Damage Detection in Flexible Propeller Beam Structures by Exploiting Impact-Induced Coupled Acceleration Signals

A local damage at the tip of a composite propeller is diagnosed by properly comparing its impact-induced free coupled dynamics to that of a pristine wooden propeller of the same size and shape. This is accomplished by creating indirectly via collocated measurements distributed information for the coupled acceleration field of the propellers. The powerful data-driven modal expansion analysis delivered by the Proper Orthogonal Decomposition (POD) Transform reveals that ensembles of impact-induced collocated coupled experimental acceleration signals are underlined by a high level of spatio-temporal coherence. Thus they furnish a valuable spatio-temporal sample of coupled response induced by a point impulse. In view of this fact, a tri-axial sensor was placed on the propeller hub to collect collocated coupled acceleration signals induced via modal hammer nondestructive impacts and thus obtained a reduced order characterization of the coupled free dynamics. This experimental data-driven analysis reveals that the in-plane unit components of the POD modes for both propellers have similar shapes-nearly identical. For the damaged propeller this POD shape-difference is quite pronounced. The shapes of the POD modes are used to compute indices of difference reflecting directly damage. At the first POD energy level, the shape-difference indices of the damaged composite propeller are quite larger than those of the pristine wooden propeller.

Design Equations for Binary Shape Memory Actuators Under Arbitrary External Forces

The paper presents the design equations for an on-off shape memory alloy actuator under an arbitrary system of external constant forces. A binary SMA actuator is considered where a cursor is moved against both conservative and dissipative force which may be different during the push or pull phase. Three cases are analyzed and differentiated in the way the bias force is applied to the primary SMA spring, using a constant force, a traditional spring, or a second SMA spring. Closed-form dimensionless design equations are developed, which form the basis of a step-by-step procedure for an optimal design of the whole actuator.

Nonlinear Stochastic Feedback Controllers for Vibratory Energy Harvesters With Power Flow Constraints

This study addresses the formulation of feedback controllers for stochastically-excited vibratory energy harvesters. Maximizing power generation from stochastic disturbances can be accomplished using LQG control theory, with the transducer current treated as the control input. For the case where the power flow direction is unconstrained, an electronic drive capable of extracting as well as delivering power to the transducer is required to implement the optimal controller. It is demonstrated that for stochastic disturbances characterized by second-order, bandpass-filtered white noise, energy harvesters can be passively tuned such that optimal stationary power generation only requires half of the system states for feedback in the active circuit. However, there are many applications where the implementation of a bi-directional power electronic drive is infeasible, due to the higher parasitic losses they must sustain. If the electronics are designed to be capable of only single-directional power flow (i.e., where the electronics are incapable of power injection), then these parasitics can be reduced significantly, which makes single-directional converters more appropriate at smaller power scales. The constraint on the directionality of power flow imposes a constraint on the feedback laws that can be implemented with such converters. In this paper, we present a sub-optimal nonlinear control design technique for this class of problems, which exhibits an analytically computable upper bound on average power generation.

Material Characterization and Mid-Span Bending Capacity With Finite Element Simulated Predictions

Intelligent materials have been the subject of research for many years. Shape memory alloys (SMAs) are a type of intelligent material that has been targeted for many different uses; such as actuators, sensors and structural supports. SMAs are attractive as actuators due to their large energy density. Although a great deal of information is available on the axial load capacity and on the tip force for SMA tweezer-like devices, there is not enough information about the load capacity at mid-span, especially at the macro-level. Imposed displacement at mid-span experimental evaluation of an SMA beam in the austenitic and martensitic regimes has been studied. To this end, a specimen of near equi-atomic nitinol was heat-treated (shape set) into a ‘U’ shape and loaded into a custom test fixture such that the boundary conditions of the beam are approximated as roller-roller; and the sample was deformed at different temperatures while reaction forces were measured. The displacement is near maximum displacement of the U shape without causing a change in concavity, thus full-scale capacity is shown. Additionally, Unified Model (finite element) predictions of the experimental response are also presented, with good agreement. Due to the robust nature of the Unified Model, geometric parameter variations (wire diameter and radius of curvature) were then simulated to encompass the design envelop for such an actuator. The material properties needed as inputs to the Unified Model were obtained from constant temperature tensile tests of a specimen subjected to the same heat treatment (shape set straight). The resultant critical stresses were then extracted using the tangent method similar to the one described in ASTM F-2082. It is worth noting that the specimen was trained before the stress value extraction, but the transversely loaded specimen was not trained due to the difficulty involved (inherent uneven stress distribution). The contribution of this work is the presentation of experimental results for transverse (mid-span) loading of a nitinol wire and the simulation results allowing for design of a proper actuator with known constraints on force, displacement or temperature (2 of 3 needed). In other words, this work could be used as a type of 3D look-up table; e.g. for a desired force/displacement, the required temperatures are given. Future work includes developing a sensor-less control strategy for simultaneous force/displacement control.

