array of piezoelectric patches that are bonded to the beam thus coupling the two waveguides. The coupling is characterized by a coincidence frequency/wavenumber corresponding to the intersection of the dispersion curves. At this coincidence frequency, the hybrid medium features attenuation of wave motion as a result of the energy transfer to the electrical network. This energy exchange is depicted in the dispersion by eigenvalue crossing, a particular case of eigenvalue veering. This paper presents the numerical investigations of the wave propagation in the considered medium, and validates the numerical findings with experimental evidence of the wave transmission characteristics. Moreover, the dispersion properties of the electrical network are further studied by varying the inductances thus exploiting the tunability of the periodic electrical domain, i.e: monoatomic and diatmomic unit cell configurations. The LC high-pass network offers several advantages over other configurations, from ease of implementation as the piezoelectric elements are not grounded, to a smaller inductance values to achieve attenuation at a given frequency. Such media could be interfaced with more complex electrical networks to create a new type of smart materials.
Proc. SPIE. 10164, Active and Passive Smart Structures and Integrated Systems 2017
KEYWORDS: Metamaterials, Resonators, Signal attenuation, Manufacturing, Kinematics, 3D modeling, Numerical analysis, Finite element methods, Structural design, Chemical elements, Vibration isolation, Systems modeling, Prototyping
In this contribution, we explore the use of locally resonant metamaterials for multi-functional structural load- bearing concepts using analytical, numerical, and experimental techniques. Locally resonant metamaterials exhibit bandgaps at wavelengths much larger than the lattice dimension. This is a promising feature for low- frequency vibration attenuation. The presented work aims to investigate highly integrated structural concepts and experimentally validated prototypes for vibration reduction in load-bearing applications. The goal is to explore and extend the design space of lightweight structural systems, by designing multi-functional periodic structural elements, preserving structural stiffness while concurrently enabling sufficiently wideband damping performance over a target frequency range of interest. Following a generalized theoretical modeling framework for bandgap design and analysis in finite structures, the focus is placed on the design, fabrication, and analysis of a load-carrying frame development with internally resonant components. Finite-element modeling is employed to design and analyze the frequency response of the frame and simplified analytical solution is compared with this numerical solution. Experimental validations are presented for a 3D-printed prototype. The effects of various parameters are reported both based on numerical and experimental findings.
Recently, the idea to exploit nonlinearity to achieve broadband energy harvesting has been introduced. Bi-stable systems have been used to realise broadband energy harvesting devices. Amongst these, harvesters constructed with bi-stable composites show great potential due to their rich dynamic behaviour. This paper studies a novel cantilevered configuration for a piezoelectric bi-stable composite device for broadband energy harvesting. The cantilevered configuration allows to exploit high strains developed close to the clamped root, further enhancing the harvesting characteristic of bi-stable composites. Furthermore, the desired broadband dynamics are obtained for lower input amplitudes when compared to previous designs constituting a significant improvement for energy harvesting applications. Several cross-well dynamic behaviours are obtained over a relatively wide range of frequencies with the proposed design. In addition, the performance of the developed concept is investigated using a switching shunt harvesting circuit suitable for conversion of broadband oscillations resulting from the cross-well dynamics exhibited by bi-stable composite laminates showing very good results.
