Vidyashankar R. Buravalla

Advances in damping materials and technology

Abstract In the continual search for better damping materials and technologies, significant advances have been made of late. Functionally gradient materials, liquid crystal polymers, magnetostrictive materials and plasma deposited damping coatings are some of the novel materials and technologies being investigated in the Dynamics Research Group at the University of Sheffield. This article presents an overview of the work being carried out in these areas.

Active and passive vibration control of flexible structures using a combination of magnetostrictive and ferromagnetic alloys

A combined passive and active damping strategy is proposed to control vibration in structures using a combination of layers of ferro-magnetic (passive) damping and smart (active) magnetostrictive material (Terfenol-D). Two types of combined damping systems are considered viz., a noninteractive system and an interactive or hybrid system. Numerical investigations on a cantilever beam model are carried out to investigate various aspects in combined damping scenario. Using variational principle, a beam Finite Element is developed to study the dynamic characteristics of a beam containing both the passive and active damping layers. It is shown that the combined system could be used effectively to dampen the structural vibration over a wide frequency range. Comparisons with only passive and only active damping schemes are also made. The influence and the mode dependence of control gain in a hybrid system is clearly brought out.

A Correction to the Brinson's Evolution Kinetics for Shape Memory Alloys

Phase diagram based approach is frequently used to investigate the shape memory alloy behavior. Typically, evolution laws for the martensitic phase fraction are defined for different local transformation regions in the stress—temperature phase diagram. In the Brinsons evolution kinetics, under certain conditions, the local evolution law for the formation of martensitic fraction leads to inadmissible phase fraction (>1). This pertains to a case where both stress- and temperature-induced martensite evolve simultaneously. A modified evolution law is presented here to overcome this anomaly.

Plasma deposition of constrained layer damping coatings

Abstract Plasma techniques are used to generate constrained layer damping (CLD) coatings on metallic substrates. The process involves the deposition of relatively thick, hard ceramic layers on to soft polymeric damping materials while maintaining the integrity of both layers. Reactive plasma sputter-deposition from an aluminium alloy target is used to deposit alumina layers, with Youngs modulus in the range 77–220GPa and thickness up to 335 μ, on top of a silicone film. This methodology is also used to deposit a 40μ alumina layer on a conventional viscoelastic damping film to produce an integral damping coating. Plasma CLD systems are shown to give at least 50 per cent more damping than equivalent metal-foil-based treatments. Numerical methods for rapid prediction of the performance of such coatings are discussed and validated by comparison with experimental results.

Model for resistance evolution in shape memory alloys including R-phase

The electrical resistance behavior of a shape memory alloy (SMA) wire can be used for sensing the state of an SMA device. Hence, this study investigates the resistance evolution in SMAs. A lumped parameter model with cosine kinetics to capture the resistance variation during the phase transformation is developed. Several SMA materials show the presence of trigonal or rhombohedral (R) phase as an intermediate phase, apart from the commonly recognized austenite and martensite phases. Most of the SMA models ignore the R-phase effect in their prediction of thermomechanical response. This may be acceptable since the changes in thermomechanical response associated with the R-phase are relatively less. However, the resistivity related effects are pronounced in the presence of the R-phase and its appearance introduces non-monotonicity in the resistivity evolution. This leads to additional complexities in the use of resistance signal for sensing and control. Hence, a lumped model is developed here for resistance evolution including the R-phase effects. A phase-diagram-based model is proposed for predicting electro-thermomechanical response. Both steady state hysteretic response and transient response are modeled. The model predictions are compared with the available test data. Numerical studies have shown that the model is able to capture all the essential features of the resistance evolution in SMAs in the presence of the R-phase.

Phenomenological Modeling of Shape Memory Alloys

Shape memory alloys exhibit two characteristic effects, viz., shape memory and superelasticity or pseudoelasticity, due to a reversible solid‐solid transformation brought about by either temperature or stress or both. The two important aspects involved in modeling the macroscopic SMA behavior are the constitutive equation describing the stress‐strain‐temperature relationship and the evolution kinetics describing the phase transformation as a function of the driving forces. Phenomenological models for macroscopic behavior of SMAs are frequently used wherein the aforementioned aspects of SMA behavior are treated independently. Using empirical data, a phase diagram is constructed to describe evolution of martensitic phase fraction (ξ) as a function of stress and temperature. A constitutive equation is derived using the appropriate form of free energy. In this paper, salient aspects in phenomenological models are discussed and a robust model for SMA behavior is presented. Using a distance based memory paramet...

Enhancement and evaluation of damping performance in layered CLD type coatings

One of the highly effective Layered damping methods is Constraining Layer Damping (CLD), wehrein a layer of viscoelastic (VE) material is sandwiched between the host structure and a stiff Constraining Layer (CL). Traditional CLD uses metallic Cls and hence all the damping comes from the VE layer. However, Cls of certain nonmetallic and/or anisotropic materials with significant inherent damping can be considered to enhance the net damping performance. A single degree of freedom (SDOF) model is presented here to investigate the performance of layered damping coatings comprising a VE layer and a stiff CL. The proposed model considers both the bending and extensional stiffness of the host structure and constraining layer separately, in addition to the shear stiffness of the VE layer. This facilitates the incorporation of some anisotropic stiffness effects and a study of its influence on the damping. The Complex stiffness is used to model the damping in individual components. Structural loss factors are obtained as a function of suitable dimensionless stiffness parameters. The useful range of modulus and the thickness of the layers/coatings are identified to obtain desired level of damping for different material loss factors enabling proper choice of the materials and/or thickness of damping treatments. The model can be reduced to represent conventional free or CLD coatings. The proposed model is validated by comparing the results with those from closed form solutions for traditional CLD systems and finite element results for anisotropic systems.

Hybrid vibration control of laminated composite structures using magnetostrictive and hard damping materials

Smart laminated composites with layers of magnetostrictive and/or piezoelectric materials have been explored for active damping applications. Active vibration control in laminated structures with low inherent damping, may lead to instability. In some other cases like flexible appendages of satellites, design considerations may impose constraints in collocated sensing and actuation in structure which would also cause degradation in system performance if only active control is used. Ideally, both structural damping and active control are desirable to achieve good dynamic performance. Of late, hard ceramic materials with hysteretic stress- strain behavior are used in passive vibration control. Because of high strain dependency of the damping in these materials, especially near the high damping regime, it is possible to enhance the amount of passive damping by active control of strain in the passive layer. Because of the layered structure in laminates, several interesting possibilities for hybrid damping exist and a few of them are proposed and explored in the present study. Smart composites are magnetostrictive (Terfenol-D) layers and strain dependent ceramic and/or ferroelectric Passive Damping (PD) layers are considered. The damping achieved in the PD layers is controlled by varying their location and/or the amount of gain in the active controller. Both frequency and time domain studies are carried out to investigate the performance of proposed hybrid damping systems. Simulations using recently developed smart beam elements are carried out to investigate the performance of proposed hybrid damping systems. Simulations using recently developed smart beam elements are carried out on laminates with several configurations of active and passive layers. The simulation brings out the significance and the exploitation of strain dependency of passive damping on the overall damping of the hybrid system.

A Generalized Three Species Model for Shape Memory Alloys

In this work, an existing three-species model for describing Shape Memory Alloy (SMA) behavior is generalized to incorporate more relevant effects like asymmetric hysteresis, two-way memory and different forms of hardening. Using the notion of multiple yield surfaces for describing different transformation paths within the existing thermodynamic framework, necessary and sufficient conditions are derived for describing phase transformation. A few case studies are provided to illustrate the efficacy of the proposed generalized model.