Unidirectional Laminates Research Articles

The three-point off-axis bending method for determining in-plane shear properties in wood

Several methods to determine in-plane shear properties of anisotropic materials (i.e. fiber reinforced composite materials and natural wood) based on flexural loading have been proposed. One of those methods is the off-axis bending test that considers unidirectional composite materials subject to three-point bending (3-PB) loading. As a result of the anisotropic behaviour, unidirectional off-axis laminates subjected to flexure present a bending–twisting coupling that may cause, in the case of three-point bending loading, a lift-off of the specimen on the fixture supports. Such specimen lift-off has been considered one of the critical features to be considered on the mentioned experimental method. Besides, geometrical parameters and material elastic constants (i.e. longitudinal, transversal, and shear elastic moduli) influence the validity to use this method for characterizing in-plane shear properties. On that sense, an analytical approach for studying the material conditions for which the application of the off-axis 3-PB test is adequate to characterize natural wood is presented in this work. The condition of small displacements and the condition of lift-off between the specimen and the fixture supports have been taken into account. Some theoretical ideas regarding stress and displacement fields are also presented. The in-plane shear characterization has been performed on two natural wood species: Norway spruce (Picea abies) and beech (Fagus sylvatica). Test conditions for the off-axis 3-PB experimental bending test, in order to ensure its applicability, are presented.

Bending in two planes of unidirectional composite laminates

In this work bending in two planes of long fiber composite laminates is proposed as a test method for knowing the behaviour of this material in case of combined loads. Different samples have been manufactured and tested using carbon/epoxy T6T/F593 material. Three cases have been considered dendending on the value of the ratio between normal stress in fiber direction and normal stress in direction perpendicular to fibre: less tan 1, equal to 1 and greater than 1. In all cases failure has been related to matrix failure, being a linear relation between stress and deformation until failure. Stress values have been analized by three failure criteria: Rankine, Tsai-Wu and maximum strain energy. Rankine criteria fits well the first two cases. The parameter of Tsai-Wu criteria between normal stresses in fiber direction and normal stresses in direction perpendicular to fibre has been obtained using experimental results. When stress in fiber direction is much greater than stress in direction perpendicular to fiber, this small stress can be ignored in the calculus of maximum strain energy.

Fracture energy for orthogonal cutting in unidirectional CFRP at different cutting directions

A unidirectional carbon fibre reinforced polymer (CFRP) laminate is a composite material made up of strong parallel carbon fibres incorporated in a polymer matrix such as epoxy to provide high stiffness and strength in the fibre direction of the laminate. Unfortunately, the interlaminar or intralaminar plane of this material has a low resistance to damages as the fracture toughness of a unidirectional CFRP laminate is related to the energy dissipation during the orthogonal cutting. The aim of this study is on cutting a unidirectional CFRP along the longitudinal or transverse directions, characterizing orthogonal cutting forces and the related fracture energy. Orthogonal cutting is performed using braised carbide tools for a range of cutting depth of 10-100 ³m with a rake angle of 30° to quantify the cutting forces and to observe the fracture mechanisms. The fibre orientations have a significant impact on surface bouncing-back. For some fibre orientations, the energy balance model is applicable, deducting the reasonable value of fracture toughness due to high normal force (F t). Fibre subsurface damage and cutting forces during cutting are found to be strongly influenced by the cutting depth. The input energy of cutting is released in form of new surface energy, fibre breakage, high bending energy, and chip fracture energy.

A stiffness-based phenomenological model to predict the fatigue life of unidirectional CFRP laminates under tensile fatigue loadings

Fibre-reinforced composite materials can withstand large number of loading cycles compared to traditional materials. However, predicting the damage progress and strength loss in a composite structure due to fatigue is difficult because of the complex interaction of different failure mechanisms. This work investigates the decrease in the stiffness and strength in a high-performance undirectional CFRP laminate subjected to on-axis fatigue loads. A stiffness-based phenomenological model is proposed in order to evalute the damage growth due to fatigue through the longitudinal stiffness degradation. Assuming that the laminate ultimate strain is independent of the distributed damage caused by fatigue, the residual strength model can be deduced, or its integral form, the S-N curve. Thanks to this assumption, the residual stiffness model is related to the residual strength model, and thus it has reduced the experimental characterization of phenomenlogical-based approaches. The accuracy of the model is verified by comparing the experimental data against the derived S-N curves.

