Free-edge Delamination Research Articles

A critical review on free edge delamination fracture criteria

Laminates experience three-dimensional singular stress near their free edges due to elastic mismatches between layers, which can cause delamination. This paper critically evaluates methods for predicting free edge delamination and highlights the limitations of conventional strength-of-materials and fracture mechanics approaches. The Theory of Critical Distances (TCD) uses a material-dependent critical distance parameter, while Finite Fracture Mechanics (FFM) employs a combined stress-energy criterion without needing a predefined length parameter. This review compares TCD and FFM, also discussing Cohesive Zone Models and Phase-Field Models, and aims to guide the selection of appropriate methods for analyzing free edge delamination.

Effect of shear stresses on fibre direction tensile failure using a new simple and reliable test method with thin plies

A new method of creating in-plane combined tension-shear stress states using only tensile loading is proposed. Thin-ply angle-ply carbon/epoxy laminates are sandwiched between unidirectional glass layers to eliminate any stress concentration around the samples ends and gripping zone. Use of the thin plies successfully suppresses early occurrence of other modes of failure i.e. matrix cracking and free-edge delamination, so the first mode of failure is fibre failure in the angle-ply carbon sub-laminate. Compared with other methods of creating combined tension-shear stresses, the proposed technique has a simpler geometry, is significantly easier and cheaper to manufacture and test. Therefore, it provides more repeatable low-scatter experimental results.The obtained experimental results showed that the presence of in-plane shear stresses did not have a significant impact on the tensile fibre-direction failure strain of the tested carbon/epoxy laminate, suggesting that even at high shear stresses, the longitudinal tensile strength of the carbon/epoxy laminate is not significantly reduced.

Hydrogen permeability of thin-ply composites after mechanical loading

Hydrogen is a sustainable alternative to conventional fuels, and it may be obtained with near zero carbon footprint. However, hydrogen storage remains a key challenge, and the use of composite tanks has gained significant interest over the last few years. In addition, thin-ply composites promote fibre damage by delaying matrix microcracking and free edge delamination. In this work, the H2 permeation/diffusion performance of virgin and mechanically loaded thin cross-ply laminates is studied. In addition, Scanning Electron Microscopy (SEM) is used to identify defects and micro-damage in the laminates and explain the experimental values. The study shows that the hydrogen (H2) barrier performances of thin-ply composites are lower than conventional metallic systems. Obtained permeability values, however, resulted well below the allowable limits for most combinations of temperature and pressure and remain unaffected despite the application of high tensile strains showing that permeation is not accelerated.

Experimental investigation of thermoset composite laminates manufactured with a novel sheet-winding compression-molding (SWCM) process

The carbon fiber-reinforced plastic (CFRP) tubular structure is suitable for aircraft and aerospace applications requiring strong axial strength and moderate thermal expansion. However, utilizing CFRP tubular constructions necessitates the addition of end-fitting components, which introduces new problems, such as adhesion. This study presented a novel two-step sheet-winding compression-molding (SWCM) process for fabricating integral thermosetting CFRP tubular-bar structure elements, which could be further drilled into a tubular-lug structural element. The influence of the proposed SWCM technique on the curing of the epoxy prepregs is demonstrated to be minor. The mechanical characteristics of SWCM specimens are much superior to those of control autoclave samples. The investigation of morphology provides a discussion and assumptions regarding the enhancement mechanism. It is considered that the advantage of SWCM specimens is that the particular wrapped edge suppresses the effect of free-edge delamination. This study provides a valuable guideline for designing and fabricating tubular-lug structural components.

An experimentally validated 3D progressive fatigue damage model for fatigue life prediction of Flax-epoxy laminates

A finite element model that incorporates a 3D progressive fatigue damage model (3D PFDM) is developed to simulate the fatigue behavior of Flax-epoxy (FE) composites. Input parameters of the model are determined from testing the material properties in all directions. A double-notched shear test is used to characterize the out-of-plane shear properties of the laminate under both static and fatigue loading conditions. The 3D PFDM, which employs a stochastic distribution of the material properties, is implemented thorough a user-defined subroutine in ABAQUS finite element software. Results of the finite element analysis of voids-containing laminates are in good agreement with experimental results. The predicted fatigue life of all layups is more influenced by voids at higher loading levels compared to laminates without voids. A further verification of the 3D PFDM model was performed using the cross ply and quasi-isotropic configurations and comparing the predicted free-edge delamination at the interface of the 90° ply and adjacent layers to the SEM micrographs of free edges of specimens. From a practical perspective, the proposed 3D PFDM could be applied to a wide range of composite materials to predict edge delamination in laminates with different fiber orientations.

