Interlaminar Strain Research Articles

Experimental Characterisation Framework for Laminate Free Edges by Digital Image Correlations and Validation of Numerical Predictions

This paper develops an accurate experimental framework to measure interlaminar strains on laminate free edges. Digital Image Correlation (DIC) is used with an ultra-fine speckle pattern and macro lens to resolve strain fields with a resolution of ∼ 15 µm, allowing for through-thickness deformation and strain mapping. Data analysis techniques are developed to denoise the strain field and discount the effect of random local fibre distribution.The major application of the framework is to validate numerical predictions, and it is demonstrated on angle-ply laminates over a range of ply orientations. A micropolar-based finite-element approach was compared to both a classical finite-element approach and the DIC-acquired interlaminar strain fields. Key improvements by the results include significantly overcoming the stark inconsistency of classical normal strains, and reducing the discrepancies of shear strains from 30 % to 3 ∼ 10 %. The outcomes can be extended to destructive failure analysis and the free-edge study of various other composite architectures.

Hybrid joint interface in composite fan blade subjected to bird strike loading

An aircraft engine’s fan blades are one of the most important parts of the engine. Bird strikes on fan blades have always been an issue, as bird parts can strike other parts of the engine, potentially causing more damage. It is impossible to avoid being struck by a bird entirely. However, finite element analysis can be used to optimize the design of blade so that the overall impact of a bird on a jet engine is reduced. Even though current composite blades can withstand the impact of a bird strike, some delamination failures have been observed on the blade’s trailing edge side, probably due to vibration bending modes. Instead of the single fiber composite blade that is currently in use, this research proposes using two fibers (Carbon and Glass). For this to be possible, two fiber joints must be designed properly at different locations on the blade. At crucial joint locations, the minimum inter-laminar strain level was used as the design criteria. Blade deformation is simulated using coupon and sub-element level finite element analysis (FEA) models with appropriate boundary conditions with in-built hybrid joints inside. The first stage of this project involved using the Ansys Parametric Design Language (APDL) and linear static analysis to create coupon models for combinations of joint positions. In the present work, dynamic bird strike analysis on sub-element level models was performed with various joint location combinations. The best joint configurations based on static and dynamic analysis results will be suggested for use in the composite blade to prevent delamination.

Analytical Analysis to Predict the Generated Stresses and Strains of Laminated Composite Tube due to Internal Pressure

The aim of this study is to build a MATLAB code to predict the interlaminar radial strains, and stresses developed in a laminated composite tube. This code is only valid for symmetrical laminated composite tubes. The code can solve for tube that is subjected to uniform internal pressure. The written MATLAB code can save time and money compared to experiments and finite element simulations. Then, this code is validated with other published mentioned articles in this study. Good agreement is achieved which it means the built code can be used for the cases that agree with a symmetrical condition. The code is run to solve for radial, hoop, and shear strains. Also, it is used to solve for axial, radial, hoop, and shear stresses.

Simulation of energy dissipation mechanisms in evaluating the critical interlaminar strain energy release rate of cross-ply carbon/epoxy laminated composites

Delamination has been introduced as the most crucial failure mechanism in laminated composites due to the absence of reinforcement along the thickness. It is necessary to investigate the phenomenon of delamination by researchers carefully to take full advantage of the composites' valuable properties. The purpose of this study is to quantify the effects of energy dissipation mechanisms on the interlaminar crack growth phenomenon in laminated composites that increase the apparent fracture toughness of these composites. The [0]12, [0/90]6 and [05/90/06] lay-ups made with carbon fiber and the epoxy matrix will be investigated with both experimental and analytical approaches. The results show an increase in the fracture toughness value of the cross-ply laminates compared to the unidirectional ones in the experimental section. According to the observations, it can be said that this increase is due to the difference in the energy absorption mechanisms (so-called toughening mechanisms), including zigzag crack propagation and fiber bridging. Using previous studies, a relationship between different parameters involved in crack growth is presented in the analytical section. Finally, using the J-integral method, the amount of energy absorbed by the fiber bridging mechanism is calculated. Also, a method for quantifying the crack zigzag growth mechanism was presented, which is estimated to increase by about 10% in increasing the fracture toughness value. By removing the effects of energy absorption mechanisms from the R-curve extracted from the experiment, it is observed that the toughening behavior of the graph is eliminated to an acceptable level, which may confirm the hypothesis of the materiality of the fracture toughness parameter in laminated composites.

