Transverse Compaction Research Articles

A friction-based method for measuring the radial compaction response of fibre tows

High performance fibre reinforced composite components are typically compacted to a high fibre volume fraction during manufacture. As such, it is important to understand the compaction behaviour of these materials to better design and model composite manufacturing processes. While most manufacturing processes apply transverse compaction pressure to planar textiles, continuous fibre composite 3D printing processes can apply a radial compaction pressure to a single tow in the printing nozzle. A similar mode of compaction is also seen in processes such as pultrusion of circular cross-sections. This work describes a new friction-based method for measuring the radial compaction response of single tows, or groups of tows. The radial compaction behaviour of a single 40K carbon fibre tow is measured and presented. Radial compaction of the characterised tow showed a steeper nonlinear relationship between compaction pressure and fibre volume fraction, than typically found for transverse compaction of 2D textile reinforcements.

Incorporation of compaction effects in the automated generation of 3D woven composites representative volume elements by geometrical modelling

Advanced 3D woven composites have become an enabling technology in many industrial applications due to their proven benefits, particularly the improvement of the out-of-plane mechanical behavior with respect to other reinforcement schemes. The latter property has been shown to be strongly affected by the effects of compaction during manufacturing. The present study proposes an automated framework, based on heuristic procedures, for the generation of representative volume elements (RVEs) of 3D woven composites, allowing to include global and local geometrical features induced by transverse compaction, such as overall thickness reduction, changes in yarn cross-sections and consistent global/local fiber fractions. The methodology is illustrated for three types of reinforcement, namely a plain weave, an angular through-thickness interlock and a 3D orthogonal RVE. The resulting geometries generated by the proposed method compare favourably with typical observations from the literature in a quantitative manner. Finally, the proposed framework is integrated with a previously developed meshing strategy to allow damage studies for a selected architecture (plain weave) under torsional loading, presenting a complete computational chain by building cohesive zone finite element models from compacted RVEs. The torque-angle response curves obtained after the damage simulations show a decrease in stiffness and an increase of damage levels with compaction, which is in line with the expectations.

Transverse compaction of twisted carbon yarns: Experiment and elasto-plastic Mohr–Coulomb modeling

The mechanical characterization of the carbon yarns, the major component of woven textiles for composite materials, is essential for optimizing the fabrication and the final properties of such materials.This work focuses on carbon yarns formed by twisting several elementary tows, each composed of thousands of carbon fibers, whose weak cohesion is ensured by a sizing agent. Different twist magnitudes are studied in dry and wet conditions. Transverse compaction tests are performed with loading–unloading cycles of increasing amplitudes. They have shown that the yarn cross section approaches a critical state where the fiber volume fraction remains constant, irrespective of twist level and humidity.The experimental results evidence an elasto-plastic behavior, analogous to that of a 2D granular medium. A simple model is proposed with a cohesive non-associated Mohr–Coulomb plasticity, having very few constitutive parameters. The effect of twist, water, and sizing can be accounted for.

An experimental investigation into single point incremental forming of glass fiber-reinforced polyamide sheet with different fiber orientations and volume fractions at elevated temperatures

Recent studies have proven the possibility of polymer sheet forming by incremental sheet forming (ISF) process, while there are limited studies on ISF of polymer matrix composites. Fiber-reinforced polymers (FRP) are significantly useful in industries due to their unique properties. In the current study, owing to the merits of ISF and the wide applicability of FRP, single point incremental forming (SPIF) of polyamide 6 (PA6) sheets reinforced by glass fibers (GF) is taken into account at various forming temperatures. In this regard, a fixture equipped with a ceramic infrared heating element was designed. The effect of some important parameters such as the volume fraction and orientation of fibers and the forming temperature on the formability, the thickness distribution, and the dimensional accuracy of PA6 and PA6/GF sheets is investigated. According to the results, an improvement in the formability of the composite sheet can be obtained using SPIF by increasing the forming temperature and decreasing the volume fraction of fibers. The formability of the composite sheet with the fiber orientation of [0/90] is far less than the one with unidirectional fibers. In the most of the successfully formed samples, the transverse compaction and the fracture of fibers at the part bottom, the buckling of fibers at the edge of the backing plate, and the draw-in of the sheet along the main direction of fibers can be detected. The thinning of the composite sheet is most affected by the volume fraction and orientation of fibers, while the forming temperature has the least effect. Moreover, the feasibility of SPIF of a circular flange with a wall angle of 90° from the composite sheet is assessed at elevated temperature.

