Unidirectional Non-crimp Fabrics Research Articles

A hyperelastic approach for modeling the membrane behavior in finite element forming simulation of unidirectional non-crimp fabrics (UD-NCF)

Unidirectional non-crimp fabrics (UD-NCFs) are highly suitable for high-performance components due to their excellent lightweight potential. However, during forming they are prone to wrinkling and gapping compared to woven or biaxial textiles. Macroscopic simulation models can be used to efficiently predict these effects as well as the global forming behavior for complex geometries. Therefore, a new hyperelastic membrane model is proposed to describe the typical deformation of UD-NCFs based on superimposed shear, transverse tension and compression perpendicular to the fiber rovings. The model is parameterized using the forces and different ratios of superimposed strains obtained in experimental off-axis-tension-tests at bias angles of 30°, 45°and 60°. The resulting approach is validated by forming simulations of a hemisphere and tetrahedron geometry in different configurations and quantitative comparison to experimental tests. The model accurately predicts the forming behavior of UD-NCF with a good agreement of the global deformation behavior and local strains.

Effect of strain rate on the in-plane orthotropic constitutive response and failure behaviour of a unidirectional non-crimp fabric composite

A comprehensive experimental campaign was performed to characterize the strain rate-dependent orthotropic constitutive response and failure behavior of a unidirectional non-crimp fabric (UD-NCF) carbon fiber/snap-cure epoxy composite. Shear and transverse compression strengths increased by 60% and 50%, respectively, over the range of strain rates considered, which is consistent with previously reported trends for unidirectional tape composites (UDTCs). The transverse tension strength increased by 18% with increasing strain rate, which is different from UDTCs. Owing to the distinct microstructure of UD-NCF composites, local damage mechanisms such as pullout of transverse supporting glass fibers and stitching were observed and are deemed to contribute to the rate dependency of transverse tension strength. The longitudinal compression strength increased by 35% with increasing strain rate. Observation of failed specimens revealed widespread inter-tow axial splitting and localized tow kinking, distinct from UDTCs. This important finding will aid in the development of failure criteria for UD-NCF composites.

Shear deformation characterization and normalized method of tricot-stitched unidirectional non-crimp fabric

Currently, the shear deformation of unidirectional non-crimp fabric (UD-NCF) is investigated sparsely, especially for the normalized method. Thus, this work systematically explored the shear behavior of UD-NCF by the off-axis extension test under different off-axis angles. The evolution rule of macro shear mechanical behavior was discussed and the effect mechanism of off-axis angle on the extension force degree and its standard deviation was revealed. Then, by analyzing the fabric morphology and deformation contour, the supporting yarn with the relatively large linear density will affect the shear deformation of UD-NCF and induce the meso buckling defect of main yarn. Most importantly, the developed normalized shear force methods suitable for the UD-NCF were derived, and the difference of normalized shear force among developed and reported methods was discussed and identified.

Suppression of compression induced delamination in tapered laminated composites using a ply scarfing method

Thickness tapering of composite laminates is a common strategy for efficient and lightweight composite structures. The termination of specific plies necessary to achieve such tapering is well known to introduce sites for delamination initiation in laminates. This propensity for delamination is highly dependent on the thickness of the terminated ply, which presents a significant challenge in using thicker materials to improve the production rate of large-scale composite structures.In this work, the effectiveness of ply scarfing in improving the compressive failure stress of tapered laminates made using thick, unidirectional non-crimp fabrics (NCF) was experimentally investigated for the first time. Unidirectional tapered laminates were manufactured using two different NCFs with fibre areal weights of 620 and 1070 g/m2 via resin infusion. Specimens made of the 1070 g/m2 material which incorporated a ply terminated using the ply scarfing method were compared with those including plies terminated using the conventional ply-drop method. which incorporated a ply terminated using the ply scarfing method. The compression failure behaviour of the specimens was analysed using a high-speed camera. The results showed that ply scarfing suppressed delamination of the belt plies under compression and increased the failure stress by ∼60%. Furthermore, the tapered geometry of the terminated ply’s end was effective in preventing the void entrapment at the ply drop and significantly reducing the manufacturing variability.

How epoxy cure profiles (may) impact performance of non-crimp fabric composites

Glass fibre reinforced polymers made from quasi uni-directional non-crimp fabrics have recently been shown to suffer from a specific damage mechanism. This damage mechanism is dominated by off-axis cracks in the off-axis backing bundles of the fabrics. In addition, research results have indicated that the onset and propagation of such off-axis cracks may be influenced by the residual stresses that arising from the manufacturing process used to produce the Glass fibre reinforced composites. The current article investigates the potential effect of residual stresses on the fatigue life through an experimental set-up that involves tailored cure profiles and image based evaluation of tensile test experiments.

