Banibrata Poddar

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.

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.

Toward identifying crack-length-related resonances in acoustic emission waveforms for structural health monitoring applications:

In this study, we focus on analyzing the acoustic emission waveforms of the fatigue crack growth despite the conventional statistics-based analysis of acoustic emission. The acoustic emission monitoring technique is a well-known approach in the non-destructive evaluation/structural health monitoring research field. The growth of the fatigue crack causes the acoustic emission in the material that propagates in the structure. The acoustic emission happens not only from the crack growth but also from the interaction of the crack tips during the fatigue loading in the structure. The acoustic emission waveforms are generated from the acoustic emission events; they propagate and create local vibration modes along the crack faces (crack resonance). In-situ fatigue and acoustic emission experiments were conducted to monitor the acoustic emission waveforms from the fatigue cracks. Several test specimens were used in the fatigue experiments, and corresponding acoustic emission waveforms were captured. The acoustic emission waveforms were analyzed and distinguished into three types based on the similar nature in both time and frequency domains. Three-dimensional harmonic finite element analyses were performed to identify the local vibration modes. The local crack resonance phenomenon has been observed from the finite element simulation that could potentially give the geometric information of the crack. The laser Doppler vibrometry experiment was performed to identify the crack resonance phenomenon, and the experimental results were used to verify the simulated results.

Identifying fatigue crack geometric features from acoustic emission signals

Acoustic emission (AE) caused by the growth of fatigue crack were well studied by researchers. Conventional approaches predominantly are based on statistical analysis. In this study we focus on identifying geometric features of the crack from the AE signals using physics based approach. One of the main challenges of this approach is to develop a physics of materials based understanding of the generation and propagation of acoustic emissions due to the growth of a fatigue crack. As the geometry changes due to the crack growth, so does the local vibration modes around the crack. Our aim is to understand these changing local vibration modes and find possible relation between the AE signal features and the geometric features of the crack. Finite element (FE) analysis was used to model AE events due to fatigue crack growth. This was done using dipole excitation at the crack tips. Harmonic analysis was also performed on these FE models to understand the local vibration modes. Experimental study was carried out to verify these results. Piezoelectric wafer active sensors (PWAS) were used to excite cracked specimen and the local vibration modes were captured using laser Doppler vibrometry. The preliminary results show that the AE signals do carry the information related to the crack geometry.

Detection and Characterization of Cracks Using Lamb Wave Propagation

The goal of a structural health monitoring system is to implement processes to detect and characterize damages in engineering structures. These systems may use many different physical phenomena to detect and characterize damages. One of these phenomena is elastic wave propagation in thin plate like structures, known as Lamb wave propagation. To develop a SHM (structural health monitoring) system based on Lamb wave propagation we first need to understand how Lamb waves interact with different damage types. The aim of this study is to develop an understanding of how Lamb waves interact with different types of damages using analytical modeling and FEM. The goal is to identify and characterize a surface breaking crack by understanding its effect on Lamb waves scattered from it. doi: 10.12783/SHM2015/75

Experimental validation of analytical model for Lamb wave interaction with geometric discontinuity

Non-destructive testing methods based on ultrasonic waves are one of the most popular methods for damage detection in structures. Of these ultrasonic waves Lamb waves are of particular interest for the inspection of large structures for various reasons. Therefore scattering of Lamb waves from flaws has generated a considerable amount of research over last couple of decades. Most of the work has been done using computational tools like Finite Element Methods and experimental technique. In this paper an analytical approach is presented to develop a fundamental understanding of the scattering of Lamb waves from geometric discontinuities in 2 dimensions. We have considered simplest of all geometric discontinuity – a step, as this fundamental understanding can easily be extended to corrosion or crack.

Guided wave crack detection and size estimation in stiffened structures

Structural health monitoring (SHM) and nondestructive evaluation (NDE) deals with the nondestructive inspection of defects, corrosion, leaks in engineering structures by using ultrasonic guided waves. In the past, simplistic structures were often considered for analyzing the guided wave interaction with the defects. In this study, we focused on more realistic and relatively complicated structure for detecting any defect by using a non-contact sensing approach. A plate with a stiffener was considered for analyzing the guided wave interactions. Piezoelectric wafer active transducers were used to produce excitation in the structures. The excitation generated the multimodal guided waves (aka Lamb waves) that propagate in the plate with stiffener. The presence of stiffener in the plate generated scattered waves. The direct wave and the additional scattered waves from the stiffener were experimentally recorded and studied. These waves were considered as a pristine case in this research. A fine horizontal semi-circular crack was manufactured by using electric discharge machining in the same stiffener. The presence of crack in the stiffener produces additional scattered waves as well as trapped waves. These scattered waves and trapped wave modes from the cracked stiffener were experimentally measured by using a scanning laser Doppler vibrometer (SLDV). These waves were analyzed and compared with that from the pristine case. The analyses suggested that both size and shape of the horizontal crack may be predicted from the pattern of the scattered waves. Different features (reflection, transmission, and mode-conversion) of the scattered wave signals are analyzed. We found direct transmission feature for incident A0 wave mode and modeconversion feature for incident S0 mode are most suitable for detecting the crack in the stiffener. The reflection feature may give a better idea of sizing the crack.