An improved semi-discrete approach for simulation of 2.5D woven fabric preforming

The geometry of the textile composite reinforcements plays an important role in the mechanical properties of the final composite part and resin injection of the RTM forming process. An improved semi-discrete approach is proposed for simulation of 2.5D woven fabric forming. Transverse compaction, in-plane shear and bending are further considered in this paper, the corresponding internal nodal loads of the user-defined element are derived based on the principle of virtual work and implemented in finite element (FE) software ABAQUS/Explicit with user element subroutine (VUEL). Three experiments are conducted to determine the mechanical behaviors of the fabric. A combined deformation of cylindrical bending and torsion of the fabric is experimentally and numerically studied. A satisfactory agreement is achieved between the simulated and the experimental results. Both indicate that this combined deformation can be equivalent to the sequential superposition of in-plane and bending. The mechanism of the combined deformation is also discussed.

Influence of yarn hybridisation and fibre architecture on the compaction response of woven fabric preforms during composite manufacturing

Fabric preforms undergo transverse compaction during composite manufacturing. This compaction changes the preform thickness, fibre volume fraction (FVF), tow geometry and voids for resin flow. In this paper, influence of yarn hybridisation and fibre architecture on the compaction response of woven fabric preforms has been studied. A series of cyclic compression tests have been carried out on both dry and wet preforms. The effect of hybridisation on compressibility has been investigated for single as well as multilayer fabrics. The influence of interlacement pattern (twill and satin fabrics) with hybrid yarns has also been investigated. Nesting efficiencies of multilayer stacks have been studied by utilising mechanical test results. Additionally, the meso-structure of single and multilayer fabrics under 1 bar pressure has been analysed using SEM images. It is observed that the thickness reduction for single layer twill hybrid fabric is 38% while thickness reduction for twill S-glass fabric is 67% at 100 kPa. Moreover, single layer hybrid twill fabrics have shown higher compressibility resistance (60% thickness reduction at 100 kPa) compared to single layer hybrid satin fabrics (which showed 67% thickness reduction at 100 kPa). Whereas opposite trend is observed for multilayer hybrid fabrics due to nesting effect.

Towards numerical prediction of flow-induced fiber displacements during wet compression molding (WCM)

Wet compression molding (WCM) provides large-scale production potential for continuous fiber-reinforced structural components due to simultaneous infiltration and draping during molding. Due to thickness-dominated infiltration of the laminate, comparatively low cavity pressures are sufficient – a considerable economic advantage. Experimental and numerical investigations prove strong mutual dependencies between the physical mechanisms, especially between resin flow (mold filling) and textile forming (draping), similar to other liquid molding techniques (LCM). Although these dependencies provide significant benefits such as improved contact, draping and infiltration capabilities, they may also lead to adverse effects such as flow-induced fiber displacement. To support WCM process and part development, process simulation requires a fully coupled approach including the capability to predict critical process effects. This work aims to demonstrate the suitability of a macroscopic, fully coupled, three-dimensional process simulation approach, to predict the process behavior during WCM, including flow-induced fiber displacements. The developed fluid model is superimposed to a suitable 3D forming model, which accounts for the deformation mechanisms including non-linear transverse compaction behavior. A strong Fluid-Structure-Interaction (FSI) enforced by Terzaghi’s law is applied to assess flow-induced fiber displacements during WCM within a porous UD-NCF stack in a homogenized manner. Accordingly, resulting local deformations are considered within the pressure field. All constitutive equations are formulated with respect to fiber deformation under finite strains. Results of a parametric study underline the relevance of contact conditions within the dry and infiltrated stack. The numerically predicted results are benchmarked and verified using both own and available experimental results from literature.

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.

Transverse compaction of 2D glass woven fabrics based on material twins – Part II: Tow and fabric deformations

In Liquid Composite Molding (LCM), compaction of the reinforcement occurs during several stages of the entire process, including before and during resin injection, which leads to significant deformations of fibrous architecture. This article aims to study by X-ray microtomography the mesoscopic deformations of 2D glass woven fabrics under transverse compaction for a range of fiber volume fractions encountered in high performance composite applications. In Part I, material twin geometric models of dry fibrous reinforcements were created at different levels of compaction to study the evolution of the morphological features and displacements of fiber tows when compressed during processing. Part II proposes a new approach to track the motion of contour points on cross-sections of fiber tows during compaction. This will allow investigating more precisely the behavior of fiber tows by calculating their mesoscopic deformations. In particular, it will also be possible to study in more details the effect of contacts between tows and of nesting between fabric plies on the mesoscopic deformations of textile preforms during compression.

