Delamination In Composite Plates Research Articles

Experimental and Numerical Study on Vibration-Based Damage Detection and Localization in Laminated Composite Plates

Damage detection in composite materials is crucial for ensuring the safety and reliability of engineering structures. Conventional methods often face challenges in accurately identifying damage in plate-like structures, particularly in scenarios involving multiple damages or small-scale delamination. This study focuses on investigating the detection and localization of delamination in composite plates by employing both experimental and numerical modal analysis. An eight-ply woven Glass-Epoxy composite laminate with and without damage was prepared with the aid of hand lamination technique. Laminate was fixed to a Clamped-Free-Free-Free (CFFF) boundary condition for experimental modal analysis by introducing controlled damage to examine its impact on modal properties. To validate the natural frequencies (NFs) of damaged and undamaged composite laminates, a numerical analysis was conducted using ANSYS Parametric Design Language (APDL). Further, to advance the understanding of using modal shapes and their spatial derivatives for damage localization in composite plates under various damage situations, post-processing of simulation results was conducted using MATLAB. Finite Difference Method has been employed to calculate the derivatives, and a novel damage index (DI) is proposed to enhance damage localization capabilities. The results affirm that the proposed DI is effective and precise in identifying damage in plate-like structures, both for individual and multiple damage scenarios. This research study presents a novel approach for identifying and pinpointing damage in composite plates, thereby making a valuable contribution to the field of structural health monitoring (SHM) applications

Adaptive time-reversal method for delamination detection of composite plates based on reconstruction algorithm for probabilistic inspection of defects

In this study, delamination in composite plates was detected by combining a baseline-free detection method based on a time-reversal method with a damage-imaging algorithm based on the reconstruction algorithm for probabilistic inspection of defects. A new adaptive time-reversal method (ATRM) for calculating the damage index values was proposed through the local adaptive analysis of the time-reversal signals of Lamb waves. The time-reversed signals were obtained by determining the maximum correlation between the forward path response and excitation signals. The forward path and time-reversal signals were intercepted using local optimal correlation coefficients to compensate for the peak offset errors that occurred during Lamb wave propagation. Multiple optimal adaptive decompositions were used to eliminate the interference of reflected waves in the forward path. The feasibility of the proposed method was verified numerically using finite element simulations, and was quantitatively compared with that of the conventional time-reversal method. Experimental studies using actual composite plates further validated the proposed method. The influence of the damage location on the performance of the proposed method was also investigated. The results show that the ATRM method significantly improved the temporal reversibility and obtained the optimal global similarity, making it more accurate and reliable for localising the delamination defects in composite plates compared with existing time reversal-based methods.

Monitoring of Damage in Composite Structures Using an Optimized Sensor Network: A Data-Driven Experimental Approach.

Due to the complexity of the fracture mechanisms in composites, monitoring damage using a vibration-based structural response remains a challenging task. This is also complex when considering the physical implementation of a health monitoring system with its numerous uncertainties and constraints, including the presence of measurement noise, changes in boundary and environmental conditions of a tested object, etc. Finally, to balance such a system in terms of efficiency and cost, the sensor network needs to be optimized. The main aim of this study is to develop a cost- and performance-effective data-driven approach to monitor damage in composite structures and validate this approach through tests performed on a physically implemented structural health monitoring (SHM) system. In this study, we combined the mentioned research problems to develop and implement an SHM system to monitor delamination in composite plates using data combined from finite element models and laboratory experiments to ensure robustness to measurement noise with a simultaneous lack of necessity to perform multiple physical experiments. The developed approach allows the implementation of a cost-effective SHM system with validated predictive performance.

Artificial neural network based delamination prediction in composite plates using vibration signals

Dynamic loading on composite components may induce damages such as cracks, delaminations, etc. and development of an early damage detection technique for delaminations is one of the most important aspects in ensuring the integrity and safety of composite components. The presence of damages such as delaminations on the composites reduces its stiffness and further changes the dynamic behaviour of the structures. As the loss in stiffness leads to changes in the natural frequencies, mode shapes, and other aspects of the structure, vibration analysis may be the ideal technique to employ in this case. In this research work, the supervised feed-forward multilayer back-propagation Artificial Neural Network (ANN) is used to determine the position and area of delaminations in GFRP plates using changes in natural frequencies as inputs. The natural frequencies were obtained by finite element analysis and results are validated by experimentation. The findings show that the suggested technique can satisfactorily estimate the location and extent of delaminations in composite plates.

