Detection Of Delaminations In Composite Laminates Research Articles

Delamination detection in composite laminates using improved surrogate-assisted optimization

Vibration-based delamination detection can be posed as an optimization problem, wherein the discrepancy of the frequency shifts obtained from a numerical model with assumed delamination parameters is minimized from target values of the frequency shifts. Unlike cracks, delamination is characterized by a mix of continuous and discrete parameters. The continuous variables comprise the in-plane locations and sizes, whereas discrete parameters correspond to the interface where the damage occurs. While the recent studies have demonstrated the effectiveness of surrogate-assisted optimization in determining the locations and sizes of the delamination, the prediction of interface has received scarce attention. To improve on this aspect, in this paper, individual surrogate models are built corresponding to each of the interfaces. Furthermore, we also attempt to improve the underlying optimization method by using multiple populations instead of one in order to reduce the likelihood of being trapped in local minima. The performance of the proposed approach is numerically investigated by considering various factors, such as the different number of samples for training the surrogate model, the combinations of different modes of frequency shifts used as input for damage detection, and the addition of artificial noise to the numerical frequency shifts to simulate the unavoidable measurement errors. Experimental investigations are also conducted on six delaminated beam specimens and one intact specimen. The proposed approach showed significant improvements in identifying the damage parameters, and in particular improving the prediction accuracy of the damage interface.

A two-step method for delamination detection in composite laminates using experience-based learning algorithm

Delamination in composite laminates reduces the structural stiffness and thus causes changes in the vibration responses of the laminates. Therefore, it is feasible to employ dynamic characteristics (such as natural frequencies and mode shapes) for delamination detection by using an optimization method. In the present study, a two-step method is proposed for the delamination detection in composite laminates using an experience-based learning algorithm. In the first step, one-dimensional equivalent through-thickness beam elements are employed to model the composite laminated beam and potential delamination locations are identified. In the second step, a typical three-dimensional finite mesh is utilized for the beam’s modeling and the detailed delamination information (including the delamination location, size, and interface layer) is detected. This two-step method combines the advantages of the two different modeling techniques and is able to significantly reduce the computational cost without reducing detection accuracy. The proposed method is applied for an eight-layer quasi-isotropic symmetric (0/-45/45/90)s composited beam with different delamination situations to verify its effectiveness and robustness. The performance of the two-step method is demonstrated by comparing with the one-step method and other three state-of-the-art algorithms (CMFOA, PSO, and SSA). Moreover, the influence of artificial noise on the accuracy of the detection performance is also investigated. Both numerical and experimental results confirm the superiority of the proposed method for delamination detection in composite laminates especially for the prediction of delamination interface.

Delamination detection in composite laminates using ensemble of surrogates

The use of surrogates to model complex engineering problems such as the vibration based delamination detection in composite laminates involves uncertainty especially in regions with high error and areas where the predictions made by the surrogates vary widely. Moreover, the choice and suitability of the surrogate for this problem is dependent on the nature of the problem. Therefore in this work the absolute errors of different surrogate modeling techniques are analyzed and the standard deviation of predictions made by the surrogate model is used to quantify the areas of uncertainty in the design space. The current delamination detection problem in composite laminates, the locations of damage, size of the damage are the variables of interest and the change in natural frequency being the response parameter is analyzed numerically and experimentally to model this prediction problem.

Delamination detection in composite laminates using low frequency guided waves: Numerical simulations

The aim of this study is to find an efficient way of finite element modelling of guided wave propagation in composite laminates to detect delaminations. A novel approach is proposed to model delaminations by locally reducing the stiffness and it is implemented in a finite element shell model. The approach is verified by comparing the results with the results of two existing approaches. Results show that the stiffness reduction approach gives reasonable accuracy for the primary wave modes and improvement in simulation time. Moreover, it is shown that new convergence criteria should be considered to simulate the guided wave propagation. Additionally, the Pearson correlation coefficient is introduced as a good criterion for delamination detection in such problems. All the conclusions are made when simulations are performed in the low frequency range and can be used to study guided wave propagation in large composite structures such as wind turbine blades.

Delamination detection in composite laminates using high-frequency P- and S-waves – Part I: Theory and analysis

A structural health monitoring (SHM) technique that exploits piezoelectric-induced ultrasonic waves propagating across the thickness of composite laminates is proposed to detect the presence of inter-layer delaminations. The damage-induced lag in the times of arrival of primary (P) and secondary (S) elastic waves is exploited as the key mechanism to enhance the detection capabilities. The theoretical aspects of the design of the proposed SHM procedure for thin-walled multi-layered structures are presented first. Two- and three-dimensional numerical analyses of the wave propagation across the thickness of composite laminates with a single delamination are carried out. A correlation between the delamination position and the percent variations of the time of flight (ToF) of P- and S-waves is found. Strong modulation of the power spectral density (PSD) of the output signals is obtained for delaminations close to the surface onto which the actuator–sensor pair is mounted. While for the latter no additional information about their position in the laminate stacking sequence is obtained, for deeper delaminations the proposed procedure delivers very accurate estimates of the damage location. The experimental validation of the described delamination detection technique is carried out in the second part of this study (Pasquali et al., in press [1]).

