Delamination Size Research Articles

A Linear Numerical Approach to Simulate the Delamination Growth Initiation in Stiffened Composite Panels

In this article, a simplified linear analysis-based approach to simulate the delamination growth initiation in stiffened composite panels, suitable as preliminary design and optimization tool implemented into a finite element code, is presented. The proposed approach is based on the determination of the delamination buckling and on the evaluation of the energy released during the delamination propagation by means of eigenvalue and linear static analyses. Stiffened composite panels with circular embedded bay delaminations, under compression loads, were adopted as a benchmark to test the simulation capabilities of the method. Obtained results, in terms of delamination growth initiation load and energy release rate distributions along the delamination front, have been compared to nonlinear results obtained by the virtual crack closure technique and experimental data for preliminary validation purposes. Comments and considerations upon the applicability of this methodology are, finally, provided with particular focus on delamination sizes and locations within the considered structural elements.

A Bayesian approach to identifying structural nonlinearity using free-decay response: Application to damage detection in composites

This work discusses a Bayesian approach to approximating the distribution of parameters governing nonlinear structural systems. Specifically, we use a Markov Chain Monte Carlo method for sampling the posterior parameter distributions thus producing both point and interval estimates for parameters. The method is first used to identify both linear and nonlinear parameters in a multiple degree-of-freedom structural systems using free-decay vibrations. The approach is then applied to the problem of identifying the location, size, and depth of delamination in a model composite beam. The influence of additive Gaussian noise on the response data is explored with respect to the quality of the resulting parameter estimates.

Machinability analysis in drilling woven GFR/epoxy composites: Part I – Effect of machining parameters

The present paper deals with the effect of machining parameters (feed, speed and drill diameter) on the thrust force and machinability of woven glass fiber-reinforced epoxy (GFRE) composites. The selected machinability parameters were delamination size, surface roughness, and bearing strength. The results show that, delamination-free in drilling GFRE composites was not observed, in the range of the investigated cutting parameters. Surface roughness instrument can be used as an indication for the position of the internal delamination damage in drilling GFRE composites. The high values of correlation coefficients between thrust force and the machinability parameters confirm the importance of reducing the thrust force to improve the load carrying capacity of composite structure assembled by rivets or bolted joints.

Finite element delamination model for vibrating composite spherical shell panels with central cutouts

The finite element model of vibrating laminated spherical shell panels with delamination around a central cutout is developed based on the third-order shear deformation theory of Sanders. In the finite element formulation for the delamination around cutout, the seven degrees of freedom per each node are used with transformations in order to fit the displacement continuity conditions at the delamination region. The numerical results obtained for plates and shells with delaminations are in good agreement with those of other preceding investigations. The new results for shell structures in this study mainly show the effect of the interactions between the radius–length ratio and other various parameters, for example, delamination size, the number of layer and location of delamination in the layer direction. Key observation points are discussed and a brief design guideline is given.

Structural collapse of a wind turbine blade. Part B: Progressive interlaminar failure models

The objective of this paper is to present a geometrical nonlinear and interlaminar progressive failure finite element analysis of a generic wind turbine blade undergoing a static flap-wise load and comparisons with experimental findings. It is found that the predictive numerical models show excellent correlation with the experimental findings and observations in the pre-instability response. Consequently, the ultimate strength of the wind turbine blade studied is governed by a delamination and buckling coupled phenomenon, which results in a chain of events and sudden structural collapse with compressive fibre failure in the delaminated flange material. Finally, a parametric study of the critical load factors with respect to various delamination sizes and positions inside the compressive flange of the wind turbine blade is presented.

Effect of Interlayer Delamination on Mechanical Behavior of Carbon/Epoxy Laminates

This article presents the results of a current study on the influence of interlayer delaminations on the static and fatigue behavior of composite laminates. The composite was manufactured by a vacuum molding method using 12 balanced bi-directional carbon fiber layers and epoxy resin. Delaminations with different length were artificially introduced. The specimens with dog bone shape were cut from the original plates having 3 mm thickness and fiber weight fraction of 0.66. Static tests were performed in order to study the influence of delamination size on the laminate stiffness and strength. Complementary finite element analysis was carried out showing the influence of angular misalignments of fiber/matrix delaminations on the laminate stiffness. Fatigue tests were performed in load control for R = 0.05 and R = —1, with a loading frequency of 10 Hz, at room temperature. The artificial interlayer delaminations have a negligible influence on the fatigue strength for tensile cycle loadings, but produce significant decreases in strength for R = —1 fatigue loadings.