Investigation of delamination effect on nonlinear vibration behaviors of a composite plate resting on nonlinear elastic foundation

The aim of the present work is to investigate the delamination effect on nonlinear vibration behaviors of composite plate resting on nonlinear elastic foundation. The composite plate is assumed to be pre-delaminated with various delamination locations and modeled with an improved layerwise theory and finite element method. The delamination is simulated by the Heaviside unit step function allowing for the discontinuity of displacement fields. By means of the variational principle, the nonlinear governing equations can be obtained and further solved by the direct iterative method. The model takes into account three foundation parameters so as to investigate their effect on the nonlinear vibration characteristics. Three longitudinal locations and three interfacial locations are considered, and both clamped-free and clamped–clamped boundary conditions are considered to investigate the nonlinear vibration behaviors. The results show that different from the linear vibration, with the existence of delamination, the nonlinear fundamental frequencies may exceed the fundamental frequency of the healthy plate when the vibration amplitude ratio reaches a certain value. This finding may help engineers understand the nonlinear vibration behaviors and further detect the delamination in composite plate resting on nonlinear foundations in presence of delamination.

Delamination detection in composite plates using random forests

Laminated composites constitute a crucial family of advanced materials for the development of high-performance components. A classic challenge with this composite family is delamination. Lately, vibration-based methods, augmented by dynamic response signal processing and soft computing techniques, have become central for the non-destructive detection of delamination. To date, several studies have been put forth in relation to tackling the problem. Despite this, the concurrent determination of the presence and severity of delamination remains a challenge for structures more complicated than simple beam-like structures. Grounded on the combination of random forests (RF) and natural frequency-shift damage assessment methods, this paper presents an alternative strategy for the simultaneous predictions of severity and location parameters of delamination in composite plates. The study commences with the establishment of a robust finite element (FE) procedure for the inclusion of delamination in composite plates. Subsequently, the FE procedure was validated with good agreement against published results. With high-dimensional datasets generated from the FE procedure, four RF models with different architectures are developed and compared (including a comparison with a deep neural network model). Relying only on four natural frequencies, the predictive strength of the RF model with optimally tuned hyperparameters is assessed via the FE-generated and experimentally obtained datasets. Results indicate that the proposed RF-based method is capable of accurately predicting five parameters quantifying the location and severity of delamination in composite plates. The best RF model produces a coefficient of correlation as high as 0.996 for the location and size parameters and up to 90% accuracy for classifying the delaminated interface.

Resonant Method for Detection and Identification of Delamination in Composite Plates by Elastic Guided Waves

By the example of a local inhomogeneity in the form of a horizontal crack, a numerical and experimental confirmation of the existence of resonance effects in an inhomogeneous elastic strip, investigated by I. I. Vorovich, is given. Examples of the use of this phenomenon for detecting delamination in composite plates and determining their size and depth based on the data of ultrasonic monitoring by traveling Lamb waves are given.

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.

Locating and imaging contact delamination based on chaotic detection of nonlinear Lamb waves

Abstract We propose a novel method for locating and imaging contact delaminations in composite plates, using a network of PZT transducers and the nonlinear Lamb waves. Being immune to noise and sensitive to weak signal, the Duffing chaotic oscillator is utilized to detect the nonlinear harmonics of Lamb waves generated at the contact delamination interfaces. The Lyapunov exponent of the Duffing system output is calculated to quantify the relative intensity of nonlinear second harmonic. Its normalized value is taken as the damage index to locate and image the delaminations by combining with the probabilistic imaging algorithm. The numerical simulation study on the proposed method is performed. Compared with the nonlinearity parameter and the conventional damage index, the Lyapunov damage index (LDI) can image the single delamination or multiple delaminations more effectively, especially under the noise interference. The experiment using the LDI is also conducted on a composite laminated plate with multiple internal delaminations. The results show that the proposed imaging method based on LDI is also valid to the practical delaminations in the composite plate. It has the following advantages: immunity to noise, needlessness of baseline data, high imaging resolution and location accuracy.