A number of adaptive structure applications call for the generation of intense electric fields (in excess of 70 MV/m). Such
intense fields across the thickness of a thin polymer dielectric layer are typically used to exploit the direct electromechanical
coupling in the form of a Maxwell stress:
Where V/d is the applied field, ε0 is the permittivity of vacuum and ε is the relative permittivity of the material. The
field that can be applied to the dielectric is limited by the dielectric strength of the material. Below the limit set by the
breakdown, the material is generally assumed to have a field independent dielectric constant and to be a perfect insulator,
i.e. to have an infinite volume resistivity. While extensive investigations about the mechanical properties of the materials
used for electronic Dielectric Elastomer Actuators (DEA) are available from literature, the results of the investigation of
the insulating and dielectric properties of these materials, especially under conditions (electric field and frequency) similar
to the ones encountered during operation are not available. In the present contribution, we present a method and a set-up
for the measurement of the electric properties of thin polymer films, such as the ones used for the fabrication of electronic
DEAs, under conditions close to operations. The method and setup where developed to investigate the properties of
'stiff' thin polymer films, such as Polyimide or Polyvinylidenefluoride, used for Electro-Bonded Laminates (EBLs). The
properties of the well known VHB 4910 acrylic elastomer are presented to illustrate how the permittivity and the leakage
current can be measured as a function of the electric field and the deformation state, using the proposed set-up. The material
properties were measured on membranes under different fixed pre-stretch conditions (λ 1, λ2=3, 4, 5), in order to eliminate
effects due to the change in sample geometry, using gold sputtered electrodes, 20nm thick. The values obtained for the
permittivity of the material are in good agreement with the work of other authors. The dissipative properties revealed by
the measurements performed at high fields, similar to the ones encountered in operation, indicate that this less investigated
aspect of VHB needs to be taken in consideration for real world applications.
Recently micro-structured solid electrodes were applied to dielectric elastomers. Compared with
common powder or liquid electrodes the electrical conductivity of the electrodes is enhanced while
the compliance necessary for large active deformations is retained. Envisaged applications range
from energy harvesting to structural damping and actuation. The compliant conducting electrodes
can be attached to the passive dielectric materials in a standardized and well controlled process. This
enhanced the general interest in the technology and more applications are expected. While some of
these applications aim for maximum actuation performance others require a superior reliability at
moderate performance. For both strategies design and optimization of the active parts are essential.
This work provides results of an extensive experimental characterization of the passive and active
response of a novel PolyPower membrane. We further developed a 3D nonlinear viscoelastic model
suitable for finite element simulation and verified the main assumptions of the modeling approach
with mechanical tests. The model is shown to provide good predictions of both passive behavior as
well as active deformation of an actuator system.
Interpenetrating polymer network reinforced acrylic elastomers (IPN) offer outstanding performance in free-standing
contractile dielectric elastomer actuators. This work presents the verification of a recently proposed material model for a
VHB 4910 based IPN . The 3D large strain material model was determined from extensive data of multiaxial
mechanical experiments and allows to account for the variations in material composition of IPN-membranes. We
employed inflation tests to membranes of different material composition to study the materials response in a stress state
different from the one that was used to extract the material parameters. By applying the material model to finite element
models we successfully validated the material model in a range of material compositions typically used for dielectric
elastomer actuator applications. In combination with a characterization of electro-mechanical coupling, this 3D large
strain model can be used to model IPN-based dielectric elastomer actuators.
The adaptive modification of the mechanical properties of structures has been described as a key to a number of new or
enhanced technologies, ranging from prosthetics to aerospace applications.
Previous work reported the electrostatic tuning of the bending stiffness of simple sandwich structures by modifying the
shear stress transfer parameters at the interface between faces and the compliant core of the sandwich. For this purpose,
the choice of a sandwich structure presented considerable experimental advantages, such as the ability to obtain a large
increase in stiffness by activating just two interfaces between the faces and the core of the beam.
The hypothesis the development of structures with tunable bending stiffness is based on, is that by applying a normal
stress at the interface between two layers of a multi-layer structure it is possible to transfer shear stresses from one layer
to the other by means of adhesion or friction forces. The normal stresses needed to generate adhesion or friction can be
generated by an electrostatic field across a dielectric layer interposed between the layers of a structure. The shear stress
in the cross section of the structure (e.g. a beam) subjected to bending forces is transferred in full, if sufficiently large
normal stresses and an adequate friction coefficient at the interface are given. Considering beams with a homogeneous
cross-section, in which all layers are made of the same material and have the same width, eliminates the need to consider
parameters such as the shear modulus of the material and the shear stiffness of the core, thus making the modelling work
easier and the results more readily understood.
