Mira Mitra

Guided wave based structural health monitoring: A review

The paper provides a state of the art review of guided wave based structural health monitoring (SHM). First, the fundamental concepts of guided wave propagation and its implementation for SHM is explained. Following sections present the different modeling schemes adopted, developments in the area of transducers for generation, and sensing of wave, signal processing and imaging technique, statistical and machine learning schemes for feature extraction. Next, a section is presented on the recent advancements in nonlinear guided wave for SHM. This is followed by section on Rayleigh and SH waves. Next is a section on real-life implementation of guided wave for industrial problems. The paper, though briefly talks about the early development for completeness,. is primarily focussed on the recent progress made in the last decade. The paper ends by discussing and highlighting the future directions and open areas of research in guided wave based SHM.

Time reversibility of a Lamb wave for damage detection in a metallic plate

In this paper, an experimental study has been carried out to develop a baseline-free damage detection technique using the time reversibility of a Lamb wave. The experiments have been carried out on a metallic plate. Time reversibility is the process in which a response signal recorded at a receiver location is reversed in time and transmitted back through the receiver to the original transmitter location. In the absence of any defect or damage in the path between the transmitter–receiver locations, theoretically the signal received back at the original transmitter location (reconstructed signal) is identical to the original input signal. The initial part of the present work is aimed at understanding the time reversibility of a Lamb wave in an undamaged metallic plate. This involves a thorough study of different parameters such as frequency, pulse frequency band width, transducer size and the effects of tuning these parameters on the quality of a reconstructed input signal. This paper also suggests a method to mitigate the effects of the frequency dependent attenuation of Lamb wave modes (amplitude dispersion) and thus achieve better reconstruction for an undamaged plate. Finally, the time reversal process (TRP) is used to detect damage in an aluminium plate without using any information from the undamaged structure. A block mass, a notch and an area of surface erosion are considered as representative of different types of damage. The results obtained show that the effect of damage on TRP is significant, contrary to the results reported earlier.

Vibrational characteristics of single-walled carbon-nanotube: Time and frequency domain analysis

In this paper, the vibrational characteristics of higher modes of single-walled carbon-nanotube (SWNT) modeled as the continuum axisymmetric cylinder are studied in both time and frequency domains. The modeling of SWNT for the high frequency dynamic analysis is done using the wavelet based spectral element method and this numerical technique involves the Daubechies scaling function approximation in time and one spatial (axial) dimension. This model is capable of capturing the coupled longitudinal-radial vibration arising due to the finiteness of SWNT. Here, first the phonon dispersion relation is obtained and validated with the atomistic and other continuum model simulations available. The effects of dimensional parameters on higher radial, longitudinal, and coupled radial-longitudinal vibrational modes are studied. Dependence of the higher mode frequencies on these parameters are much different from that of the first mode frequencies. Further time domain responses for broadband excitations are simulated and effects of the radius and thickness of the SWNT are studied. The prominent influence of the above geometrical parameters are observed in the time domain results.

Wavelet based spectral finite element modelling and detection of de-lamination in composite beams

In this paper, a model for composite beam with embedded de-lamination is developed using the wavelet based spectral finite element (WSFE) method particularly for damage detection using wave propagation analysis. The simulated responses are used as surrogate experimental results for the inverse problem of detection of damage using wavelet filtering. The WSFE technique is very similar to the fast fourier transform (FFT) based spectral finite element (FSFE) except that it uses compactly supported Daubechies scaling function approximation in time. Unlike FSFE formulation with periodicity assumption, the wavelet-based method allows imposition of initial values and thus is free from wrap around problems. This helps in analysis of finite length undamped structures, where the FSFE method fails to simulate accurate response. First, numerical experiments are performed to study the effect of de-lamination on the wave propagation characteristics. The responses are simulated for different de-lamination configurations for both broad-band and narrow-band excitations. Next, simulated responses are used for damage detection using wavelet analysis.

Vibration control in a composite box beam with piezoelectric actuators

This work presents a theoretical and experimental investigation of vibration control of composite box beams using distributed, surface mounted piezoelectric (PZT) patches as actuators. The finite element modeling of the box beam is done by formulating a first-order shear deformable active composite thin walled beam element. The degrees of freedom at each node are axial, bending in spanwise and chordwise directions, corresponding shears and twist. The superconvergent element derived uses higher order interpolating polynomials obtained by solving electromechanically coupled static governing differential equations. The open loop response is validated with experimental and analytical results available in the current literature. A system equivalent reduction expansion process (SEREP) is implemented for reduced order modeling. Open and closed loop responses to electrical bending actuation are obtained both experimentally and analytically using state space modeling. A proportional-integral (PI) controller using acceleration feedback is implemented for control of vibration due to single-frequency excitations. Experimental and numerical results correlate very well in the above cases. Eigenstructure assignment through output feedback is designed for multimodal control of transient responses.

