A new method to detect delamination in composites using chirp-excited Lamb wave and wavelet analysis

The guided wave technique is commonly used in structural health monitoring as the guided waves can propagate far in the structures without much energy loss. The guided waves are conventionally generated by the surface-mounted piezoelectric wafer active sensor (PWAS). However, there is still lack of understanding of the wave propagation in layered structures, especially in structures made of anisotropic materials such as carbon fiber reinforced polymer (CFRP) composites. In this paper, the Rayleigh-Lamb wave strain tuning curves in a PWAS-mounted unidirectional CFRP plate are analytically derived using the normal mode expansion (NME) method. The excitation frequency spectrum is then multiplied by the tuning curves to calculate the frequency response spectrum. The corresponding time domain responses are obtained through the inverse Fourier transform. The theoretical calculations are validated through finite element analysis and an experimental study. The PWAS responses under the free, debonded and bonded CFRP conditions are investigated and compared. The results demonstrate that the amplitude and travelling time of wave packet can be used to evaluate the CFRP bonding conditions. The method can work on a baseline-free manner.

There are two major problems with fiber-reinforced polymer (FRP) composites during their machining that need to be addressed. The first concern is the delamination and formation of burrs at machined edges, and the second is the effects of aging leading to mechanical deterioration. In this study, carbon FRP (CFRP) and aramid FRP (AFRP) composites were manufactured by vacuum infusion method and aged for 2 years under natural environmental conditions. Piercing with three different clearances (1%, 5%, and 10% of sheet thickness) and speed of 4 m/s were performed. Additionally, conventional drilling was carried out at a feed rate of 0.2 m/min. The highest delamination factor difference between piercing and drilling processes was calculated as 7.3% and 13.9% for CFRP and AFRP, respectively. The highest burr amounts for AFRP and CFRP composites were obtained as 91.5% and 39% at 10% clearance for piercing process and 123% and 32.1% for drilling process, respectively. Compared with drilling, piercing generates less burr formation except for CFRP composites in the case of 10% clearance and more precise hole production. It is understood that piercing results significantly improve when smaller clearances up to 5% of the sheet thickness are utilized.

Carbon fiber-reinforced polymers (CFRPs) are widely used in the fabrication of solid rocket motor casings due to their exceptional performance. However, the bonding interface between CFRP and viscoelastic materials (rubber) is prone to debonding damage during service and storage under complex environmental conditions, which poses a significant threat to the structural integrity and reliability of the engine. Existing nondestructive testing (NDT) methods, such as X-ray imaging, infrared thermography, and ultrasonic testing, although somewhat effective, exhibit significant limitations in detecting interfacial defects in deep or multilayered composite materials, particularly under the challenging conditions of service and storage. This study proposes an innovative method based on active Lamb wave energy analysis and introduces the Damage Evolution Factor (DEF), specifically designed to detect and evaluate interfacial debonding defects in CFRP-rubber bonded structures within solid rocket motors during service and storage. Through numerical simulations and experimental validation, we selected the A0 mode Lamb wave, which is more sensitive to interfacial damage, as the incident wave and excited it on the surface of the structure. Displacement time-history response signals at observation points under different damage models were extracted and analyzed, and DEF values were calculated. The results show that DEF values increase with the size of the interfacial debonding damage. Similar trends were observed in experimental studies, further validating the effectiveness of this method and demonstrating that DEF can be used for the quantitative evaluation of interfacial debonding defects in CFRP-rubber bilayer bonded structures.