Modeling and detection of delamination in a composite beam: A polyspectral approach

This work considers the difficult problem of detecting delamination in a composite beam structure based on a polyspectral analysis of the structure's vibrational response. First, a low-dimensional model of the structure is presented that captures the delamination-induced nonlinearity and shows how it influences the beam's dynamic response. A Volterra series solution is then used to approximate the nonlinear beam response. The associated Volterra kernels are derived and then substituted into previously obtained expressions for the power spectral, auto-bispectral and auto-trispectral density functions for multi-degree-of-freedom systems subject to Gaussian distributed, random inputs. The influence of the delamination on the polyspectra is then explored with respect to both delamination size and depth. Specifically, it is shown how the dominant peaks of the polyspectra relate directly to these delamination parameters. The limits of polyspectral approaches to delamination detection are presented.

Buckling and Delamination Growth Analysis of Composite Laminates Containing Embedded Delaminations

The objective of this work is to study the post buckling behavior of composite laminates, containing embedded delamination, under uniaxial compression loading. For this purpose, delamination initiation and propagation is modeled using the softening behavior of interface elements. The full layer-wise plate theory is also employed for approximating the displacement field of laminates and the interface elements are considered as a numerical layer between any two adjacent layers which delamination is expected to propagate. A finite element program was developed and the geometric non-linearity in the von karman sense is incorporated to the strain/displacement relations, to obtain the buckling behavior. It will be shown that, the buckling load, delamination growth process and buckling mode of the composite plates depends on the size of delamination and stacking sequence of the laminates.

Stress analysis of microwedge indentation-induced delamination

The indentation stress is the key and fundamental parameter in indentation delamination tests, which are widely used in characterization of the interfacial fracture toughness (or adhesion) between a thin film and its substrate. The indentation stress was analyzed in the present work by using the finite element method for microwedge indenters of different wedge angles and with various other geometrical and mechanical parameters including the penetration depth, film thickness, delamination size, Young's modulus, and yield strength of the indented film. The analysis exhibited the stress field under an indentation load and the stress field after unloading caused by plastic deformation, which resulted in the loading indentation stress and the unloading indentation stress, respectively, expressed by empirical formula in terms of the indenter geometry, the indentation depth, and the film thickness and mechanical properties. The energy release rate was also calculated for the indentation-induced interfacial crack.

Effect of Helical Drill Points on Delamination in Drilling Carbon Fiber Reinforced Plastics (CFRP)

A study of helical drill points for drilling carbon fiber reinforced plastics (CFRP) is presented. A helical drill point has an “S” contour with a radiused crown chisel that reduces the thrust force and make the drills self-centering. The S-shape chisel edge has a lower negative rake angle than a conventional chisel edge, and therefore may cut rather than extrude material. Experiments of drilling CFRP with helical drill points and conventional drill points were conducted. The results indicate that the helical drill points can reduce delamination significantly as compared to the conventional drill points under same cutting condition. Otherwise, delamination size decreases with increasing the cutting speed and increases with increasing the feed

Parametric instability of delaminated composite spherical shells subjected to in-plane pulsating forces

Abstract The dynamic stability analysis of delaminated spherical shell structures subjected to in-plane pulsating forces is carried out based on the higher-order shell theory of Sanders. In the finite element (FE) formulation, the seven degrees of freedom per each node are used with transformations in order to fit the displacement continuity conditions at the delamination region. The boundaries of the instability regions are determined using the method proposed by Bolotin. The numerical results obtained for plates and shells are in good agreement with those reported by other investigators. The new results for delaminated shell structures in this study mainly show the effect of the interactions between the radius–length ratio and other various parameters, for example, delamination size, the fiber angle of layer and location of delamination in the layer direction. The effect of the magnitude of the periodic in-plane load on the instability regions is also investigated.