Characterising the shear resistance of a unidirectional non-crimp glass fabric using modified picture frame and uniaxial bias extension test methods

The forming behaviour of a unidirectional non-crimp fabric (UD-NCF) consisting of polyamide stitches with a tricot-chain stitching pattern is explored. Notably, there are no stabilising tows orientated transverse to the main tow direction in this fabric, a common feature in many ‘quasi’ UD-NCFs, this allows extension of the stitch in the transverse direction under certain loading conditions. The lack of stabilising tows introduces a possible low-energy deformation mode to the UD-NCF, which is absent in biaxial fabrics and to a large extent in ‘quasi’ UD-NCFs. The in-plane shear behaviour is initially investigated using both standard ‘tightly-clamped’ picture frame tests and uniaxial bias extension tests. Preliminary results show a dramatic difference in results produced by the two test methods. During the picture frame test, fibres can be subjected to unintended tension due to sample misalignment in the picture frame rig. To mitigate error arising from this effect, the picture frame test procedure is modified in two different ways: by using an intentional pre-displacement of the picture frame rig, and by changing the clamping condition of test specimen. Results show that the modified picture frame test data contain less error than the standard ‘tightly-clamped’ test but also that the shear stiffness of the UD-NCF is notably lower when measured in the bias extension test compared to the picture frame test, mainly due to the difference in loading conditions imposed during the two tests.

Characterizing and modelling the coupled in-plane shear-biaxial tension deformation response of unidirectional non-crimp fabrics

Characterizing and modelling the coupled in-plane shear-biaxial tension deformation response of unidirectional non-crimp fabrics

Experimentally verified dual-scale modelling framework for predicting the strain rate-dependent nonlinear anisotropic deformation response of unidirectional non-crimp fabric composites

A new hierarchical dual-scale modelling framework is developed for predicting the strain rate-dependent nonlinear deformation response of a unidirectional non-crimp fabric (NCF) carbon fiber/snap-cure epoxy composite. A linear Drucker-Prager model that considers tension–compression asymmetry of the yield surface was used to capture the elastic–plastic response of the epoxy within the microscale finite element (FE) model, while randomly dispersed carbon fibers were treated as linear elastic. For the mesoscale FE model, the effective elastic–plastic response of the impregnated tow was modelled using Hill’s anisotropic yield function. The post-yield strain rate dependency of the epoxy and impregnated tow was captured with the Johnson-Cook model, with the former being calibrated using available experimental data. The predicted post-yield response of the impregnated tow was found to be dependent on the applied strain rate. An excellent agreement was observed between the predicted and experimentally measured stress–strain response for the NCF composite at quasi-static strain rates. The predicted in-plane and out-of-plane shear stress–strain responses revealed a strong dependence on the applied strain rate. The newly developed dual-scale modelling framework can be utilized as a tool to effectively predict the strain rate-dependent non-linear response for other fabric-reinforced composite material systems.

Prediction of Damage in Non-Crimp Fabric Composites Subjected to Transverse Crushing: A Comparison of Two Constitutive Models

Non-crimp fabrics (NCFs) are increasingly used in industry for manufacturing of composite structures due to a combination of high mechanical properties and excellent manufacturability. As with other composites, in-service damage can be a cause for severe reduction in load-carrying capacity of NCF-reinforced plastics. In this experimental and numerical study, two constitutive material models previously used only for damage prediction in unidirectional (UD) tape and woven fabric-reinforced materials (LS-DYNA’s *MAT_ENHANCED_COMPOSITE_DAMAGE—MAT54 and *MAT_LAMINATED_COMPOSITE_FABRIC—MAT58) were evaluated for simulating transverse crushing of composite parts processed from a non-crimp carbon fabric. For this purpose, UD NCF components of tubular shape were subjected to transverse crushing through a controlled indentation of a metallic cylinder, and results of the experiment were compared with numerical modeling. Considered verification metrics included the observed and the predicted patterns of interlaminar damage, the extent of delamination, as well as the ability of the models to replicate force-displacement response exhibited by the tested specimens.

