Adjacent Plies Research Articles

Effect of fiber orientation of adjacent plies on the mode I delamination fracture of carbon fiber reinforced polymer multidirectional laminates

AbstractThe extensive use of multidirectional composite laminates requires to understand the delamination behavior at interfaces between plies with different fiber orientations. In this study, double cantilever beam testing was performed to investigate the mode I interlaminar fracture of carbon fiber reinforced polymer laminates with different central interfaces (0//0, 0//+45 and +45//−45). X‐ray microtomography revealed the curved crack front shape that was incorporated to calculate fracture toughness. The fracture toughness varies with the crack length in a typical delamination resistance curve, which can be quantified by an empirical equation. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate, compared to constant toughness. It was found that the delamination mechanism is independent of central interfaces. However, compared to unidirectional laminates, multidirectional laminates are less resistant to crack initiation, but more resistant to propagation with higher toughness and shorter fiber bridging zone.Highlights Mode I interlaminar fracture of CFRP laminates with different central interfaces was investigated experimentally and numerically. Curved crack front shape was incorporated to calculate fracture toughness. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate. Multidirectional laminates are less resistant to crack initiation but more resistant to propagation with higher toughness and shorter fiber bridging zone.

Enhanced LaRC05 failure criteria for investigating low-velocity impact on fiber-reinforced composites: An experimental and computational study

A finite element model was developed using both continuum and discrete damage modeling techniques to provide detailed predictions for ply-by-ply damage progression in composite laminates during low-velocity impact (LVI) events. A new fiber failure model was incorporated into the LaRC05 failure criteria to predict fiber pull-out and fiber crushing during the fiber damage evolution. In addition, the selective range golden section search (SRGSS) algorithm was implemented to efficiently predict fiber breakage, pull-out, splitting, kinking and crushing, and matrix cracking. The delamination was captured by cohesive element layers embedded between every adjacent composite ply. The interactions of intralaminar matrix cracking and delamination were modeled by deploying cohesive elements within each composite ply. The prediction results were validated by 30 J and 75 J drop-weight tests with different-sized impactors, as well as X-Ray CT inspections on 254 mm by 304.8 mm [0/45/90/-45]4s IM7/977–3 laminates. The model predicted the maximum deflection and contact duration with <2 % error, and the peak load, damaged areas, and absorbed energy with <8 % error. The matrix fracture plane and the fiber kink band angle were found with 1° precision 48 % faster via the SRGSS algorithm. The detailed sequences of damage occurrence were predicted by analyzing the energy dissipation histories through various damage modes. Although this modeling methodology was developed for LVI scenarios, it has broad applications for predicting failures in composites.

J-Integral Experimental Reduction Reveals Fracture Toughness Improvements in Thin-Ply Carbon Fiber Laminates with Aligned Carbon Nanotube Interlaminar Reinforcement.

The Mode I, Mode II, and mixed-mode interlaminar failure behavior of a thin-ply (54 gsm) carbon fiber-epoxy laminated composite reinforced by 20 μm tall z-direction-aligned carbon nanotubes (CNTs), comprising ∼50 billion CNT fibers per cm2, is analyzed following J-integral-based data reduction methods. The inclusion of aligned CNTs in the ply interfaces provides enhanced crack resistance, resulting in sustained crack deflection from the reinforced interlaminar region to the intralaminar region of the adjacent plies, i.e., the CNTs drive the crack from the interlaminar region into the plies. The CNTs do not appreciably increase the interlaminar thickness or laminate weight and preserve the intralaminar microfiber morphology. Improvements of 34 and 62% on the Mode I and Mode II initiation fracture toughness, respectively, are observed. This type of interlaminar nanoreinforcement effectively drives crack propagation from the interface to within the ply where the crack propagates parallel to the interlaminar region, providing new insight into previously reported strength and fatigue performance increases. These findings extend to industries where lightweight and durable materials are critical for improving the structural efficiency.