Microcrack damage was detected in orthotropic carbon fiber-reinforced polymers (CFRPs) repaired with a single-sided patch during the tensile process via nonlinear mixing Lamb wave detection technology. An integrated numerical model of tensile displacement–devibration–nonlinear mixing Lamb wave detection was established to explore the relationship between the patch parameters and the nonlinear mixing Lamb wave response. A static tensile test and nonlinear mixing Lamb wave detection were designed to verify the exactness of the simulation model. On the basis of the optimal Latin hypercube design, a response surface mathematical model with the patch radius, patch thickness and patch rotation angle as independent variables and mixing relative acoustic nonlinearity parameters (MRANPs) as the response value was constructed. An archiving microgenetic algorithm was used to identify the optimal combination of patch design parameters. The results revealed that under the same tensile displacement conditions, the tensile load on the patch-repaired CFRP plate was significantly greater than that on the unrepaired sample, indicating a greater bearing capacity. The MRANPs initially decreased but then increased with increasing patch radius and patch thickness; they first increased but then decreased with increasing patch rotation angle. After multiobjective optimization, the sum RANP and difference RANP were 0.4754 and 0.2434, respectively, representing reductions of 8.29% and 30.75%, respectively, compared with the single–factor simulation results. This significant optimization effect provides reference value for CFRP structural health monitoring in aircraft safety engineering.

Quantitative damage detection in cross-ply laminates using Lamb wave method

In this study, we investigate the interactions of Lamb wave A0 mode with different sizes of delaminations in composites using finite element code Abaqus®. According to Lamb wave dispersion curves, the group velocity of A0 mode increases rapidly as the frequency-thickness increases in the relatively low frequency region. In the delamination region, the frequency-thickness product decreases compared to the healthy laminate since the damage causes ply separation at the lamina interface. In the current study this observation is investigated in detail using finite element simulations. The resulting phase delay is analyzed by Empirical Mode Decomposition (EMD) and instantaneous phase approach. Finite element simulations are performed using Abaqus® and signal processing is performed in joint time-frequency domain using Hilbert-Huang Transform (HHT) method. The unwrapped instantaneous phase difference is correlated with the extent of delamination (quantitative level of damage).

Baseline-free assisted lamb wave-based damage detection in CFRP composites using graph convolutional networks and Transformer models

Delaminations in composite laminates resulting from impact events may be accompanied by minimal indication of damage at the surface. As such, inspection techniques are required to ensure defects are within allowable limits. Conventional ultrasonic scanning techniques have been shown to effectively characterize the size and depth of delaminations but require physical contact with the structure. Alternatively, a noncontact scanning laser vibrometer may be used to measure guided wave propagation in the laminate structure. A local Fourier domain analysis method is presented for processing guided wavefield data to estimate spatially-dependent wavenumber values, which can be used to determine delamination depth. The technique is applied to simulated wavefields and results are analyzed to determine limitations of the technique with regards to determining defect size and depth. Finally, experimental wavefield data obtained in quasi-isotropic carbon fiber reinforced polymer (CFRP) laminates with impact damage is analyzed and wavenumber is measured to an accuracy of 8.5% in the region of shallow delaminations.

Drilling of carbon fiber reinforced polymer (CFRP) is a challenging task in modern manufacturing sector and machining induced delamination is one of the major problems affecting assembly precision. In this work, a new three-dimensional (3D) finite element model is developed to study the chip formation and entrance delamination in drilling of CFRP composites on the microscopic level. Fiber phase, matrix phase and equivalent homogeneous phase in the multi-phase model have different constitutive behaviors, respectively. A comparative drilling test, in which the cement carbide drill and unidirectional CFRP laminate are employed, is conducted to validate the proposedmodel in terms of the delamination and the similar changing trend is obtained. Microscopic mechanism of entrance delamination together with the chip formation process at four special fiber cutting angles (0°, 45°, 90° and 135°) is investigated. Moreover, the peeling force is also predicted. The results show that the delamination occurrence and the chip formation are both strongly dependent on the fiber cutting angle. The length of entrance delamination rises with increasing fiber cutting angles. Negligible delamination at 0° is attributed to the compression by the minor flank face. For 45° and 90°, the delamination resulted from the mode III fracture. At 135°, serious delamination which is driven by the mode I and III fractures is more inclined to occur and the peeling force reaches its maximum. Such numerical models can help understand the mechanism of hole entrance delamination further and provide guidance for the damage-free drilling of CFRP.