Investigation of the Membrane Behavior of UD-NCF in Macroscopic Forming Simulations

Unidirectional non-crimp fabrics (UD-NCF) provide an exceptionally high lightweight potentialcompared to other dry fabrics. However, their defect-free formability is limited compared towoven or biaxial fabrics due to their susceptibility to draping effects like wrinkling, gapping andfiber waviness. To predict these local effects and the global forming behavior efficiently, macroscopicmodelling approaches require the consideration of the mesoscopic material structure. A very detailedmacroscopic approach was proposed by Schirmaier et al. [1] and its prediction accuracy validatedwith qualitative and quantitative comparisons to component forming results. However, the approachcouples several deformation modes and therefore requires a very high number of inversely determinedmaterial parameters. In the present work a new macroscopic forming model for the membrane behaviorof UD-NCF is introduced based on superimposed shear, transverse tensile and perpendicular to thecarbon fiber tows oriented compressive strains. The model is parametrized with experimental resultsof different off-axis-tension-tests (OATs) (30°, 45° and 60°) and compared to the model proposed bySchirmaier et al. [1]. The results of the new membrane model agree well with the experimental andsimulative results in large areas, while utilizing a significantly reduced number of material parameters.However, some limitations are identified due to the reduced complexity of the model.

Comprehending the Bending: A Comparison of Different Test Setups for Measuring the Out-of-Plane Flexural Rigidity of a UD Fabric

To simulate the forming process of a unidirectional (UD) glass fiber non-crimp fabric (NCF) for wind turbine blades, the fabric material needs to be characterized. In this way, input to the material model of the simulation can be generated. One important deformation to characterize is the out-of-plane bending. A number of test setups have been described in the literature but it is not clear which setup is more appropriate for the UD NCF under consideration. Therefore, five different out-of-plane bending test setups are compared and discussed, namely Peirce's cantilever test (with an inclined plane), the free-hanging cantilever test, a vertical cantilever test, a rotary rheometer bending test and a buckling test. For the latter an inverse modeling approach is developed. The test results shows that similar flexural rigidities can be computed from the different test setups but that some values are sensitive to the parameters used in the data processing techniques.

Design guidelines for hybrid continuous/discontinuous carbon fibre laminates

Combining discontinuous carbon fibre moulding compounds with unidirectional (UD) non-crimp fabric has been studied as a potential solution for producing cost effective composite structures, with short cycle times via compression moulding. The tensile stiffness and strength of a composite with a hybrid fibre architecture were found to be up to 60% and 110% higher, respectively, than the baseline discontinuous fibre moulding compound, when 20% (by vol.) of UD fibres were aligned in the primary loading direction. However, stress concentrations arising from the dissimilar material properties can result in the strength of the hybrid architecture being reduced to below that of the baseline composite if the UD fibres are not incorporated effectively. This paper focusses on optimising the stress transfer within the transition zone (inter-material interface) between the UD material and the discontinuous fibre moulding compound, when local isolated patches of continuous fibre are used rather than full coverage plies. This is key if hybrid architectures are to be adopted for structural applications, minimising the stress concentration effects from the UD ply ends, whilst maintaining the low cycle times and costs associated with compression moulding. Following aerospace laminate design guidelines, a range of step sizes and joint configurations have been considered for different ply drop strategies, minimising the magnitude of the stress concentration at the UD ply ends. Results indicate that the step length must be at least 20 times the thickness of the UD ply to ensure the bending strength at the joint is the same as the baseline discontinuous fibre material. An ‘alternating joint' yields 18% higher bending strengths than a conventional ‘stepped joint’ design, as the stress concentrations at the dropped ply ends are reduced in comparison to the inherent stress concentrations within the discontinuous fibre material.

Characterizing the macroscopic response and local deformation mechanisms of a unidirectional non-crimp fabric

The macroscopic response and local deformation mechanisms of a warp stitched heavy-tow unidirectional non-crimp fabric (UD-NCF) was characterized. Quasi-static uniaxial tests captured the fabric in-plane longitudinal, transverse, and shear deformation response where digital image correlation was employed. The fabric bending response was captured through modified cantilever tests. Off-axis extension tests conducted along 30°, 45° and 60° bias directions revealed that extension and contraction of the polyester stitching web and resulting transverse compression of the carbon fiber tows and inter-tow sliding were the dominant local deformation mechanisms. At a bias angle of 30°, the extent of inter-tow sliding increased when compared to a bias angle of 45° leading to earlier onset of fabric shear locking, while at a bias angle of 60° shear locking also occurred sooner. Shear angle maps and strain profiles at different bias angles further revealed the notable inter-dependency between the fabric shear, longitudinal and transverse deformations, which was governed by the interactions between the stitching and carbon fiber tows. The distinct out-of-plane bending behavior of the fabric reinforcement along longitudinal and transverse directions was also captured and quantified. The study provided a comprehensive characterization of the UD-NCF, which is critical for the development of constitutive models.