The influences of ply interference on ballistic impact response and energy absorption behavior of multilayered woven fabrics

This paper presents a detailed experimental and numerical analysis aiming to investigate the ballistic impact response and energy absorption behavior of multilayered woven fabrics with different ply interference. Different levels of ply interference are provided by controlling the influencing factors such as angle-plying, stitching the constituent plies and spacing the adjacent plies. The three aforementioned designs were evaluated for their energy absorption capacity against ballistic impact with the impact velocities ranging from 300∼400 m/s. The energy absorption mechanism was studied through the combination of high-speed camera and numerical simulations in terms of fabric deformation, and evolution of energy absorption for each ply at different impact velocity. It was found that stitching the aligned systems showed the most pronounced beneficial effect on energy absorption, showing approximate 50 % ∼ 110 % increase than the quasi-isotropic system with a mismatched lay-up angle of 22.5º.This is probably because that thread stitching enables the panel to be more efficient in spreading energy to a more global extent, involving a more defined transverse deformation and a larger area of stress distribution, which is beneficial for energy absorption.

Modeling the interaction between intra and interlaminar cracking in FRP composites using phase-field regularized interface fracture

Fiber-reinforced polymer composites (FRPCs) exhibit a complex range of failure mechanisms, including matrix cracking, fiber breakage, fiber–matrix debonding, and delamination. Among these failure modes, interlaminar fracture or delamination across the interfaces between the different layers (plies) is commonly observed in experiments. Hence, the fracture behavior of these interfaces plays a critical role in determining the overall fracture performance of the FRPCs. In this work, our aim is to model the effect of interface on the final failure of FRPC laminates. Specifically, the present work focuses on modeling the complex interactions between the bulk FRPC damage and interfacial delamination in the carbon fiber reinforced composites (CFRP) within the realm of phase-field fracture. In the present formulation, the phase fields smear the sharp interface between FRP plies as regularized cohesive zones and also describe the bulk crack surface density eventually allowing interaction between bulk damage and interfacial delamination. For the composite’s constitutive description, the FRPC is treated as a homogenized material, with effective mechanical properties depending on the properties of the individual matrix and fibers with their respective volume fractions. The elastic matrix is assumed to be isotropic, while the fibers are characterized by their orientations. Further, an anisotropic modification is considered in the fracture energy of the FRPC to promote crack growth along the fiber direction. The model is implemented in a commercial finite element package and several numerical examples are presented to illustrate the complex crack path behavior in CFRP. Specifically, crack growth and interactions are simulated when there is a mismatch in the fiber orientation between the adjacent plies, resulting in dissimilar material properties on either side of the interface. The utilization of the regularized interface phase field model in an anisotropic framework to account for the mismatch in fiber orientation and hence material properties between the laminas makes it capable of predicting complex modes of fracture in FRPC laminates as demonstrated in the numerical simulations. We have also compared the model predictions with the experiments to validate their accuracy.

Low-velocity impact damage detection in cfrp laminates based on ultrasonic phased-array ndt technique

To investigate the delamination characteristics of CFRP laminates in Barely Visible Impact Damage (BVID) state, Low-Velocity Impact Tests (LVIT) and Non-Destructive Testing (NDT) were employed. The energy thresholds of six diverse ply-stacking sequences of laminates in the BVID state were established utilizing visual inspection and contact measurement methods. Ultrasonic phased-array NDT techniques were employed to evaluate quantitatively the magnitude of delamination damage from different viewpoints, including delamination area, shape, and trend variation. Furthermore, the impact resistance of laminates was qualitatively evaluated by analyzing the distinctive delamination damage forms of various specimens to identify the effects of the ply-stacking sequence. The findings of this study demonstrate that reducing the angle disparity between adjacent plies and minimizing the repetition of identical plies can mitigate delamination damage. Additionally, enhancing the ratio of ±45 and 0° plies can improve the flexural stiffness of the structure. Specifically, the ultrasonic phased-array NDT technique was shown to effectively detect delamination damage within the laminate in the BVID state.

