Fully automated operational modal analysis using multi-stage clustering

The interest for robust automatic modal parameter extraction techniques has increased significantly over the last years, together with the rising demand for continuous health monitoring of critical infrastructure like bridges, buildings and wind turbine blades. In this study a novel, multi-stage clustering approach for Automated Operational Modal Analysis (AOMA) is introduced. In contrast to existing approaches, the procedure works without any user-provided thresholds, is applicable within large system order ranges, can be used with very small sensor numbers and does not place any limitations on the damping ratio or the complexity of the system under investigation. The approach works with any parametric system identification algorithm that uses the system order n as sole parameter. Here a data-driven Stochastic Subspace Identification (SSI) method is used. Measurements from a wind tunnel investigation with a composite cantilever equipped with Fiber Bragg Grating Sensors (FBGSs) and piezoelectric sensors are used to assess the performance of the algorithm with a highly damped structure and low signal to noise ratio conditions. The proposed method was able to identify all physical system modes in the investigated frequency range from over 1000 individual datasets using FBGSs under challenging signal to noise ratio conditions and under better signal conditions but from only two sensors

Operational Modal Analysis of a wing excited by transonic flow

Operational Modal Analysis (OMA) is a promising candidate for flutter testing and Structural Health Monitoring (SHM) of aircraft wings that are passively excited by wind loads. However, no studies have been published where OMA is tested in transonic flows, which is the dominant condition for large civil aircraft and is characterized by complex and unique aerodynamic phenomena. We use data from the HIRENASD large-scale wind tunnel experiment to automatically extract modal parameters from an ambiently excited wing operated in the transonic regime using two OMA methods: Stochastic Subspace Identification (SSI) and Frequency Domain Decomposition (FDD). The system response is evaluated based on accelerometer measurements. The excitation is investigated from surface pressure measurements. The forcing function is shown to be non-white, non-stationary and contaminated by narrow-banded transonic disturbances. All these properties violate fundamental OMA assumptions about the forcing function. Despite this, all physical modes in the investigated frequency range were successfully identified, and in addition transonic pressure waves were identified as physical modes as well. The SSI method showed superior identification capabilities for the investigated case. The investigation shows that complex transonic flows can interfere with OMA. This can make existing approaches for modal tracking unsuitable for their application to aircraft wings operated in the transonic flight regime. Approaches to separate the true physical modes from the transonic disturbances are discussed

A hybrid embedded cohesive element method for predicting matrix cracking in composites

The complex architecture of many fibre-reinforced composites makes the generation of finite element meshes a labour-intensive process. The embedded element method, which allows the matrix and fibre reinforcement to be meshed separately, offers a computationally efficient approach to reduce the time and cost of meshing. In this paper we present a new approach of introducing cohesive elements into the matrix domain to enable the prediction of matrix cracking using the embedded element method. To validate this approach, experiments were carried out using a modified Double Cantilever Beam with ply drops, with the results being compared with model predictions. Crack deflection was observed at the ply drop region, due to the differences in stiffness, strength and toughness at the bi-material interface. The new modelling technique yields accurate predictions of the failure process in composites, including fracture loads and crack deflection path

A study on the uniaxial tension of FCC metals at nano level using MD

Molecular Dynamics (MD) are now having orthodox means for simulation of matter in nano-scale. It can be regarded as an accurate alternative for experimental work in nano-science. In this paper, Molecular Dynamics simulation of uniaxial tension of some face centered cubic (FCC) metals (namely Au, Ag, Cu and Ni) at nano-level have been carried out. Sutton-Chen potential functions and velocity Verlet formulation of Noise-Hoover dynamic as well as periodic boundary conditions were applied. MD simulations at different loading rates and temperatures were conducted, and it was concluded that by increasing the temperature, maximum engineering stress decreases while engineering strain at failure is increasing. On the other hand, by increasing the loading rate both maximum engineering stress and strain at failure are increasing