Crack suppression in glass epoxy hybrid L-bend composites through drawdown coating technique using nano and micro fillers

L-bend composite laminates are fundamentally weak against the crack growth in the through-thickness direction, particularly at the curvature region. To deal with this problem, an effort was made to suppress the crack growth by incorporating MWCNT, Al2O3, and SiC Micro fillers at the interface of unidirectional (UD) Glass fiber lamina, using a draw-down coating technique. The L-Bend Glass-Epoxy (GE) laminates were manufactured with the aid of a Hot pressing machine with a holding time being 10 min. The laminates were prepared with a manufacturing temperature of 130 °C and a pressure of 10 kg/cm2. GE curved laminates added with Nano and Micro fillers in the range of 0.5–2 wt% were manufactured. Four-point bending fixtures were designed and manufactured to facilitate the holding and loading of the specimen in the uni-axial tensile machine. Tests were conducted in accordance with ASTM D6415 and load-displacement plots were carefully recorded.GE laminates added with 0.5 wt% fillers greatly influence Curved Beam Strength (CBS) and interlaminar stresses for delamination as compared to pristine GE laminates. The laminates loaded with 0.5 wt% SiC exhibited higher interlaminar radial stresses (108.6 MPa) compared to other GE laminates.

Effect of Graphene Coating of Carbon Fibers on the Fracture of Uni-directional Laminates

High specific strength and stiffness of carbon fiber reinforced polymer (CFRP) composites make them the preferred structural materials for a wide range of applications. The interfacial region between the matrix and reinforcing phases plays a critical role in determining the performance of CFRPs under mechanical loading. Hence, enhancing the interfacial bonding between the epoxy and the fibers is expected to improve the strength of the resulting composites. In the current study, a coating of graphene nano-platelets (GNPs) on unidirectional carbon-fibers has been made with the aim to strengthen the interface/interphase between the fiber and the matrix. Vacuum assisted resin transfer molding technique was used to manufacture the laminates after the solvent spray coating of the GNPs onto carbon fiber. The uncoated and GNPs coated CFRPs have been subjected to tensile tests followed by detailed fractographic analysis. Uniaxial tensile strength and Young’s modulus of the laminates have been found to improve with GNP addition. The fractographs reveal debonding gaps between fiber and epoxy in cross-sectional fracture surfaces and adherence of the epoxy with fibers in longitudinal fracture surfaces. Laminates made with fibers with a coating of GNPs showed smaller debonding gaps and larger epoxy adherence on the fibers than the pristine laminates. A larger interphase for the former laminate was found using carbon linear elemental mapping with the help of SEM equipped with energy dispersive X-ray spectroscopy.

Graphene fibre film/polydimethylsiloxane nanocomposites for high-performance electromagnetic interference shielding.

Exploration of high-performance electromagnetic interference (EMI) shielding materials has become a trend to address the increasing electromagnetic (EM) wave pollution environment. In this paper, oriented graphene fibre film (GFF)/polydimethylsiloxane (PDMS) nanocomposites with one-ply unidirectional, two-ply cross-ply, and two-ply unidirectional configurations were prepared using wet-spinning and hot-pressing techniques in a two-step process. Due to the anisotropic electrical performance of GFF, the one-ply laminate exhibits EMI shielding anisotropy that is affected by fibre orientation relative to the electric field component in EM waves. The maximum shielding difference at 8.8 GHz is up to 32.0 dB between the fibre orientation parallel to and perpendicular to the electric field component. In addition, we found that adding a layer of GFF is an intuitive method to enhance the shielding efficiency (SE) of GFF/PDMS nanocomposites by providing more interfaces to enhance absorption losses. An optimal EMI shielding performance of a two-ply unidirectional laminate is observed with an SE value of 50.6 dB, which shields 99.999% of EM waves. The shielding mechanisms are also discussed and clarified from the results of both experimental and theoretical analyses by adjusting the GFF structural parameters, such as the fibre orientation, areal density, number of plies and stacking sequence.