On the ultrasonic characterization of the stacking sequence of CFRP laminates

Ultrasound in pulse-echo scan mode has already been demonstrated for the extraction of local fiber angle distribution in composite laminates. Depending on the ultrasonic (scan) parameters employed, different sensitivity, spatial resolution, and dynamic depth range can be reached.This study investigates the use of pulse-echo ultrasound testing in different frequency ranges to characterize the in-plane fiber angle distribution and ply stacking sequence in a multi-layer composite laminate. Minimization of the Mumford-Shah functional is performed as an edge-preserving smoothing procedure for the recorded ultrasonic dataset, followed by the application of a Gabor Filter-based Information Diagram approach to extract a 3D tomographic image of the fiber angles. The performance of the method is experimentally studied on ultrasonic datasets, recorded at various center frequencies, of a 5.5 mm thick 24-layer quasi-isotropic carbon fiber reinforced polymer laminate with stacking sequence [+45/0/−45/90]3s. For quantitative analysis, statistical metrics are employed to evaluate the accuracy and precision of the extracted angles over depth, and to better understand the interfering influence of adjacent plies. For the given case study, a 15 MHz ultrasound frequency is recommended as it provides a good balance between precision, accuracy, and depth range. To understand the influence of scan parameters on the fiber angle characterization, the 15 MHz ultrasonic dataset is analyzed for various noise conditions as well as different spatial step size of the scan.

Micro-mechanical analysis of the effect of ply thickness on curing micro-residual stresses in a carbon/epoxy composite laminate

During the manufacturing of thermoset-based carbon fiber reinforced polymers (CFRPs), micro-residual stresses are developed in the material, due a mismatch of the chemical-thermal-mechanical properties of the fibers and the polymer matrix. This ultimately leads to a reduction in the mechanical performance of the composite material. In this work, a representative volume element (RVE) of a composite sub-laminate with a 90° ply discretized at micro-scale level, stacked within homogenized plies, is proposed to study the micro-residual stresses and the ply mechanical response considering the in-situ effect caused by the constraining adjacent layers. A model framework is developed using finite elements in Abaqus®/Standard with user-subroutines. The retrieved micro-residual stresses in the 90° layer are generated by the dissimilar chemical shrinkage and thermal expansions, plus the additional residual meso-scale stresses caused by the constraining effect of adjacent plies. The residual stresses in the constrained condition can achieve almost 80 % of the ply failure load for the 0/90/0 sub-laminate, meaning lower sub-laminate strain to failure. Thick laminates retrieve more residual stresses in the transverse 90° layer and exhibit lower constrained strength and more unstable post-failure response, demonstrating that thin-ply laminates exhibit beneficial constraining effects during the curing process.

Interaction of compression failure mechanisms in composites with ply waviness defects

This article presents results from experimental and computational investigations of composite laminates with embedded ply waviness defects of different geometries under compression. The effect of the ply waviness defect on the initiation, interaction, and evolution of failure mechanisms of surrounding plies are studied. Using high-resolution imaging and digital correlation analysis, the strain localization phenomenon is monitored, and the sequence of failure mechanisms is established. The experimental observations are supported with computational finite element progressive failure analyses of the same composites with ply waviness defects using an energy based continuum damage mechanics approach. For the case of laminates with ply waviness defects of high angles, a delamination crack between the wavy plies and adjacent transverse plies is the dominant mode of failure, which interacts strongly with the fiber compression and matrix tension modes. For the case of laminates with ply waviness defects of low angles, failure does not initiate at the waviness defect but at locations where the laminate strength was locally reduced due to other microstructural defects. The combined experimental and computational analyses presented here advanced the understanding of the interaction of compression failure mechanisms in composites with ply waviness defects, which is highly challenging to experimentally measure.

