Delamination In Composite Laminates Research Articles

Optimised dynamic line scan thermographic detection of CFRP inserts using FE updating and POD analysis

The detection of delaminations in composite laminates using automated thermographic scanning is a quite challenging task. The set-up parameters are not only dependent on the equipment, but on the inspected component as well. In this work, a methodology is discussed to use Finite Element (FE) model updating to automatically establish the most suitable inspection parameters for a given combination of the structure and the investigated delamination depths. The optimised results are compared using binary Probability of Detection analysis and are benchmarked with parameter sets retrieved by an expert using the regular trial & error approach. The results show an improvement of the accuracy and scanning speed which significantly increases as the POD decreases and the complexity of the samples increases.

Characterization of low-velocity impact-induced damages in carbon/epoxy composite laminates using a poly(vinylidene fluoride–trifluoroethylene) film sensor

Abstract In this work, low-velocity impact-induced damages such as matrix cracking and delamination in composite laminates made of unidirectional carbon fiber/epoxy and woven carbon prepregs with different stacking sequences were investigated by using a poly(vinylidene fluoride–trifluoroethylene) (P(VDF-TrFE)) film sensor. The effect of insertion of film sensor having various areal ratios on the structural integrity of the host composite was investigated by conducting tensile and short beam shear tests. The findings revealed that the degradation rates of the Young's modulus, strength, and inter-laminar shear strength were in the acceptable range; the degradation rate was 7–8% even under unrealistically harsh conditions. Subsequently, low-velocity impact tests were performed by using a drop-weight impact machine (11.77–28.25 J). For damage characterization of the composite laminates, four P(VDF-TrFE) film sensors were inserted at different sites in each composite laminate at the impact point. By considering the relationships between impact energy, voltage output from the sensors, and the corresponding material failure modes, an estimation technique for the potential failure of the composite laminates that experienced low-velocity impacts was suggested.

Fibre bridging effect on the Paris relation of mode I fatigue delamination in composite laminates with different thicknesses

Fatigue delamination with fibre bridging in composite laminates with different thicknesses was investigated. The experimental results clearly demonstrated fibre bridging had significant retardation effects on fatigue delamination behavior, making it insufficient to use a single Paris resistance curve to determine fatigue crack growth. To address this problem, the coefficients of the Paris relation were correlated to the normalized crack extension (a−a0)/Lpz. It was found that the exponent n was independent on the normalized crack extension and specimen thickness, whereas the parameter log(c) bi-linearly decreased with the normalized crack extension and kept constant once fibre bridging became saturation. And the magnitude of log(c) was independent on specimen thickness. Thus, it was concluded that fatigue delamination behavior and fibre bridging significance were independent on specimen thickness at a given normalized crack extension (a−a0)/Lpz. With substitutions of these correlations into the Paris relation, an empirical power law relation was developed to characterize fatigue delamination behavior. And its validation was verified by a comparison between predictions and experiments.

Robust quantitative depth estimation on CFRP samples using active thermography inspection and numerical simulation updating

A quantitative evaluation of delaminations in composite laminates encounters multiple difficulties due to the anisotropic behaviour of the laminate. Extensive calibrations for each structure are required and each depth needs certain manual modifications for optimal performance of the estimation routines. In this manuscript, a robust technique is developed using a numerical model to estimate the thermal diffusivity through the anisotropic material which improves defect depth estimation between each layer. Three different calibration depths are necessary to compute the diffusivity through a certain stacking sequence of a multi-layered composite laminate. The results are compared with the state-of-the-arts experimental evaluation techniques and with a regular numerical model. It is seen that especially for deeper defects, the optimised numerical model delivers more accurate results due to the considered anisotropic diffusivity.

A progressive analysis of matrix cracking-induced delamination in composite laminates using an advanced phantom node method

Matrix cracking-induced delamination in composite laminates is qualitatively and quantitatively investigated in a finite element framework. The phantom node method is extended to incorporate breakable interfaces at transverse matrix crack tips. New user-defined element types in Abaqus improve the numerical stability in a geometrically nonlinear analysis. The new formulation allows for accurate prediction of matrix crack density and stiffness reduction in a number of composite laminates. Furthermore, the advanced phantom node method is able to simulate progressive matrix cracking-induced delamination with good accuracy.