3D explicit simulation of bearing failure in metal–composite bolted joints

Hybrid bolted joints between composite materials and metals are an example of composites’ sensitivity to notches. The current research paper presents an experimental test setup on single-lap shear bolted joints of a unidirectional (UD) composite laminate with steel plate exposed to a tensile load. A 3D explicit finite element (FE) simulation is developed using ABAQUS/Explicit and a 3D physically-based progressive damage model in the VUMAT subroutine. The model can identify the locations of delamination represented by the fully damaged plies among other degrading properties. Then “damage and cohesive” contact surfaces are defined in the respected damaged plies, for the material interface, to simulate interlaminar damage and delamination. The predicted failure load was significantly influenced by the combination of a 3D physically-based progressive damage model and damage and cohesive surfaces. The combined model can reproduce with accuracy the experimental load–displacement test curves up to the point where bearing damage occurs. Experimental results using DIC measurement are presented to demonstrate the feasibility of the methodology. The study shows that numerical models can be used to help in the stiffness and strength design of the bolted joints of composites and metals, but further improvements are necessary to predict the energy absorption.

Effects of inter-ply mismatch angle on interlaminar properties and their influence in numerical simulations

In this study, the interlaminar shear strength and mode II fracture toughness of carbon-epoxy laminates with respect to different inter-ply mismatch angles are determined and shown to generally decrease as inter-ply mismatch angle increases. Despite this dependency, most numerical simulations of multi-directional laminates involving delamination adopt interlaminar properties measured from unidirectional laminates only. Using the derived angle-dependent shear strength and fracture toughness, computational simulations of short-beam shear and out-of-plane loading of multi-directional laminate plates were performed and compared with physical experiments. It is shown that delamination tends to form intensively at interfaces with larger mismatch angles where delamination resistance is lower. This characteristic is captured by assigning interlaminar properties corresponding to the inter-ply mismatch angles in the numerical simulation. The practice of applying interlaminar properties obtained from tests on unidirectional samples may lead to erroneous predictions of varying severity for the simulation of multi-directional laminates.

Low-velocity impact behaviours of AFP manufactured fibre metal laminate structures

This study reports the findings on the impact behaviour of metal-composite structures. Carbon fibre/Epoxy composite laminates were manufactured by using the Automated Fibre Placement (AFP) technique. Subsequently, hybrid structures were fabricated by bonding fibre laminate and aluminium plate together. The hybrids were tested at low velocity impact energies to probe the role of AFP composite laminates on the performance of the hybrid system. Our findings show that the bonding of the composite onto aluminium plate increased the natural frequency (stiffness) of the system and its energy absorption capability by almost 40 %. It was also found that the fibre layout of AFP composite significantly improved the central deflection of the hybrid. For instance, the deflection of cross-ply hybrids was 15% less in comparison to their counterparts with unidirectional fibrous composites. Nevertheless, under the investigated energies, the fibre layout appeared to have a marginal influence on the peak contact force. Post-mortem examination showed that at an impact energy of 150 J there was no fibre failure observed in asymmetric FMLs with unidirectional composite laminates. Conversely, the fibre failure in cross-ply hybrids occurred at 150 J. This suggests that less energy is required to induce fibre failure energy in cross-ply hybrids when compared to their unidirectional hybrid counterparts. Furthermore, it was also found that substrate treatment was successful in preventing excessive delamination, even though its influence on the peak contact force was not significant.

Mechanical characterisation of pneumatically-spliced carbon fibre yarns as reinforcements for polymer composites

An investigation into the mechanical response of pneumatically-spliced carbon fibre yarns as a potential reinforcing material for polymer composites is presented. High strength mechanical connections between carbon fibre yarns are produced by joining short discontinuous tows into longer lengths via fibre entanglement. The effect of altering the number of high-pressure air pulses fired by a commercially available (Airbond 701H) splicing machine, to form the tow-tow connection, on load bearing capacity and linear stiffness is first evaluated on splices between virgin T700SC-24K-50C carbon fibre tows. The best performing spliced configuration is subsequently utilised in reinforcing unidirectional epoxy laminates, which are mechanically characterised, and their properties compared to those of various continuous fibre and chopped strand mat panels. Results presented in this study demonstrate that pneumatic splicing provides a high strength and sustainable solution for reinforcing polymers with discontinuous (approx. >50 mm in length) virgin, off-cut or waste carbon fibre yarns. It is speculated that with further research, quasi-continuous yarns remanufactured by splicing waste fibres could provide a novel material for weaving, braiding, non-crimp fabrics, or use in 3D printing applications.

