Physics-Based Approaches For Structural Health Monitoring And Nondestructive Evaluation With Ultrasonic Guided Waves

The engineering infrastructures have a growing demand for damage monitoring systems to avoid any potential risk of failure. Proper damage monitoring solutions are of a great interest to this growing demand. The structural health monitoring (SHM) and nondestructive evaluation (NDE) offer appropriate online and offline damage monitoring solutions for a variety of mechanical and civil infrastructures that includes unmanned aerial vehicles (UAV), spaceships, commercial aircraft, ground transportation, wind turbines, nuclear spent fuel storage tanks, bridges, naval ships, and submarines. The fundamentals of the ultrasonic SHM and NDE consist of multi-disciplinary fields. The dissertation addresses SHM and NDE using ultrasonic guided waves, with an emphasis on the development of an analytical solution for non-axisymmetric guided wave propagation, multiphysics simulation, and experimental study of acoustic emission from the structural fatigue damage. An analytical solution for non-axisymmetric coupled guided wave propagation in plate-like structures was developed based on the equations of motion and elasticity relations. A general non-axisymmetric solution of guided wave propagation inplateis needed to analyze the guided wave-scatter from non-axisymmetric damage as encountered in practice. Under non-axisymmetric conditions, the problem is highly coupled and no potential based analytical solution has been reported in the literature so far. Helmholtz decomposition theorem was applied to the Navier-Lame equations that yielded a set of four coupled partial differential equations in four unknowns, the scalar potential Φ and the three components of the vector potential Hr, Hz, HΘ. A fourth equation, the ‘gauge condition’ was then added to the decomposition. A particular interpretation of the gauge condition is proposed. Our proposed approach decouples the governing equations and reduced the number of unknowns from four to three thus allowing one to express the solution in an elegant straight-forward way. The Rayleigh-Lamb characteristic equations were recovered and a general normal-modes expression for the solution was obtained. A hybrid global analytical and local finite element method was used to solve a practical aerospace rivet hole crack detection. The scatter cube of complex-valued wave damage interaction coefficients (WDICs) was developed to analyze any rivet hole of a multiple-rivet-hole lap joint system. It had been shown that not all parameters such as actuator-sensor locations, and frequencies were equally sensitive to the damage scatter. The optimum combination of parameters could better detect the crack in the rivet hole. The simulated time domain signals were produced for the optimum combination of parameters. Multiphysics simulations for fatigue crack generated acoustic emission (AE) were performed and the results were validated by the experiments. A novel application of inexpensive piezoelectric wafer active sensors (PWAS) has been explored. It has been shown that PWAS transducers successfully captured the fatigue-crack generated acoustic emissions in the thin plate-like aerospace materials. PWAS performance was compared with existing commercial AE sensors. It was found that PWAS captured richer frequency content than the existing AE sensors. Various AE waveform signatures were found from the fatigue crack advancement during the fatigue load evolution. Some AE waveform signatures were found to be related to the fatigue-crack extension while some of them were related to the fatigue-crack fretting, rubbing, and clapping. This observation was confirmed viii by synchronizing the fatigue loading with AE measurement by the same AE instrument. The in-situ microscopic measurement was performed during fatigue loading in MTS which provided the insights of the AE waveform evolution. It was hypothesized that the crack length estimation could be related the AE waveform signatures. FEM simulations and experiments were conducted using laser Doppler vibrometer (LDV) to verify our hypothesis. Two case studies are discussed showing the implementation of SHM and NDE approach in practical applications: (1) horizontal crack detection, size, and shape estimation in stiffened structures, (2) impact damage detection in manufactured aerospace composite structures. The dissertation finishes with conclusions, major contributions, and suggestions for future work

Using the Gauge Condition to Simplify The Elastodynamic Analysis of Guided Wave Propagation

In this article, gauge condition in elastodynamics is explored more to revive its potential capability of simplifying wave propagation problems in elastic medium. The inception of gauge condition in elastodynamics happens from the Navier-Lame equations upon application of Helmholtz theorem. In order to solve the elastic wave problems by potential function approach, the gauge condition provides the necessary conditions for the potential functions. The gauge condition may be considered as the superposition of the separate gauge conditions of Lamb waves and shear horizontal (SH) guided waves respectively, and thus, it may be resolved into corresponding gauges of Lamb waves and SH waves. The manipulation and proper choice of the gauge condition does not violate the classical solutions of elastic waves in plates; rather, it simplifies the problems. The gauge condition allows to obtain the analytical solution of complicated problems in a simplified manner

Guided Wave Inspection Of Cracks In The Rivet Hole Of An Aerospace Lap Joint Using Analytical-Fem Approach