High-frequency Dispersion Characteristics of Smart Delaminated Composite Beams

Many promising techniques for structural health monitoring rely on the usage of in situ piezoelectric actuators and sensors, which provide the ability of self-excitation and monitoring of the damage effect on the structural vibration and wave propagation. This article presents layerwise mechanics and a finite element capable of describing the response of delaminated composite beams with piezoelectric actuators and sensors. The layerwise beam theory approximates the through-thickness, in-plane displacements, and electrical field as a continuous assembly of piecewise linear layerwise fields through the thickness. This theory further describes the field discontinuities across the delamination as additional degrees of freedom. The introduction of a variable transverse displacement field provides the capability to capture important symmetric and antisymmetric high-frequency modes, guided waves, and other complex phenomena. Pseudo Wigner-Ville distributions provide the frequency dispersion characteristics of predicted and measured time responses of beams with single delaminations and an active piezoelectric sensor pair. Time—frequency plots obtained from the experimental and numerical data are correlated and their ability to reveal the presence and size of delamination is investigated.

Acousto-ultrasonic sensing for delaminated GFRP composites using an embedded FBG sensor

In this paper, an active damage detection system for composite laminates is introduced. A fiber Bragg grating (FBG) was employed as an embedded sensor for detecting ultrasonic Lamb wave generated by a piezoelectric (PZT) actuator, inside the laminates. There were two different configurations for the system, making use of a broadband light source and a laser source, respectively. It was found that the use of a laser source could provide a higher sensitivity, so it was preferred in an ultrasonic signal acquisition unit for damage detection. When a delamination is developed in composite structures, the propagation characteristics of Lamb wave is then changed. According to altered signals, the types and natures of delamination inside the structures can be evaluated. In this experiment, glass fiber-reinforced epoxy (GFRP) composite beams were used to study the feasibility of our proposed detection technique. FBG sensors were embedded in different layers of the composite beams. The responses of FBG sensors from both intact and delaminated composite beams were then compared. Finally, the acquired Lamb wave signals corresponding to different delamination sizes and locations were examined.

Low-velocity impact damage on dispersed stacking sequence laminates. Part II: Numerical simulations

This paper is the follow-up on the previous work by the authors on the experimental evaluation of the impact damage resistance of laminates with dispersed stacking sequences. The current work focuses on the evaluation of the impact performance of the tested laminates by innovative numerical methods. Constitutive models which take into account the physical progressive failure behaviour of fibres, matrix, and interfaces between plies were implemented in an explicit finite element method and used in the simulation of low-velocity impact events on composite laminates. The computational effort resulted in reliable predictions of the impact dynamics, impact footprint, locus and size of delaminations, matrix cracks and fibre damage, as well as the amount of energy dissipated through delaminations, intraply damage and friction. The accuracy achieved with this method increases the reliability of numerical methods in the simulation of impact loads enabling the reduction of the time and costs associated with mechanical testing.

Scaled models for predicting buckling of delaminated orthotropic beam-plates

This study investigates the applicability of scaled models for predicting the buckling behavior of delaminated composites. Such a study is important since it provides the necessary scaling laws, and the factors which affect the accuracy of the scale models. Employment of similitude theory to establish similarity among structural systems can save considerable expense and time, provided that the proper scaling laws are found and validated. In this study a number of parametric studies are performed. The limitations and acceptable intervals of all parameters and corresponding scale factors are investigated. Particular emphasis is placed on the case of delamination buckling of orthotropic plates. Both complete and partial similarities are discussed. This study indicates that models with a different delamination size, depth and number of delaminations than those of the prototype are capable of predicting buckling loads of the prototypes with good accuracy.

Improvement of Out-of-Plane Impact Damage Resistance of CFRP Due to Through-the-Thickness Stitching

The present study investigated, both experimentally and numerically, the improvement of low-velocity impact damage resistance of carbon fiber reinforced plastic (CFRP) laminates due to through-the-thickness stitching. First, we conducted drop-weight impact tests for stitched and unstitched laminates. The results of damage inspection confirmed that stitching did improve the impact damage resistance, and revealed that the improvement effect became greater as the impact energy increased. Moreover, the stitching affected the through-the-thickness damage distribution. Next, we performed FEM analysis and calculated the energy release rate of the delamination crack using the virtual crack closure technique (VCCT). The numerical results revealed that the stitching affected the through-the-thickness damage distribution because the stitch threads had a marked effect on decreasing both the modes I and II energy release rate around the bottom of the laminate. Comparison of the results for models that contained delaminations of various sizes revealed that the energy release rate became lower as delamination size increased; therefore the stitching improved the impact resistance more effectively when the impact energy was higher.

