An improved nonlinear ultrasonic technique for detecting and monitoring impact induced damage in composite plates

Monitoring fatigue cracks in riveted plates using a sideband intensity based nonlinear ultrasonic technique

Aluminum structures are routinely used in aircraft due to their lightweight and corrosion resistance properties. Multi-layered aluminum plates are generally joined by rivets forming regions which are prone to fatigue crack formation in an aircraft. Therefore, the detection and monitoring of fatigue cracks at rivet joints in aluminum structures are crucial for ensuring flight safety. In this study, piezoelectric sensors were utilized to generate and detect Lamb waves on aluminum plates with rivet joints to investigate the feasibility of a newly developed Sideband Peak Count (SPC) technique for detecting fatigue cracks around these joints. To overcome the limitations of existing SPC-I (Sideband Peak Count - Index) and SPI (Sideband Peak Intensity) techniques in capturing harmonic and modulating wave frequencies due to material nonlinearity, a comprehensive index, the Sideband Intensity Index (SII) is introduced. Comparative analysis with existing SPC-I and SPI techniques confirm the effectiveness of the SII technique. This investigation shows that the SII technique significantly improves the detection capability of initial fatigue cracks around rivet joints on aluminum plates. This study offers a more efficient method for detecting critical fatigue cracks in rivet joints.

Monitoring damage growth and topographical changes in plate structures using sideband peak count-index and topological acoustic sensing techniques

Some topographies in plate structures can hide cracks and make it difficult to monitor damage growth. This is because topographical features convert homogeneous structures to heterogeneous one and complicate the wave propagation through such structures. At certain points destructive interference between incident, reflected and transmitted elastic waves can make those points insensitive to the damage growth when adopting acoustics based structural health monitoring (SHM) techniques. A newly developed nonlinear ultrasonic (NLU) technique called sideband peak count - index (or SPC-I) has shown its effectiveness and superiority compared to other techniques for nondestructive testing (NDT) and SHM applications and is adopted in this work for monitoring damage growth in plate structures with topographical features. The performance of SPC-I technique in heterogeneous specimens having different topographies is investigated using nonlocal peridynamics based peri-ultrasound modeling. Three types of topographies - "X" topography, "Y" topography and "XY" topography are investigated. It is observed that "X" and "XY" topographies can help to hide the crack growth, thus making cracks undetectable when the SPC-I based monitoring technique is adopted. In addition to the SPC-I technique, we also investigate the effectiveness of an emerging sensing technique based on topological acoustic sensing. This method monitors the changes in the geometric phase; a measure of the changes in the acoustic wave's spatial behavior. The computed results show that changes in the geometric phase can be exploited to monitor the damage growth in plate structures for all three topographies considered here. The significant changes in geometric phase can be related to the crack growth even when these cracks remain hidden for some topographies during the SPC-I based single point inspection. Sensitivities of both the SPC-I and the topological acoustic sensing techniques are also investigated for sensing the topographical changes in the plate structures.

Acoustic source localization in composite plates using Sideband Peak Count - Index technique

In this study the Non-Linear Ultrasonic Sideband Peak Count-Index (SPC-I) technique is used as the foundation for anovel approach towards acoustic source localization (ASL) in orthotropic composite plates. The SPC-I based technique proposed here does not require the signal attenuation information or any knowledge on the time of arrival of the signal. It should be noted that since individual sensors can have varying sensitivities, the signal attenuation measured from the recorded signal amplitude is not very reliable. In addition, it is not necessary to have any prior knowledge of the mechanical properties of the composite plate material. All these are achievable by attaching 25 sensors that are well-scattered on the surface of the plate. The signals that are generated by an acoustic source are recorded by these 25 sensors. The recorded signals are then analyzed to derive the SPC-I value for each signal. The calculated SPC-I values are run through an optimization algorithm to predict the acoustic source location. Such localization is possible because the composite plate is inherently a non-linear material. Hence, as the signal travels longer distances through a composite plate, the recorded signal should show increasing level of distortion due to material non-linearity and dispersion. This phenomenon manifests itself primarily as a consequence of signal scattering and frequency modulation. Because of this, the phenomena of increasing distortion in the signal with increasing propagation distance can be exploited and utilized to predict the location of the acoustic source by solely utilizing the SPC-I values. This acoustic source localization technique is experimentally verified on a Carbon Fiber Reinforced (CFR) composite plate of dimension 500 mm x 500 mm with a thickness of 1 mm. The experimental results confirmed the feasibility of the proposed technique.

Numerical modeling with experimental verification investigating the effect of various nonlinearities on the sideband peak count-index technique

A newly developed nonlinear ultrasonic (NLU) technique called sideband band peak count-index (or SPC-I) measures the degree of nonlinearity in materials by counting the sideband peaks above a moving threshold line - larger the SPC-I values, higher is the material nonlinearity. In various published papers, the SPC-I technique has shown its effectiveness in structural health monitoring (SHM) applications. However, the effects of different types of nonlinear phenomenon on the sideband peak generation is yet to be investigated in depth. This work addresses this knowledge gap and investigates the effects of different types of nonlinearity on the SPC-I technique. Three types of nonlinearity (material nonlinearity, structural nonlinearity and contact nonlinearity) are investigated separately through numerical modeling. In this investigation the material nonlinearity and the contact nonlinearity are modeled by finite element method (FEM) using the commercial Abaqus/CAE software. The structural nonlinearity arising from stationary cracks is modeled using nonlocal peridynamics based peri-ultrasound modeling technique. Numerical modeling shows that the sideband peak values do not increase proportional to the input signal strength thus indicating nonlinear response, and different types of nonlinearities affect the SPC-I measurements differently. For the experimental verification a composite plate with impact-induced damage is considered for investigating the material nonlinearity and structural nonlinearity while a linear elastic aluminum plate is used to examine the contact nonlinearity between the transducers and the plate. The trends observed in the experimental observations matched with the numerical model predictions.