Lightweight sandwich and composite beam analysis using improved higher-order theory with respect to strain energy fidelity in ply-wise approach

ABSTRACT This approach present improved higher-order layer-wise theory for the investigation of flawlessly wedged sandwich-composite beams with general laminate configurations. Our analysis incorporates the continuity assumption of interlaminar shear stresses and in-plane and flexural displacements between interfaces. In addition, interlaminar shear stresses are constrained using the Lagrange multiplier technique by introducing new unknown variables. These unknown variables are expressed with interlaminar strain energy, assuming that the strain energy is continuous throughout the overall thickness of the beam. To govern the newly introduced and other unknown variables, the total potential energy (TPE) is minimised using variational calculus. The numerical analysis results show that our approach provides enhanced accuracy to examine sandwich-composite beams.

Experimental Study of AFRP-Confined Square and Circular Concrete Columns Using Fiber Bragg Grating Sensors

An experimental investigation on square and circular high-strength concrete short columns confined with aramid fiber-reinforced polymer (AFRP) sheets was conducted in this study. Fiber Bragg grating sensors have been applied successfully in monitoring of the strains of the AFRP-confined square and circular concrete columns. The experimental results demonstrate that two types of axial force-strain curves were observed depending on the form of the column. Results show fiber Bragg grating sensors have good repeatability and the ultimate load of the circular concrete column is larger than that of the square concrete column. The interlaminar strains of AFRP and high-strength concrete have also been attained. It helps to analyze the constraint effect of the concrete column and compute the ultimate load of the square and circular concrete column.

An Investigation on Mechanical Behavior of Tooth-Plate-Glass-Fiber Hybrid Sandwich Beams

Tooth-plate-glass-fiber hybrid sandwich (TFS) is a type of sandwich composites fabricated by vacuum-assisted resin infusion process, in which glass fiber facesheets reinforced by metal plate are connected to foam core through tooth nails. Bending properties and interlaminar properties of TFS beams with various foam densities were investigated by flexural tests and DCB (double cantilever beam) tests. The test results showed that by increasing the foam core density from 35 kg/m3 to 150 kg/m3, the peak strength of TFS beams significantly increased by 168% to 258% compared with similar sandwich beams with fibrous composite facesheets. With the change of foam density and span length, the main failure modes are core shear and facesheet indentation beneath the loading roller. The interlaminar strain energy release rates of TFS specimens also increased by increasing the density of the foam. In addition, an analytical model was used to predict the ultimate bending strength of TFS beams, which were in good accordance with the experimental results.

Improvement of Interlaminar Fracture Properties of Out of Autoclave Manufactured Carbon Fiber Reinforced Polymers Using Multi-Walled Carbon Nanotubes

The present study aims to the development of Out of Autoclave (OoA) Carbon Fiber Reinforced Polymers (CFRPs) with increased interlaminar fracture toughness by using MWCNTs. The introduction of MWCNTs into the structure of CFRPs has been succeeded by using carbon nanotube-enriched sizing agent for the pretreatment of the fiber preform using an in-house developed methodology that can be easily scaled up. The positive effect of the proposed methodology on the interlaminar fracture toughness of the CFRP laminate was assessed by the increase of Mode I and Mode II interlaminar fracture toughness of the composites. Different wt% MWCNTs concentrations were used (namely 0.5%, 1%, 1.5% and 2.5%). It was found that the nanomodified composites exhibit a significant increase of the interlaminar critical strain energy release rate GIC and GIIC of the order of 103% and 62% respectively, in the case of 1.5 wt% MWCNTs weight content. Scanning Electron Microscopy (SEM) of the fracture surfaces of CFRP samples revealed the contribution and the associated synergistic mechanisms of MWCNTs to the increase of the crack propagation resistance in the case of nano-modified CFRPs compared to the reference material.