A novel ultrasonic signal detection and characterization technique is presented in this dissertation. The basic tool is a simplified time-frequency (scale) projection called a dynamic wavelet fingerprint. Take advantage of the matched filter and adaptive time-frequency analysis properties of the wavelet transform, the dynamic wavelet fingerprint is a coupled approach of detection and recognition. Different from traditional value-based approaches, the dynamic wavelet fingerprint-based technique is patternor knowledge-based. It is intuitive and self-explanatory, which enables the direct observation of the variation of non-stationary ultrasonic signals, even in complex environments. Due to this transparent property, efficient detection and characterization algorithms can be customized to address specific application problems. Furthermore, artificial intelligence can be integrated and expert systems can be built on it. Several practical ultrasonic applications were used to evaluate the feasibility and performance of this new idea. The echoes from the surface of five different plates were characterized by the dynamic wavelet fingerprint. After learning, 100% correct identification ratio was achieved. The second application was ultrasonic periodontal probing. The dynamic wavelet fingerprint technique was used to expose the hidden trend of the complex waveforms. Based on the experiment results of a simplified phantom, the regular variation of the inclination of the dynamic wavelet fingerprints was related to reflection regions of interest. The depth of the periodontal pocket was then estimated by finding the third significant peak in the inclination curves. Taking the manual probing data as the “gold standard,” a 40% agreement ration is achieved with the tolerance of 1mm. Statistically however, a lack of agreement was found in terms of the “limits of agreement” of Bland and Altman. The third application was multi-mode Lamb wave tomography. The dynamic wavelet fingerprint technique was used to detect and characterize each suspect through transmitted code. The area of the dynamic wavelet fingerprint was then used as the feature to identify false modes caused by noise and other interference. The overall qualities of the arrival times are acceptable in terms of smooth distribution and variation pattern corresponding to specific defects. The tomographic images generated with the estimated arrival times were also fine enough to indicate different defects in four plates. The last application is ultrasonic thin multi-layers inspection. The dynamic wavelet fingerprint was generated at each sample point to achieve extreme time resolution. Based on observation of the simulated signals, a distinctive dynamic wavelet fingerprint was found to differentiate actual echoes from noise. High precision and robustness were proved by simulated signals. When applied to practical data obtained from a plastic encapsulated IC package, it was able to detect multiple interfaces in the package.

This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. An Acoustic Emission system is used jointly with the Lamb wave analysis in order to discuss about damage development. Finally, it is demonstrated that both methods can work together and that the results obtained are in good keeping.

Mean local frequency-wavenumber estimation through synthetic time-reversal of diffuse Lamb waves

Using passive and active acoustic methods for impact damage assessment of composite structures

Modeling lamb wave propagation in visco-elastic composite plates using a fifth-order plate theory

ABSTRACTIn this review paper, a methodology for measuring the interface fracture toughness of a crack between two isotropic, homogeneous materials and a delamination between two laminae of unidirectional composite materials of differing directions is presented. Four cases are considered. Two isotropic material pairs are described: glass/epoxy and two ceramic clays. Similar studies are presented for two cross‐ply laminates: 0°/90° and +45°/−45. The Brazilian disk specimen was used to carry out mixed mode fracture tests. The load and crack or delamination length at fracture were measured and used in a finite element analysis to determine the displacement field. An interaction energy orM‐integral was used to obtain the stress intensity factors at failure. These in turn were employed to calculate the critical interface energy release rateand two phase anglesψandφ, which measure the mode mixity. For theM‐integral and for each interface crack or delamination, the first term of the asymptotic solution of the fields is required. For two isotropic materials, these solutions are well known. For the laminates described here, they were determined by the Stroh and Lekhnittski formalisms. A failure criterion determined from first principles is presented. The values of,ψandφare used to specify the criterion for each material pair. A statistical analysis is presented, which predicts a 5% probability of failure.