Delamination In Composite Laminates Research Articles

Sequential quantification strategy of delamination in composite laminate via collaborative active and passive monitoring

The weak interlaminar strength of composite laminate and rough load environment cause unavoidable invisible delamination and make online delamination identification challenging. In this paper, a sequential quantification method for delamination in thin-walled fiber-reinforced composite laminate by combining active piezoelectric sensors with passive strain sensors is proposed. First, based on the guided wave generated by the alternate active excitation, we accurately estimate the center and boundary points of the delamination sequentially according to the wave propagation characteristics. The reliable boundary characterization of delamination can be achieved. Secondly, we build the mathematical model under the framework of Bayesian posterior inference to match the numerical and the physical damage model. By updating the posterior probability model through the strain measurement, the depth of delamination can be identified finally. Numerical and experimental cases are presented to demonstrate the feasibility, accuracy, and efficiency of the proposed methodology. The robustness is elaborated in detail by the discussion about key parameters and noisy interference.

A hysteresis cohesive approach for predicting mixed-mode delamination onset of composite laminates under cyclic loading: Part I, model development

A new approach is proposed to predict onset of fatigue-induced delamination in composite laminates under cyclic loading by progressive interface degradation. Based on the hysteresis cohesive zone approach, a new damage parameter is defined to construct an irreversible cohesive zone model. Residual displacement jump is introduced and used along with mixed-mode bilinear traction-separation law parameters for calculating the damage parameter. Fatigue-induced delamination onset is then predicted by proposing a damage criterion. 3-D Finite Element (FE) formulation is developed with proper elements for plies and also proper elements for the interface. Cyclic loading is also simulated mimicked by the MIN-MAX method. The current research is conducted in two parts as Part I and Part II. While this article focuses on the theoretical framework of the developed modeling, the numerical implementation of the modeling and also a comparison of the outputs with experimental observations will be presented in the companion paper as Part II.

Assessment of delamination location in composite laminates based on a chaotic oscillator method

The work aims to study the assessment of delamination location in composite laminates using vibration measurement with a chaotic oscillator method. Delamination is a type of damage that commonly occurs in composite laminates, which can cause a severe degradation of their material properties. The traditional vibration-based methods can encounter difficulties in detecting and locating these delamination-type damages especially when the size of delamination is relatively small and there is a significant level of noise in its vibration measurement. With this particular consideration, a vibration-based method using a non-linear chaotic oscillator was used in this study due to its sensitivity to the change in vibration signal characteristics. A numerical model of composite laminates with delamination damage under harmonic excitation was developed and the vibration signal obtained from composite laminates was processed using the chaotic oscillator method. A feature named Lyapunov Exponent (LE) was used as a delamination damage index to describe the characteristics of the chaotic oscillator for cases with delamination at varying structural locations. The effects of delamination locations on the developed damage index were analyzed in this work. The results showed that there was a strong correlation between the delamination location and the LE feature, even for the case with a relatively high level of measurement noise. The results demonstrated the effectiveness of the method to identify delamination in composite laminates, which has also the potential to be used to detect other types of damages.

Guided wave scattering at a delamination in a quasi-isotropic composite laminate: Experiment and simulation

Carbon fibre composite laminates are increasingly being used for aerospace structures due to their low weight and improved mechanical performance. Impact damage can cause delaminations below the visible surface of the structure due to limited interlaminar strength. Guided ultrasonic waves can detect and characterize delaminations in composite laminates. The scattering of the A0 Lamb wave mode at an artificial delamination, located at an asymmetric depth in a quasi-isotropic laminate, was investigated. Full field non-contact laser measurements were used to visualise wave trapping and scattered waves. A three-dimensional finite element model was developed and validated against the experiments. The influence of delamination shape and depth on guided wave scattering were studied. Small variations in delamination shape significantly affected the interference pattern on top of the delamination, but had limited effect on the scattered wave outside the delamination. Delamination depth was found to strongly influence the angular direction and amplitude of scattered waves. Implications for structural health monitoring were discussed.

Influence of curing processes on the development of fiber bridging during delamination in composite laminates

Fiber bridging can significantly influence the interlaminar toughness of laminated composites. However, very little is known about to what extent it depends on the processing conditions. In this paper, we performed a back-bonded double cantilever beam (BB-DCB) test to study the curing process influence on the development of fiber bridging for composite laminates with various thicknesses. The BB-DCB sample included a middle core plate and two backing adherends bonded to each side of the core plate. The total thickness of the BB-DCB samples was kept the same to guarantee the same structural effect due to beam curvature. Thus, only the core plate thickness effect was varied and studied. The experimental results showed that the core plate thickness significantly influences fiber bridging, as the samples with thicker core plates triggered more fiber bridging. Differential scanning calorimetry (DSC) was used to evaluate the degree of cure (DoC) for the different thicknesses, and a correlation was shown between the DoC and the fiber bridging extent. An embedded cell approach was also adopted to show that more fiber bridging was triggered with a larger variance of the fiber/matrix interface toughness and strength, which might have resulted from the change in the DoC with the plate thickness.