A study on the nanoindentation behaviour of single crystal silicon using hybrid MD-FE method

Developing new techniques for the prediction of materials behaviors in nano-scales has been an attractive and challenging area for many researches. Molecular Dynamics (MD) is the popular method that is usually used to simulate the behavior of nano-scale material. Considering high computational costs of MD, however, has made this technique inapplicable as well as inflexible in various situations. To overcome these difficulties, alternative procedures are thought. Considering its capabilities, Finite Element Analysis (FEA) seems to be the most appropriate substitute for MD simulations in most cases. But since the material properties in nano, micro, and macro scales are different, therefore to use FEA methods in nano-scale modeling one must use material properties appropriate to that scale. To this end, a previously developed Hybrid Molecular Dynamics-Finite Element (HMDFE) approach was used to investigate the nanoindentation behavior of single crystal silicon with Berkovich indenter. In this study, a FEA model was developed based on the material properties extracted from molecular dynamics simulation of uniaxial tension test on single crystal Silicon. Eventually, by comparison of FEA results with experimental data, the validity of this new technique for the prediction of nanoindentation behavior of Silicon was concluded

Durability of structural health monitoring systems under impact loading

This paper reports an experimental study on the degradation of Piezoelectric Wafer Active Sensors (PWAS) under impact loading. Carbon/epoxy laminates with surface bonded / embedded sensors were subjected to different levels of impact energy, and the performance of PWAS was monitored using impedance analysis. The effect of direct and indirect impact loading on the degradation of piezoelectric sensors was also examined. Using the force history data, a reduction in the flexural stiffness of the embedded specimens was identified. Examining the capacitance, output voltage and electromechanical impedance of PWAS, it was found that the degradation of the sensors under impact loading depends on the level of impact energy, location of the impactor as well as the number of impacts. Identification of the structural damage and sensor's degradation was verified using SEM micrographs

Evaluation of mechanical and piezoelectric properties of boron nitride nanotube: A novel electrostructural analogy approach

A new continuum mechanics approach based on electrostructural elements is proposed which is capable of studying and predicting the piezoelectric as well as mechanical properties of nanostructures. To evaluate the capabilities of the proposed approach, several properties of zigzag boron nitride nanotubes (BNNTs) including elastic constants and piezoelectric coefficients are evaluated. Comparisons are made between the results obtained in this paper and those available in the open literature

On the evaluation of in-plane elastic behaviour of woven fibre metal laminates under uniform loading

This article deals with the non-linear behaviour of woven fibre metal laminates (WFMLs). A theoretical micromechanical model has been proposed for the plain-wave fabric-reinforced flexible composite with two surface-bonded metal layers under biaxial loading. The constitutive equations are derived through a strain energy approach and energy variation theorem based on the microstructure of composites. The modelling strategy starts with a geometrical description of the yarn and the unit cell and fibres are assumed to be in a sinusoidal shape. Meanwhile, a simple and conventional analytical technique is applied to predict the tensile properties of WFMLs. Stress-strain behaviour of such structure plates under uniform uniaxial loading are illustrated in figures. The proposed integrated micromechanical model shows excellent agreement with the three-dimensional finite-element results. Finally, a parametric study is performed using the presented models to investigate the effect of thickness of metal layers on the elastic properties of the composite

Micromechanical modeling of fiber reinforced metal laminates under biaxial deformation

This presentation examines theoretically the elastic behavior of fiber reinforced metal laminates composed of layers of two types. Woven flexible fabric and metal, in which woven flexible fabric layer includes of sinusoidal shaped fibers. The composite is subjected under biaxial/uniaxial deformation. The theoretical analysis is based upon the Lagrangian description of deformation and the strain-energy density which is assumed to be a function of the Lagrangian strain components referring to the principle material coordinates. The micromechanical model has been obtained using strain energy of components. Finally, the model was solved numerically and then results were compared with published literatures