Transverse compaction of 2D glass woven fabrics based on material twins – Part I: Geometric analysis

In Liquid Composite Molding (LCM), compaction of the reinforcement occurs during several stages of the entire process, including before and during resin injection, which leads to significant deformation of the fibrous architecture. This affects not only the manufacturing process, but also the mechanical properties of final parts. This article aims to study by X-ray microtomography the mesoscopic deformations of 2D glass woven fabrics under transverse compaction for a range of fiber volume fractions encountered in high performance composite applications. The analysis is based on the recently proposed Micro-CT Aided Geometric Modeling (AGM) technique Huang et al. (2019) [1], which is used to create, from the three-dimensional images of dry textile preforms obtained by microtomography, “material twin” geometric models representative of fiber architecture. Three “material twin” mesostructural geometric models of a stack of multiple layer 2D woven fabrics are generated from microtomographic images at different compaction levels. Since all the fiber tows are reconstructed from real fabrics and are labeled separately, the deformation and displacement of fiber tows during compaction can be subsequently studied, which is not necessarily possible with other reconstruction methods. The results show that contacts between fiber tows have an effect on the evolution of their morphological features, which is related to the material variability of fiber tows in real fabrics. Tracking of the fiber tows positions also reveals that the vertical displacement of fiber tows are significant, while their horizontal movements are negligible.

A reference specimen for compaction tests of fiber reinforcements

Compaction behavior of textiles has a major influence on the outcome of various manufacturing processes for fiber reinforced polymer composites. Nevertheless, no standard exists up to date which specifies test methods or test rigs. A recent international benchmark revealed high variation associated with the result data. This work is a very first step toward a reference specimen, allowing for an isolated view on variations attributed to the test rig mechanics. A specimen design is proposed, intended to show compaction characteristics similar to technical textiles in terms of transverse compaction pressure and corresponding displacement. The reference specimen was tested in a round-robin study comprising test rigs at four different European research institutions. While reproducibility of the compaction behavior on each of the test rigs was high, clear variations between the results gained with different test rigs were observed.

Evaluation of compaction uniformity of the paving layer based on transverse and longitudinal measurements

ABSTRACT Compaction uniformity is essential to the successful performance of asphalt pavement. Therefore, understanding the characteristic and full evaluation on the overall density distribution in field construction is necessary. An available method capable to assess compaction uniformity of the hot paving layer was proposed by considering both longitudinal and transverse density distribution. Field tests of density distribution behind pavers and rollers were conducted on two test sections, respectively. Then, the quantitative evaluation indexes of uniformity (i.e. the range (R), coefficient of variation (CV), repeatability standard deviation in detection track (Sr), etc.) were used to analyse compaction uniformity and the severity of non-uniformity of the asphalt mat. Moreover, the uniformity variation from paving to rolling was evaluated through the tests and a compaction uniformity model. Results indicated that rolling compaction could intensify the degree of non-uniformity of the paving mat, although all the density after rollers is higher than the density behind pavers, and simulation analysis corresponds with the test results. The uniformity of density distribution in transverse is worse than in longitudinal. Tamper seemed to be more effective than other parameters at improving the initial density, while frequency of vibrator appeared to have the most impact on transverse compaction uniformity.

Construction of new enriched beam models accounting for cross-section deformation and pinching

The consideration of nonclassical beam theories beyond Timoshenko beam model is necessary in applications involving complex yarns subjected to transverse compaction, for which the section dilatation and in-plane shear require a detailed kinematic and static modeling. In the present work, new enriched beam elements with additional degrees of freedom are derived from the mechanics of generalized continua, allowing a representation of the in plane-shear and dilatation of sections. As a novel aspect, enriched beam model called macrodilatation, macroshear, and macrostrain have been constructed on the basis of generalized beam theories but with a more condensed kinematics, assuming uniform hyperstress tensor within the beam. The choice of the adequate extended continuum for a given beam depends on its macroscopic deformation mechanisms. As a result, the developed beam models include modified material parameters (the tensile, shear and bulk moduli for an isotropic beam) involving additional microstructural parameters. These macrobeam models are expected to better describe the in plane-shear and dilatation of the beam sections along its mean line in comparison to classical beam theories. Numerical illustrative examples show that the microdilatation beam model has the ability to capture the local deformation field around holes, in contrast to Timoshenko beam model. The parameters of the microdilatation beam model provide a very good estimate of the overall porosity.

Effect of Compressive and Shear Deformation of 2.5D Preform on its Stiffness of Composites

Transverse compaction and in-plane shear deformartion are the dominative deformation mode for woven preform during forming process. A full finite element model of the 2.5D woven composites has been established by the computed tomography (CT) in this paper. Based on the energy method, the effective orthotropic/anisotropic stiffness coefficientsCijare calculated by performing a finite element analysis (FEA) of this full cell model. Using this model, the effects of the compaction and shear deformation of the 2.5D woven preform on the composites stiffness are investigated in detail. Compared the results of the static tensile tests, the rationality of the model and the method is verified.