Consideration of interlaminar strain–energy continuity in composite plate analysis using improved higher order theory

The main goal of this paper is to suggest an improved higher order refined theory for analysing perfectly bonded stacked composite laminates with the usual lamination configurations. The analysis incorporates continuous flexural and in-plane displacements at the interfaces. Furthermore, the transverse shear stress is continuous and constrained with the Lagrange multiplier technique by introducing 14 new unknown variables that are expressed in terms of the interfacial strain energy, which is assuming to be continuous throughout the thickness of the laminate. To determine the newly introduced flexural and in-plane unknown variables, the total potential energy is minimised using variational calculus. The numerical results are compared with those from existing reliable published papers. In general, the proposed approach is sufficient for analysing laminate structures with the required accuracy.

Mixed mode embedded circular delamination propagation in spar wingskin joint made with curved FRP composite laminates

This paper presents the mixed mode propagation of circular delamination pre-embedded at the interface of 1st and 2nd plies of wingskin adherend of the Spar Wingskin Joint made with curved fibre reinforced plastic composite panels. Three dimensional finite element analyses of these joints under uniformly applied out of plane transverse load have been carried out using contact and multi point constraint elements. The growth of embedded delamination is simulated by sequential release of the constraints of these elements ahead of the delamination front. The inter-laminar stresses and strain energy release rate values being indicative parameters in delamination growth studies are computed in the vicinity of the curved delamination fronts. These values are evaluated using virtual crack closure technique. It has been observed that the circular delamination size significantly influences the magnitudes of inter-laminar stresses and the three components of SERR values in the vicinity of delamination front. Maximum values of peel stress and mode I SERR occur at orthogonal locations to the loading direction i.e. at θ = 90° and 270° measured counter clockwise from positive X-axis along the periphery of the delamination fronts. The variations of inter-laminar stresses and the values of SERR are non-uniform along the circular delamination front indicating variable rates of propagation. This simulation procedure will be used for the assessment of loss of structural integrity of the SWJs having circular delamination when the embedded delamination undergoes propagation upon increase of loading.

On the interlaminar fracture toughness of carbon fiber composites enhanced with graphene nano-species

Abstract The present study concerns the development of a new class of carbon fiber reinforced polymers (CFRPs) with a nano-modified matrix based on graphene nano-species. The premium target is the increase of the interlaminar fracture toughness of carbon fiber composites enhanced with graphene nano-species under mode I loading. An in-house developed methodology for the dispersion of the nano-fillers and the manufacturing of nano-doped prepregs is analytically described. Three different kinds of CFRP plates, one with neat matrix and two doped with graphene nano-platelets or graphene oxide were manufactured. Several double-cantilever beam coupons were tested for interlaminar mode I fracture toughness characterization under simultaneous recording of Acoustic Emission (AE) activity. The nano-doped composites exhibit a significant increase of the interlaminar strain energy release rate GIC of the order of 50% for the case of graphene nano-platelets. AE findings combined with extensive scanning electron microscopy (SEM) attempt to delineate the fracture process variations and the involved micro-mechanisms at a meso-scale level that explain the differences in the crack propagation process.

