Guided waves in anisotropic and quasi-isotropic aerospace composites: three-dimensional simulation and experiment

Diagnostic-Quality Guided Wave Signals Synthesized Using Generative Adversarial Neural Networks

Guided waves are a potent tool in structural health monitoring, with promising machine learning algorithm applications due to the complexity of their signals. However, these algorithms usually require copious amounts of data to be trained. Collecting the correct amount and distribution of data is costly and time-consuming, and sometimes even borderline impossible due to the necessity of introducing damage to vital machinery to collect signals for various damaged scenarios. This data scarcity problem is not unique to guided waves or structural health monitoring, and has been partly addressed in the field of computer vision using generative adversarial neural networks. These networks generate synthetic data samples based on the distribution of the data they were trained on. Though there are multiple researched methods for simulating guided wave signals, the problem is not yet solved. This work presents a generative adversarial network architecture for guided waves generation and showcases its capabilities when working with a series of pitch-catch experiments from the OpenGuidedWaves database. The network correctly generates random signals and can accurately reconstruct signals it has not seen during training. The potential of synthetic data to be used for training other algorithms was confirmed in a simple damage detection scenario, with the classifiers trained exclusively on synthetic data and evaluated on real signals. As a side effect of the signal reconstruction process, the network can also compress the signals by 98.44% while retaining the damage index information they carry.

Feasibility of Model-Assisted Probability of Detection Principles for Structural Health Monitoring Systems Based on Guided Waves for Fiber-Reinforced Composites

In many industrial sectors, structural health monitoring (SHM) is considered as an addition to nondestructive testing (NDT) that can reduce maintenance effort during the lifetime of a technical facility, structural component, or vehicle. A large number of SHM methods are based on ultrasonic waves, whose properties change depending on structural health. However, the wide application of SHM systems is limited due to the lack of suitable methods to assess their reliability. The evaluation of the system performance usually refers to the determination of the probability of detection (POD) of a test procedure. Up until now, only a few limited methods exist to evaluate the POD of SHM systems, which prevents them from being standardized and widely accepted in the industry. The biggest hurdle concerning the POD calculation is the large number of samples needed. A POD analysis requires data from numerous identical structures with integrated SHM systems. Each structure is then damaged at different locations and with various degrees of severity. All of these are connected to high costs. Therefore, one possible way to tackle this problem is to perform computer-aided investigations. In this work, the POD assessment procedure established in NDT according to the Berens model is adapted to guided wave-based SHM systems. The approach implemented here is based on solely computer-aided investigations. After efficient modeling of wave propagation phenomena across an automotive component made of a carbon-fiber-reinforced composite, the POD curves are extracted. Finally, the novel concept of a POD map is introduced to look into the effect of damage position on system reliability.

A time-frequency analysis of the correlation between the electromechanical impedance (EMI) of surface bonded piezoelectric wafer active transducers (PWaTs) and the pitch-catch signal

Ultrasound based Structural Health Monitoring (SHM) commonly uses surface-bonded piezoelectric wafer active transducers (PWaTs) for ultrasound wave generation and sensing. Both the electromechanical impedance (EMI) of the surface bonded PWaT and the ultrasound pitch-catch signal have been studied extensively for damage detection. However, these two signals were studied separately. The correlation between the EMI and the pitch-catch signal has not been studied in detail. In this paper, a broadband spectral analysis method is presented to analyze the influence of the EMI resonances on the fundamental symmetric (S0) pitch-catch signal. First, the broadband responses of the PWaT actuator and sensor are measured and analyzed in the time-frequency domain. The results clearly demonstrate that the S0 pitch-catch signal can deviate significantly from the excitation signal when the excitation frequency is above a threshold. Next, a simulation model was implemented to explain the observed distortions. The simulation model was first validated by adjusting the adhesive parameters to reproduce the experiment measurements. The resonant characteristics of the PWaT actuator and sensor were then analyzed separately. The study reveals that the S0 deviations are due to the resonances and anti-resonances of the PWaT EMI. Furthermore, this study demonstrates that the resonance characteristics of surface-bonded PWaTs are more complicated than previously known. The research framework presented in this paper lays the theoretical foundation for future more in-depth analysis of the PWaT resonances.