An experimental dynamic analysis of Z-pinned unidirectional CFRP beams

The application of composites in mechanical components has been growing gradually over the years. In general, laminated composite material have excellent mechanical resistance to in-plane loads, however, for out-of-plane loads this resistance decreases considerably due the weakness of matrix, that is responsible to keep layers mechanically connected. This may lead the structure to present a high vulnerability to delamination between the layers. The delamination is a common problem in composite materials and frequently occurs due the appearance of interlaminar shear stresses or loads under impact spreading damage along the matrix area of composite. To increase the resistance to delamination in composite laminates, different techniques for inserting reinforcements through thickness are applied including stitching, z-anchoring, tufting and z-pinning. This work aims to evaluate the influence of z-pinning on the modal responses. The z-pinning process was made considering machined rectangular carbon fiber/epoxy z-pins with different dimensions at different insertion spacing densities on the structure. Both the insertion density and the dimensions of the pin were obtained by RSM analysis to evaluate the influence on the mechanical modal properties (natural frequency, loss factor and vibration amplitude) and to obtain an ideal value of insertion area density and pin width. Experimental results have shown that the pin has a statistically significant influence on modal responses, allowing a reduction of up to 110% in vibration levels.

Imaging delamination in composite laminates using perturbation to steady-state wavefields

Delamination in composite laminates, though barely visible, can jeopardize integrity and safety of the composite structures. Interaction of guided waves with delamination in full wavefields have been widely used to graphically characterize the delamination. Recently, steady-state wavefields (SWs) in place of transient wavefields have been used for imaging delamination. However, interaction with delamination can be largely obscured by steady-state waves and hardly observed. Addressing this problem, this study proposes a novel approach for imaging delamination using perturbation to SWs. In particular, a new concept of delamination-caused perturbation to a SW is proposed. Compared to scattered waves from delamination, it is determined by delamination only and naturally sensitive to delamination. Delamination-caused perturbation to a SW is concentrated within the delamination region and rapidly attenuates at undamaged locations. On the basis of this new concept, a delamination index (DI) is established for imaging delamination, by which the delamination can be graphically characterized. The capability of the approach is experimentally validated on composite laminates with delamination, whose SWs are acquired through laser scanning. The experimental results validate that delamination imaging can be achieved by DIs, by which the occurrence, locations, and sizes of the delamination can be graphically characterized.

Analytical study and experimental investigation on delamination in drilling of CFRP laminates using twist drills

Delamination in composite laminates is one of the most common and critical defects, it would significantly reduce the stiffness and strength of components. This paper aims at reducing the drilling-induced delamination to improve hole quality. First, drilling tests are carried out to examine the distribution and amplitude of thrust force on drill under different cutting conditions. Empirical thrust force models are developed using experimental data. Then, analytical models of critical thrust force (CTF) are separately established considering two drilling states, considering the actual interactions between the drill and laminate. Anisotropy of composite, tool geometry and load distribution on drill are considered. Comparisons with experimental data and existing models show that the proposed models are capable of predicting CTFs under different loading conditions. In addition, the critical feed rates at different ply locations are determined. This study can provide guidance for the selection of cutting parameters to reduce drilling-induced delamination.

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.

An XFEM-VCCT coupled approach for modeling mode I fatigue delamination in composite laminates under high cycle loading

In this study, virtual crack closure technique (VCCT) and extended finite element method (XFEM) are coupled to each other as XFEM-VCCT approach to simulate mode I fatigue delamination growth in composites, employing the direct cyclic method in Abaqus. Both two-dimensional plane strain and three-dimensional finite element models under force and displacement control are considered. Numerical simulation results are compared with the existed experimental test data for double cantilever beam (DCB) specimens and validated. Finally, challenges ahead of VCCT and XFEM-VCCT are discussed in detail and the appropriate method for modeling fatigue delamination growth in laminated composites under high cycle loading is suggested. It is found that simulation of the DCB fatigue delamination via the displacement control loading leads to more accurate results in comparison to the force control. VCCT was found as a suitable method for simulation of fatigue delamination growth in 2D and 3D-shell models. While XFEM-VCCT shows high accuracy and low computational time in 3D-solid finite element models. The key conclusion is that the XFEM-VCCT coupled approach is independent of time increment, whereas the time increment is more effective on the results of VCCT analysis, and it affects the run-time significantly.

