Sauvik Banerjee

A Conceptual Structural Health Monitoring System based on Vibration and Wave Propagation

Development of efficient methodologies to determine the presence, location, and severity of hidden damage in critical structural components is an important task in the design and construction of structural health monitoring systems in aging as well as new structures. In this article, a methodology for automatic damage identification and localization is presented. The structure is assumed to be instrumented with an array of actuators and sensors to excite and record its dynamic response, including vibration and wave propagation effects. In the vibrational approach, the data consist of the modal response of the structure produced by the actuators while in the wave propagation approach, they are the broadband signals due to ultrasonic waves propagating in the structures. Both types of signals are affected by the presence of defects. The approximate location and severity of an unknown defect is determined using a damage correlation index calculated from the frequency response function (FRF) of the structure. The damage index is a relative measure whose value depends on the differences in the dynamical properties of the undamaged (baseline) and damaged structures. The method is applied to simple structural components involving aluminum beams and plates with reduced local stiffness and a composite plate with impact damage. The potential application of the technique to practical health monitoring problems is discussed.

An automated damage identification technique based on vibration and wave propagation data.

This paper is concerned with the detection and characterization of hidden defects in advanced structures before they grow to a critical size. A novel method is developed using a combination of vibration and wave propagation data to determine the location and degree of damage in structural components requiring minimal operator intervention. The structural component is to be instrumented with an array of actuators and sensors to excite and record its dynamic response. A damage index, calculated from the measured dynamic response of the structure in a reference state (baseline) and the current state, is introduced as a determinant of structural damage. The index is a relative measure comparing the two states of the structure under the same ambient conditions. The indices are used to identify damages in the forms of delaminations and holes in composite plates for different arrangements of the source and the receivers. The potential applications of the approach in developing health monitoring systems in defects-critical structures are discussed.

Identification of impact force on a thick plate based on the elastodynamic and higher-order time-frequency analysis

Abstract The current paper presents a novel approach to precisely locate and characterize an impact load in thick plates. The approach is based on the analysis of the acoustic waveforms measured by a sensor array located on the plate surface in combination with the theoretical Greens function for the plate. The Greens functions are derived based on either the exact elastodynamic theory or an approximate shear deformation plate theory. For accurate estimation of the location of the impact source, the time differences in the arrival times of the waves at the sensors and their propagation velocities are determined first. This is accomplished through the use of a combined higher-order time-frequency (CHOTF) method, which is capable of detecting signals with lower signal to noise ratio compared with other available methods. Since most of the energy in the wave is carried by the flexural waves (A 0 mode), the group velocity of this mode is extracted using the CHOTF technique for estimating the impact source location. The estimates are shown to be in excellent agreement with the actual locations and time histories of the applied impact loads.

Guided wave propagation in a honeycomb composite sandwich structure in presence of a high density core

A coordinated theoretical, numerical and experimental study is carried out in an effort to interpret the characteristics of propagating guided Lamb wave modes in presence of a high-density (HD) core region in a honeycomb composite sandwich structure (HCSS). Initially, a two-dimensional (2D) semi-analytical model based on the global matrix method is used to study the response and dispersion characteristics of the HCSS with a soft core. Due to the complex structural characteristics, the study of guided wave (GW) propagation in HCSS with HD-core region inherently poses many challenges. Therefore, a numerical simulation of GW propagation in the HCSS with and without the HD-core region is carried out, using surface-bonded piezoelectric wafer transducer (PWT) network. From the numerical results, it is observed that the presence of HD-core significantly decreases both the group velocity and the amplitude of the received GW signal. Laboratory experiments are then conducted in order to verify the theoretical and numerical results. A good agreement between the theoretical, numerical and experimental results is observed in all the cases studied. An extensive parametric study is also carried out for a range of HD-core sizes and densities in order to study the effect due to the change in size and density of the HD zone on the characteristics of propagating GW modes. It is found that the amplitudes and group velocities of the GW modes decrease with the increase in HD-core width and density.

Ultrasonic guided wave propagation and disbond identification in a honeycomb composite sandwich structure using bonded piezoelectric wafer transducers

A coordinated theoretical, numerical, and experimental study is carried out in an effort to understand ultrasonic guided wave propagation and interaction with disbond, as well as, to identify disbond in a honeycomb composite sandwich structure using surface-bonded piezoelectric wafer transducers. In contrast to most of the work done previously, a fast and efficient two-dimensional semi-analytical model based on global matrix method is used to study dispersion characteristics as well as transient response of the healthy honeycomb composite sandwich structure subjected to relatively high-frequency piezoelectric wafer transducer excitations. Numerical simulations are then conducted using commercially available finite element package, ABAQUS, in order to explore guided wave propagation mechanisms due to the presence of disbond. Numerical simulations are further broadened to investigate the effect of disbond size on the amplitudes and group velocities of propagating guided wave modes. A good agreement is observed between the theoretical, numerical and experimental results in all cases studied. It is noticed that the presence of disbond, in particular, amplifies the first anti-symmetric (A0) mode and increases its group velocity. Finally, based on these modal behaviors, the location of an unknown disbond, within the piezoelectric wafer transducer array is experimentally determined by applying a probability-based damage detection algorithm.