Quantifying adhesive thickness and adhesion parameters using higher-order SH guided waves

A method to quantify the interface shear stiffness, adhesive shear modulus and adhesive thickness in an aluminium-epoxy-aluminium joint is presented. Shear horizontal guided waves are considered to infer the properties. A numerical model that employs spring stiffness boundary conditions at the aluminium-epoxy interface was developed to generate dispersion curves. The sensitivity of the first four SH-like modes to epoxy thickness, interface shear stiffness, and adhesive shear modulus are analyzed. The dispersion analysis reveals that higher-order anti-symmetric modes are sensitive to all three parameters, whereas the symmetric modes are sensitive only to adhesive thickness. Hence to prevent false alarms that might arise while assessing the bond conditions, symmetric and anti-symmetric modes should be simultaneously generated. Periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) are used to generate and detect SH-like modes. Utilizing the constant wavelength property of PPM-EMATs, SH₂-like and SH₃-like modes are generated. Short-time Fourier transform (STFT) is used to separate the modes merged in the received time response. By overlaying the dispersion curves of SH₂-like mode on STFT, the thickness of epoxy is quantified. The dispersion curves of SH₃-like mode are generated using the measured thickness and overlaid on STFT to measure the interface shear stiffness and epoxy shear modulus. The proposed method is experimentally demonstrated on aluminium-epoxy-aluminium samples of different surface treatments. The study demonstrates a reliable nondestructive evaluation of adhesive bonds that reduces possible false alarms.

Ultrasonic Guided Wave Testing on Cross-Ply Composite Laminate: An Empirical Study

Structural health monitoring comprises a set of techniques to detect defects appearing in structures. One of the most viable techniques is based on the guided ultrasonic wave test (UGWT), which consists of emitting waves throughout the structure, acquiring the emitted waves with various sensors, and processing the waves to detect changes in the structure. The UGWT of layered composite structures is challenging due to the anisotropic wave propagation characteristics of such structures and to the high signal attenuation that the waves experience. Hence, very low amplitude signals that are hard to distinguish from noise are typically recovered. This paper analyzes the propagation of guided waves along a cross-ply composite laminate following an empirical methodology. The research compares several implementations for UGWT with piezoelectric wafer active sensors. The reference for comparison is set on a basic mode, which considers the application of nominal voltage to a single sensor. The attenuation and spreading of the waves in several directions are compared when more energy is applied to the monitored structure. In addition, delayed multiple emission is also considered in multisensor tests. The goal of all the UGWT configurations is to transmit more energy to the structure such that the echoes of the emission are of greater amplitude and they ease the signal processing. The study is focused on the realization of viable monitoring systems for aeronautical composite made structures.

Sparse Wavenumber Recovery and Prediction of Anisotropic Guided Waves in Composites: A Comparative Study

Guided wave methodologies are among the established approaches for structural health monitoring (SHM). For guided wave data, being able to accurately estimate wave properties in the absence of ample measurements can greatly facilitate the often time-consuming and potentially expensive data acquisition procedure. Nevertheless, inherent complexities of the guided waves, including their multimodal and frequency dispersive nature, hinder processing, analysis, and behavior prediction. The severity of these complexities is even higher in anisotropic media, such as composites. Several methods, including sparse wavenumber analysis (SWA), have been proposed in the literature to characterize guided wave propagation by extracting wave characteristics in a particular medium from the information contained in a few measurements, and subsequently using this information for full wavefield prediction. In this paper, we investigate the efficacy of guided wave reconstruction techniques, based on SWA, for predicting the behavior of guided waves in composite materials. We implement these techniques on several experimental and simulation data sets. We study their performance in estimating the frequency-dependent (dispersive) and anisotropic velocities of guided waves and in reconstructing full wavefields from limited available information.

Multi-mode reverse time migration damage imaging using ultrasonic guided waves

The sensitivity of Lamb wave modes to a particular defect or instance of damage is dependent on various factors (e.g., the local strain energy density due to that wave mode). As a result, different modes will be more useful than others for damage detection and quantification, dependent on damage type and location. For example, prior work in the field has shown that out-of-plane modes may have a higher sensitivity than in-plane modes to surface defects in plates. The excitability of a certain data acquisition system and the corresponding resolution for damage imaging also varies with frequency. The aim of the present work was to develop a multi-mode damage imaging technique that enables characterization of damage type and size, general sensitivity to unknown damage types, higher resolution imaging, and detectability regardless of the data acquisition system used. A reverse-time migration (RTM) imaging algorithm was combined with a numerical simulator-the three-dimensional (3D) elastodynamic finite integration technique (EFIT)-to provide multi-mode damage imaging. The approach was applied to two simulated case studies featuring damaged isotropic plates. Sensitivities of damage type to wave mode were investigated by separating the A₀ and S₀ Lamb wave modes obtained from the resultant RTM wavefields.

