Biaxial Non-crimp Fabrics Research Articles

Assessment of Secondary Fiber Print-Through Effects on Class-A CFRP Parts Produced with Highly Cost Efficient Processes

Cost-optimized materials and processes are the key to high-performance components at attractive production costs. This study shows that non crimp fabrics (NCF) used as inner layers of high performance Class-A cfrp parts can lead to unwanted print-through effects on Class-A composite surfaces, even though they are not the surface layer. This surface distortion that is expressed in scattered lines in the direction of inner NCF layers can lead to high reject rates and is normally first noticed in the painted state. The presented methodology is able to quantify this secondaryprint-through effect for cured composites as well as for the dry textile intermediates. The surface can be measured with conventional measuring techniques, such as laser triangulation or interferometers, and characterized with the standardized values Sz and Sz25. The results show that the visible and measurable more uneven surface of a 50k biaxial NCF leads to significantly higher Sz and Sz25 values in the dry textile and the cured component. Also the regularity of the measured textiles can be detected by determining the variation of different areas of a 800 by 800mm sized sample. The presented methodology has the potential to optimize the incoming goods inspection of high volume Class-A composite manufacturers, as well as the requirement-orientated and cost-efficient development of textile intermediates by suppliers.

Algorithm-based Verification of Manufacturing Constraints for a Loadpath Reinforced Fabric

Lightweight construction has become increasingly important in recent decades. The fundamental idea of lightweight design is not to save weight at any price, but to use resources responsibly. Less waste and the right material in the right place can save costs and reduce energy consumption. Lightweight construction often increases design complexity. Algorithms can help to solve highly complicated design tasks effectively. This paper shows how load paths can be used to reinforce a component optimally. The load path method is an efficient way of structural reinforcement. It also fits perfectly to a new manufacturing method which will be shortly discussed: A textile machine for biaxial NCF (non crimp fabrics) with a special unit for offsetting warps of carbon fibers. This novel technology allows the reinforcing material to be placed at exactly the right place at high production speeds. The base material can be a cheaper fiber, such as glass fiber, the local reinforcement material can be a high performance fiber, such as carbon fiber. The manufacturing process is only considered to the extent that all relevant manufacturing restrictions can be extracted. Five key restrictions are being studied (e.g. minimal curvature radius or minimal and maximal stacking angle). All restriction can be broken down to geometrical information and formulated as various mathematical problems. These problems can be solved efficiently by known algorithms.

Analysis of the 3D draping behavior of carbon fiber non-crimp fabrics with eddy current technique

Abstract Assessing and controlling the complex deformation behavior of textile reinforcements fabrics remains one of the major challenges in the production of fiber-reinforced plastics. In this paper, the draping of +45°/-45° biaxial non-crimp fabrics to a hemispherical shape is investigated with an eddy current imaging technique. After an automated draping process, the textiles are scanned with a robot-guided eddy current measurement system. From the resulting conductivity maps of the samples, the local yarn directions are extracted by image analysis and the paths of individual yarns are reconstructed for both the upper and the lower layer. Experiments are carried out for different forming speeds, blank holder forces and different non-crimp fabric parameters (stitch length, stitch type, stitch tension and yarn count). The influences of these parameters are compared and discussed, with the conclusion that blank holder forces have by far the most significant influence on the draping result. Sample experimental results are compared to results from an FEM draping simulation.

Characterisation of the draping behaviour of unidirectional non-crimp fabrics (UD-NCF)

Thin composites shell structures manufactured from stitched unidirectional non-crimp fabrics (UD-NCF) in a liquid composite moulding process provides high lightweight design capabilities. However, draping behaviour of UD-NCF has been investigated only sparsely, in contrast to research on woven fabrics or biaxial non-crimp fabrics. Hence, this contribution focuses on fundamental investigations of the draping behaviour of UD-NCF. Within this investigation picture frame tests and uniaxial bias extension tests are performed to examine the in-plane shear behaviour of UD-NCF. Furthermore, a new method is presented to examine ambivalent tensile behaviour of UD-NCF transverse to the carbon fibre roving orientation. In particular, the influence of thin glass fibres on transverse tensile behaviour of UF-NCFs is investigated using a new clamping mechanism in tensile testing. Finally, hemisphere tests are performed to observe the forming behaviour of UD-NCF in a realistic forming process and to evaluate the proposed material characterisation methods regarding its suitability for UD-NCFs.

A mesoscopic model for coupled drape-infusion simulation of biaxial Non-Crimp Fabric

Modelling fabric draping can be performed using simplified mapping or more detailed finite element methods that allow greater accuracy and features such as tooling, blankholders and friction to be included. The materials laws, however, invariably treat the fabric as a homogenized continuum in which stretch, shear and bending stiffness are calibrated against coupon fabric tests. Certain fabrics such as stitched Non-Crimp Fabrics (NCFs) are particularly difficult to characterize using this macro approach since important deformation mechanisms occur at the tow and stitch (meso) level. Consequently, a meso-scale modelling methodology for drape of NCF is presented that allows individual tow and stitch deformations and their interaction. The approach uses deliberately coarse modelling so that structural scale problems may be tackled. A further aim is subsequent coupling of results to a meso-scale resin infusion simulation of the draped fabric. Experimental testing at the tow and fabric levels provides necessary fabric deformation and permeability data to calibrate the models. One aim of the work is to develop a methodology for coupled drape and infusion simulation at the meso-scale. It is recognized some aspects still require improvement; never-the-less the studies presented and a final validation study for draping of a hemisphere and coupled infusion simulation have provided encouraging results compared to test measurements.

