Local Shear Angles Research Articles

Three-dimensional shear angle determination with application to shear-frame test

The shear frame test is very often used in the context of plastics engineering to examine only the fiber construction for its deformability. Here, it is common to assume a homogeneous deformation, which apparently exists as a global deformation due to the shear frame. This results in the assumption of a constant overall shear angle. In this paper, we compare two analytical approaches to describe the deformation and the resulting strains in detail, particularly, the local shear angle calculation. As a second step, the extension of the shear angle computation based on curvilinear, three-dimensional surface deformations is applied to optical results of the experimental deformation behavior. For this purpose, a three-dimensional image correlation system is drawn on to determine the local, tangential shear angle even for folded fabrics. This requires an approach which is different to the classical digital image correlation method since the speckle varnish influences the deformation of the very compliant fiber bundles. In a third step, the same procedure is applied to the results of three-dimensional finite element computations since the programs do not provide the shear angle measure. The shear angle determination is discussed at various examples. As a side result of the investigations, the material parameters for orthotropic elasticity are determined using a representative volume element approach exploiting μ-CT data. Although the wrinkling prediction calculations depend on the pre-deformation, finite element discretization, and uncertainties of the specimens, calculation results might be used as an indicator to improve the specimen geometries.

Optimisation of intra-ply stitch removal for improved formability of biaxial non-crimp fabrics

Automated fabric forming solutions are required to meet the demand of liquid moulding processes, but wrinkling is a common problem for double-curvature parts due to a combination of the reinforcement type, manufacturing parameters and the part geometry. Local intra-ply stitch removal is introduced in the current work to improve the formability of a pillar-stitched biaxial NCF. An optimisation method is developed to remove stitches selectively, using a genetic algorithm coupled with a finite element model. Two criteria are defined to reduce the occurrence of forming defects whilst maintaining the integrity of the fabric. The first is to minimise the local shear angle across the surface of the ply and the second is to minimise the total stitch removal area. These criteria are combined into a single objective function and validated using a hemisphere forming case study. Experimental results confirm that macro-scale wrinkling can be successfully eliminated when intra-ply stitches are removed according to the optimised pattern. The stitch removal regions are distributed across both the positive and negative shear areas of the optimised NCF blank, indicating that local stitch removal can have a global effect on the formability. Perimeter shapes show that the optimum local stitch removal pattern enables a more balanced global material draw-in, demonstrating that the effect of stitch removal is not limited to the high shear regions. Removing stitches from just the over-sheared regions is therefore insufficient to fully mitigate wrinkles, justifying the need for the optimisation algorithm, as the optimised stitch removal pattern appears to be non-intuitive.

Measurement of local shear deformation in fabric drape using three-dimensional scanning

We propose a measuring method of shear deformation in drape using three-dimensional (3D) scanning. We measured the local shear angles in fabric drape based on the Fabric Research Laboratories (FRL) drape test for woven fabrics using the proposed method. We investigate the effects of the relative positions of the node to the center grainlines that cross at the fabric center, and the bending and shear properties of fabric on the shear angles. To measure the local shear deformation, we obtained 3D drape shapes of four different fabrics with three to six nodes. We covered the obtained drape shapes using a fabric model composed of square cells that allowed shear deformation. By calculating the shear angles of the cells, we obtained the local shear deformation. We found that the FRL drape can be characterized by three areas, except for the flat areas of the support disks: (a) areas along the center grainlines with zero or small shear angles within 3°, which could result from single curvature bending; (b) areas in the bias directions with relatively large shear angles over 3°, which could result from double curvature bending; and (c) polygon edges connected by tangents of the support disk with relatively larger shear angles than their surroundings, which could result from both bending and shear deformation, such as folding and wrinkles. By investigating the relationships between areas with large shear angles and the bending rigidity/shear stiffness, we clarified that the bending rigidity indirectly affects the local shear deformation of drape.

Testing, characterizing, and forming of glass twill fabric/polypropylene prepregs

This paper investigates the mechanical behaviors of thermoplastic woven prepregs via testing and forming experiments. Glass twill fabric/polypropylene prepregs are produced by chemical treatment on fabric surface and a hot pressure molding approach. Then, mechanical tests including uniaxial tensile and bias extension of the glass twill fabric and its prepregs are carried out to provide basic data set for material modelling. An anisotropic hyperelastic model based on strain energy decomposition is proposed. And its material parameters are obtained by fitting these experimental data. Hemispherical thermo-stamping experiments are implemented for model verification. Very good agreements between forming simulation results and experimental data including boundary profiles, local shear angles, and forming force magnitude are obtained. The present work provides a complete data set for the model development and verification of thermoplastic woven fabric prepregs.

Experimental and numerical investigation of the shear behaviour of infiltrated woven fabrics

Wet compression moulding (WCM) as a promising alternative to resin transfer moulding (RTM) provides high-volume production potential for continuously fibre reinforced composite components. Lower cycle times are possible due to the parallelisation of the process steps draping, infiltration and curing during moulding. Although experimental and theoretical investigations indicate a strong mutual dependency arising from this parallelisation, no material characterisation set-ups for textiles infiltrated with low viscous fluids are yet available, which limits a physical-based process understanding and prevents the development of proper simulation tools. Therefore, a modified bias-extension test set-up is presented, which enables infiltrated shear characterisation of engineering textiles. Experimental studies on an infiltrated woven fabric reveal both, rate- and viscosity-dependent shear behaviour. The process relevance is evaluated on part level within a numerical study by means of FE-forming simulation. Results reveal a significant impact on the global and local shear angle distribution, especially during forming.