Experimental process pressure analysis for model-based manufacturing of composites by resin transfer moulding

An experimental study of the process pressure characteristics prevailing in an RTM mould during the preform impregnation stage is presented. The pressure data is analysed with respect to a superposition of preform compaction and hydraulic fluid pressure. A methodology is proposed to isolate the hydraulic fluid pressure from the preform compaction pressure involving knowledge about the compressibility behaviour of the reinforcing material. For this purpose, the preform compressibility and relaxation behavior was studied by means of transverse compaction tests on both, unsaturated as well as the saturated sample stacks. The resulting isolated hydraulic fluid pressure is finally compared with numerical predictions derived from flow simulations, revealing good overall agreement. The findings of this work will contribute to realize the concept of model-based processing of fibre-reinforced polymer composites by means of the RTM process.

Experimental analysis of the planar compaction and preforming of unidirectional flax reinforcements using a thin paper or flax mat as binder for the UD fibers

Transverse compaction and geometric conformation of fiber reinforcements have a significant importance in the production of complex dry fiber preforms for the resin transfer molding (RTM) process. This work investigates for the first time the planar compaction and preforming of two reinforcements made from aligned flax yarns held together by a thin layer of short kraft pulp or flax fibers. Planar compaction tests have been carried out in order to evaluate the influence of temperature, pressure, humidity and number of layers of reinforcement on the creep percentage, thickness recovery and permanent deformation of reinforcements. Next, the effect of process parameters on the quality of a complex 3D preform was evaluated. It appears that temperature, pressure and humidity increase the permanent deformation while the number of layers acts inversely. In addition, hot and wet made preforms have better dimensional stability (dimensions and geometry) than preforms obtained under other conditions.

Transverse Compaction Analysis of 2.5D Preform Composite

Transverse compaction is an important pattern of deformation during the composite resin transfer molding (RTM) process. Reasonable compaction rate is related to both the composite mould design and the fiber volume fraction of the final composite. In this paper, a mesoscopic geometry model based on CT scanning of 2.5D preform reinforcements is presented. Applying this model to the FEM simulation of transverse compaction, we prove the validation on simulating transverse compaction property of 2.5D preform by comparing to results of compaction test. Orientation angle during the progressive compaction is studied. Using this geometry structure, we build internal and surface RUC respectively then combining together to predict the in-plane mechanical property of 2.5D preform composite. Prediction result is acceptable corresponding to the mechanical properties calculated by homogenized method and compressive stiffness tested by combined loading compression (CLC) experiment.

Influence of Tow Architecture on Compaction and Nesting in Textile Preforms

Transverse compression response of tows during processes such as vacuum infusion or autoclave curing has significant influence on resin permeability in fabrics as well as the laminate thickness, fibre volume fraction and tow orientations in the finished composite. This paper reports macro –scale deformations in dry fibre assemblies due to transverse compaction. In this study, influence of weave geometry and the presence of interlacements or stitches on the ply-level compaction as well as nesting have been investigated. 2D woven fabrics with a variety of interlacement patterns - plain, twill and sateen- as well as stitched Non-crimp (NCF) fabrics have been investigated for macro-level deformations. Compression response of single layer and multilayer stacks has been studied as a function of external pressure in order to establish nesting behaviour. It appears that the degree of individual ply compaction and degree of nesting between the plies are influenced by tow architectures. Inter-tow spacing and stitching thread thickness appears to influence the degree of nesting in non-crimp fabrics.

3D composite reinforcement meso F.E. analyses based on X-ray computed tomography

Meso-FE modelling of 3D textile composites is a powerful tool, which can help determine mechanical properties and permeability of the reinforcements or composites. The quality of the meso FE analyses depends on the quality of the initial model. A direct method based on X-ray tomography imaging is introduced to determine finite element models based on the real geometry of 3D composite reinforcements. The method is particularly suitable regarding 3D textile reinforcements for which internal geometries are numerous and complex. The approach used for the separation of the yarns in different directions is specialised because the fibres flow in three-dimensional space. An analysis of the image’s texture is performed. The homogeneity parameter has proved to be the most efficient criterion to separate the yarns numerically. A hyperelastic model developed for fibre bundles is used for the simulation of the deformation of the 3D reinforcement. Meso-FE simulations based on this approach are performed in the event of transverse compaction of a 3D orthogonal reinforcement. The deformation of the yarns’ geometry in the simulation is compared with real conditions.

Analysis of filament arrangements and generation of statistically equivalent composite micro-structures

An efficient method to describe and quantify the filament arrangement in fibre bundles based on the analysis of micrographs was developed. Quantitative measurement of relative filament positions indicated that the initially random arrangement of filaments shows increasingly strong characteristics of square and hexagonal configurations with increasing level of transverse compaction. An existing micro-structure generator was extended to incorporate the measured data allowing statistically equivalent filament arrangements to be generated at any fibre volume fraction. These can be used to determine micro-structural properties of actual fibre reinforced composites.