STRUCTURAL HEALTH MONITORING OF A CANTILEVER BEAM USING SUPPORT VECTOR MACHINE

In this article, the effectiveness of support vector machine (SVM) is examined for health monitoring of beam-like structures using vibration-induced modal displacement data. The SVM is used to predict the intensity or location of damage in a simulated cantilever beam from displacements of the first mode shape. Twelve levels of damage intensities have been simulated at 12 locations, and six levels of white Gaussian noise have been added, thereby obtaining 1,008 simulations. About 90% of these are used for training the SVM, and the remaining are used for testing. The trained SVM is able to predict damage intensity and location of all the training set data with nearly 100% accuracy. The test set data reveal that SVM is able to predict damage intensity and damage location with errors varying from 0.28% to 4.57% and 0% to 20.3%, respectively, when there is no noise in the data. Addition of noise degrades the performance of SVM, the degradation being significant for intensity prediction and less for damage location prediction. The results demonstrate the use of SVM as a powerful tool for structural health monitoring without using the data of healthy state.

Effects of Partial Edge Loading and Fibre Configuration on Vibration and Buckling Characteristics of Stiffened Composite Plates

In this work, the influence of uniaxial and biaxial partial edge loads on buckling and vibration characteristics of stiffened laminated plates is examined by using finite element method. As the initial pre-buckling stress distributions within an element are highly non-uniform in nature for a given loading and edge conditions, the critical loads are evaluated by dynamic approach. Towards this, a nine-node heterosis plate element and a compatible three-node beam element are developed by employing the effect of shear deformation for both the plate and the stiffeners respectively. In the structural modeling, the plate and the stiffener elements are treated separately, and then the displacement compatibility is maintained between them by using a transformation matrix. Effect of different parameters such as loaded edge width, position of loads, boundary conditions, ply-orientations and stiffener factors are considered in this study. Buckling results show that the uniaxial loaded stiffened plate with around (+30o/-30o)2 layup can withstand higher load irrespective of boundary conditions and loading patterns, whereas the maximum load resisting layup for the bi-axially loaded stiffened plate is purely dependent on edge conditions and loading patterns.

Stress Analysis of Laminated Composite and Sandwich Beams using a Novel Shear and Normal Deformation Theory

A novel Normal and Shear Deformation Theory (NSDT) for analysis of laminated composite and sandwich beams, taking into account shear deformation as well as normal deformation, is developed. The paper investigates flexural behaviors of thick laminated and sandwich beams under plane stress conditions using NSDT. A generalized displacement-based refined formulation is elucidated with inclusion of various warping functions in terms of thickness coordinates to represent shear and normal deformation effects. These effects become pronounced in thick laminated beams and particularly in sandwich beams with transversely flexible core. Present formulation satisfies the shear stress free surface conditions at the top and bottom surfaces of the beam with realistic through-the-thickness variation of transverse shear stresses. The results obtained are compared with higher order theories available in literature. It is observed that NSDT predicts displacement and stresses accurately compared to other higher order theories.

Acoustic emission waveform simulation in multilayered composites

In this paper, an efficient and accurate semi-analytical method is developed to calculate the elastodynamic field produced by localized dynamic loads in a relatively thick composite plate. Two types of loads are considered: a pencil lead break source located on the surface and a localized shear delamination within the interior of the plate. In the case of the pencil lead break source, the calculated results for the surface motion are compared with those obtained in laboratory experiments on a 4.4mm thick 32 layered cross-ply graphite/epoxy using high-fidelity broadband transducers. The waveforms consist of both flexural and extensional modes; the amplitude variations of these modes are found to be strongly dependent on their propagation direction. For the delamination source, the results from the exact calculation are compared with those from an approximate laminate theory with a shear correction factor and ‘moment tensor’ representation of the source. The results obtained by the two methods are shown to have excellent agreement in the low-frequency ranges. Although the motion due to the delamination is dominated by flexural waves of lower frequency in both thin and thick plates, the presence of extensional waves are observed in thicker laminates.

Performance of damage detection algorithms for health monitoring of joints in steel frame structures using vibration-based techniques

Steel frame structures are widely used for industrial buildings, bridges, off-shore structures, etc., due to their high reliability and less construction time. In these types of structures, beams and columns are often connected by welding or fasteners forming rigid joints. In this study, viability of several damage detection algorithms for health monitoring of joints in planar frame structures using vibration-based techniques is addressed. Planar frames are modelled in the finite element framework with two node beam elements having two degrees of freedom at each node. The joint damage is introduced by reducing the cross section of an element attached to the joint making it semi-rigid. Several damage detection algorithms, with and without baseline information, are applied to mode shapes and frequency response functions (FRFs) of the structures. Synthetically zero mean Gaussian noise of different level is added to the simulated displacement data to examine the effectiveness of the algorithms for identification of joint damage in both the single and two storied planar frames. Experiment is then performed in a laboratory model of a signal storied planer frame. FRF curvature-based damage detection algorithm is found to be most effective among all the algorithms addressed.