Delamination detection in composite laminates using high-frequency P- and S-waves – Part II: Experimental validation

In this work, the delamination detection procedure presented in part I is experimentally investigated (Pasquali and Lacarbonara, 2015). Attention placed on the through-the-thickness propagation direction of composite laminates undergoing delaminations. As observed in the numerical tests, a precise correlation between the delamination position and the variations of the Time of Flight (ToF) of primary (P) and secondary (S) waves is found. A substantial modulation of the power spectral density (PSD) of the acquired output signals is recorded in the case of delaminations close to the surface onto which the actuator/sensor transducers are bonded. The experimental validation of the proposed SHM procedure is carried out through extensive testing on different types of isotropic and composite specimen. A good agreement between the experimental results and the theoretical predictions shown in part I [1] is found, together with determination of limitations on the delamination position offset with respect to the actuator–sensor pair position.

Delamination detection in composite laminates using dispersion change based on modeconversion of Lamb waves

A new ultrasonic propagation system has been constructed using macrofiber composite(MFC) actuators and fiber Bragg grating (FBG) sensors. The MFCs and FBGs can beintegrated into composite laminates because of their small size and high fracture strain.The developed system can send and receive broadband Lamb waves. In this research, thissystem was used to detect delamination damage in composite laminates. First, the multiplemodes of Lamb waves in a carbon-fiber-reinforced plastic (CFRP) quasi-isotropic laminatewere identified by transmitting and receiving the symmetric and antisymmetric modesseparately. Then, the mode conversions at both tips of a delamination were investigatedthrough an experiment and a two-dimensional finite element analysis (FEA). A newdelamination detection method was proposed on the basis of the mode conversions,and experiments were carried out on laminates with an artificial delamination.When antisymmetric modes were excited, the frequency dispersion of the receivedA1 mode changed, depending on the delamination length owing to the mode conversion between theA1 modeand the S0 mode. This phenomenon was confirmed through the FEA and these results prove thatthis new method is effective in detecting a delamination in CFRP laminates.

J0402-3-1 広帯域ラム波のモード変換による周波数分散性変化を利用した複合材料中の剥離損傷検知([J0402-3]超音波計測・解析法の新展開(3))

J0402-3-1 広帯域ラム波のモード変換による周波数分散性変化を利用した複合材料中の剥離損傷検知([J0402-3]超音波計測・解析法の新展開(3))

Optimization of piezoelectric sensor location for delamination detection in composite laminates

The optimal placement of sensors is a critical issue in detecting damage in laminated composite structures. The aim is to use a minimum number of sensors, placed at the correct locations, so that the voltage signals received from the sensor set can be used to detect both the presence and the extent of damage. In this study, an optimization procedure is developed to detect arbitrarily located discrete delamination in composite plates using distributed piezoelectric sensors. The probability of damage distribution in the plate is determined using a statistical model. A genetic algorithm (GA) is used to detect the number and location of the sensors. The analysis uses a Monte Carlo method to generate the initial population. The simulation and signal processing is performed using a finite element procedure based on the refined layer-wise theory, which is capable of modelling ply-level stresses, and seeded delaminations are modelled with Heaviside step functions. A two-way electromechanical coupled field formulation is used to describe the induced strain. The objective function is a damage index which compares the voltage signals from a healthy (no delamination) and a statistically determined delaminated model. The voltage signals are affected by the local changes in the strain induced by the presence of delamination. The optimization solutions are verified by numerical simulation as well as with experiments conducted using customized piezoelectric sensors and a laser scanning vibrometer. The results presented show that the optimum sensor pattern is capable of detecting discrete seeded delaminations in moderately thick composite plates.

Delamination Detection in Composite Laminates from Variations of Their Modal Characteristics

As a first step in developing a health monitoring system, the effect of delamination on the modal frequencies of laminated composite beams has been investigated. A piezoceramic patch driven with a linear rapid frequency sweep was used to induce vibrations on the structure and its response registered via piezoelectric sensors. Modal frequencies were obtained using concepts of resonant ultrasound spectroscopy (RUS). Changes of the modal frequencies after delamination initiation, compared to those of a non-delaminated specimen, gave a good indication of the degree of damage, demonstrating the feasibility of using measured changes in the vibration characteristics to detect damage.

Application of the Rapid Frequency Sweep Technique for Delamination Detection in Composite Laminates

As a first step in developing a health monitoring system, the effect of delamination on the natural frequencies of laminated composite beams has been investigated. A piezoceramic patch driven with a linear rapid frequency sweep was used to induce vibrations on the structure and its response registered via piezoelectric film sensors. Modal frequencies were obtained through a method defined here as the maximum amplitude at resonance (MAR), which has proved to be a simple and effective procedure. Changes of the modal frequencies after delamination initiation, compared to those of an uncracked (non-delaminated) specimen, gave a good indication of the degree of damage, especially at high modal frequencies (9-12 kHz).