Research on delamination monitoring for composite structures based on HHGA–WNN

Due to the deficiencies of the training algorithms for available wavelet neural network used for structural health monitoring, a new hybrid hierarchy genetic algorithm was introduced by combining hierarchy genetic algorithm and least-square method to improve the learning procedure of wavelet neural network. The hybrid algorithm was able to determine the structure and parameters of the wavelet neural network simultaneously. In this algorithm, adaptive crossover and mutation probability were used to accelerate the genetic speed and avoid the occurrence of prematurity. The modal frequencies of a glass/epoxy laminates beam with varying assumed delamination sizes and locations were computed using finite element method and fed into the wavelet neural network to predict the delamination location and its extent. The simulation demonstrates that the wavelet neural network based on hybrid hierarchy genetic algorithm is robust, promising and converges very fast.

Simulation of multiple delaminations in impacted cross-ply laminates using a finite element model based on cohesive interface elements

The paper investigates the capability of a finite element model based on cohesive interface elements to simulate complex three-dimensional damage patterns in composite laminates subjected to low-velocity impact. The impact response and the damage process of cross-ply laminated plates with grouped ([0 3/90 3]s and [90 3/0 3]s) and interspersed ([0/90] 3s) ply stacking was simulated using a FE model developed by the authors in a previous study and the numerical results were compared to experimental observations. The model provided a correct simulation of size, shape and location of the principal fracture modes occurring in impacted [0 3/90 3]s and [90 3/0 3]s laminates. In [0/90] 3s laminates, characterized by a complex spatial damage distribution, the model was able to predict the approximately circular shape of the overall projected damage area and to capture the typical shape features of individual delaminations; significant discrepancies between experiments and predictions were however observed in terms of delamination sizes at single interfaces. Further investigations are needed to clarify the main reasons of these discrepancies.

Determination of Critical Width of Multiple Delamination in Laminated Composite Plate Under Buckling Load

In order to investigate the effects of multiple delaminations' width on the buckling loads of the simply supported woven steel-reinforced thermoplastic-laminated composite plates, having the strip and lateral delamination regions between each layers, 3-D finite elements models have been established. The critical delamination representing the maximum delamination sizes and tolerate value in a structure without decreasing compressive strength has been obtained from the evalution of the buckling analyses. There are four layers and two different delamination shapes, vertical and horizontal, in models. The orientation angles of the fibers have been choosen as [0°]4, [15°/-15°] s, [15°/-15°]2, [30°/-30°]s, [30°/-30°]2 , [45°]4 and [45°/-45°]s. Firstly, the harmony between theoretical and finite element solutions' results of the plate without delamination has been shown. Then, the buckling loads have been determined for each model having different delamination width. There is no important decrease in buckling loads until certain values of delamination width exactly. For the symmetrical and antisymmetrical cases the obtained results show that the changing ratios are the same for each angle. By comparing orientation angle, it can be said that the values and changing ratios of the buckling loads of high-value orientation angles are higher than the low values.

Buckling behavior and compressive failure of composite laminates containing multiple large delaminations

In this paper, the effects of the delaminations size on the critical buckling load and compressive failure load of E-glass/epoxy composite laminates with multiple large delaminations are studied. A numerical and experimental study is carried out to determine the buckling load of rectangular composite plates . For the experiments (0 ° /90 ° /0 ° /90 ° )s oriented cross-ply laminated plates with multiple large delaminations and without delamination are produced by using hand lay-up technique. The experimental buckling loads of plates are found by clamping from the two edges and then these results are compared with the results obtained by ANSYS11.0 package. In addition, the compressive failure loads of composite laminates containing multiple large delaminations are found by compression test fixture . It is found that the longest and near-surface delamination size influences the buckling load and compressive failure load of composite laminates. However, the size of beneath delaminations has no significant effect on the buckling load and compressive failure load of composite laminates.