Virtual nondestructive evaluation for anisotropic plates using Symmetry Informed Sequential Mapping of Anisotropic Green's function (SISMAG)

In this article, a generalized computational method to simulate virtual nondestructive evaluation (NDE) of anisotropic composite plates is presented. The ultrasonic wave fields were computed using a modified and generalized version of Distributed Point Source Method (DPSM), a semi-analytical mesh-free technique. The anisotropic Green's functions required for DPSM implementation were calculated using Fourier transform method and Radon transform method and compared. It is established that the Green's functions obtained from two different methods are identical. Applying generalized mathematical formulations, NDE of different degrees of anisotropic: transversely isotropic, orthotropic and monoclinic material are simulated and reported in this article. To boost the computational efficiency, a Symmetry Informed Sequential Mapping of Anisotropic Green's function (SISMAG) is introduced with DPSM and discussed in detail. To prove the above claims, virtual NDE experiments of anisotropic plates with normal and angle incidence of the ultrasonic wave are simulated using a circular transducer of central frequency ∼1 MHz. Wave fields inside both the fluid and the solid media were calculated.

Lebedev scheme for ultrasound simulation in composites

The growing use of composite materials for aerospace applications has resulted in a need for quantitative nondestructive evaluation (NDE) methods appropriate for characterizing damage in composite components. NDE simulation tools, such as ultrasound models, can aid in enabling optimized inspection methods and establishing confidence in inspection capabilities. In this paper a mathematical approach using the Lebedev Finite Difference (LFD) method is presented for ultrasonic wave simulation in composites. Boundary condition equations for implementing stress-free boundaries (necessary for simulation of NDE scenarios) are also presented. Quantitative comparisons between LFD guided wave ultrasound simulation results, experimental guided wave data, and dispersion curves are described. Additionally, stability tests are performed to establish the LFD code behavior in the presence of stress-free boundaries and low-symmetry anisotropy. Results show that LFD is an appropriate approach for simulating ultrasound in anisotropic composite materials and that the method is stable in the presence of low-symmetry anisotropy and stress-free boundaries. Studies presented in this paper include guided wave simulation in hexagonal, monoclinic, triclinic and layered composite laminates.

Simulation of guided-wave ultrasound propagation in composite laminates: Benchmark comparisons of numerical codes and experiment

Ultrasonic wave methods constitute the leading physical mechanism for nondestructive evaluation (NDE) and structural health monitoring (SHM) of solid composite materials, such as carbon fiber reinforced polymer (CFRP) laminates. Computational models of ultrasonic wave excitation, propagation, and scattering in CFRP composites can be extremely valuable in designing practicable NDE and SHM hardware, software, and methodologies that accomplish the desired accuracy, reliability, efficiency, and coverage. The development and application of ultrasonic simulation approaches for composite materials is an active area of research in the field of NDE. This paper presents comparisons of guided wave simulations for CFRP composites implemented using four different simulation codes: the commercial finite element modeling (FEM) packages ABAQUS, ANSYS, and COMSOL, and a custom code executing the Elastodynamic Finite Integration Technique (EFIT). Benchmark comparisons are made between the simulation tools and both experimental laser Doppler vibrometry data and theoretical dispersion curves. A pristine and a delamination type case (Teflon insert in the experimental specimen) is studied. A summary is given of the accuracy of simulation results and the respective computational performance of the four different simulation tools.