Nonlinear Interactions of Lamb Waves With a Delamination in Composite Laminates

AbstractIn view of their higher sensitivity in localizing an incipient damage, methods of non-destructive evaluation based on the nonlinear wave-damage interactions have been of continued interest in the recent past. In this paper, the propagation of guided waves through a delamination with contacting interfaces is studied numerically using a finite element-based framework. In particular, influence of the interlaminar location of the delamination on the nonlinear acoustic features in the response spectrum is investigated in detail. The numerical framework is validated by an in-house experimentation performed on a unidirectional glass fiber reinforced polymer (GFRP) laminate containing a through-width delamination. A parameter, referred to as the nonlinearity index (NI), is defined for quantifying the strength of the nonlinear wave-damage interactions and its dependence on the interlaminar location of the delamination is studied across a range of interrogation frequencies. The notion of contact energy intensity is introduced and further used for justifying the trends of variation of the NI obtained numerically and observed experimentally. Results indicate that two fundamental parameters govern the underlying contact phenomenon; they are the phase difference between the wave packets passing through the two sub-laminates and the flexural rigidities of the two sub-laminates present at the site of the delamination defect. While the former controls the relative displacement between the two sub-laminates, the latter governs the propensity of collisions between the two sub-laminates. Finally, a diametric effect of these two parameters on the generation of nonlinear harmonic signals with varying interlaminar locations of the delamination is brought out.

Scattering analysis of nonlinear Lamb waves at delaminations in composite laminates

This study investigates the higher harmonic generation of the Lamb wave at a delamination due to contact acoustic nonlinearity, which is a clapping phenomenon between sublaminate surfaces due to the Lamb wave interaction with the delamination. In this study, higher harmonics of the Lamb wave induced at the delamination in composite laminates are studied in detail. This study performs both numerical and experimental studies. A three-dimensional finite element model is proposed for predicting the propagation of nonlinear Lamb waves in composite laminates and is verified using experimentally measured data. The results show the proposed numerical model can reasonably predict higher harmonic generated by contact acoustic nonlinearity. It is found that the delamination is the major source of contact acoustic nonlinearity in the composite laminates. A mode conversion study is also carried out to gain further physical insight into the higher harmonic generation of the Lamb wave at the delamination.

Buckle-driven delamination models for laminate strength prediction and damage tolerant design

Two state-of-the-art analytical compression after impact (CAI) modelling approaches are presented and evaluated for the problem of thin-film buckle-driven propagation of a delamination in composite laminates. Characteristic phenomena are investigated by evaluating the behaviour of the energy release rate of an anisotropic sublaminate above a 2D embedded delamination. These characteristics include extension-bend, shear-bend and bend-twist coupling as well as contact of sublaminate and base laminate. A holistic approach with the aid of a detailed analysis of deformation characteristics from artificial delamination experiments and finite element analysis provide strong validation of the modelling approaches. Suggestions are made regarding analytical methods suitable for use in the initial aerospace structural design stage. It is found that models which capture the mode-mixity and post-buckled energy terms accurately will allow for better design decisions to be made that are not overly conservative. Whereas methods, which do not account for such mixity and post-buckling, can nevertheless be used to design for damage tolerance.

Loading rate dependency of strain energy release rate in mode I delamination of composite laminates

This work aims at studying the loading rate dependency of mode I delamination growth in CFRPs, using typical fracture toughness analysis through both the R-curve and the crack tip opening rate. The average SERR is a method of data reduction based on energy balance which has been previously introduced to characterize delamination growth under different types of loading conditions in a similar manner. In the present research, the application of this method was extended to further analyze the results of delamination experiments at different loading rates. Mode I delamination tests on double cantilever beam specimens were performed at displacement rates varying from standard quasi-static testing up to 400 mm/s. A clear decrease in the propagation fracture toughness as well as in the average SERR was observed at high loading rates. The reduced fracture resistance at elevated rates was physically explained in correlation with fiber bridging, fiber breakage, and matrix cleavage observed in fracture surfaces via scanning electron microscopy.

Numerical Simulation of Progressive Delamination in Composite Laminates Under Mode I and Mode II Loadings

A progressive interlaminar damage model, based on the Northwestern University theory and a trapezoidal trilinear traction-separation law, is presented to predict the initiation and propagation of delamination in layered composites under Mode I and Mode II loadings. The capability and reliability of the damage model proposed is assessed by comparing its results with experimental data. It was found that the model was able to accurately predict the maximum load and postfailure behavior of composite laminates. The model proposed was also compared with the Abaqus inbuilt cohesive surface model, which is also based on a bilinear tractionseparation law, and our model was found to give more accurate results.