The goal of the present work is to describe a numerical model of a homogeneous multi-layer beam. The model is
validated against analytical solutions for the extreme cases of interaction at the interface (no friction and a high level of
friction allowing for full shear stress transfer). The obtained model is used to better understand the processes taking place
at the interfaces between layers, demonstrate the existence of discrete stiffness states and to find guidance for the selection
of suitable dielectric layers for the generation of the electrostatic normal stresses needed for the shear stress transfer at the
The main cables of suspension bridges are often wrapped with a steel wire, in order to compact the cable and hold it in
shape. If a non-destructive evaluation by means of magnetic methods is performed on such a cable, disturbances due to
the wrapping can be expected in the measured signal. In the presented work, these disturbances shall be quantified and
compared to the flaw signals. Different approaches for the separation of the disturbance and the flaw signal are discussed.
Additionally, the possibility to detect wire breaks and corrosion within an unwrapped steel cross-section could be shown
in laboratory measurements. The influence of the wrapping was investigated using finite element (FE) simulations and
experimental laboratory measurements. A parameter study was performed in order to obtain data in which the components
from a flaw and the wrapping can be separated. The parameters varied in this study were chosen depending on the prospect
of success and the cost of the realization. Using these data sets different filtering methods, such as wavelet analysis, were
implemented. A final comparison of the different methods suggests the most efficient way to assess the condition of such
cable systems using magneto-inductive testing. Finally, it can be concluded that the use of FE simulation is a very useful
tool for the development of new data analysis methods, even if a real set-up and data from measurements exist.
The suppression of vibrations of a structure is commonly considered a necessary measure for the extension of its
lifetime, when high amplitude vibrations are observed. As an alternative to the introduction of discrete damping
devices, the modification of the stiffness of a beam is proposed as a means to suppress vibrations due to resonance,
thank to the ability to reject mechanical energy input at specific frequencies. Previous work has outlined the
principle and the potential advantages of such an approach based on the behavior of a small scale system. In
order to confirm the feasibility of the approach on macro-scale systems, such as a light weight pedestrian bridge,
experiments for the tuning of a 2.5 m long glass fiber reinforced polymer I-beam were performed. The results
of the experiments show that it is possible to modify the bending stiffness of structural elements that can be
used for real life engineering applications. Measurements show that it is possible to shift the resonance peak of
a beam while maintaining a reasonably good q-factor in the transfer function, thus indicating that the change
in behavior happens in connection with an increased stiffness rather than with the introduction of substantial
damping. Based on the presented feasibility study, the development of an adaptive bridge deck will be considered.
Vibration control and suppression in structures plays a central role in the extension of their service life and improvement of their reliability. While in many cases the solution of this problem implies the introduction of external damping devices, it is also conceivable to adaptively modify their vibratory properties, so that the occurrence of severe vibrations due to resonance phenomena can be curbed at its origin. The modification of the shear stress transfer at the interface between the core and the faces of a sandwich beam has been shown to have a remarkable effect on the bending stiffness of the structure. Such modification can be obtained by applying a normal stress between the core and the un-bonded, electrically insulated faces of the sandwich by means of a strong electrical field.
An intermediate behavior between fully bonded and un-bonded layers in terms of inter-laminar shear stress can be achieved by temporary electrostatic bonding of the components. The outlined approach to the reduction of transversal vibrations in thin multi-layer beams is promising and can in principle be applied to multi-layer plates.
Experimental work performed on several full-scale stay-cable models as well as on RAMA IX Bridge in Bangkok has confirmed that the application of magnetic flux leakage (MFL) methods is a viable approach to the non-destructive evaluation of large diameter steel cables. Such method allows for a high sensitivity and high-resolution detection of fractured wires in stay cable systems.
So far, the information obtained from the recorded data (intensity of the MFL on the surface of a cable) was limited to the accurate position of detected flaws along the axis of the cable and a qualitative indication of the position of the flaws within the cross-section. The ability to accurately determine the position of flawed wires within the cross-section of a cable is especially useful in the case of multi-strand systems, in which individual strands can be replaced if damaged.
Such information can be obtained by computation with finite element models or sophisticated dipole approximations. An alternative to such computing intensive approach, based on a simple mathematical model of the MFL function is proposed in this work. The function is used for a non-linear fit of the measured data. The method has been tested successfully on simulated and measured data.