Experimental observation and modeling of resin pocket cracking in unidirectional laminates with ply discontinuity

Experimental observations revealed that a resin pocket in a unidirectional laminate with a discontinuous ply may form several cracks prior to initiation of delamination cracks. The observed cracks typically occur within the resin pocket, rather than at the interfaces with the discontinuous ply, and are curved towards the center of the pocket. To study this unintuitive cracking behavior, a simple variational stress analysis based on the principle of minimum complementary energy is developed. It allowed to estimate the stress field in the laminate with a discontinuous ply, with or without cracks in the resin region. The observed cracking pattern is explained by assuming a higher level of residual stresses due to chemical shrinkage of the resin during curing in the confined conditions of the pocket. It is shown that the crack locations are best determined using the principle of maximum energy release rate, while the curved crack path can be governed by the distribution of the maximum principle stress. The predicted cracking sequence and the crack curvatures agree well with experimental observations.

Fatigue delamination behaviour of carbon fibre/epoxy composites interleaved with thermoplastic veils

Interleaving thermoplastic veils has proved to enhance the interlaminar fracture toughness of carbon fibre/epoxy composites under static loading conditions. However, the fatigue delamination behaviour has yet to be investigated. Herein, meltable Polyamide-12 (PA) veils and non-meltable Polyphenylene-sulphide (PPS) veils were used for interlay toughening of unidirectional (UD) and non-crimp fabric (NCF) laminates that were manufactured using a prepreg process and resin transfer moulding process, respectively. The results of Mode-I fatigue delamination tests demonstrated a significant improvement in the fatigue life of the laminates due to interleaving. Additionally, the fatigue resistance energy has been maximumly increased by 143% and 190% for the UD and NCF laminates, respectively. The microscopy analysis revealed that the toughening mechanisms of thermoplastic veils were affected by the form of the thermoplastic veils in the laminates (melted or non-melted), the fracture mechanisms of the reference laminates and the adhesion/miscibility between the thermoplastic veils and the epoxy.

Simulating delamination in composite laminates involving large scale fiber bridging based on the mixed-mode three-linear cohesive zone model

A three-linear cohesive zone model (CZM) superposed by two bilinear laws for modelling mixed-mode delamination with the effect of large scale fiber bridging is proposed in this study. The parameters in the formulation can be obtained from pure mode delamination tests or selected by reliable guidelines, making the CZM easy to be implemented in the finite element model. A novel method to determine the bridging strength by applying varied values in the different regions of the prearranged delamination path is developed. The division of the region is based on the concept of the length of the fracture process zone (FPZ). Simulations of mode I delamination in unidirectional and multidirectional composite laminates and mixed-mode I/II delamination in multidirectional laminate are performed in the finite element analysis software. Good agreement between the predicted load–displacement responses and the corresponding experimental results are achieved by adopting the established CZM and the bridging strength determination method, validating their efficiency in predicting the delamination behaviors under both pure mode and mixed-mode loadings.

An X-ray computed tomography analysis of damage induced by thermal cycling in non-crimp fabric composites

In this work, the micro-crack formation and propagation mechanisms are characterized by X-ray computed tomography (XCT) in non-crimp fabric (NCF) laminates produced by resin transfer moulding (RTM) and subjected to thermal cycling between −55 °C and 72 °C. The low crimped textile structure has comparable in-plane mechanical properties to their traditional unidirectional angle-ply laminates. However, the presence of the stitching thread induces the generation and progression of damage in the form of matrix cracks triggered by the temperature drop. In this study, matrix cracks are extracted from X-ray computed tomography volumes, using machine learning (ML) algorithms, which helps to deliver robust and reliable crack segmentation as well as geometrical features of the damage shape, size, spatial distribution and density. This information allows understanding damage progression.

An ABAQUS® plug-in for generating virtual data required for inverse analysis of unidirectional composites using artificial neural networks

This paper presents a robust ABAQUS® plug-in called Virtual Data Generator (VDGen) for generating virtual data for identifying the uncertain material properties in unidirectional lamina through artificial neural networks (ANNs). The plug-in supports the 3D finite element models of unit cells with square and hexagonal fibre arrays, uses Latin-Hypercube sampling methods and robustly imposes periodic boundary conditions. Using the data generated from the plug-in, ANN is demonstrated to explicitly and accurately parameterise the relationship between fibre mechanical properties and fibre/matrix interphase parameters at microscale and the mechanical properties of a UD lamina at macroscale. The plug-in tool is applicable to general unidirectional lamina and enables easy establishment of high-fidelity micromechanical finite element models with identified material properties.