Operational Risk Assessment of Wind Turbine Structures Using Probabilistic Analysis of Aerodynamically Induced Vibrations

The study of efficiency and safety for wind turbine structures under variable operating conditions is increasingly important for wind turbine design. Optimum aerodynamic performance of a wind turbine demands that serviceability effects and ultimate strength loads remain under safety design limits. From the perspective of wind turbine efficiency, variations in wind speed causes bluffing effects and vortex shedding that lead to vibration intensities in the longitudinal and transversal direction that can negatively impact aerodynamic performance of the turbine. From the perspective of wind turbine safety, variations in loading may lead to transient internal loads that threaten the safety of the structure. Inertial effects and asynchronous delays on rotational-force transmission may generate similar hazards. Monitoring and controlling displacement limits and load demands at critical tower locations can improve the efficiency of wind power generation, not to mention the structural performance of the turbine from both a strength and serviceability point of view. In this study, a probabilistic monitoring approach is developed to measure the response of the combined tower/nacelle/blade system to stochastic loading, estimate peak demand, and compare that demand to building code-derived estimates of structural resistance. Risk assessment is performed for the effects of along and across-wind forces in a framework of quantitative risk analysis with the goal of developing a near real-time estimate of structural risk that may be used to monitor safety and serviceability of the structure as well as regulate the aggressiveness of the controller that commands the blade angle of attack. To accomplish this goal, a numerical simulation of the aerodynamic performance of a wind turbine (including blades, the nacelle and the tower) is analyzed to study the interaction between the structural system and incoming flow. A model based on distributed-stationary random wind load profile for the combined along-wind and across-wind responses is implemented in Matlab to simulate full aero-elastic dynamic analysis to simulate tower with nacelle, hub, rotor and tower substructures. Self-weight, rotational, and axial effects of the blades, as well as lateral resistance of substructure elements are incorporated in the finite element model, including vortex-shedding effects on the wake zone. Reliability on the numerical solution is inspected on the tower structure by comparing the numerical solution with established experimental-analytical procedures.

Computer-Aided Development of Shape Memory Actuators as an Approach Towards a Standardized Developing Method

Shape memory alloys (SMA) are thermally activated smart materials. Due to their ability to change into a previously imprinted actual shape through the means of thermal activation, they are suitable as actuators for mechatronical systems. Despite of the advantages shape memory alloy actuators provide (lightweight-actuators, lower costs…etc.) these elements are only seldom integrated by engineers into mechatronical systems. The reason for this phenomenon is the insufficiently described dynamic behavior, especially at different boundary conditions. Also the lack of empirical data (like fatigue behavior and thermal balances) is a reason why development projects with shape memory actuators lead often to failures. Therefore a need of developing methods, standardized testings of empirical properties and computer aided actuator development systems is motivated. Based on an analysis of energy fluxes into and out of the actuator, a numerical model, implemented in MATLAB/SIMULINK is presented. The numerical model includes also a configuration and design tool which allows simulating different solutions to a problem. Additionally, this paper describes a development method for SMA which is fitted to uniqueness of these smart materials. In conclusion, this paper compares the conventional developing process to the presented method applying a mechatronical SMA-device.