High-fidelity simulation of low-velocity impact damage in fiber-reinforced composite laminates using integrated discrete and continuum damage models

A high-fidelity finite element (FE) model based on integrated discrete and continuum damage modeling techniques was developed to predict damage modes, locations, sizes, and damage occurrence sequence in a composite laminate during a low-velocity impact event. Fiber breakage and transverse matrix cracking were captured using the three-dimensional Hashin’s criteria, and delamination was captured by inserting a layer of cohesive elements between every adjacent composite ply. Delamination progression from one layer to another was captured by embedding cohesive elements inside each composite ply aligned with the fiber orientation and with a 45° angle to the thickness direction. The refined mesh and intralaminar cohesive elements were only used in the predicted damage region based on an analytical solution. The FE model predicted impact response was validated by drop-weight tests, and the layer-by-layer damage areas were validated and characterized by X-Ray CT inspections on 254 mm by 304.8 mm IM7/977-3 laminates with a stacking sequence of [0/45/90/-45]4s. The absorbed energy, damage areas, maximum deflection, contact time, and peak load were predicted with less than 10% error. The detailed damage occurrence sequence during the impact event was predicted through histories of energies dissipated by different damage modes. The modeling methodology developed in this work highlights the effects of modeling parameters on convergence, solution accuracy and computational efficiency, and provides an effective approach for progressive damage modeling of composite laminates.

Damage mechanisms of bismaleimide matrix composites under transverse loading via quasi-static indentation

Abstract With the application of carbon fiber reinforced plastics (CFRPs), the damage behaviors and mechanisms are desirable to understand. In this work, the Bismaleimide (BMI) matrix composites were employed to study the damage mechanism under transverse loading through quasi-static indentation, where optical microscopy, ultrasonic C-scan, and scanning electron microscope were subjected to examine the damaged regions. The results show that cracks first take place in a resin-rich interlaminar region before the loading reached to a threshold value. Then, the cracks would develop into intralaminar cracks in the fiber bundle region consisting of the shear and bending cracks. Subsequently, the cracks randomly extend to adjacent plies and result in delamination. Finally, the back surface of composites presents a remarkable ply splitting and fiber breakage under increasing loading. Furthermore, conical-shaped indentation patterns in cross-sectional and the undamaged zone under the indenter suggest the BMI matrix composites possess potential applications in engineering materials with a superior stress-releasing structure.

Mechanical characterization of natural and synthetic fibre based penta layered hybrid polymer composite

The goal of the current study is to determine how the mechanical performance of hybrid composites is impacted by the fibre composition, layering arrangement and sequencing. Hand-layup approach was used to create four distinct hybrid composites with five lamina comprising varied stacking sequences of jute/coir/carbon fibres. The skin layers only encapsulate bio-fibres orientated horizontally. CJKJC, CKJKC, CJCJC and CKCKC were the stacking sequences, with C, J and K representing carbon fibre mat, jute fibres (unidirectional) and coir fibres (unidirectional). The fibres in the core layer were oriented perpendicular to the enclosing adjacent plies and they may be jute/coir fibres depending on the stacking sequence. Stress vs. strain curves shows that all the proposed composites fail suddenly exhibiting a linear trend except for the sample with a significant concentration of coir fibres. Jute fibre layers added strength and energy-absorbing capability to the composites, whereas coir fibre layers added strain and toughness. It is found from the present study that incorporating the fibres of carbon in the core and skin enhances the mechanical properties compared to its counterparts. The fractography of the proposed composites are studied using scanning electron microscope.

Prediction of a complex delamination front using a general cohesive model

The influence of the delamination direction with respect to the fiber orientation in adjacent plies on the apparent interlaminar toughness is considered by proposing a general cohesive model. A local delamination-direction definition method is adopted. Based on the mismatch between the fiber orientation and delamination direction, the linear evolution of the apparent toughness is developed. The general cohesive model is implemented in a user-defined element subroutine in ABAQUS and is validated on a case study with a single cohesive element. Second, a square sample under Mode I fractures is designed, where multiple delamination directions, ranging from 0° to 90°, form a complex delamination front. Nonlocal crack branching can be observed where high-delamination direction angles exist. Using the segmented linear evolution of toughness can significantly improve the accuracy of predicting the delamination fronts and the reaction-force-opening curve. This work provides a framework to assess the intra/interlaminar damage coupling during the multidirectional delamination propagation.

