Delamination In Composite Laminates Research Articles

Delamination inspection in composite laminates with S0 Lamb wave using flexible macro-fiber composite transducers

Abstract Delamination in composite laminates poses a serious threat to structural integrity in aerospace applications due to its potential to cause structural failure. Effective structural health monitoring (SHM) methods are needed to identify such damage. The objective of this paper is to measure and localize delamination in a glass fiber reinforced polymer (GFRP) cross-ply composite laminate based on the S0 Lamb wave detection method, using the Macro-Fiber Composite (MFC) as both actuator and receiver. To analyze the interaction behaviors between the excited S0 Lamb wave and delamination, the finite element models were established to simulate the Lamb wave propagation in composites, incorporating the local stiffness reduction method to model the delamination region. The simulation results indicate that the interaction behaviors between the excited S0 wave and the delamination occur mainly at the end of the delamination, and the generated new wave packets in the received signals can be used to identify the delamination. Furthermore, the S0 wave reflected from the end edge of delamination was used to localize the delamination. Reflection coefficients were found to vary significantly with delamination depth, and the delamination's longitudinal position was accurately localized using Time-of-Flight (ToF) extraction. Finally, a pitch-catch experiment with MFC transducers was conducted to detect an artificial delamination, demonstrating that it is a promising approach to inspect delamination accurately with end-edge reflected waves.

Predicting delamination in composite laminates through semi-analytical dynamic analysis and vibration-based quantitative assessment

Delamination is a frequent failure mode of laminated fiber-reinforced polymer (FRP) composites structure in aeronautical and other industries, leading to changes in vibration characteristics. Vibration-based techniques evaluate and compare the dynamic response between damaged and undamaged structures, and guarantee the non-destructive measurement with reliability and repeatability. Previous studies typically concentrate on using finite element method to obtain vibration characteristics and enhance the database for intelligent algorithms. This paper presents a semi-analytical result using the Chebyshev−Ritz method to expand delamination prediction. Vibration frequency serves as a global damage indicator, and multi-order frequency characteristics are utilized to identify the delamination length and location of FRP composite plates. A database of natural frequencies corresponding to damage parameters for FRP laminated plates is generated based on the established model using the region approach. An intelligent approach, known as a genetic algorithm optimization-based back-propagation (GA-BP) artificial neural network, is utilized for system identification. The network model is subjected to a sensitivity analysis, where artificial noise is added to vibration frequency to distinguish between the actual structure and the numerical model. The results indicate that the GA-BP algorithm shows good accuracy and stable performance against the standard neural networks for delamination analysis.

Numerical investigation of two-dimensional Mode-II delamination in composite laminates

Existing standards for delamination tests on composite materials typically employ one-dimensional (1D) beam specimens. However, such specimens may not represent real delamination scenarios in composite structures, where cracks tend to propagate in two dimensions. To address this limitation and compare the delamination behavior under both 1D and two-dimensional (2D) Mode-II fracture conditions, a numerical investigation was carried out based on previous experiments. A novel cohesive zone model, considering both microcracking and fiber bridging within the fracture process zone, was developed using a semi-experimental approach and incorporated in three-dimensional finite element simulations. According to the numerical results, the investigated laminates exhibited similar maximum strain energy release rates in both 1D and 2D delamination; however, different traction-separation responses were obtained. Practical methods of locating the tip of an embedded crack were proposed based on curvature and strain measurements on the laminate surface, demonstrating potential for application in structures with irregular crack shapes.

Detection of Delamination in Composite Laminate Using Mode Shape Processing Method and YOLOv8

In this study, a novel delamination detection method for composite materials is proposed through the innovative use of You Only Look Once v8 (YOLOv8), vibration analysis, and 2D continuous wavelet transform techniques. The method detects the location and size of damage more accurately than existing methods and avoids manual intervention in the detection process. Damage detection performed on the simulation dataset shows that the method is able to accurately identify the delamination location with IoU = 0.9906 and an average accuracy of 91.32%. The proposed method is then compared with the widely used YOLOv5 model, and the superior performance of the YOLOv8 model is verified, with a 37.93% improvement in training speed and 0.81% improvement in detection accuracy. In addition, an experimental dataset of four composite laminates with delamination damage is constructed. By using transfer learning, the performance of the pretrained network achieves a good precision up to 1. The method proposed in this study expands the range of tasks that can be accomplished by mode shape analysis and is very effective in real experiments.