Ultrasonic guided waves are very attractive for the inspection of large structures using nondestructive evaluation (NDE) and structural health monitoring (SHM) technique. Combined analytical and finite element analysis (CAFA) has been introduced for the detection of butterfly cracks in the rivet hole of the aerospace lap joint. Finite element analyses have been performed on the local damage area in spite of the whole large structure. Fundamental Lamb wave modes (S0 and A0) have been strike on the local damage from multiple directions to analyze the cracks of multiple-rivet-hole lap joint. The rivet hole cracks (damage) in the plate structure gives rise to the non-axisymmetric scattering of Lamb wave as well as shear horizontal (SH) wave although the incident Lamb wave source (primary source) is axisymmetric. Hence, the damage in the plate acts as a non-axisymmetric secondary source of Lamb wave and SH wave. The non-axisymmetric scattering of Lamb and SH waves are described using the wave damage interaction coefficient (WDIC). The WDIC of scattered Lamb and SH waves depends on the azimuth directions of the rivet hole as well as the frequencies of excitation. The WDIC involves scattering and mode conversion of Lamb waves occurred due to local damage. WDIC is captured around the damage for each direction of incidence over the frequency domain and “scatter cube” is formed for each incident Lamb mode. By analyzing the scattered cube of WDICs over the frequency domain and azimuth directions, the optimum parameters (frequency and location of sensor) can be determined for each angle of incidence. The scatter cubes are fed into the exact analytical framework to produce the time domain signal. This analysis enables us to obtain the optimum design parameters for better detection of the cracks in the rivet holes. The optimum parameters can be obtained for all possible cases of incident Lamb waves that would help to analyze the multiple-rivet-hole problem. Some examples of obtaining the optimum parameters are illustrated based on the most prominent time domain signal. The optimum parameters provide the guideline of the design of the sensor installation to obtain the most noticeable signals that represent the presence of cracks in the rivet hole. The thesis finishes with conclusions, and suggestions for future work

Guided Wave Based Crack Detection in the Rivet Hole Using Global Analytical with Local FEM Approach

In this article, ultrasonic guided wave propagation and interaction with the rivet hole cracks has been formulated using closed-form analytical solution while the local damage interaction, scattering, and mode conversion have been obtained from finite element analysis. The rivet hole cracks (damage) in the plate structure gives rise to the non-axisymmetric scattering of Lamb wave, as well as shear horizontal (SH) wave, although the incident Lamb wave source (primary source) is axisymmetric. The damage in the plate acts as a non-axisymmetric secondary source of Lamb wave and SH wave. The scattering of Lamb and SH waves are captured using wave damage interaction coefficient (WDIC). The scatter cubes of complex-valued WDIC are formed that can describe the 3D interaction (frequency, incident direction, and azimuth direction) of Lamb waves with the damage. The scatter cubes are fed into the exact analytical framework to produce the time domain signal. This analysis enables us to obtain the optimum design parameters for better detection of the cracks in a multiple-rivet-hole problem. The optimum parameters provide the guideline of the design of the sensor installation to obtain the most noticeable signals that represent the presence of cracks in the rivet hole

Vibration-Based In-Situ Detection and Quantification of Delamination in Composite Plates

This paper presents a new methodology for detecting and quantifying delamination in composite plates based on the high-frequency local vibration under the excitation of piezoelectric wafer active sensors. Finite-element-method-based numerical simulations and experimental measurements were performed to quantify the size, shape, and depth of the delaminations. Two composite plates with purpose-built delaminations of different sizes and depths were analyzed. In the experiments, ultrasonic C-scan was applied to visualize the simulated delaminations. In this methodology, piezoelectric wafer active sensors were used for the high-frequency excitation with a linear sine wave chirp from 1 to 500 kHz and a scanning laser Doppler vibrometer was used to measure the local vibration response of the composite plates. The local defect resonance frequencies of delaminations were determined from scanning laser Doppler vibrometer measurements and the corresponding operational vibration shapes were measured and utilized to quantify the delaminations. Harmonic analysis of local finite element model at the local defect resonance frequencies demonstrated that the strong vibrations only occurred in the delamination region. It is shown that the effect of delamination depth on the detectability of the delamination was more significant than the size of the delamination. The experimental and finite element modeling results demonstrate a good capability for the assessment of delamination with different sizes and depths in composite structures

Experimental Investigation of Impact Localization in Composite Plate Using Newly Developed Imaging Method

This paper focuses on impact localization of composite structures, which possess more complexity in the guided wave propagation due to the anisotropic behavior of composite materials. In this work, a composite plate was manufactured by using a compression molding process with proper pressure and temperature cycle. Eight layers of woven composite prepreg were used to manufacture the composite plate. A structural health monitoring (SHM) technique was implemented with piezoelectric wafer active sensors (PWAS) to detect and localize the impact on the plate. There were two types of impact event that were considered in this paper (a) low energy impact event (b) high energy impact event. Two clusters of sensors recorded the guided acoustic waves generated from the impact. The acoustic signals were then analyzed using a wavelet transform based time-frequency analysis. The proposed SHM technique successfully detected and localized the impact event on the plate. The experimentally measured impact locations were compared with the actual impact locations. An immersion ultrasonic scanning method was used to visualize the composite plate before and after the impact event. A high frequency 10 MHz 1-inch focused transducer was used to scan the plate in the immersion tank. Scanning results showed that there was no visible manufacturing damage in the composite plate. However, clear impact damage was observed after the high-energy impact event