Additive binding layers to suppress free edge delamination in composite laminates under tension

An additive laminate applied to the free-edges of composite laminates is proposed to stop the development of delamination. Numerical analyses were conducted to find the optimum additive binding layup and the experimental results proved that the free-edge delamination was successfully suppressed. When it was used to bind the edges of a [(202/-202)2]s angle-ply laminate, the tensile failure strain and load of the laminate were increased by about 50%, and the failure mode changed from free-edge delamination to in-plane shear. However, it did not change the failure load and final failure mode of a [452/02/-452/902]s quasi-isotropic laminate, as this substrate is less susceptible to free-edge delamination. Thus, this technique is an efficient solution for suppressing free-edge delamination in real structural applications and in the characterisation of coupon tests where damage at the free-edges undermines the real potential of the composite substrate.

High crack self-healing efficiency and enhanced free-edge delamination resistance of carbon fibrous composites with hierarchical interleaves

Carbon nanotube (CNT), poly(ethylene-co-methacrylic acid) (EMAA) and epoxy (EP) are used to prepare a hierarchical interleaf, consisting of [CNT/EP/EMAA]s sandwich structure with periodic EP column arrays, which can act as crack self-healing interface within carbon fiber reinforced plastic (CFRP) by in-situ electrical-heating the CNTs. Results show that the introduced [CNT/EP/EMAA]s interleaf has little effect on the free-edge delamination initiation stress (<±3.2%) and the in-plane strength (<±1.2%) of the baseline CFRP. But, the interleaved CFRP shows high self-healing efficiencies of 99.1–111.3% to the delamination initiation stress and 97.9–99.4% to the elastic modulus, respectively, from two delamination initiation-healing circles activated by electrical-heating the embedded CNTs, as well as with a maintenance of 92.6–99.7% to the tensile ultimate strength. This study reveals that the [CNT/EP/EMAA]s interleaf and the electrical-heating activated self-healing method are promising to effectively restore the initial damage of CFRP without decreasing the in-plane strength, unlike the traditional pure EMAA interleaf.

Prediction of laminate delamination with no iteration

To simulate laminate delamination using least input parameters and with no iteration is still a challenge. As the laminate is essentially bonded from individual laminae by the matrix, there exists a matrix-layer between two adjacent lamina ones, and a delamination of them must be resulted from a matrix-layer failure. However, a three-dimensional finite element analysis indicates that the stresses of the matrix layer are generally so small that it cannot fail before one of the adjacent laminae assumes a failure. This contradiction suggests that there must be something missing from the elemental stresses of the matrix-layer near the delamination front. We introduce a modifying coefficient (MC) to the matrix-layer stresses, and the modified stresses are checked with a strength-failure criterion. Once a tensile or shear failure is attained, delamination occurs to the adjacent laminae. At any load step, either the layers of an element are completely bonded together or completely separated from each other. No iteration is necessary to identify a delamination in the element. Only an additional critical energy release rate from a DCB test is required. Reasonable correlation between predicted delamination propagations and free-edge delamination initiations and measured counterparts of a number of laminates confirm the efficiency of this work.

Effect of out-of-plane wrinkles in curved multi-directional carbon/epoxy laminates

Defects such as out-of-plane wrinkles are known to strongly affect in-plane strength but there has been very little research on their effect on out-of-plane properties. Experimental and numerical studies of multi-directional curved-beam laminates were thus carried out to understand the effects of out-of-plane wrinkles on through-thickness tensile strength. The initially selected layup saw free-edge delamination interacting with transverse cracking, which is undesirable. After suppressing the free-edge delamination by dispersing the plies near the specimen surfaces, through-thickness tensile failure was observed near the mid-plane. The effects of out-of-plane wrinkles could be studied with this appropriate layup, showing a 16% reduction in strength. A High-fidelity Finite Element Method (Hi-FEM) has been used to distinguish between the different failure modes and to understand the effects of wrinkles. Good agreement was achieved between the numerical and experimental results in terms of through-thickness tensile strengths and delamination locations.

In-situ imaging of inter- and intra-laminar damage in open-hole tension tests of carbon fibre-reinforced composites

This paper investigates damage progression during Open-Hole Tension (OHT) test of carbon fibre-reinforced composites with different Quasi-Isotropic (QI) layups. Digital Image Correlation (DIC) and in-situ edge microscopy are utilized simultaneously to detect damage on the specimen’s surface and in the inner plies, respectively. It is found that damage does not always initiate in the 90° plies, but also in the surface 45° plies, depending on the orientation of the adjacent ply. DIC resulting strain field on the surface does not necessarily have the same butterfly-shape concentration around the hole, in all layups. Some laminates show large free edge delaminations at 45°/90° interfaces. This study adds in-situ evidences to the existing data on damage during OHT test of QI laminates, providing quantitative and comparative information on different damage modes, and reveals new features of the damage phenomena.