Experimental investigation of two-dimensional Mode-II delamination in composite laminates

Experimental investigation of two-dimensional Mode-II delamination in composite laminates

On the Cover: A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

On the Cover: A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

PENGARUH SUDUT PAHAT PADA PENGGURDIAN KOMPOSIT SERAT DAUN NANAS / TERMOPLASTIK PADA PEMBENTUKAN DELAMINASI

This study aims to determine the effect of the Gurdi tool angle on delamination in composite laminates in the formation of delaminations. The spacing of the selected reinforcing fibers was 2.4 and 6 mm in a composite laminate of ethylene vinyl acetate (EVA) polymer matrix with reinforcing fibers from pineapple leaves. The method used in composite fabrication is hand lay-up, namely by direct casting on three layers of fiber that has been woven and tied to the sides of the mold. For cutting speed, Vf 72 mm / min, using a chisel with a diameter of 8 mm with a tool angle of 70o and 118o. Delamination was determined using the Feda method. The delamination sections were observed using a digital microscope and then processed using AutoCAD to measure Dmax and Ad. The delamination ratio in the top hole for the tool diameter of 8mm with a 70° angle at the variation of the distance of 2 mm, 4 mm and 6 mm were 1.268, 1.224 and 1.192 for the bottom hole were 1.078, 1.042 and 1.029, respectively. Meanwhile, the ratio of delamination in the upper hole for a tool diameter of 8mm with an angle of 118° at a distance variation of 2mm, 4mm and 6mm were 1.194, 1.194, 1.158 for the bottom hole, respectively, 1.046, 1.042 and 1.038.

A numerical investigation of the interaction between interlaminar and intralaminar damages in a fatigued composite panel

AbstractThe fatigue behavior of composite materials is still a very challenging issue for the scientific community. So far, several numerical methodologies allow us to faithfully simulate the propagation of delamination due to fatigue. However, the reduction of material properties due to the applied cyclic load should be considered. The main objective of this work is to investigate the role of the material property degradation and the intralaminar damages on the propagation of delamination in composite laminates under fatigue loads. The numerical tool FT‐SMXB, based on the Paris Law and virtual crack closure technique, for the mimic of fatigue‐driven delamination, has been integrated with a user material subroutine, based on the generalized residual material property 7degradation model and the Hashin fatigue failure criteria. Typical aeronautical stiffened panel has been considered as test case. Matrix damages and strength degradation due to fatigue cycles have been found to influence the propagation of delamination.

Qualification of distributed optical fiber sensors using probability of detection curves for delamination in composite laminates

Despite the promising application of Distributed Optical Fiber Sensors (DOFS) in monitoring damage in composite structures, their implementation outside academia is still unsatisfactory due to the lack of a systematic approach to assessing their damage detection performance. The existing tool developed for non-destructive evaluation, Probability of Detection (POD) curves, needs to be adapted for structural health monitoring applications to account for spatial and temporal dependency. Damage detection performance with DOFS is deeply related to the inherent variability sources of the system, the strain transfer properties of the optical fiber, and the loading conditions, which determine the damage-induced strain on the structure. This paper establishes a systematic approach based on the Length at Detection (LaD) method to qualify DOFS for damage detection in composites under different scenarios. Specifically, this study considers two DOFS with different strain transfer properties for monitoring delamination in carbon fiber reinforced polymers double-cantilever beam specimens under mode I quasi-static and fatigue loading. The POD curves derived from the LaD method confirm that this methodology can quantify the change in the detection performance due to the DOFS type and the loading conditions. The study also proposes a practical solution to compare POD curves obtained with different sample sizes, by introducing the concept of virtual specimens to simulate the lower confidence bound convergence.

Lamb wave mode conversion and multiple-reflection mechanisms for simply and reliably evaluating delamination in composite laminates

Lamb wave propagation must be understood comprehensively for simply evaluating delamination during ultrasonic testing. However, the difference between wave propagation, visualized using laser Doppler vibrometer and pulsed-laser scanners, has not been sufficiently investigated, and knowledge of optimal conditions for evaluating delamination is limited. Thus, in this study, the mode conversion and multiple reflections of Lamb waves propagating in a delaminated cross-ply laminate were visualized using different laser scanners, delamination depths, and wave incident angles. Delamination was characterized using maximum-amplitude map postprocessing under specific conditions. Further numerical analysis revealed that owing to multiple reflections of the antisymmetric mode in incident and mode-converted waves, standing waves were generated in the delaminated sublaminate. Dispersion curve and flexural stiffness calculations confirmed the conditions required for high-amplitude standing waves, thereby providing guidelines for simply and reliably evaluating delamination during inspections.