Uncertainty Evaluation in the Design of Structural Health Monitoring Systems for Damage Detection

The validation of structural health monitoring (SHM) systems for aircraft is complicated by the extent and number of factors that the SHM system must demonstrate for robust performance. Therefore, a time- and cost-efficient method for examining all of the sensitive factors must be conducted. In this paper, we demonstrate the utility of using the simulation modeling environment to determine the SHM sensitive factors that must be considered for subsequent experiments, in order to enable the SHM validation. We demonstrate this concept by examining the effect of SHM system configuration and flaw characteristics on the response of a signal from a known piezoelectric wafer active sensor (PWAS) in an aluminum plate, using simulation models of a particular hot spot. We derive the signal responses mathematically and through the statistical design of experiments, we determine the significant factors that affect the damage indices that are computed from the signal, using only half the number of runs that are normally required. We determine that the transmitter angle is the largest source of variation for the damage indices that are considered, followed by signal frequency and transmitter distance to the hot spot. These results demonstrate that the use of efficient statistical design and simulation may enable a cost- and time-efficient sequential approach to quantifying sensitive SHM factors and system validation

Vibration-Based In-Situ Detection and Quantification of Delamination in Composite Plates

This paper presents a new methodology for detecting and quantifying delamination in composite plates based on the high-frequency local vibration under the excitation of piezoelectric wafer active sensors. Finite-element-method-based numerical simulations and experimental measurements were performed to quantify the size, shape, and depth of the delaminations. Two composite plates with purpose-built delaminations of different sizes and depths were analyzed. In the experiments, ultrasonic C-scan was applied to visualize the simulated delaminations. In this methodology, piezoelectric wafer active sensors were used for the high-frequency excitation with a linear sine wave chirp from 1 to 500 kHz and a scanning laser Doppler vibrometer was used to measure the local vibration response of the composite plates. The local defect resonance frequencies of delaminations were determined from scanning laser Doppler vibrometer measurements and the corresponding operational vibration shapes were measured and utilized to quantify the delaminations. Harmonic analysis of local finite element model at the local defect resonance frequencies demonstrated that the strong vibrations only occurred in the delamination region. It is shown that the effect of delamination depth on the detectability of the delamination was more significant than the size of the delamination. The experimental and finite element modeling results demonstrate a good capability for the assessment of delamination with different sizes and depths in composite structures

Multiphysics Simulation of Low-Amplitude Acoustic Wave Detection by Piezoelectric Wafer Active Sensors Validated by In-Situ AE-Fatigue Experiment

Piezoelectric wafer active sensors (PWAS) are commonly used for detecting Lamb waves for structural health monitoring application. However, in most applications of active sensing, the signals are of high-amplitude and easy to detect. In this article, we have shown a new avenue of using the PWAS transducer for detecting the low-amplitude fatigue-crack related acoustic emission (AE) signals. Multiphysics finite element (FE) simulations were performed with two PWAS transducers bonded to the structure. Various configurations of the sensors were studied by using the simulations. One PWAS was placed near to the fatigue-crack and the other one was placed at a certain distance from the crack. The simulated AE event was generated at the crack tip. The simulation results showed that both PWAS transducers were capable of sensing the AE signals. To validate the multiphysics simulation results, an in-situ AE-fatigue experiment was performed. Two PWAS transducers were bonded to the thin aerospace test coupon. The fatigue crack was generated in the test coupon which had produced low-amplitude acoustic waves. The low-amplitude fatigue-crack related AE signals were successfully captured by the PWAS transducers. The distance effect on the captured AE signals was also studied. It has been shown that some high-frequency contents of the AE signal have developed as they travel away from the crack

Interaction of Lamb waves with rivet hole cracks from multiple directions

This paper presents the interaction of Lamb waves with rivet hole cracks from multiple directions of incident using the finite element approach. Lamb waves undergo scattering and mode conversion after interacting with the damage. Shear horizontal waves appear in the scattered waves because of the mode conversion. Instead of analyzing the whole large structure, the local damage area is analyzed using finite element analyses and analytical formulation is used to analyze the whole structure. The scatter fields are described in terms of wave damage interaction coefficients that involve scattering and mode conversion of Lamb waves. Lamb wave mode (S0 and A0) hit the damage from multiple directions and corresponding wave damage interaction coefficients are obtained around the damage. Harmonic analysis has been performed over the fundamental frequency domain and “scatter cubes” of complex-valued wave damage interaction coefficients are formed. The scatter cube provides the information of relative amplitude and phase of scattered waves around the damage that can be used for designing the sensor installation. An application based on real time domain signal has been illustrated for the problem of multiple-rivet-hole cracks using the scatter cubes with the analytical framework