Detection of delamination in composite laminates using Lamb wave based nonlinear method

In this study, a Lamb wave based nonlinear method is used to detect delamination in a composite laminate. For this purpose, an artificial delamination is created in a composite laminate using a thin Teflon sheet. The research includes an experimental study as well as finite element simulation. Two higher harmonics are observed in the Lamb wave response as a result of contact nonlinearity at the delamination. A spectral damage index is extracted from the spectral information of the fundamental and higher harmonics. It is found to be invariant to the sensor location and its values show decreasing trend with increase in the delamination width. Further, a new hybrid method is introduced, wherein the spectral and temporal data are used together in order to locate a delamination.

Characterization of delaminations in composite laminates by laser ultrasonics

Abstract The influence of delaminations in the anisotropic carbon fiber reinforced plastics (CFRP) on laser- generated ultrasound is investigated. A 3D-finite element method is used to simulate the propagation of ultrasonic waves in CFRP with delaminations. The asymmetric properties and distortions as well as the wave reflection and attenuation at delamination interfaces are clearly shown. Measurements on CFRP specimens with circular delaminations above 2 mm in diameter are imaged by C-scans and correlated to the modeling results. The effects of ultrasound frequency, defect size and depth location are discussed. The results show that it is possible to decrease the influence of anisotropy in CFRP composites by choosing appropriate ultrasonic frequencies and controlling the laser parameters.

Modelling interfacial cracking with non-matching cohesive interface elements

Interfacial cracking occurs in many engineering problems such as delamination in composite laminates, matrix/interface debonding in fibre reinforced composites etc. Computational modelling of these interfacial cracks usually employs compatible or matching cohesive interface elements. In this paper, incompatible or non-matching cohesive interface elements are proposed for interfacial fracture mechanics problems. They allow non-matching finite element discretisations of the opposite crack faces thus lifting the constraint on the compatible discretisation of the domains sharing the interface. The formulation is based on a discontinuous Galerkin method and works with both initially elastic and rigid cohesive laws. The proposed formulation has the following advantages compared to classical interface elements: (i) non-matching discretisations of the domains and (ii) no high dummy stiffness. Two and three dimensional quasi-static fracture simulations are conducted to demonstrate the method. Our method not only simplifies the meshing process but also it requires less computational demands, compared with standard interface elements, for problems that involve materials/solids having a large mismatch in stiffnesses.

Interaction of Z-pins with Multiple Mode II Delaminations in Composite Laminates

The application of Z-pinning is a subject of great interest in the field of through-thickness reinforcement (TTR) of composite laminates. To date, the majority of Z-pin characterisation work has been conducted on fracture coupons containing a single embedded delamination, which is often not representative of real failure of reinforced composite structures in service. In this investigation a test procedure to produce two independent Mode II delaminations was developed to analyse their interaction with a region of Z-pin reinforcement. Initially numerical models were used to optimise the chosen configuration. Experimental results show in detail the response of Z-pins to two independent delaminations. These results highlight the ability of the Z-pins to effectively arrest mode II delaminations at multiple levels through the sample thickness. Additionally they provide a much needed data set for validation and verification of Z-pin numerical modelling tools.

A hybrid method based upon nonlinear Lamb wave response for locating a delamination in composite laminates

A new hybrid method based upon nonlinear Lamb wave response in time and frequency domains is introduced to locate a delamination in composite laminates. In Lamb wave based nonlinear method, the presence of damage is shown by the appearance of higher harmonics in the Lamb wave response. The proposed method not only uses this spectral information but also the corresponding temporal response data, for locating the delamination. Thus, the method is termed as a hybrid method. The paper includes formulation of the method and its application to locate a Barely Visible Impact Damage (BVID) induced delamination in a Carbon Fiber Reinforced Polymer (CFRP) laminate. The method gives the damage location fairly well. It is a baseline free method, as it does not need data from the pristine specimen.

Detection of low-velocity impact-induced delaminations in composite laminates using Auto-Regressive models

In this paper, the detection of delaminations in carbon-fiber-reinforced-plastic (CFRP) laminate plates induced by low-velocity impacts (LVI) is investigated by means of Auto-Regressive (AR) models obtained from the time histories of the acquired responses of the composite specimens. A couple of piezoelectric patches for actuation and sensing purposes are employed. The proposed structural health monitoring (SHM) routine begins with the selection of the suitable locations of the piezoelectric transducers via the numerical analysis of the curvature mode shapes of the CFRP plates. The normalized data recorded for the undamaged plate configuration are then analyzed to obtain the most suitable AR model using five techniques based on the Akaike Information Criterion (AIC), the Akaike Final Prediction Error (FPE), the Partial Autocorrelation Function (PAF), the Root Mean Squared (RMS) of the AR residuals for different order p, and the Singular Value Decomposition (SVD). Linear Discriminant Analysis (LDA) is then applied on the AR model parameters to enhance the performance of the proposed delamination identification routine. Results show the effectiveness of the developed procedure when a reduced number of sensors is available.