Non-Contact Detection of Delamination in Composite Laminates Coated with a Mechanoluminescent Sensor Using Convolutional AutoEncoder

Delamination is a typical defect of carbon fiber-reinforced composite laminates. Detecting delamination is very important in the performance of laminated composite structures. Structural Health Monitoring (SHM) methods using the latest sensors have been proposed to detect delamination that occurs during the operation of laminated composite structures. However, most sensors used in SHM methods measure data in the contact form and do not provide visual information about delamination. Research into mechanoluminescent sensors (ML) that can address the limitations of existing sensors has been actively conducted for decades. The ML sensor responds to mechanical deformation and emits light proportional to mechanical stimuli, thanks it can provide visual information about changes in the physical quantity of the entire structure. Many researchers focus on detecting cracks in structures and impact damage with the ML sensor. This paper presents a method of detecting the delamination of composites using ML sensors. A Convolutional AutoEncoder (CAE) was used to automatically extract the delamination positions from light emission images, which offers better performance compared to edge detection methods.

Lamb wavefield-based monogenic signal processing for quantifying delamination in composite laminates

Delamination is one of the common damages affecting the safety of composite structures. In this paper, a Lamb wavefield-based monogenic signal processing algorithm is proposed to quantify the delamination parameters in composite laminates, including location, size, shape, and depth. A quality-guided fast phase unwrapping algorithm is developed to solve the problem of phase wrapping after Riesz transform-based monogenic signal processing. Then, space distribution of the phase of Lamb wavefield can be extracted for calculating wavenumber distribution, which is related to the structural thickness or delamination depth and can be used for delamination imaging. Simulated Lamb wavefield signals calculated by finite element simulation are employed to evaluate the parameters of delamination in composite laminates. Compared with other traditional methods, the damage identification algorithm based on Riesz transform has excellent identification effect and shorter calculation time. The results show that the algorithm can be used not only for single delamination recognition but also for multi-delamination recognition with good accuracy. In particular, the interaction between incident waves along different ply directions and delamination is explored, and its influence on delamination quantification is studied, whose results are worthy of attention in engineering application. Finally, a completely non-contact laser ultrasonic system is established to obtain the Lamb wavefield with delamination. Experiments show that the algorithm can accurately quantify the location, size, shape, and depth of delamination.

Experimental Identification of Frictional Effects on Interlaminar Toughness of Composite Laminates in 4ENF Test

BackgroundUnderstanding and characterizing delamination in composite laminates is fundamental for analysing their structural integrity, since operational loads may promote the propagation of interlaminar defects. Propagation often occurs in mode II and is driven by shear stress. However, the methods used to characterise this propagation mode are affected by frictional effects between crack surfaces.ObjectiveThis work aims to build up an experimental method to identify the effect of friction in a 4 points End Notched Flexure test, which does not require the use of analytical or numerical models of the specimen.MethodsThis goal is achieved by performing a series of loading–unloading cycles before the delamination test, which helps to calibrate an analytical expression that estimates the energy dissipated by friction, and the analytical expression is then inserted into the formulation of the Irwin-Kies equation. Experimental validation is carried out considering tests on different materials and different friction conditions between the crack surfaces, as well as validation by means of a virtual experiment being performed for comparison with an analytical model presented in literature. The novelty of this method lies in the fact that it does not require the development of any analytical or numerical model of the specimen and consequently no calibration between models and the experiment is required.ResultsTests on composite specimens show good results, the friction contribution estimated by the method is comparable with those presented in the literature. Moreover, the virtual experiment shows that there is a good match between the results obtained using this method and those obtained using an analytical model presented in the literature.ConclusionsThis method seems to provide satisfactory results for both real and virtual experiments, moreover, the procedure is relatively simple, making it a suitable method for the evaluation of frictional effects in the 4 point End Notched Flexure test.

Continuum damage analysis of delamination in composite laminates using a stress-based layerwise plate model

This contribution addresses the modelling of delamination in a stress-based layerwise plate model. Such models benefit from a good representation of the local 3D stress field, especially inter-laminar stresses which are natural generalized stress unknowns arising in their derivation. We extend their construction to situations involving onset and propagation of interfacial delamination. We propose a variational formulation relying on a continuous damage description of the opening and sliding interfacial degradation processes using two damage variables. A mixed-mode delamination propagation criterion can be obtained through proper coupling between both variables. Finally, the model predictions are validated against classical delamination tests in pure and mixed modes.