Performance evaluation of damage detection algorithms for identification of debond in stiffened metallic plates using a scanning laser vibrometer

While several studies have focused on the detection and localization of delamination in composite plates, few comprehensive studies have been performed for the identification of debond in stiffened metallic plates using vibration-based approaches. Therefore, this study is motivated by the need to evaluate the qualitative performance of existing damage detection algorithms, namely modal curvature, the gapped smoothing method (GSM), the generalized fractal dimension (GFD) and the wavelet transform coefficient (WTC), in detecting debond in stiffened metallic plates. Extensive experimental investigation is performed using laser Doppler vibrometer as a noncontact sensing device and LDS Permanent Magnetic Shaker as an actuator. The obtained results show high susceptibility to noise and lesser accuracy in locating the debond zone, except the WTC and GFD. However, the WTC fails to provide good results for higher debond lengths, and the GFD shows prominent false alarms at the free edges of the plates. To circumvent these difficulties, two different modifications of the fractal dimension algorithm, namely the modified GFD (MGFD) and the GFD with GSM (GFD-GSM), have been proposed. Extensive numerical simulations are further carried out using commercially available finite element package ANSYS 14.0 in order to examine the experimental findings. In contrast to most previous work, the signal-to-noise ratio (SNR) in the experimental data has been appropriately quantified and noise of the same SNR level has been synthetically generated and applied on the modal data obtained from numerical simulations. Comprehensive studies for different debond locations and lengths suggests a similar trend as that obtained from the experimental investigations. Finally, a study on damage severity has been performed using the WTC and proposed modifications of the GFD. It is found that the proposed modifications of the fractal dimension perform outstandingly well in all circumstances, and can be used as an excellent tool for debond localization and quantification.

Lamb Waves for Detection of Delamination in Composite Plates

Lamb Waves for Detection of Delamination in Composite Plates

Numerical study of the response of dynamic parameters to defects in composite structures

The purpose of this work is to study three different models of delamination in composite plate (the free mode model, the constrained model and the contact model) and applicability of this models to the vibrational method of damage detection based on frequency shifts. The results of numerical simulation have shown that the free mode model leads to abrupt changes in natural frequencies due to non-physical condition of mutual penetration of adjacent volumes in the defect zone. The constrained and the contact models yield qualitative agreement in shifts of natural frequencies with a change of the defect size. All models have shown the necessity of analyzing shifts of high frequencies to detect small size delamination.

Detection and characterization of delaminations in composite plates via air-coupled probes and warped-domain filtering

In this work, a novel baseline-free procedure aimed at detecting and sizing delaminations in composite plates is proposed. To such a purpose an air-coupled probe is used to measure the sound pressure in air that is generated by leaky guided waves traveling along the plate and scattered by defects. In particular, by scanning the plate surface with a proper probe tilt angle and lift-off, wavefields dominated by a single guided mode can be obtained. Then, a processing procedure based on warped time–frequency operators is used to filter out the main wavefield generated by the acoustic source in order to enhance the one scattered by defects. Such operators are designed basing on the dispersion curves of the dominant guided mode considered. Finally, it is shown how the scattered wavefield can be further processed to infer the defect position and size by means of the Circular Hough Transform.The presence of a barely visible delamination on a 4.9mm thick composite laminate was detected by exploiting the proposed approach. The obtained results are in good agreement with those obtained by using a commercial phased-array ultrasonic C-scan device.

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.

Effects of Piezoelectric Actuation on Delamination in Transversely Loaded Composite Plates

Fiber reinforced composite materials are known to have poor tolerance to impact loads. Damages can be observed in the forms of matrix crack, fiber failure and delamination. In the case of low velocity impact, delamination is often a major concern due to its hidden nature. In this work, the effects of piezoelectric actuation on delamination in composite plates subjected to low velocity impact have been studied using LS-DYNA. It was found that, piezoelectric actuators can be used to reduce delamination in composite laminates. This was achieved by actuating the laminate to curve in the opposite direction of the incoming impact load.

Non-baseline identification of delamination in plates using wavelet-aided fractal analysis of two-dimensional mode shapes

Delamination is a typical form of damage in composite plates. Identification of delamination in plates has been a focus of increasing research interest in relatively recent years. This study develops a new approach, termed a scale waveform dimension analysis of two-dimensional mode shapes, to identify delamination in composite plates. The scale waveform dimension analysis comprises two components: decomposition of a mode shape into scale mode shapes and waveform dimension analysis of scale mode shapes. The first component acts as a splitter that broadly splits a noisy mode shape into three sets of trend-, noise-, and damage-scale mode shapes, from which the damage scale mode shapes can be selected for use in damage characterization; the second component functions as a detector to detect abnormalities of damage scale mode shapes, to indicate the presence and location of delamination. The efficacy of the method is numerically studied and experimentally examined using composite plates containing small delamination. The results show that the proposed method can identify delamination with great accuracy in noisy conditions, needing no intact baseline mode shapes nor any prior knowledge of either the material properties or boundary conditions of the plate being inspected.