Strain monitoring of composite elements by fibre Bragg grating sensors during a quasi-static indentation

The damage resistance of a fibre reinforced composite materials subjected to a concentrated indentation force has been investigated. Composite samples were manufactured by vacuum infusion process and then tested according to the concentrated quasi-static indentation test standard (QSI). The composite coupons were sensorized with embedded Bragg grating fibres in various positions, to monitor the indentation-induced deformations during the loading event. Numerical simulations, performed by using a commercial FEM software, were carried out and predictions results compared with experimental data.Final results showed that Bragg wavelength changes sensitively depending on the applied quasi-static forces and it identifies correctly the non-linear pattern of the force–displacement curve. It was determined that strain transfer to the Bragg fibre is strongly influenced by the embedding conditions and final location of the optoelectronic sensors in term of shifting and in-plane rotation. Comparison between simulated and experimental data showed some discrepancy mainly due to the displacement of the embedded fibre sensors from their original location, caused by the manufacturing. However, by an inverse procedure based on iso-strain curves both shifting and rotation of the FBG sensors respect to their original position could be evaluated.This work confirms that although Bragg fibres are effective and reliable to monitor deformations of composite elements during an impact event but unwanted and unpredictable effects, due to manufacturing, should be carefully taking into account to analyse correctly the sensor signal and quantitatively evaluate the inter-laminar strains.

Analytical modelling of the electromechanical behaviour of surface-bonded piezoelectric actuators including the adhesive layer

The behaviour of a piezoelectric actuator is strongly affected by the bonding condition along the interface between the actuator and the host structure. The current paper represents an analytical study of the static effect of the mechanical and geometrical properties of the adhesive layer on the coupled electromechanical behaviour of a thin piezoceramic actuator bonded to an elastic medium. An actuator model with an imperfect adhesive bonding layer, which undergoes a shear deformation, is proposed to simulate the two dimensional electromechanical behaviour of the integrated system. Analytical solution of the problem is provided by solving the resulting integral equations in terms of the interfacial stress. Numerical simulation is conducted to study the effect of the bonding layer upon the actuation process. The effect of interfacial debonding on the response of the layered structure and on the interlaminar strain and stress transfer mechanisms is discussed.

The Effect of Adhesive Layers on the Dynamic Behavior of Surface-bonded Piezoelectric Sensors with Debonding

The behavior of a piezoelectric sensor is strongly affected by the bonding condition along the interface between the sensor and the host structure. This article presents an analytical study of the effect of the mechanical and geometrical properties of the adhesive layer on the coupled dynamic electromechanical behavior of a thin piezoceramic sensor bonded to an elastic medium. A sensor model with an imperfect adhesive bonding layer is proposed to simulate the two dimensional electromechanical behavior of the integrated system. The analytical solution of the problem is provided by solving the resulting integral equations in terms of the interfacial stress. Numerical simulation is conducted to study the effect of the bonding layer upon the dynamic response of the sensor under different loading frequencies. The interfacial debonding and its effect on the interlaminar strain and stress transfer mechanisms are discussed in detail.

The dynamic behaviour of surface-bonded piezoelectric actuators with debonded adhesive layers

The performance of smart structures depends on the electromechanical behaviour of piezoelectric actuators and the bonding condition along the interface, which connects the actuators and the host structures. This paper provides a theoretical study of the effect of partially debonded adhesive layers on the coupled electromechanical behaviour of piezoelectric actuators subjected to high-frequency electric loads. An actuator model with an imperfect adhesive bonding layer, which undergoes a shear deformation, is proposed to simulate the two-dimensional electromechanical behaviour of the integrated system. An analytical solution of the problem is provided by solving the resulting integral equations in terms of the interfacial stress. Numerical simulation is conducted to study the effect of the bonding layer upon the actuation process. The effect of interfacial debonding on the dynamic response of the layered structure and on the interlaminar strain and stress transfer mechanisms is discussed.