Free‐edge delamination in composite laminates under tensile loading: An analytical closed‐form approach

AbstractIn the present study, interlaminar crack initiation in symmetric angle‐ply laminates under tensile loading is investigated using the coupled stress and energy criterion, whereby the required stress and energy quantities are determined by means of a new analytical closed‐form model. The identification of unknown interface fracture toughnesses is addressed by a parameter fit within physical reasonable limits. The resulting effective failure loads are compared to experimental data and show the high potential of the coupled criterion on the one hand and of the proposed analytical model on the other hand.

Nonlinear pseudo-force in “breathing” delamination to generate harmonics: A mechanism and application study

Subject to coupled vibro-acoustic excitation, the opening–closing motion of a “breathing” delamination in a composite laminate can create nonlinear harmonics, i.e., higher and sideband harmonics, in its steady-state vibration responses. Nonlinear harmonics have attracted increasing attention in the field of nondestructive testing because they can be sensitive indicators of barely visible delamination that is difficult to detect by conventional linear approaches. Although vibro-acoustic modulation has been acknowledged as the cause of nonlinear harmonics, the intrinsic force in a delamination that generates harmonics is not yet clear. Addressing this problem, this study analytically formulates a novel concept of nonlinear pseudo-force (NPF) in “breathing” delamination of composite laminates, by which the mechanism for generating nonlinear harmonics by vibro-acoustic modulation can be explicitly expounded. In the application aspect, as the NPF in delamination can cause local changes in operating deflection shapes (ODSs), this study proposes a novel approach using ODSs at nonlinear harmonics for locating “breathing” delamination of composite laminates, which is superior to current approaches that can only manifest the occurrence of delamination by nonlinear harmonics. Numerical simulations using finite element method are used to validate the mechanism and explore the application potential of ODSs for locating delamination. In particular, an array of coupled vibro-acoustic excitation is proposed to avoid wave attenuation of acoustic excitations. Thereby, vibration and acoustics can interact in the “breathing” delamination to generate nonlinear harmonics.

Development of a standardized test procedure and an improved data reduction method for the mixed-mode I/II delamination in composite laminates

The original data reduction method in the ASTM standard D6671/D6671M-13 is established for unidirectional laminates free of fiber bridging. However, for multidirectional laminates, large-scale fiber bridging occurs in the wake of the crack-tip. Whether the original data reduction method can be applied for multidirectional laminates is questionable. This paper examines the effects of the complex failure mechanism on the flexural modulus of the multidirectional laminates, and how to avoid potential incorrect calculations of the fracture toughness in multidirectional laminates. To achieve this, mixed-mode bending delamination experiments are designed and conducted on specimens with two types of material combinations. An unloading-loading test procedure is proposed to determine the flexural modulus at various delamination lengths. Experimental results show that the flexural modulus increases with the delamination length rather than a constant. And a nearly linear relation between each other exists. An improved data reduction method is thus proposed to consider the effects of the fiber bridging on the flexural modulus, which can provide an accurate characterization for the fracture toughness and thus lay the foundation for the development of standard test procedures for the mixed mode I/II delamination in composite multidirectional laminates.

Analyses of criticality for multiple-site delaminations in the flap spar of Finnish F/A-18 aircraft

AbstractMetal-composite airframes will suffer various defects during their lifetime. One category of defects is composite laminate delamination. This study evaluates the criticality of delaminations existing around adjacent fastener holes in the carbon-fibre-reinforced plastic spar web of the F/A-18 aircraft’s trailing-edge flap. The evaluation is based on experiments and analyses. First, an intensive experimental program for determining necessary material values of F/A-18 is described. Multiple delaminations of the flap spar web are then modelled by varying the set of delaminated hole edges and the interface of delamination. The interaction of defects at the start of delamination propagation is studied via the developed interaction parameter. The results suggest that the interaction parameter can show significant differences in the interaction per delamination case and that the interface of delamination is an important variable. Finally, operator-dependent control parameters are studied, and it is found that the criticality of a delamination case is merely dependent on true material parameters.

An incremental-onset model for fatigue delamination propagation in composite laminates

We propose to treat propagation of a fatigue-driven delamination in composite laminates as a sequence of incremental delamination onsets and develop a cohesive zone model (CZM) to simulate it. The model employs a fatigue damage evolution law that is based on the reduction of critical strain energy release rate. Unlike existing fatigue CZMs, which assume and use Paris' law derived from propagation tests as input, the proposed model relies solely on the experimental data from fatigue onset tests (namely the G-N curve, where G is the supplied strain energy release rate and N is the number of cycles to onset). The model is implemented within an implicit finite element code using three-dimensional cohesive elements and is demonstrated to predict fatigue delamination propagation in unidirectional carbon fibre/epoxy composites under Mode I loading. This incremental onset model predicts the Paris’ law, which compares well with experimental data for this case. Composites with toughened inter-laminar interface and under other modes of deformation and their combinations may exhibit a different fatigue behaviour.