Wave Propagation Analysis in Anisotropic Plate Using Wavelet Spectral Element Approach

In this paper, a 2D wavelet-based spectral finite element (WSFE) is developed for a anisotropic laminated composite plate to study wave propagation. Spectral element model captures the exact inertial distribution as the governing partial differential equations (PDEs) are solved exactly in the transformed frequency-wave-number domain. Thus, the method results in large computational savings compared to conventional finite element (FE) modeling, particularly for wave propagation analysis. In this approach, first, Daubechies scaling function approximation is used in both time and one spatial dimensions to reduce the coupled PDEs to a set of ordinary differential equations (ODEs). Similar to the conventional fast Fourier transform (FFT) based spectral finite element (FSFE), the frequency-dependent wave characteristics can also be extracted directly from the present formulation. However, most importantly, the use of localized basis functions in the present 2D WSFE method circumvents several limitations of the corresponding 2D FSFE technique. Here, the formulated element is used to study wave propagation in laminated composite plates with different ply orientations, both in time and frequency domains.

Time reversed Lamb wave for damage detection in a stiffened aluminum plate

According to the concept of time reversibility of the Lamb wave, in the absence of damage, a Lamb wave signal can be reconstructed at the transmitter location if a time reversed signal is sent back from the receiver location. This property is used for baseline-free damage detection, where the presence of damage breaks down the time reversibility and the mismatch between the reconstructed and the input signal is inferred as the presence of damage. This paper presents an experimental and a simulation study of baseline-free damage detection in a stiffened aluminum plate by time reversed Lamb wave (TRLW). In this study, single Lamb wave mode (A0) is generated and sensed using piezoelectric (PZT) transducers through specific transducer placement and amplitude tuning. Different stiffening configurations such as plane and T-stiffeners are considered. Damage cases of disbonding of stiffeners from the base plate, and vertical and embedded cracks in the stiffened plate, are studied. The results show that TRLW based schemes can efficiently identify the presence of damage in a stiffened plate.

Wave propagation analysis in carbon nanotube embedded composite using wavelet based spectral finite elements

In this paper, elastic wave propagation is studied in a nanocomposite reinforced with multiwall carbon nanotubes (CNTs). Analysis is performed on a representative volume element of square cross section. The frequency content of the exciting signal is at the terahertz level. Here, the composite is modeled as a higher order shear deformable beam using layerwise theory, to account for partial shear stress transfer between the CNTs and the matrix. The walls of the multiwall CNTs are considered to be connected throughout their length by distributed springs, whose stiffness is governed by the van der Waals force acting between the walls of nanotubes. The analyses in both the frequency and time domains are done using the wavelet-based spectral finite element method (WSFEM). The method uses the Daubechies wavelet basis approximation in time to reduce the governing PDE to a set of ODEs. These transformed ODEs are solved using a finite element (FE) technique by deriving an exact interpolating function in the transformed domain to obtain the exact dynamic stiffness matrix. Numerical analyses are performed to study the spectrum and dispersion relations for different matrix materials and also for different beam models. The effects of partial shear stress transfer between CNTs and matrix on the frequency response function (FRF) and the time response due to broadband impulse loading are investigated for different matrix materials. The simultaneous existence of four coupled propagating modes in a double-walled CNT-composite is also captured using modulated sinusoidal excitation.

Damage detection in a woven-fabric composite laminate using time-reversed Lamb wave

Time reversibility is the process in which a response signal recorded at a receiver location is reversed in time and transmitted back through the receiver to the original transmitter location. In the absence of any defect or damage in the path between the transmitter and the receiver locations, theoretically, the signal received back at the original transmitter location (reconstructed signal) is identical to the original input signal. Therefore, differences in the transmitted and reconstructed signals are an indication of the possibility of a defect being present. An experimental study of a baseline-free damage detection technique using time reversibility of Lamb wave for a woven-fabric composite laminate is presented in this article. The initial part of the study is aimed towards obtaining the best possible reconstruction of the input signal by tuning various parameters of interest, including an experimental study of the frequency-dependent attenuation of Lamb wave modes (amplitude tuning). A finite element simulation has also been carried out to study the effect of amplitude tuning. Finally, the time-reversal concept is used to detect damage in woven composite laminates without using any information from the undamaged structure. In this study, a small block mass bonded to the surface, surface erosion and local impact are considered as representative of different types of damage. The results obtained show that the Lamb wave technique using time-reversal concept identifies correctly the presence of damage in woven-fabric composite laminates, thus providing a basis for baseline-free damage detection in composite structures.

BUCKLING BEHAVIOR OF COMPOSITE LAMINATES (WITH AND WITHOUT CUTOUTS) SUBJECTED TO NONUNIFORM IN-PLANE LOADS

Critical buckling loads of composite laminates are usually calculated using analytical solutions based on the assumption of uniform in-plane loads, despite of the fact that real structures are often subjected to various nonuniform loads. The present work is focused on the buckling behavior of composite laminates, with and without cutouts, subjected to various nonuniform in-plane loads. The effect of the size of the cutouts, on the buckling behavior, has been studied using finite element method. Furthermore, parametric studies on the effects of plate aspect ratio, location of the cutout and the application of a nonuniform load combined with a shear load have also been studied. Higher buckling loads were observed in pure in-plane bending compared to uniformly/nonuniformly distributed loads. Consequently, it is important to consider nonuniformly distributed loading whenever applicable, to utilize complete strength of the composite laminate and to avoid premature failure of the composite laminate due to structural instability.