Dynamic Thermo-Mechanical Properties of Shape Memory Alloy Nanowires Upon Multi-Axial Loading

In this paper, we study the behavior of shape memory alloy (SMA) nanowires subjected to multi-axial loading. We use the model developed in our earlier work to study the microstructure and mechanical properties of finite length nanowires. The phase field model with the Ginzburg-Landau free energy is used to model the phase transformation based on the chosen order parameter. The governing equations of the thermo-mechanical model are solved simultaneously for different loading cases. We observe that nanowire behaves in a stiff manner to axial load with complete conversion of the unfavorable martensite to the favorable one. The bending load aids the phase transformation by redistributing the martensitic variants based on the local axial stress sign. The nanowire behavior to multi-axial (axial and bending together) is stiffer axially than the axial loading case. The understanding of the behavior of nanowire to multi-axial loading will be useful in developing better SMA-based MEMS and NEMS devices.

Modeling the Conductance of the Peptide Alamethicin With and Without Ion Gradients

Alamethicin is an antibiotic peptide from the fungus Trichoderma viride that forms ion channels in bilayer lipid membranes. Each peptide consists of 20 amino acids that can form larger channels with the congregation of multiple monomers of the peptide. These formed ion channels have some voltage dependent characteristics when a potential is induced across the bilayer. This potential can be from an applied voltage source or from an ion concentration gradient inducing a transmembrane potential across the membrane. The peptide alamethicin can be modeled as a conductor that allows the flow of ions through the membrane. The formed channels have distinct conductance level states caused by accumulation of additional alamethicin monomers being added to an individual ion channel. The voltage dependence of the accumulation of multiple ion channels can be modeled for the average response. A probabilistic model is used to capture the statistics of the state changes of individual channels. This type of model can be summed to simulate the conductance of multiple channels within a bilayer. This work focuses on obtaining the statistic for individual ion channels and using those statistics to show that a probabilistic model of the peptide’s conductance can capture some of the dynamics seen in aggregated responses. The Nernst equation is used to estimate the transmembrane potential caused by an ion gradient of a bilayer in equilibrium. This potential is used in the model to assist in determining the current conductance states of an individual channel of the peptide in the presence of an ion gradient. This paper will show the experimental results of ion currents across a bilayer induced by membrane potentials and the ion currents induced by ion gradients. The statistics of the measurements are used in a probabilistic conductance model of the peptide alamethicin.

Simulation-Based Optimization of a Piezoelectric Sound Generator by Combining a Finite-Element and Network Model

In this paper network methods and Finite-Element methods are combined to optimize the design of a piezoelectric sound generator. The FE-model is used to determine network parameters of the transducer model and finally the behavior of the chosen design variant. For the design optimization the network model is used. One design variant reaches a nominal sound pressure level of 93 dB at a center frequency of 497 Hz and a bandwidth of 242 Hz in the simulation. The computation of the dynamic behavior of a single design variant using the network model was approx. 600k times faster than using the FE-model.

Design of a Bio-Inspired Smart Ear Prototype

The outer ears (pinnae) of many bat species are smart structures that undergo non-rigid deformations controlled through an intricate muscular actuation system. It is hypothesized that such non-rigid changes in the physical shape of the pinnae provide a substrate for adaption of the spatial sensitivity (reception beampattern) of the animals’ biosonar system on a short time scale. In the research presented here, a simplified biomimetic baffle shape was developed to investigate the functional properties of non-rigidly deforming baffles. This prototype had the shape of an obliquely truncated cone that was augmented with local shape features that aided in achieving a biomimetic deformation pattern and may also have direct acoustic effects on the device beampattern. The prototype was manufactured from a thin sheet of rubber and actuated parsimoniously through a single linear actuator. Despite its comparative simplicity, the prototype device was able to reproduce the deformation pattern seen in the ears of horseshoe bats qualitatively. Biomimetic baffle deformations resulted in profound, qualitative, and quantitative changes to the beampattern. Future research will investigate how the time-variant beampatterns relate to the specifics of the deformation patterns and how they could be controlled and used in an engineering context.