Guided wave propagation in a double-layer plate with a nonlinear spring-interface

This article derives the solution to the guided wave fields in a double-layer plate consisting of two sublayers. It is assumed that the two sublayers are linearly elastic. They are bonded together at their interface by a nonlinear adhesive layer of infinitesimal thickness. This allows us to propose a nonlinear spring-interface model. Based on such an idealized model for the double-layer plate, guided wave fields in the plate are solved using the modified normal mode expansion method. It is found that the nonlinearity of the spring-interface can generate resonant guided waves in the double-layer plate. Specifically, when certain conditions are met, mixing of two primary guided waves will generate resonant guided waves whose frequencies are either the sum or difference of those of the two primary waves. Amplitudes of such resonant mixed waves are proportional to the compliance of the nonlinear spring-interface. As a special case, if the two primary waves have the same frequency, a resonant second harmonic guided wave may be generated. In addition, the conditions that generate resonant mixed waves are identified. We believe that the results of this work provide the theoretical foundation on which nondestructive evaluation techniques using nonlinear guided waves can be developed to nondestructively evaluate, for example, the bond strength of thin coating on a substrate.

Impact monitoring on complex structure using VMD-MPE feature extraction and transfer learning

Impacts are common damage events in aviation scenarios that can cause damage to the structural integrity ofan aircraft and pose a threat to its safe operation. Therefore, it is crucial to monitor impact events. A region-to-point monitoring method is proposed to address the challenges posed by the large area of monitored aircraft structures and the long distance between sensors. Firstly, to fully use the information in the original impact signal and reduce the aliasing effect caused by the reinforced structure, the original signal is decomposed into several modes with different frequency bands by Variational Mode Decomposition (VMD). The Multi-scale Permutation Entropy (MPE) value is then calculated to reflect the various characteristics of each mode, which is used as a basis for classification. Secondly, Transfer Component Analysis (TCA) is selected as a transfer learning method to reduce the difference between the features of the source domain and the target domains' features. Thirdly, the TCA-transformed source domain data are used to train the Probabilistic Neural Network model (PNN), and the unfamiliar target domain data are used to verify the impact area identification. Finally, based on regional location, the system identification technology and weighted centroid algorithm can be used to obtain the history of impact force and the precise coordinates of the impact location.

Modeling and simulation of zero-group velocity combined harmonic generated by guided waves mixing

In this paper, modelling and numerical perspective of zero-group velocity (ZGV) combined harmonic generated by guided waves mixing are investigated. The conditions for the generation of the ZGV combined harmonic are analyzed by S₀-S₀ and SH₀-SH₀ guided waves mixing in an isotropic plate, respectively. The generation of ZGV combined harmonics at sum frequency caused by counter-directional guided waves mixing is observed. It is confirmed that the ZGV combined harmonic with a considerable magnitude can be generated by this counter-directional guided waves mixing when both the internal resonant condition and non-zero power flux are satisfied. The application of generated ZGV combined harmonics for localized material degradation assessment is numerically examined in the given plate. The obtained results indicate that the generated ZGV combined harmonic induced by the counter-directional guided waves mixing can be used to assess the localized material degradation with improved signal-to-noise ratio. This study provides an insight into the physical process of the ZGV combined harmonic generation, and meanwhile offer a promising means for localized material degradation assessment by ZGV combined harmonics generated by guided waves mixing.

A modified sideband peak count based nonlinear ultrasonic technique for material characterization

The ultrasonic Non-Destructive Testing and Evaluation (NDT&E) has been widely used for Structural Health Monitoring (SHM). The conventional linear ultrasonic technique which is suitable for detecting macro-scale defects is routinely used in industry; however, it often fails to detect the micro-scale defects. Generally, micro-defects in a material appear first due to dislocations at grain boundaries. These micro-defects then grow and coalesce to form macro-defects. The crack growth rate is much faster for macro-defects than micro-defects. Therefore, monitoring micro-defects is important to avoid catastrophic failures of structures. Nonlinear ultrasonic techniques help to detect micro-defects. A recently developed nonlinear ultrasonic technique called Sideband Peak Count - Index (SPC-I) technique has some inherent advantages over other nonlinear techniques for monitoring progression of micro-defects. In this research, the SPC-I technique is further modified. This modified technique, Sideband Peak Intensity (SPI) technique, is shown to be more robust and easier to implement. Both SPC-I and SPI techniques are used to monitor the damage progression in impact induced damages in metals. Similarities and dissimilarities between these two techniques are investigated. Then it is concluded that the SPI technique is good as a general-purpose robust damage monitoring tool that can be used by less skilled users while the SPC-I technique although requires more skills has more sensitivity and has the flexibility for an in-depth damage analysis in materials.

Sideband Peak Count in a Vibro-Acoustic Modulation Method for Crack Detection

This paper presents a new method of signal processing for vibro-acoustic modulation (VAM) methods in order to detect damage accumulation in steel samples. Damage in the tested samples was produced by cycle loading, which, with a small amplitude, was used as a pump wave to modulate an ultrasonic probe wave. Multiple sideband peaks were observed, which were used to characterize the modulation effect. We propose the effectiveness sideband peak number (SPN) method as an indicator of any damage accumulation when the load cycle is applied. Moreover, after comparing the SPN with the previously used modulation index (MI), we concluded that, for some of the samples, the SPN provided better damage indication than the MI. The presented results can be explained by a simple model of bilinear crack nonlinearity. This model demonstrates that the amplitude dependences of the sideband components on the pump and the probe wave amplitudes are very different from the quadratic crack model that is usually used for MI test explanation.