Thickness effect of carbon nanotube interleaves on free-edge delamination and ultimate strength within a symmetric composite laminate

This paper utilizes carbon nanotube (CNT) buckypaper (BP) as a modified interleaf and incorporates it into the potential delaminated interfaces within the [±θn]S laminates, and then how the CNT BP affects the interlaminar stress and induces free-edge delamination is considered. According to the measured in-put mechanical parameters, a numerical model is firstly developed. Results show that the thickness of the CNT-based interleaf has an obvious effect on the interlaminar stress and corresponding delamination. It just changes the relative value of stress or strength but doesn’t change the patterns of stress distribution or delamination. More importantly, the CNT-based interleaf with a reasonable thickness would significantly decrease the interlaminar stresses (less than 16 μm), delay initiation of delamination and enhance the ultimate strength (less than 40 μm) of the laminates. Simultaneously, the obtained experimental results are coherent with the numerical results, which further prove the thickness effect of the CNT-based interleaf.

Notch insensitive orientation-dispersed pseudo-ductile thin-ply carbon/glass hybrid laminates

Notch sensitivity, free edge delamination and brittle failure are limiting factors for the wider use of conventional composite laminates. In our previous study, a hybrid layup concept with the different materials blocked together but with dispersed orientations was successfully used to design pseudo-ductile hybrid composites with no free-edge delamination. This study introduces a comprehensive set of designed and characterised orientation-dispersed pseudo-ductile thin-ply hybrid composites to address notch sensitivity, another important limiting factor in conventional composite laminates. Un-notched, open-hole and sharp notched tension tests were performed on three different thin-ply carbon/glass hybrid configurations. The investigated laminates showed a successful pseudo-ductile un-notched behaviour with improved notch-insensitivity and suppression of free-edge delamination that was an undesirable damage mode in previously investigated hybrids with plies of the same orientation blocked together. This notch insensitivity results from subcritical damage in the laminates due to the pseudo-ductile damage mechanisms, i.e. dispersed delamination and fragmentation. These damage mechanisms can eliminate stress concentrations near the notch and suppress the conventional damage mechanisms that govern the notched response of the laminates.

Orientation-dispersed pseudo-ductile hybrid composite laminates – A new lay-up concept to avoid free-edge delamination

Multi-Directional hybrid composites can suffer from free-edge delamination, a damage mode that doesn't exist in Uni-Directional hybrid composites and can hinder the pseudo-ductility that can be achieved with thin-ply hybrids. This paper presents a new lay-up concept called ‘orientation-dispersed’ laminates to avoid this mode of damage in quasi-isotropic hybrids. It is shown that the energy release rates at the free-edges of orientation-dispersed layups are significantly lower than those in ‘orientation-blocked’ laminates. Finally, the experimental results from two quasi-isotropic layups with π/3 and π/4 intervals are presented showing a good pseudo-ductility with no free-edge delamination.

Modeling framework for free edge effects in laminates under thermo-mechanical loading

Abstract In this paper, a novel Quasi-2D (Q-2D) plane strain formulation for predicting interlaminar stresses in multi-directional laminates is developed and implemented within the finite element method framework. In particular, analyses of free edge stresses in composite laminates subjected to uniform axial and/or thermal loading are conducted. The Q-2D modeling approach presented here is validated by comparing the predicted interlaminar stresses with corresponding 3D models and previously published data. Computational time required for determining the interlaminar stresses using Q-2D model is approximately 30 times lower than the 3D analysis of the same laminate. Also, the Q-2D model is implemented within a commercially available software by modifying a 3D model to behave like a 2D model. This is particularly advantageous over developing an in-house finite element method code to capture the complex free edge stress states in multi-directional composites. Hence, the current framework can potentially be used as a computational tool for efficiently predicting the interlaminar stresses in different laminates subjected to thermo-mechanical loading, which can assist in determining interlaminar regions susceptible to free edge delamination.

A quick procedure to predict free-edge delamination in thin-ply laminates under tension

Delaminations at the free-edges of a laminated composite under tension can be triggered by transverse cracks or by high interlaminar stresses. The capability for predicting these phenomena when the ply thickness is reduced (thin-ply laminates) is particularly challenging because damage mechanisms are delayed or even suppressed. In this work, an existing energy-based failure criterion and a simplified finite element model with cohesive elements are combined to develop a computationally inexpensive predictive tool. Its comparison with experimental data demonstrates that this approach captures the trends of the critical strain for delamination with respect to ply thickness and ply location and the quantitative agreement with the predictions is satisfactory.