Influence of the bevel and steeple triggers on crashworthiness of carbon fiber-reinforced composite channel section structure

To induce the progressive crushing of composite channel section structure (CCSS), the bevel and steeple trigger are proposed and designed. A nonlinear damage model considering both damage initiation and evolution of intra-ply and inter-ply is developed. The damage initiation and stiffness degradation processes of intra-ply are predicted by the maximum stress criterion and an exponential evolution law, respectively. The adhesion behavior between adjacent plies is modeled following the surface-based cohesive contact method, and the delamination failure behaviors of inter-ply are predicted using the traction-separation model. The effects of different configurations of bevel and steeple trigger on the energy absorption characteristics of CCSSs subjected to quasi-static axial loading are compared and analyzed. The results show the initial peak load of CCSS using the bevel and steeple trigger has a negative correlation function with the bevel angle. The steeple trigger can induce progressive crushing of the structure and significantly enhance the specific energy absorption (SEA).

Automatic ply detection and finite element model generation for composite laminates

This paper presents a novel methodology to detect and isolate individual ply information contained within a relatively low-resolution cross-section image of thick composite laminate specimens. The proposed method can process laminate sample images and construct detailed geometric models in a fast and automated manner with minimal user interaction. The finite element models can be used directly for structural and strength simulations to analyse the effect of waviness defects. The algorithm processes the greyscale sample image and splits it into multiple slices. The initial starting points for each ply were identified by analysing the pixel brightness of the image. The pixel brightness variation was used to identify the different plies in all image slices and a list of possible ply centreline coordinate is generated. The ply centreline points are grouped and connected by selecting the points with minimal distance to the previous one in the ply. A finite element mesh is created for each ply by creating a boundary at the midpoint between two adjacent plies. The geometric information of the isolated plies is then used to create structured finite element models using an in-house meshing algorithm.

Numerical Study on the Rebound of Low-Velocity Impact-Induced Indentation in Composite Laminate

Indentation is an effective indication of LVI damage in PMCs. However, indentation can rebound partly with time. Thus, a good understanding of the rebound behavior of the impacted pit is helpful in damage assessment for composites. In this paper, a transverse isotropic viscoelastic model and a viscoelastic cohesive interface model are proposed to represent the viscoelastic properties of ply and the interface between adjacent plies, respectively. In these models, we implement the in-plane 3D Hashin failure criterion to simulate ply level failures and the stress-based quadratic failure criterion and linear softening mixed-mode BK law to simulate cohesive interface failure initiation and propagation, respectively. LVI testing was performed on specimens at different impact energies (30 J, 40 J, and 50 J). Dents induced by impact will eventually rebound due to the viscoelastic behavior of plies and cohesive interfaces. This results in a decrease in depth with time. This indentation and its rebound phenomenon were simulated in ABAQUS by considering viscoelasticity with user-defined material subroutines. The simulation results show good agreement with the experimental observations and are validated accurately in terms of the indentation’s initial depth upon impact and its final rebound with time. From experiments, it was observed that the decrease in the original depth of indentation initially becomes faster with time after impact; then, it slows down with time and eventually stops due to viscoelasticity. While this decrease in the original depth of indentation remains invariable with time in simulation, it has a different rebound path.

Verification of Puck's criterion for CFRP laminates under multiaxial loads at ambient and cryogenic temperatures

Objective of the present study is the verification of Puck's failure criterion for multiaxial mechanical loading situations in the ambient and cryogenic thermal regimes. The assessment is based on the material data and the failure envelope determined in a previous study on uniaxially loaded single-ply specimens study. Here, the pre-existing experimental data base is complemented by experiments on specimens made from angle ply laminates featuring holes, tapered sections and combinations thereof. The specimens were tested at ambient temperature and in a liquid Helium environment at 4.2 K. For determination of the local stress states in the individual plies at the stress concentrations induced by the holes and tapered sections, the experiments were simulated numerically by the finite element method. It is found that Puck's criterion in most cases provides conservative results. Unresolved additional safety margins may develop if a structural failure in an inter-fiber mode requires development of larger delaminations of adjacent plies to form a through crack.