Damage Behaviour of Ncf Carbon / Epoxy Laminates Under Tension

Composites reinforced by non-crimp fabrics (NCF) reach stiffness level close to the ideal cross-ply laminates with unidirectional plies, but damage behaviour of NCF-composites exhibit significant differences from the behaviour of the laminates with unidirectional plies (non-stitched laminates). The paper presents results of experimental studies of the initiation and development of damage in NCF-composites. Some of the observed phenomena can be explained and predicted using FE modelling of the composite deformation. Others are difficult to explain; they present a challenge for understanding the behaviour of NCF-composites. The materials are carbon/epoxy composites, reinforced by biaxial NCF 0°/90° and ±45°. Two different sets of fabrics (different producers) were used for the reinforcement, and the fibre volume fraction of the plates had two different levels: about 45% and about 55%. The samples were loaded in different directions in tension. Damage initiation and development was studied using acoustic emission and X-ray investigation.

Bubble motion through non-crimp fabrics during composites manufacturing

Bubbles motion through interbundle channels in biaxial non-crimp fabrics is modelled. The scenario is that formed bubbles move with the resin through these channels and are trapped if the channels become too narrow. By usage of a permeability network model, existing criteria on bubble deformation and a variety of analytical and probabilistic methods it is found that the paths of the bubbles depend significantly on the position of the threads keeping the fabric together and the number of fibres crossing the interbundle channels. Another result is that the pressure difference over a trapped bubble increases with 50% in a 3D geometry possible helping the bubble to escape. A third result is that, on average, the bubbles move biased to the direction of the tows. Finally, it is found that the predicted void distribution of bubbles after a major part of bubbles have moved through the system are in qualitative agreement with experimental data.

Effect of Multi-Scale Porosity in Local Permeability Modelling of Non-Crimp Fabrics

The influence of multi-scale porosity of fibre reinforcements on local permeability is investigated, in order to determine the possibility of simplifying permeability models for more efficient permeability calculations. Unit cell models of a biaxial Non-Crimp Fabric are developed and used to investigate, whether or not the porous bundles can be excluded, when modelling the local permeability. Numerical accuracy of calculations is controlled to guarantee the quality of the results and the conclusions drawn from them. It is found that fibre bundles with high fibre density can be excluded from permeability models, while bundles with low fibre volume fractions need to be included. A new method to model the local permeability of multi-scale reinforcements is developed and verified for low fibre density in the bundles. In this method, the effects of the flow inside the fibre bundles are included through modifications of the boundary conditions of a single-scale model representing the interbundle regions. The local permeability of multi-scale reinforcements can, therefore, be calculated by models with simplified fluid domains for all fibre bundle porosities, instead of being calculated by models consisting of the entire multi-scale geometry.

Particle deposition mechanisms during processing of advanced composite materials

Liquid composite moulding of advanced composite materials often comprises infiltration of a particle-filled resin into a multi-scale porous fabric. These injections/infusions are subject to severe particle depositions inside the reinforcement, leading to undesired inhomogeneous mechanical and functional properties. Hence, the mechanisms for particle depositions are investigated by detailed meso-scale experiments, analysed by microscopic imaging and micro-particle image velocimetry, and macroscopic infusions of a biaxial non-crimp fabric. It is shown that two main particle deposition mechanisms are filtration during fibre bundle impregnation and filtration induced by stationary flow through fibre bundles. It is also clarified where in the reinforcement the particles will deposit. Finally, a number of suggestions on how to process advanced composite materials with a more homogeneous particle distribution are launched.

Development of a Representative Unit Cell Model for Bi-axial NCF Composites

Composite materials fabricated from non-crimp fabric (NCF) are showing potential in primary aerospace components and structures. They can be used in common RFI, RIFT, and RTM manufacturing processes, and are showing promise to replace traditional pre-preg systems. The composite designer hence needs a technique, which can be used to predict the elastic properties of the NCF laminate including manufacturing defects from these infusion processes. The present study develops a representative unit cell (RUC) model of the NCF blanket, which can include potential variability in the NCF structure. Representative unit cells have been developed to predict the two- and three-dimensional mechanical properties in the bi-axial NCF composite. The model is based on the two-dimensional methods of Naik and Scida for woven fiber composites, incorporating the elastic anisotropy theory of Lekhnitskii to extend the predictions to three dimensions. Continuous tow layers with uniform thickness and fiber volume fraction are assumed, and the effects of crimp due to stitching and surrounding resin regions are simulated. The input data are based on measurements and observations of a ±45° carbon-epoxy NCF composite having a 63% fiber volume fraction and less than 1% voids. Output from the model is obtained in terms of the stiffness and compliance matrices as well as traditional engineering properties. For the tensile and compressive moduli, comparisons with data from the carbon/epoxy NCF composite show the values are over-predicted by 11.5—11.6% in tension and 13.5—13.9% in compression. Improved predictions are obtained using a simple stiffness model that allows for different behaviors of the resin as continuous layers or gaps between tows. The resin layers, compared to the resin as gaps, significantly decrease the in-plane moduli, and the model is considered to simulate local through-thickness shear deformation in the resin layers.

Bending, compression, and shear behavior of woven glass fiber–epoxy composites

The mechanical properties and failure mechanisms of through-the-thickness stitched plain weave glass fabric‐epoxy composites were studied. Unstitched plain weave and biaxial non-crimp fabrics were used for comparison. Composite panels were fabricated using Resin Transfer Molding. Z-directional stitching increased the delamination resistance and lowered the bending strength of the composites. Composites made from through-the-thickness stitched fabrics demonstrated improved compression after impact behavior as compared to the unstitched fabrics. The results presented in this investigation should be useful in tailoring textile composites to achieve specific property goals. q 2000 Elsevier Science Ltd. All rights reserved.