Free Edge Mixed Mode Delamination Analysis of Laminated Composites with Wrap-Around Configuration: A Finite Element Study

Finite element analyses of laminated composites were done in the present study with respect to suppression of free edge delamination by an innovative technique. Wrap-around configuration was considered to determine its effectiveness over the wrapper-less laminated configuration on laminated composites. Nodal stresses were generated ahead of the crack tip through finite element analysis. This was used for determining interlaminar normal stress and inter laminar shear stress distributions at the critical interface. Later virtual crack closure technique was used to estimate the strain energy release rate components for several sizes of virtual crack extensions through a single finite element analysis. Computational analysis predicts Mode-I delamination as dominant mode of failure. This mode of delamination was significantly suppressed with wrap-around configuration on laminated composites.

Damage occurrence at edges of non-crimp-fabric thin-ply laminates under off-axis uniaxial loading

Thin-ply based laminates are a promising development in composite materials and are expected in the near future to outperform conventional laminates in mechanical performance. A rational design with thin plies requires understanding the effect of ply thickness on each damage mechanism. This paper presents an experimental investigation into damage occurrence in a quasi-isotropic laminate made from thin-ply, bi-axial, Non-Crimp-Fabric (NCF), under different off-axis uniaxial loadings. The NCF layers are positioned through the laminate thickness creating two regions, namely THICK and THIN (with and without ply clustering). Then, the onset and progress of three damage mechanisms (transverse matrix cracking, matrix crack induced delamination and free-edge delamination) for both regions are analyzed by monitoring the specimen’s free-edge. The results show that the critical region where damage occurs is that with ply clustering (THICK), whereas delamination originating from matrix cracks or free edge effects are delayed or even suppressed in the THIN region.

Delamination damage analysis of laminated bonded tubular single lap joint made of fiber-reinforced polymer composite

Onset and growth of delamination damages in laminated fiber-reinforced polymer composite made bonded tubular single lap joints have been thoroughly studied through a finite element analysis-based simulation technique developed in the present research. The model provides sufficient scope for a comprehensive parametric study of the bonded structure in terms of three-dimensional stress analyses within the joint, studying the delamination damage onset and growth characteristics, and investigating its effect on stress distributions in the joint and both the adherends (outer and inner). Quadratic failure criterion has been used to depict delamination damage-induced cohesion failure within the adhesive layer. Whereas Tsai–Wu coupled stress criterion has been used for locating the delamination and adhesion failure prone regions at ply-interfaces of the adherends and adherend–adhesive interfaces within the joint, respectively. Three-dimensional stress analyses revealed that outer adherend of the bonded tubular single lap joints experiences comparatively higher interlaminar peel ([Formula: see text]) and shear stress ([Formula: see text]) concentrations at the free edge of the first ply-interface and is likely to delaminate at this edge. Accordingly, pre-embedded through-the-circumference delamination damages have been simulated at this edge, and contact finite element analyses have been performed in order to avoid interpenetration of delaminated surfaces. Strain energy release rate calculated using modified crack closure integral vis-à-vis virtual crack closure technique has been used as the characterizing parameter for assessing the growth of the damages. Free edge delamination in the outer adherend has been observed to propagate in a self-similar manner mainly in the in-plane shearing mode. It causes peel and shear stress concentrations in regions of the adherend–adhesive interfaces and first ply-interface of the inner adherend, localized to the clamped edge of the joint making the joint and the adherend prone to adhesion failure and delamination damage, respectively.

Tiled Composite Laminates

This article describes our efforts to develop a meso-scale in-plane tiling technique for fiber-reinforced composite laminates. Such a technique expands the design space of the laminate, and the ability to tailor local laminate properties may provide a means, for example, to mitigate stress concentrations that arise in places such as along free edges of the laminate. Preliminary fabrication of tiled laminates produced material with an elastic modulus that is nearly equal to that of continuously reinforced laminates; however, the strength of these first specimens was significantly reduced. Finite element analyses were then performed to characterize the effects of features unique to tiled composite laminates, such as the existence of resin-rich tile-to-tile interfaces, and to explore the effects of relative arrangement of tiles through the thickness of the laminate. This led to a novel composite joint geometry as well as recommendations to minimize strength reduction. Strength retention of laminates fabricated using the new design guidelines was experimentally found to exceed 92% in comparison with the traditional analogs. Finally, we discuss the potential application of composite tiling for the suppression of free-edge delamination.

A twofold strength and toughness criterion for the onset of free-edge shear delamination in angle-ply laminates

Free edge delamination in composite structures results from very localised stress fields which induce a stress concentration promoting the nucleation of an interfacial crack. To predict such a delamination onset at the free edge of a ( ± θ ) s laminate in traction, use is made of a strength and toughness criterion which combines a stress condition with an energy analysis. A generalised plane strain model allows to determine the stress distribution near the free edge and the energy released by the nucleation of an interfacial crack. The results show that this approach can predict the delamination onset for ( ( ± 10 ) s , ( ± 20 ) s ) laminates provided the interfacial fracture energy and interlaminar shear strength are known. These characteristic values can be identified with the help of traction tests performed on samples with different thicknesses.