A multi-step biaxial bias extension test for wrinkling/de-wrinkling characterization of woven fabrics: Towards optimum forming design guidelines

Wrinkling is one of the most common flaws taking place during the forming process of textile composites with complex mould geometries. It is of importance to designers to be able to predict the minimum amount of forces that are required to diminish the wrinkles in useful regions of the final product, through applying appropriate boundary conditions enforced at the blank holders. Towards this goal, herein a multi-step biaxial bias extension (MBBE) characterization test has been proposed to assess the magnitude of the required transverse de-wrinkling forces that can smooth out wrinkles of different sizes, formed at different shear angles of a plain weave fabric. Employing image processing and 3D scanning during the MBBE test, the local shear angles and dimensions of wrinkles were characterized and correlated to the measured forces. It was shown that applying excessive transverse loads to yarns can result in a very narrow formability window and eventually cause fiber breakage, hence suggesting critical role of forming optimization, given different part shapes and fabric types.

Double diaphragm forming simulation for complex composite structures

A finite element (FE) model has been developed to simulate the double diaphragm forming (DDF) process, to identify potential defects when forming complex 3D preforms from 2D biaxial non-crimp fabric plies. Three different metrics have been introduced to predict and characterise defects, which include local shear angles to determine ply wrinkling induced by over-shear, compressive strains in the primary fibre directions to determine bundle wrinkling, and tensile stresses in the primary fibre directions to determine fabric bridging. The FE simulation is in good agreement with experiments performed on a demonstrator component. Results indicate that fabric bridging occurs in large-curvature regions, which is the dominant defect in DDF, as wrinkling is generally lower than in matched-tool forming due to relatively low forming pressures (up to 1bar). The axial tensile stress in fibres has been used as a measure to identify suitable positions and orientations for darts, to alleviate fabric bridging and improve surface conformity, whilst minimising the effect on the mechanical performance of the component.

Defect formation during preforming of a bi-axial non-crimp fabric with a pillar stitch pattern

To capture the asymmetrical shear behaviour of a bi-axial NCF with a pillar stitch, a non-orthogonal constitutive model was developed and implemented in finite element forming simulations. Preforming experiments indicate that the local distribution of defects is significantly different on both sides of each bi-axial ply, with two different defect mechanisms observed. Correlation with simulation results indicates that one defect type is caused by excessive shear, inducing out-of-plane wrinkling in regions of positive shear (macro-scale wrinkling). The other defect type is caused by fibre compression, inducing in-plane wrinkling in regions of negative shear (meso-scale wrinkling). Local distributions of shear angle and wrinkling strain were used to determine the wrinkling mode and to confirm the corresponding defect mechanism. Results indicate that simulations based on the advanced constitutive model can predict local shear angles within ±5° of experimental values and that predicted wrinkling positions and defect types correlate well with the experiments.

Draping of plain woven carbon fabrics over a double-curvature mold

Abstract This paper investigates the complex fiber reorientation and redistribution of plain woven carbon fabrics draping over a double-curvature mold through experimental and numerical approaches. Firstly, uni-axial tensile, bias-extension and picture-frame tests are carried out to obtain the material properties of the woven carbon fabrics. Material parameters for a previously developed simple anisotropic hyperelastic model are accordingly obtained. Secondly, draping experiments are implemented at room temperature using samples with different original fiber orientation. Deformed fabric boundary profiles and local shear angle variations are recorded and analyzed. The draping process is then simulated by using the anisotropic hyperelastic model taking into account material and geometric non-linearity. Very good agreement between simulation and experimental results is obtained. The ultimate goal of the investigation is to develop a design tool for the numerical simulation and processing optimization of woven composites forming.

Experimental studies and analysis of the draping of woven fabrics

This study investigates and compares the draping and forming of four types of woven fabrics, namely a loose plain weave (basket weave), a tight plain weave, a satin and a twill weave. The fabrics were draped over a hat mould consisting of a hemispherical dome surrounded by a flat base. The draping of each fabric was examined in terms of wrinkle formation, boundary profile of the draped fabric, distribution of fibre orientation and local shear angles. A theoretical analysis of the experimental results involved the calculation of the distributions of the fibre volume fraction and mechanical properties, in terms of components of the reduced stiffness matrix, from the experimental data of local shear angles.

FABRIC MECHANICAL AND PHYSICAL PROPERTIES RELEVANT TO CLOTHING MANUFACTURE —

The mechanical properties of fabric longitudinal extension and compression, shear and bending have an important influence on the performance of fabrics during tailoring and also on the performance of garments during use. Fabrics are overfed or underfed in tailoring during the sewing operations in such a way that longitudinal compression and extension are allowed in the fabric plane in order to produce the three‐dimensional shape or fullness of the garment. Fabric buckling or puckering at the seams should not occur, or if it does, should be removed during subsequent steam pressing operations. An experiment is described to measure the maximum limit of overfeeding that is possible during seaming without the subsequent formation of seam puckers. The relationships are studied between the maximum degree of overfeed, the bias angle between the feed direction and the fabric warp or weft, fibre type and fabric mechanical properties, especially fabric formability defined as the product of fabric bending rigidity and fabric longitudinal compressibility. When fabrics are extended or compressed longitudinally at a bias angle to the warp or weft direction during seaming in order to produce garment fullness, the warp and weft threads are rotated relative to each other in such a way that a local shear deformation is applied to the fabric adjacent to the seam‐line. Measurements are reported of the variations in the local shear angle along the shoulder seam‐line of a men's jacket and the measured values are related to the high degree of overfeed required in the bias direction in this area of the garment. Finally, hygral expansion measurements for wool fabrics and yarns unravelled from the fabrics are measured and compared for six different wool fabrics.