Ballistic resistance and energy dissipation of woven-fabric composite targets: Insights on the effects of projectile shape and obliquity angle

This study aims at investigating the ballistic resistance and energy absorption in woven E-glass composite panels, considering different projectile nose shapes and oblique incidence angles. To that scope, three-dimensional finite element (FE) models of both projectiles and the laminated target are developed and numerical investigations are carried out using Abaqus Explicit solver. The composite damage model's constitutive law encompasses nonlinear material response, material properties degradation, progressive failure, and an element deletion strategy. The cohesive surface technique is used to represent the interface between two adjacent plies in the laminate, and the traction-separation law is used to characterize the behaviors of interlaminar degradation and failure. Material responses attributable to fiber rupture, matrix cracking, and plasticity caused by micro-matrix cracking due to shear loading are taken into account with suitable damage evolution laws. The computational framework is first validated against the experimental results reported in the literature by performing ballistic impact tests on the target laminate with conical, hemispherical and blunt-ended projectile, and the numerical results showed a good comparison in terms of residual velocity. Subsequently the framework is explored in simulating more complex failure mechanisms, with particular emphasis on the influence of the impact angle of obliquity, a parameter that is not usually analyzed in the literature. In that regard, the effects of normal and oblique impact on the damage morphologies and ballistic behavior of the fabric composite target in terms of energy absorption, impact contact force, and projectile residual velocity are conducted and analyzed, comparatively. The findings showed that the ballistic impact behavior of target composite is substantially influenced by projectile nose shape and incidence angle obliquity.

Numerical study to investigate the influence of CNTs on delamination resistance in broken ply carbon/epoxy composites

In fibre reinforced polymer (FRP) composite laminates subjected to low velocity impact, one of the common mode of failure is ply breakage followed by interfacial delamination at the site of ply break. Improving the delamination resistance at the interface is therefore important and the present work examines quantitative understanding of how the addition of CNTs to epoxy might enhance the resistance to such delamination at the interface. A laminate made of plies with CNT-epoxy as the matrix and carbon fibre as the reinforcement has been considered to be broken through the full width. A full three dimensional FE analysis was carried out for such a carbon/CNT-epoxy laminate for different wt% of CNTs. Resistance to delamination has been assessed by computing critical strain energy release rate (Gc ) using quadratic stress criterion (QSC) and virtual crack closure integral (VCCI). From the results, by adding CNTs to epoxy, a significant improvement in the delamination resistance could be observed. However, the improvement is till a certain specific CNT wt% and adding CNTs further leads to a significant reduction in delamination resistance. In addition, the effect of various factors such as fiber orientation of the adjacent continuous ply and resin layer thickness on critical strain energy release rate (Gc ) have been studied. The results show that Gc also depends upon the fiber orientation of the adjacent ply and on the resin layer thickness. As the resin layer thickness increases, a significant increase in Gc is observed.

Progressive Failure Analysis of Composite Laminates Subjected to Transverse Loading with Augmented Finite Element Method

In this paper, two-dimensional (2D) orthotropic augmented finite element method (A-FEM) is applied to account for progressive failure of composite laminates under transverse loading, which considers all major cracking modes (delamination, fiber kinking/rupture matrix cracking). High-fidelity simulations of different stacking composite laminates under transverse loading are implemented. Both predicted load−deflection curves and damage evolution are in good agreement with that of experimental results, which demonstrates the numerical capability of A-FEM. In addition, the influence of stacking sequence on the failure mechanism is also studied by predicted damage evolution of laminates with different stacking sequence [Formula: see text], [Formula: see text] and [Formula: see text]. Results show that the tensile matrix crack in the bottom laminar is always the first damage mode for the composite laminate, and the subsequent crack propagation is related to ply orientation of adjacent plies which have a blocking effect on crack propagation.