Interlaminar shear strain measurement on angle-ply laminate free edge using digital image correlation

Because only coarse polishing is required in order for Digital Image Correlation (DIC) to be used, such a technique has been used to study interlaminar shear strain on the free edges of carbon/epoxy laminates. Pure UD and quasi UD have been considered. The measurement uncertainties are small when compared to measured fields. The correlation parameters of DIC have been optimized to highlight the strain gradients. DIC results for pure-UD samples show high interlaminar strain gradients with a non-linear behaviour and local damage close to inter-ply interfaces. Measurements of quasi-UD samples highlight the microstructure influence on the spatial repartition of free edge strain gradients.

Moiré interferometry measurements of composite laminate repair behavior: Influence of grating thickness on interlaminar response

Bonded scarf-lap and step-lap specimens representative of fully repaired composites were fabricated from two quasi-isotropic sixteen ply panels made from IM6/3501-6 material. The full-field optical displacement measuring technique, moire interferometry, was used to gather detailed information about the distribution of interlaminar strain on the edge of these specimens. Highly detailed 3D numerical models were constructed for both specimen geometries and results obtained. Instead of modeling idealized bondline geometries, the geometries were obtained through digitization of micrographs of the actual test specimens. As a further enhancement on a second set of models, a thin resin layer representing the moire interferometry diffraction grating was added to each model. Comparisons between the model and experimental results showed that a localized effect over the bondline was more accurately represented when the grating layer was modeled. Surprisingly, it was found that in regions away from the bondline, the through-thickness interlaminar normal strain was found to be amplified by the presence of the modeled grating. A series of parametric models were conducted on a laminated composite while varying grating thickness and modulus to further examine this phenomenon. A mechanism for the strain amplification is proposed.

Numerical Simulation of Elastic-Plastic Behavior of Thermoplastic Composites

In this paper, a one-parameter plasticity model is used for characterizing the plastic behavior of AS4-PEEK thermoplastic composites. Using the finite element automatic generating system (FEPG), a three-dimensional one-parameter plasticity model which is not presented in the commercial codes is introduced into this system. Taking the unidirectional composite as an example, computing is carried out using a procedure developed. By comparing the numerical simulating results with the experimental ones, it can be found that the three-dimensional plasticity model introduced can simulate the nonlinear behavior of AS4-PEEK well. Furthermore, the precision of this procedure is confirmed. This achievement has laid the foundation for the analysis of inter-laminar stresses and strains of the three-dimensional elastic-plastic PEEK composite laminates.

Residual interlaminar deformation analysis in the carbon/epoxy composites using micro-moiré interferometry

In this paper, a new micro-moiré interferometer is proposed to measure the residual interlaminar deformation due to the cooled down state of carbon fibre reinforced epoxy composites. The principle of the micro-moiré interferometry method and the measurement techniques are discussed in detail. Some 16-ply laminates with ply orientations of [90° 2, 0° 2, +45° 2, −45° 2 were manufactured using carbon fibre reinforced epoxy prepregs. The residual interlaminar deformation in the composite was measured from the moire fringes and the corresponding residual interlaminar strain components ε x , ε y , γ xy were calculated. The experimental results showed that this micro-moiré interferometer can resolve moiré fringes in a range from tens to a few hundreds of a micrometer, and thus this measurement method has great potential in many other areas of application.

Interlaminar fracture characterization for plain weave fabric composites

For the analysis of laminated composite plates under transverse loading and drilling of composites, all the elastic, strength and fracture properties of the composite plates are essential. Interlaminar critical strain energy release rate properties in mode I, mode II, mixed mode I/II and mode III have been evaluated for two types of plain weave fabric E-glass/epoxy laminates. The double cantilever beam test and the end notch flexure test have been used for mode I and mode II loading. The mixed mode bending test and split cantilever beam test have been used for mixed mode I/II and mode III loading. It is observed that the plain weave fabric composite with lesser strand width has higher interlaminar fracture properties compared to the plain weave fabric composite with more strand width. Further, crack length versus crack growth resistance plots have been presented for mode III loading. In general, it is observed that total fracture resistance is significantly higher than the critical strain energy release rate.