Ajit Achuthan

Development of a FBG based distributed strain sensor system for wind turbine structural health monitoring

The development of a fiber Bragg grating (FBG) based distributed strain sensor system for real time structural health monitoring of a wind turbine rotor and its validation under a laboratory scale test setup is discussed in this paper. A 1 kW, 1.6 m diameter rotor, horizontal axis wind turbine with three instrumented blades is used in this study. The sensor system consists of strain sensors, surface mounted at various locations on the blade. At first the sensors are calibrated under static loading conditions to validate the FBG mounting and the proposed data collection techniques. Then, the capability of the sensor system coupled with the operational modal analysis (OMA) methods to capture natural frequencies and corresponding mode shapes in terms of distributed strains are validated under various non-rotating dynamic loading conditions. Finally, the sensor system is tested under rotating conditions using the wind flow from an open-jet wind tunnel, for both a baseline wind turbine and a wind turbine with a structurally modified blade. The blade was modified by attaching a lumped mass at the blade tip simulating structural damage or ice accretion. The dynamic characteristics of the baseline (healthy) blade and modified (altered) blade are compared to validate the sensor systems ability for real time structural health monitoring of the rotor.

Shape control of coupled nonlinear piezoelectric beams

This paper deals with the shape control of composite laminated beams with nonlinear piezoelectric patch actuators. The governing electromechanical coupled equations for nonlinear material behaviour are developed for a general three-dimensional structural element using Gibbs free energy formulation. In the analysis, nonlinearity is accounted for by incorporating enough nonlinear terms in the Gibbs free energy expression. Since many of the higher-order material properties are not available for most of the piezoelectric materials, the experimental data that are available for the nonlinear relationship between the electric field and the electric displacement are used. A finite element model is developed for the beam with piezoelectric actuators using a modified bilinear four-noded quadratic element. The expression for the actuation voltages required for shape control is then obtained by minimizing an error function, which is a measure of the area between the achieved and desired shape. The final system of coupled equations is solved by an iterative finite element procedure. Numerical results are obtained for several piezoelectric patch configurations on beams with various boundary conditions. The results show the significance of nonlinear effects in the shape control of beams with piezoelectric actuators, which has hitherto not been accounted.

Domain switching in ferroelectric ceramic materials under combined loads

An experimental study to investigate the various characteristics of the nonlinear behavior of a ferroelectric ceramic material under different electrical and mechanical loading conditions is reported. The nonlinear strain behavior under different loading conditions is obtained and used for studying the effect of residual stress, developed in the ferroelectric ceramic material during manufacture, on domain switching under applied loads. First, the behavior at low electric field and mechanical stress are studied by comparing the linear piezoelectric properties measured from the strain responses under mechanical and electrical loading with those measured by resonance method. This is followed by an investigation on the mechanism of polarization reversal due to cyclic electric field. Based on the observed large magnitude of strain and the comparison of the magnitude of the sum of transverse strains with the magnitude of strain in the poling direction it is concluded that polarization reversal due to cyclic ele...

Domain switching criteria for piezoelectric materials

Experimental results reported by many researchers showed that the coercive electric field for ferroelectric switching depends on mechanical stresses present in the material. Similarly, the coercive stress for ferroelastic switching depends on the electric field. To model these dependences, several domain switching criteria based on different considerations have been proposed in earlier studies. In this paper, these domain switching criteria are briefly reviewed and the predictions based on these domain switching criteria are compared with the available experimental data for 180 degree(s) domain switching in PZT. It is found that the predictions do not match the experimental results. Motivated by this observation, a new domain switching criterion based on internal energy density is proposed. This new criterion is found to yield very good predictions compared with the existing experimental results for 180 degree(s) domain switching in PZT. To verify the new domain switching criterion for 90 degree(s) switching, experiments were conducted using PZT-5H. The new experimental result indicates that the new domain switching criterion gives a much better prediction than other existing criteria.

The effect of work-hardening and pile-up on nanoindentation measurements

Nanoindentation is performed on the cross-section of copper samples subjected to surface mechanical attrition treatment (SMAT). The cross-section of the SMAT samples provides a unique microstructure with varying amounts of work-hardening depending on the distance from the SMAT surface. Results show that for a given indentation load the pile-up height decreases and the indentation depth increases as the distance from the SMAT surface increases, both following a power law relationship. Based on image analysis of the indented surface this increase in the pile-up height and decrease in indentation depth is attributed to the localization of plastic strain due to the increased resistance to dislocation motion in the work-hardened region. For a given amount of work-hardening (in terms of distance from SMAT surface), the indentation depth increased with the indentation load obeying a power law relationship with the exponent ranging from 0.58 to 0.68. However, the pile-up height increased linearly with the load, with the rate (slope) increasing with the amount of work-hardening. The observed linear increase in pile-up height with indentation load would naturally introduce an indentation size effect (ISE) if the hardness is corrected for the pile-up. Interestingly, this ISE associated with pile-up increased with an increase in indentation depth, in contradiction to the ISE associated with strain gradient. Deviation of the hardness values corrected for pile-up from the bulk behavior due to surface effect is highlighted and a method to obtain a bulk-equivalent hardness quantity representing the bulk behavior is proposed.