Fracture Mechanics Approach for Analysis of Delamination in Composite Plates

Delamination can be a substantial problem in designing composite structures. Modelling of delamination by finite element (FE) codes is limited. Previous efforts to model delamination and debonding failure modes using FE codes have typically relied on ad hoc failure criteria and quasi-static fracture data. Improvements to these modelling procedures can be made by using an approach based on fracture mechanics. A study of modelling delamination using the FE code ANSYS was conducted. This investigation demonstrates the modelling of composites through improved delamination modelling. Further developments to this approach may be improved.

Ultrasonic characterization of delamination in aeronautical composites using noncontact laser generation and detection

The characterization of delamination in composite plates with ultrasonic waves generated and detected by lasers is presented. Composite materials have become one of the most important structural materials in the aviation industry because of their excellent mechanical properties, such as high specific stiffness and antifatigue. This paper reports a new application of the laser ultrasonic technique to perform nondestructive detection of carbon-fiber-reinforced plastic (CFRP) and continuous-fiber-reinforced ceramic matrix composites (CFCCs) containing artificial internal defects, based on propagation characteristic of ultrasonic waves generated by pulse laser with a wavelength of 1064 nm and pulse duration of 10 ns. A laser interferometer based on two-wave mixing is used to measure ultrasonic wave signals. The main advantage of this technique over conventional ultrasonic testing techniques is the ability to carry out detection without using coupling agents. The research results prove that the laser ultrasonic technique is effective for the detection of internal defects in both CFRP and CFCC composite components, which should promote and expand the application of the technique in the aviation industry.

Instantaneous delamination detection in a composite plate using a dual piezoelectric transducer network

This study proposes a new damage detection technique so that delamination in composite plates can be detected by comparing pitch-catch Lamb wave signals obtained from a piezoelectric transducer (PZT) network without using their own baseline signals obtained from the pristine condition. The proposed technique is based on the premise that the fundamental anti-symmetric (A0) mode slows down when it passes through a delamination area while the speed of the fundamental symmetric mode is little affected by delamination. First, the A0 mode in each path is isolated using a mode extraction technique. This mode extraction technique is able to isolate the A0 mode without frequency or transducer size tuning using dual PZTs composed of concentric ring and circular PZTs. Once the A0 modes are extracted from all paths in the transducer network, the relative time delay of the A0 mode in each path with respect to the other paths is defined as a delamination sensitive feature. Then, an instantaneous outlier analysis is developed and performed on the damage sensitive feature to identify the path(s) affected by the delaminated region(s). Because the relative time delays of the A0 modes are instantaneously compared, robust delamination detection is achieved even under varying temperature conditions.

Extended Analysis of a Damage Prognosis Approach Based on Interval Arithmetic

Abstract: In the previous work by the authors, an approach to damage prognosis which incorporates the effects of uncertainties has been proposed. This approach is based on the idea of integrating the laws for damage progression within the framework of interval arithmetic, a framework that naturally accommodates uncertainty. The modest purposes of this study are to extend/modify the approach in a small but significant manner and also to consider a more complex problem than the previous benchmark. The developments in the paper are illustrated through two case studies. The first case study revisits the benchmark of the initial paper – an isotropic finite plate under harmonic uniaxial loading, where the damage is assumed to be a central, mode I, through‐crack. The damage propagation model for the cracked plate is the Paris‐Erdogan law. The second example considers the growth of internal delaminations in composite plates subjected to cyclic compression. Under compressive loading, laminated composite plates experience repeated buckling‐unloading of the delaminated layer with a consequent reduction in interlayer resistance. The state of stress near the delamination tip is of mixed mode I and II. A graphite‐epoxy unidirectional specimen has been assumed here and the thin‐film model of Chai and colleagues is used; the latter is a closed‐form solution for the initial post‐buckling and growth behaviour in the ideal case of a surface delamination in an infinitely thick plate. The delamination growth law considered is the one proposed by Kardomateas and co‐workers. As the objective of the study is to examine the effects of uncertainties, the various models are considered within the framework of interval arithmetic; Monte Carlo analysis is also carried out to provide a basis for comparison.