XFEM simulation of delamination in composite laminates

In this paper, the extended finite element method (XFEM) is extended to simulate delamination problems in composite laminates. A crack-leading model is proposed and implemented in the ABAQUS® to discriminate different delamination morphologies, i.e., the 0°/0° interface in unidirectional laminates and the 0°/90° interface in multidirectional laminates, which accounts for both interlaminar and intralaminar crack propagation. Three typical delamination problems were simulated and verified. The results of single delamination in unidirectional laminates under pure mode I, mode II, and mixed mode I/II correspond well with the analytical solutions. The results of multiple delaminations in unidirectional laminates are in good agreement with experimental data. Finally, using a recently proposed test that characterizes the interaction of delamination and matrix cracks in cross-ply laminates, the present numerical results of the delamination migration caused by the coupled failure mechanisms are consistent with experimental observations.

Numerical Modeling of Combined Matrix Cracking and Delamination in Composite Laminates Using Cohesive Elements

Sub-laminate damage in the form of matrix cracking and delamination was simulated by using interface cohesive elements in the finite element (FE) software ABAQUS. Interface cohesive elements were inserted parallel to the fiber orientation in the transverse ply with equal spacing (matrix cracking) and between the interfaces (delamination). Matrix cracking initiation in the cohesive elements was based on stress traction separation laws and propagated under mixed-mode loading. We expanded the work of Shi et al. (Appl. Compos. Mater. 21, 57–70 2014) to include delamination and simulated additional [45/−45/0/90]s and [02/90n]s {n = 1,2,3} CFRP laminates and a [0/903]s GFRP laminate. Delamination damage was quantified numerically in terms of damage dissipative energy. We observed that transverse matrix cracks can propagate to the ply interface and initiate delamination. We also observed for [0/90n/0] laminates that as the number of 90° ply increases past n = 2, the crack density decreases. The predicted crack density evolution compared well with experimental results and the equivalent constraint model (ECM) theory. Empirical relationships were established between crack density and applied stress by linear curve fitting. The reduction of laminate elastic modulus due to cracking was also computed numerically and it is in accordance with reported experimental measurements.

An XFEM approach for modelling delamination in composite laminates

A First-order Shear Deformation Theory (FSDT) is chosen to simulate composite laminates in the linear and the geometrically non-linear regimes. The formulation is based on the Equivalent Single Layer (ESL) theory that fails to predict the delamination onset in composite laminates. The lack of resolving three-dimensional states and correct transverse stresses in this model is principally improved using post-processing. In order to precisely compute interlaminar stresses, a non-frictional linear adhesive contact model is applied in the context of the eXtended Finite Element Method (XFEM). The discontinuities are imposed within any arbitrary interface by enriching the displacement field. Thus two sub-domains define the plane of the discontinuity. Thereafter, the aforementioned adhesive contact can be formulated at the discontinued interface. Stress values are retrieved at nodal points using the interface constitutive equation. Consequently, the interface formulations are extended into the softening regime to model the delamination growth as a mixed-mode cohesive effect. The accuracy of the proposed method in predicting the interlaminar stresses and the delamination propagation is demonstrated by comparing the results with the ones available in literature. By combining the lower-order plate theory and the novel XFEM technique, the model is able to accurately calculate the delamination onset and the propagation with less computational effort.

Sparse sensing detection of impact-induced delaminations in composite laminates

The detection of delaminations in composite structures due to impacts is a critical issue for any structural health monitoring (SHM) programme. In this work, an experimental campaign is carried out to investigate the presence of delamination induced in carbon-fibre-reinforced-plastic (CFRP) plates by low velocity impacts (LVI). A reduced number of piezoelectric devices for actuation and sensing purposes are employed. The proposed study starts with the choice of the proper positions of the piezoelectric sensor and actuator through the analysis of the numerical curvature mode shapes of the composite laminated plate. A wavelet packet transform (WPT)-based algorithm is applied on the vibrating response of the plates to extract wavelet-based damage sensitive features. Linear Discriminant Analysis (LDA) is then applied on the extracted damage sensitive features to enhance the performance of the proposed delamination identification procedure. Results show the effectiveness of the developed SHM routine when a reduced number of sensors is either desirable or mandatory.