Analysis of acoustic emission waveforms from fatigue cracks

Acoustic emission (AE) monitoring technique is a well-known approach in the field of NDE/SHM. AE monitoring from the defect formation and failure in the materials were well studied by the researchers. However, conventional AE monitoring techniques are predominantly based on statistical analysis. In this study we focus on understanding the AE waveforms from the fatigue crack growth using physics based approach. The growth of the fatigue crack causes the acoustic emission in the material that propagates in the structure. One of the main challenges of this approach is to develop the physics based understanding of the AE source itself. The acoustic emission happens not only from the crack growth but also from the interaction of the crack lips during fatigue loading of the materials. As the waveforms are generated from the AE event, they propagate and create local vibration modes along the crack faces. Fatigue experiments were performed to generate the fatigue cracks. Several test specimens were used in the fatigue experiments and corresponding AE waveforms were captured. The AE waveforms were analyzed and distinguished into different groups based on the similar nature on both time domain and frequency domain. The experimental results are explained based on the physical observation of the specimen.

Fast and accurate analytical model to solve inverse problem in SHM using Lamb wave propagation

Lamb wave propagation is at the center of attention of researchers for structural health monitoring of thin walled structures. This is due to the fact that Lamb wave modes are natural modes of wave propagation in these structures with long travel distances and without much attenuation. This brings the prospect of monitoring large structure with few sensors/actuators. However the problem of damage detection and identification is an “inverse problem” where we do not have the luxury to know the exact mathematical model of the system. On top of that the problem is more challenging due to the confounding factors of statistical variation of the material and geometric properties. Typically this problem may also be ill posed. Due to all these complexities the direct solution of the problem of damage detection and identification in SHM is impossible. Therefore an indirect method using the solution of the “forward problem” is popular for solving the “inverse problem”. This requires a fast forward problem solver. Due to the complexities involved with the forward problem of scattering of Lamb waves from damages researchers rely primarily on numerical techniques such as FEM, BEM, etc. But these methods are slow and practically impossible to be used in structural health monitoring. We have developed a fast and accurate analytical forward problem solver for this purpose. This solver, CMEP (complex modes expansion and vector projection), can simulate scattering of Lamb waves from all types of damages in thin walled structures fast and accurately to assist the inverse problem solver.

Experimentation and Adaptive Modeling for Temperature Effect Quantification in PVP Structural Health Monitoring With PWAS

The thermal effects at elevated temperatures mostly exist for pressure vessel and pipe (PVP) applications. The technologies for diagnosis and prognosis of PVP systems need to take the thermal effect into account and compensate it on sensing and monitoring of PVP structures. One of the extensively employed sensor technologies has been permanently installed piezoelectric wafer active sensor (PWAS) for in-situ continuous structural health monitoring (SHM). Using the transduction of ultrasonic elastic waves into voltage and vice versa, PWAS has been emerged as one of the major SHM sensing technologies. However, the dynamic characteristics of PWAS need to be explored prior its installation for in-situ SHM. Electro-mechanical impedance spectroscopy (EMIS) method has been utilized as a dynamic descriptor of PWAS and as a high frequency local modal sensing technique by applying standing waves to indicate the response of the PWAS resonator by determining the resonance and anti-resonance frequencies. Another SHM technology utilizing PWAS is guided wave propagation (GWP) as a far-field transient sensing technique by transducing the traveling guided ultrasonic waves (GUW) into substrate structure. The paper first presents EMIS method that qualifies and quantifies circular PWAS resonators under traction-free boundary condition and in an ambience with increasing temperature. The piezoelectric material degradation was investigated by introducing the temperature effects on the material parameters that are obtained from experimental observations as well as from related work in literature. GWP technique is also presented by inclusion of the thermal effects on the substrate material. The MATLAB GUI under the name of Wave Form Revealer (WFR) was adapted for prediction of the thermal effects on coupled guided waves and dynamic structural change in the substrate material at elevated temperature. The WFR software allows for the analysis of multimodal guided waves in the structure with affected material parameters in an ambience with elevated temperature.Copyright