Fatigue and Damage Assessment of CFRP Material Using Digital Image Correlation

Fatigue testing of a Carbon Fiber Reinforced Polymer (CFRP) in tension-tension loading has been conducted. In-situ surface strain measurements were performed to examine the gradual elongation of the specimen as this relates to stiffness loss and fatigue damage. A methodology capturing the specimen at peak load has been developed, including an automated trigger mechanism that activates the camera at the desired cycle count. The material tested was a Unidirectional (UD) Non-Crimp Fabric (NCF) with carbon fibers and an epoxy matrix. The fatigue test results revealed a wide scatter in the mid-range of the high cycle fatigue region. By studying the strain in the early fatigue loading cycles and the stiffness loss over time, benchmark of the fatigue performance between different material samples could be carried out, explaining the scatter in the fatigue testing. It could be observed that the fatigue limit of the UD CFRP material in the fiber direction is in the magnitude of 80 % of the material's Ultimate Tensile Strength (UTS).

Effect of Strain Rate on the Transverse Tension and Compression Behavior of a Unidirectional Non-Crimp Fabric Carbon Fiber/Snap-Cure Epoxy Composite.

The strain rate-dependent behavior of a unidirectional non-crimp fabric (UD-NCF) carbon fiber/snap-cure epoxy composite loaded along the transverse direction under quasi-static and dynamic conditions was characterized. Transverse tension and compression tests at quasi-static and intermediate strain rates were performed using hydraulic testing machines, while a split Hopkinson pressure bar (SHPB) apparatus was used for transverse compression tests at high strain rates. A pulse shaper was used on the SHPB apparatus to ensure dynamic equilibrium was achieved and that the test specimens deformed homogenously with a nearly constant strain rate. The transverse tensile strength at a strain rate of 16 s−1 increased by 16% when compared to that at quasi-static strain rates, while distinct localized fracture surface morphology was observed for specimens tested at different strain rates. The transverse compressive yield stress and strength at a strain rate of 325 s−1 increased by 94% and 96%, respectively, when compared to those at quasi-static strain rates. The initial fracture plane orientation for the transverse compression tests was captured with high-speed cameras and found to increase with increasing strain rate. The study provides an important data set for the strain rate-dependent response of a UD-NCF composite material, while the qualitative fracture surface observations provide a deeper understanding of the failure characteristics.

Development of a Shape Replicating Draping Unit for Continuous Layup of Unidirectional Non-Crimp Fabrics on Complex Surface Geometries

The manufacturing of large-scale structural components is still dominated by manual labor in many sectors of the modern composite industry. Efforts are being made to establish an automated layup technology for complex structural elements. Processing dry non-crimp fiber fabrics (NCF) offers great cost opportunities and high deposition rates, compared to prepreg-based technologies like automated fiber placement (AFP). Here, the fabric architecture is considered during the draping of the plane textile on curved surfaces. In this paper, the development of a draping unit for balancing fabric tension and consolidating continuously across the layup width is presented. We introduce a geometrical process model to achieve a fabric-friendly draping of the used unidirectional NCF. The shape of the resulting draping front depends on the surface geometry, the shearing of the previously laid-up textile, and the positioning of the material feed. To consolidate the fabric at the altering draping front in an automated layup process, the shape of the continuous consolidation element can be controlled by the elongation of serial soft actuators, manipulated by parallel robot kinematics. The shape replication ability of the draping unit is promising for the implementation of a continuous, fabric-friendly draping process for complex surface geometries.

Prediction of forming effects in UD-NCF by macroscopic forming simulation – Capabilities and limitations

Unidirectional non-crimp fabrics (UD-NCF) provide the highest lightweight potential among dry textile materials. Compared to multiaxial NCF, the fabric layers in UD-NCF enable a more targeted tailoring. Compared to woven fabrics, the fibres of UD-NCF are straight without weakening undulations. However, the formability of UD-NCF is more challenging compared to woven fabrics. The yarns are bonded by a stitching and the deformation behaviour highly depends on this stitching and on the slippage between the stitching and the fibre yarns. Moreover, distinct local draping effects occur, like gapping and fibre waviness, which can have a considerable impact on the mechanical performance. Such local effects are particularly challenging or even impossible to be predicted by macroscopic forming simulation. The present work applies a previously published macroscopic UD-NCF modelling approach to perform numerical forming analyses and evaluate the prediction accuracy of forming effects. In addition to fibre orientations and shear angles, as investigated in previous work, the present work also provides indication for fibre area ratios, gapping, transverse compaction and fibre waviness. Moreover, the prediction accuracy is validated by comparison with experimental tests, where full-field strains of inner plies are captured by prior application of dots onto the fibre yarns, by measuring them via radiography and applying a photogrammetry software. The modelling approach provides good prediction accuracy for fibre orientations, shear strains and fibre area ratio. Conversely, normal fibre strains, indicating fibre waviness, and transverse strains, indicating gapping, show some deviations due to the multiscale nature of UD-NCF that cannot be captured entirely on macroscopic scale.