Additive binding layers to suppress free edge delamination in composite laminates under tension

An additive laminate applied to the free-edges of composite laminates is proposed to stop the development of delamination. Numerical analyses were conducted to find the optimum additive binding layup and the experimental results proved that the free-edge delamination was successfully suppressed. When it was used to bind the edges of a [(202/-202)2]s angle-ply laminate, the tensile failure strain and load of the laminate were increased by about 50%, and the failure mode changed from free-edge delamination to in-plane shear. However, it did not change the failure load and final failure mode of a [452/02/-452/902]s quasi-isotropic laminate, as this substrate is less susceptible to free-edge delamination. Thus, this technique is an efficient solution for suppressing free-edge delamination in real structural applications and in the characterisation of coupon tests where damage at the free-edges undermines the real potential of the composite substrate.

Experimental and numerical investigations on the mode I delamination growth behavior of laminated composites with different z-pin fiber reinforcements

Z-pinning can effectively improve delamination growth resistance of fiber reinforcing laminated composites and attracts much attention. Material and geometrical characteristics of z-pins have effects on the delamination behavior. Numerous studies have been conducted to investigate the effect of volume fraction, diameter, shape, surface treatment and angle of z-pin, insertion length and areal density. However, the effect of elastic modulus of z-pins on the delamination behavior is still lack of investigation. In this work, the delamination behavior of two kinds of z-pinned laminates is experimentally and numerically studied. The laminates are reinforced by z-pins with different elastic moduli, including carbon fiber pin and polyimide fiber pin. It is found the elastic modulus of z-pins has effects on the steady-state fracture toughness, the bridging zone length and the micro failure mechanism. A bilinear cohesive zone model is applied for the simulation of delamination in composite laminates and discrete nonlinear spring elements are used for the characterization of the mechanical behavior of z-pins. The FE model is validated by comparing the predictions with the experimental results. Validated FE model is further applied to numerically investigate the density, the distribution distance and the distribution form of z-pins on the mode I delamination behavior.

Thermo-mechanical progressive analysis on multiple delaminations in composite laminates

Thermo-mechanical progressive analysis on multiple delaminations in composite laminates

Propose a generic framework for probabilistic prediction of the onset and growth of matrix cracking induced delamination in composite laminates

Matrix cracking induced delamination (MCID) is one of the catastrophic modes that can occur in composite laminates. In this paper, an efficient algorithm based on the concept of energy release rate for delamination due to matrix cracking and mode II interlaminar fracture toughness is developed and by applying this proposed framework, the reliability analysis of MCID damage is performed. The limit state function is formulated using a general and effectual failure criterion which is developed by the authors of this article, previously. To represent the performance of the proposed algorithm, the probability of the occurrence and growth of MCID in a quasi-isotropic laminate including 45°, 90°, −45° and 0° plies under different loading conditions and various stacking sequences is extracted by using first and second order reliability methods. The obtained results are verified using Monte Carlo simulation. In addition, some significant probability results are compared with several experimental data for validation of the proposed method, qualitatively. For the first time, stochastic failure envelopes called reliability based failure envelops are generated to attain deeper and clear insight about laminate random behavior under MCID mode. The effect of some variables such as the ply thickness, different stress level and stress states and matrix cracking density on the probability of occurrence and growth of MCID is investigated.

Simulating delamination in composite laminates involving large scale fiber bridging based on the mixed-mode three-linear cohesive zone model

A three-linear cohesive zone model (CZM) superposed by two bilinear laws for modelling mixed-mode delamination with the effect of large scale fiber bridging is proposed in this study. The parameters in the formulation can be obtained from pure mode delamination tests or selected by reliable guidelines, making the CZM easy to be implemented in the finite element model. A novel method to determine the bridging strength by applying varied values in the different regions of the prearranged delamination path is developed. The division of the region is based on the concept of the length of the fracture process zone (FPZ). Simulations of mode I delamination in unidirectional and multidirectional composite laminates and mixed-mode I/II delamination in multidirectional laminate are performed in the finite element analysis software. Good agreement between the predicted load–displacement responses and the corresponding experimental results are achieved by adopting the established CZM and the bridging strength determination method, validating their efficiency in predicting the delamination behaviors under both pure mode and mixed-mode loadings.