Peri-Elastodynamic Simulations of Guided Ultrasonic Waves in Plate-Like Structure with Surface Mounted PZT

Peridynamic based elastodynamic computation tool named Peri-elastodynamics is proposed herein to simulate the three-dimensional (3D) Lamb wave modes in materials for the first time. Peri-elastodynamics is a nonlocal meshless approach which is a scale-independent generalized technique to visualize the acoustic and ultrasonic waves in plate-like structure, micro-electro-mechanical systems (MEMS) and nanodevices for their respective characterization. In this article, the characteristics of the fundamental Lamb wave modes are simulated in a sample plate-like structure. Lamb wave modes are generated using a surface mounted piezoelectric (PZT) transducer which is actuated from the top surface. The proposed generalized Peri-elastodynamics method is not only capable of simulating two dimensional (2D) in plane wave under plane strain condition formulated previously but also capable of accurately simulating the out of plane Symmetric and Antisymmetric Lamb wave modes in plate like structures in 3D. For structural health monitoring (SHM) of plate-like structures and nondestructive evaluation (NDE) of MEMS devices, it is necessary to simulate the 3D wave-damage interaction scenarios and visualize the different wave features due to damages. Hence, in addition, to simulating the guided ultrasonic wave modes in pristine material, Lamb waves were also simulated in a damaged plate. The accuracy of the proposed technique is verified by comparing the modes generated in the plate and the mode shapes across the thickness of the plate with theoretical wave analysis.

A model based bayesian solution for characterization of complex damage scenarios in aerospace composite structures

Ultrasonic damage detection and characterization is commonly used in nondestructive evaluation (NDE) of aerospace composite components. In recent years there has been an increased development of guided wave based methods. In real materials and structures, these dispersive waves result in complicated behavior in the presence of complex damage scenarios. Model-based characterization methods utilize accurate three dimensional finite element models (FEMs) of guided wave interaction with realistic damage scenarios to aid in defect identification and classification. This work describes an inverse solution for realistic composite damage characterization by comparing the wavenumber-frequency spectra of experimental and simulated ultrasonic inspections. The composite laminate material properties are first verified through a Bayesian solution (Markov chain Monte Carlo), enabling uncertainty quantification surrounding the characterization. A study is undertaken to assess the efficacy of the proposed damage model and comparative metrics between the experimental and simulated output. The FEM is then parameterized with a damage model capable of describing the typical complex damage created by impact events in composites. The damage is characterized through a transdimensional Markov chain Monte Carlo solution, enabling a flexible damage model capable of adapting to the complex damage geometry investigated here. The posterior probability distributions of the individual delamination petals as well as the overall envelope of the damage site are determined.

Scattering of guided waves at delaminations in composite plates

Carbon fiber laminate composites are increasingly employed for aerospace structures as they offer advantages, such as a good strength to weight ratio. However, impact during the operation and servicing of the aircraft can lead to barely visible and difficult to detect damage. Depending on the severity of the impact, fiber and matrix breakage or delaminations can occur, reducing the load carrying capacity of the structure. Efficient nondestructive testing and structural health monitoring of composite panels can be achieved using guided ultrasonic waves propagating along the structure. The scattering of the A0 Lamb wave mode at delaminations was investigated using a full three-dimensional (3D) finite element (FE) analysis. The influence of the delamination geometry (size and depth) was systematically evaluated. In addition to the depth dependency, a significant influence of the delamination width due to sideways reflection of the guided waves within the delamination area was found. Mixed-mode defects were simulated using a combined model of delamination with localized material degradation. The guided wave scattering at cross-ply composite plates with impact damage was measured experimentally using a non-contact laser interferometer. Good agreement between experiments and FE predictions using the mixed-mode model for an approximation of the impact damage was found.

Multi-frequency local wavenumber analysis and ply correlation of delamination damage

Wavenumber domain analysis through use of scanning laser Doppler vibrometry has been shown to be effective for non-contact inspection of damage in composites. Qualitative and semi-quantitative local wavenumber analysis of realistic delamination damage and quantitative analysis of idealized damage scenarios (Teflon inserts) have been performed previously in the literature. This paper presents a new methodology based on multi-frequency local wavenumber analysis for quantitative assessment of multi-ply delamination damage in carbon fiber reinforced polymer (CFRP) composite specimens. The methodology is presented and applied to a real world damage scenario (impact damage in an aerospace CFRP composite). The methodology yields delamination size and also correlates local wavenumber results from multiple excitation frequencies to theoretical dispersion curves in order to robustly determine the delamination ply depth. Results from the wavenumber based technique are validated against a traditional nondestructive evaluation method.