Quantitative identification of delamination at different interfaces using guided wave signals in composite laminates

This article presents the development of a quantitative method for identifying interfacial delamination in composite laminates. Splitting and merging occur as Lamb waves pass through the delamination. It is demonstrated from the dispersion curve of Lamb waves that the group velocities of A0 and S0 modes in the sub-laminates above and below the delamination area are different when the delamination interface is not at the thickness center of a symmetrical layup laminate. Changes in the time of flight (ToF) of A0 and S0 modes are caused by the parallel propagation of Lamb waves. Based on those changes in the ToF, delamination size can be evaluated even though the delamination location through the thickness is not pregiven. Numerical and experimental studies are conducted to assess this method. Continuous wavelet transform is applied to extract the ToF of A0 mode waves from the overlapped signals. Mode conversion is illustrated for the interaction of Lamb waves and delamination. The sensitivity and applicability of the proposed method are validated numerically and experimentally in two asymmetric layup composite laminates. The limitation of this method for symmetric layup composite laminates is analyzed and a potential improvement approach is further discussed.

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.

An experimental study on matrix crack induced delamination in composite laminates

Matrix crack induced delamination is one of the predominant failure mechanisms in laminated composites under off-axis loading. However, there exist a reduced number of experimental studies on the literature specifically devoted to this failure mechanism. In this work, an experimental campaign focused on the occurrence of matrix cracking and delamination induced by matrix crack is presented. Five different carbon–epoxy lay-ups have been tested under tensile load, leading to different damage evolutions. The experimental data has been compared with a failure criterion recently developed by the authors. A good agreement between the experimental data and the predictions of the failure criterion has been obtained.

Efficient modelling of forces and local strain evolution during delamination of composite laminates

FEM analyses based on cohesive zone models are a well-assessed methodology to predict onset and propagation of delamination in composites. In this work, a specific modelling technique based on a cohesive zone model is applied to analyse Double Cantilever Beam (DCB) and 4-point bending End Notched Flexure (4-ENF) tests, focusing on the evolution of forces as well as of internal local strains, which have been monitored by Fibre Bragg Grating sensors embedded in the specimens. The numerical approach is based on explicit FEM computations and presents some appealing advantages with respect to conventional models, since it does not use zero-thickness cohesive elements and does not require a non-physical penalty stiffness to be introduced between adjacent plies. In the cases presented, such approach is applied to model both force response and local strain evolution during the stable propagation of delamination in mode I and mode II, in the presence of fibre bridging phenomena and taking into account frictional effects between crack faces. The paper presents the experimental results and analyses the data acquired by the sensors embedded in the specimens. Then, the general accuracy and the computational advantages of the numerical approach proposed are evaluated considering numerical benchmarks. Models of the tests are developed at different levels of through-the-thickness mesh refinement and sensitivity analyses are performed to point out the effects on the overall and local response of significant model parameters, such as the length attributed to the process zone in the cohesive zone model and the friction coefficient in the contact interaction between crack faces. Numerical results and numerical–experimental correlation prove that the modelling technique and the methodologies applied to represent fibre bridging and frictional effects represent efficient tools to reliably model complex delamination processes.

Instantaneous and local wavenumber estimations for damage quantification in composites

The continued and expanded use of composite materials in aerospace applications necessitates structural health monitoring and/or nondestructive evaluation techniques that can provide quantitative and detailed damage information for layered plate-like components (such as composite laminates). Guided wavefield methods are at the basis of a number of promising techniques for the detection and the characterization of damage in plate-like structures. Among the processing techniques that have been proposed for guided wavefield analysis, the estimation of instantaneous and local wavenumbers can lead to effective metrics that quantify the size and the depth of delaminations in composite laminates. This article reports the application of both instantaneous and local wavenumber damage quantification techniques to guided wavefield data for delaminated composite laminates. The techniques are applied to experimental data for a simple single delamination case and to simulated data for a more complex multi-ply delamination case. The two techniques are compared in terms of accuracy in damage characterization and computational demand. The proposed methodologies can be considered as steps toward a hybrid structural health monitoring/nondestructive evaluation approach for damage assessment in composites.