Finite element modeling of crushing of CFRP cylindrical tubes under low-velocity axial impact

The dynamic axial crushing response of cylindrical tubes made of unidirectional (UD) carbon fiber-epoxy material is investigated numerically with emphasis on generating realistic crushing modes and crushing morphologies besides predicting crushing force with good accuracy. The emphasis is on the crushing modes since only the strong similarity between predicted crushed modes by the FE model and experimental observations can assure that the FE model dissipates the energy in the same manner that real composite absorbers dissipate it during the experimental trial. The impact behavior of two multi-layer models of laminated cylindrical tubes having different stacking sequences, namely cross-ply and angle-ply, were simulated using stacked layers of conventional 2D shell elements (S4R) bonded with the cohesive element interface model in ABAQUS. The FE progressive failure model of UD composite layers, including the damage initiations, damage evolution law, and element removal, is executed through a user-defined material subroutine (VUMAT) in ABAQUS/Explicit code. The implemented progressive failure material model represented well the various crushing modes, including fragmentation mode, splaying mode, and axial splitting of UD lamina in the splaying crushing mode. The FE model was validated by experimental test results obtained by the authors for cross-ply and angle-ply tubes subjected to axial impact.

An experimental investigation on bearing behavior and failure mechanism of bolted composite interference-fit joints under thermal effects

This paper reports an experimental investigation on bearing behavior and failure mechanism of single-lap bolted composite interference-fit joints under thermal effects. To achieve this target, testing temperatures ranged from −25 to 110 ℃ were designed to simulate thermal service environments. The unidirectional composite lamina was tested longitudinally and transversely to figure out potential change of material properties induced by thermal loading. The residual effects of stacking sequence, interference-fit size and tightening torque on bearing strength and stiffness of composite joints after exposed to thermal conditions were evaluated. Microscopic studies applying X-ray technique on damage areas to monitor damage evolution were conducted to comprehend the thermal failure mechanism. The results evidence that the bearing performance of composite joints was slightly enhanced by low temperature due to cold hardening of matrix resin but greatly weakened by high temperature owing to the combined effects of heat softening of resin, residual stress relaxation and thermal expansion of joint components. Tightening torque to enhance bearing performance is the most beneficial at room temperature, whereas the interference-fit remains the dominant influence at sub-freezing environments. In conclusion, thermal loading plays an important role in bolted composite interference-fit joints. It is an unneglectable factor that would enlarge damage area, alter failure mode and should be understood thoroughly.

Fatigue life prediction of high-speed composite craft under slamming loads using progressive fatigue damage modeling technique

A reliable predictive method is developed based on the progressive fatigue damage modeling technique to estimate the fatigue life of high-speed composite craft. In this method, at each loading cycle, the occurrence of different failure types would be inspected and based on the failure type, either gradual or sudden degradation rules would be followed to update the material properties. This procedure continues until the final failure of the structure. To implement this method, a MATLAB code is developed based on material degradation rules which integrating with the Abaqus it can be used for complex geometries. This method enables us to predict the fatigue life of composite structures with different layup configurations under arbitrary loading conditions only by knowing the static and fatigue behavior of unidirectional laminates. To even more reduce the number of required tests, the energy-based unified fatigue life model is employed. Owing to the low number of required tests, this method can be very cost-effective in the design stage of composite structures that are intended to endure cyclic loads.

Mode I translaminar fracture toughness of high performance laminated biocomposites reinforced by sisal fibers: Accurate measurement approach and lay-up effects

The present work performs a systematic experimental analysis of the translaminar fracture behavior of high performance biocomposites constituted by green epoxy reinforced by sisal fibers, by varying the main influence parameters as fiber concentration and lay-up. Despite the corrective function properly introduced to take into account the anisotropy as well as the use of the equivalent crack length, the study shows that the LEFM does not give accurate estimations of the fracture toughness, because the extension of the near tip damaged zone is higher than the singular dominated one. Accurate estimations can be obtained instead by the proposed modified area method that takes into account both the local damage and the fiber bridging that occurs during crack propagation, that lead to R-curves whose asymptotic values constitute the true fracture toughness of the biocomposites examined. The constancy of the damage mechanisms observed by varying the fiber concentration, allows the user to compute the fracture toughness of a generic laminate from the specific fracture energy of the unidirectional lamina. Finally, the relatively high fracture toughness of the examined laminates allows to state that they can advantageously replace not only other composites having lower toughness, but also metals as steel, aluminum and titanium.