A Multiscale Computational Model Combining a Single Crystal Plasticity Constitutive Model with the Generalized Method of Cells (GMC) for Metallic Polycrystals

A multiscale computational model is developed for determining the elasto-plastic behavior of polycrystal metals by employing a single crystal plasticity constitutive model that can capture the microstructural scale stress field on a finite element analysis (FEA) framework. The generalized method of cells (GMC) micromechanics model is used for homogenizing the local field quantities. At first, the stand-alone GMC is applied for studying simple material microstructures such as a repeating unit cell (RUC) containing single grain or two grains under uniaxial loading conditions. For verification, the results obtained by the stand-alone GMC are compared to those from an analogous FEA model incorporating the same single crystal plasticity constitutive model. This verification is then extended to samples containing tens to hundreds of grains. The results demonstrate that the GMC homogenization combined with the crystal plasticity constitutive framework is a promising approach for failure analysis of structures as it allows for properly predicting the von Mises stress in the entire RUC, in an average sense, as well as in the local microstructural level, i.e., each individual grain. Two–three orders of saving in computational cost, at the expense of some accuracy in prediction, especially in the prediction of the components of local tensor field quantities and the quantities near the grain boundaries, was obtained with GMC. Finally, the capability of the developed multiscale model linking FEA and GMC to solve real-life-sized structures is demonstrated by successfully analyzing an engine disc component and determining the microstructural scale details of the field quantities.

Effect of residual stresses on domain switching in ferroelectric ceramic materials

In this article, an experimental investigation to study the effect of residual stresses on the nonlinear behavior of ferroelectric ceramic material is reported. The effect of residual stresses on the behavior at low electric field and mechanical stress is demonstrated first by showing the large difference in the linear properties measured from strain behavior under mechanical and electrical loading, and resonance method. This is followed by an investigation on the mechanism of polarization reversal due to cyclic electric field. Based on the observed large magnitude of strain and the comparison of the magnitude of the sum of transverse strains with the magnitude of strain in the poling direction it is concluded that polarization reversal due to cyclic electric field in the ferroelectric material at morphotropic phase boundary is the result of two successive 90o domain switchings. Finally, two types of combined loading experiments were conducted to investigate the residual stress and electric field effect on the mechanism of domain switching. The behavior under combined loading showed many new interesting characteristics, and possible mechanisms for such behavior is discussed. While most of the characteristics of the ferroelectric behavior observed in the present experimental study could be explained based on the residual stress state, the understanding of others need further studies.

A Subscale Experimental Test Method to Characterize Extrusion-Based Elastomer Seals

Extrusion based elastomer seals are used in many applications, such as the seal in a variable bore ram valve used in offshore oil and gas drilling. Performing full-scale closing pressure experiments of such valves to characterize the seal performance and material failure of elastomer, especially under various temperature conditions, are quite expensive and time consuming. Conversely, simple coupon tests to characterize the elastomer mechanical properties and failure do not capture the complex deformation associated with the extrusion and subsequent sealing type that these material undergo in the valves. In view of this, a simple sub-scale experimental test method capable of simulating the extrusion and sealing type deformation is developed. The extrusion and sealing deformation is realized by bonding the rectangular elastomer sample to metal pieces on top and bottom surfaces, and then compressing the sample in the vertical direction, with the deformation of the the three lateral surfaces kept constrained. As a result, sample deforms and extrudes out of the front surface, eventually forming the seal against a flat rigid metal plate placed at an appropriate distance. Simple scaling rules to determine the appropriate sample size and initial sealing gap, equivalent to the full-scale valve in terms of similar strain conditions, are derived and then verified using finite element analysis (FEA). Finally, the experimental test method is demonstrated by characterizing the contact pressure of Nitrile (NBR) samples under different operating temperatures, ranging from 21 ? C to 160 ?C using pressure sensitive film sensor.

Modal response of a computational vocal fold model with a substrate layer of adipose tissue

This study demonstrates the effect of a substrate layer of adipose tissue on the modal response of the vocal folds, and hence, on the mechanics of voice production. Modal analysis is performed on the vocal fold structure with a lateral layer of adipose tissue. A finite element model is employed, and the first six mode shapes and modal frequencies are studied. The results show significant changes in modal frequencies and substantial variation in mode shapes depending on the strain rate of the adipose tissue. These findings highlight the importance of considering adipose tissue in computational vocal fold modeling.

Efficient Multiscale Plasticity Model for Polycrystalline Materials Based on Micromechanical Homogenization

Development of a multiscale computational model for analyzing plasticity in polycrystal materials is reported in this article. The model is based on crystal plasticity theory along with the Generalized Method of Cells (GMC) micromechanics theory for homogenization. A preprocessor tool is developed to create polycrystal grain geometry and to assign random orientations to the grains. This preprocessor also generates appropriate material orientations for finite elements and analogous GMC subcells based on their location in the polycrystal grain structure. Several cases of polycrystal grain configuration, namely 1, 2, 3, 4 and 5 grains per dimension, were studied for uniaxial tensile loading, both using MAC/GMC and Abaqus FEA, independently. Results show excellent global agreement between these two models. In addition, MAC/GMC requires much smaller computational time, for example, more than two orders of magnitude lower computational time per iteration for a model with 1000 elements that contains 27 grains.