Deformation characteristics and formability of a tricot-stitched carbon fiber unidirectional non-crimp fabric

Deformation characteristics and formability of a heavy-tow unidirectional non-crimp fabric (UD-NCF) was studied. The fabric response under in-plane shear and biaxial tension loading (experienced during forming operations) was first investigated. Local deformations captured by measuring tensile/compressive strains in the stitching segments traversing the carbon fiber tows was correlated to the fabric three-stage macroscopic response during shear tests. Biaxial tension tests were performed at room and elevated temperatures with different displacement ratios to mimic typical preforming processes. Variations in biaxial displacement ratio revealed the interdependency of the fabric orthogonal tensile deformation modes, while temperature did not influence the response. Next, single layer hemispherical and flat punch draping experiments were performed to monitor defects (wrinkling) and capture strain contours, which correlated with in-plane shear and biaxial tension deformations. The results yield an improved understanding of the complex multiscale deformation modes for the heavy-tow UD-NCF and represent important data sets for constitutive models.

A multiscale framework for predicting the mechanical properties of unidirectional non-crimp fabric composites with manufacturing induced defects

The aim of this study was to evaluate the effect of manufacturing induced in-plane tow misalignment and out-of-plane tow crimp on the mechanical properties of a heavy-tow unidirectional non-crimp fabric (UD-NCF) composite. The elastic constants and failure onset (strength) are predicted by employing a multiscale computational approach. Micro-scale finite element (FE) models that explicitly represent the fibers and matrix within the tow microstructure were used to predict the effective properties of the tow. Meso-scale FE models comprised of the homogenized tows and surrounding matrix were used to predict the properties of a UD NCF composite lamina. Four meso-scale models, identified as ideal, crimp, misalignment and real, were considered in this study. No manufacturing defects were represented in the ideal model, while out-of-plane crimp, in-plane misalignment and both out-of-plane crimp and in-plane misalignment were accounted for in the crimp model, misalignment model and real model, respectively. Predicted lamina stiffness based on the real model are found to be in an excellent agreement with available experimental data, which was not always the case for the other three models. The longitudinal and transverse strength predictions are found to be dependent on the chosen local failure criteria for each model. Max-stress and Puck’s fiber failure criteria provide an excellent estimate of longitudinal strength while the Puck’s inter-fiber failure and Tsai-Hill criteria predict transverse strength with good accuracy. The feasibility to accurately predict the mechanical properties of heavy tow non-crimp fabric composites by incorporating their inherent micro-structural defects is demonstrated in this study.

The Impact of Draping Effects on the Stiffness and Failure Behavior of Unidirectional Non-Crimp Fabric Fiber Reinforced Composites.

Unidirectional non-crimp fabrics (UD-NCF) are often used to exploit the lightweight potential of continuous fiber reinforced plastics (CoFRP). During the draping process, the UD-NCF fabric can undergo large deformations that alter the local fiber orientation, the local fiber volume content (FVC) and create local fiber waviness. Especially the FVC is affected and has a large impact on the mechanical properties. This impact, resulting from different deformation modes during draping, is in general not considered in composite design processes. To analyze the impact of different draping effects on the mechanical properties and the failure behavior of UD-NCF composites, experimental results of reference laminates are compared to the results of laminates with specifically induced draping effects, such as non-constant FVC and fiber waviness. Furthermore, an analytical model to predict the failure strengths of UD laminates with in-plane waviness is introduced. The resulting stiffness and strength values for different FVC or amplitude to wavelength configurations are presented and discussed. In addition, failure envelopes based on the PUCK failure criterion for each draping effect are derived, which show a clear specific impact on the mechanical properties. The findings suggest that each draping effect leads to a “new fabric” type. Additionally, analytical models are introduced and the experimental results are compared to the predictions. Results indicate that the models provide reliable predictions for each draping effect. Recommendations regarding necessary tests to consider each draping effect are presented. As a further prospect the resulting stiffness and strength values for each draping effect can be used for a more accurate prediction of the structural performance of CoFRP parts.