Intra-ply shear characterization of unidirectional fiber reinforced thermoplastic tape using the bias extension method

Press forming of thermoplastic unidirectional (UD) carbon fiber reinforced laminates is an attractive production method in the aerospace industry for cost-effective manufacturing of high-performance parts. The possible formation of wrinkling defects in the formed parts has led to the development of predictive, finite element based, process simulation tools. The material behavior during the forming process is described based on the governing deformation mechanisms, being intra-ply shear, inter-ply and tool-ply slippage and bending. Intra-ply shear is especially important when forming parts having double curvature. The intra-ply shear behavior of fabric-based composite materials is often characterized using the bias extension method but has not successfully been applied to thermoplastic UD tapes yet. This work describes the application of bias extension experiments on cross-ply UD laminates at forming conditions to characterize the intra-ply shear material behavior. The test procedure was designed to prevent deconsolidation and improve load introduction, promoting specimen integrity and reduce shear buckling during testing. Preliminary results show that the material exhibits rate-dependent behavior. A video extensometer was used to measure the shear deformation in the center of the specimen. Additionally, a deformation analysis was performed using a grid of lines on the specimen, where the theoretical areas of constant shear according to a pin-jointed net can be recognized but are not fully uniform. In particular, shear banding parallel to the fiber direction is observed on the outer ply at a length scale below the grid size used for the deformation analysis suggesting a yield point and softening behavior on the meso scale which is not directly evident from the macroscopic response.

Effect of Self-stitched Double Fabric’s Properties on Tensile and Permeability Performances

The double fabrics having a complex structure are used in different areas both technically and aesthetically. In this study, tensile and air permeability properties of self-stitched double fabrics were investigated. Firstly, six different self-stitched double fabrics having the same yarn type, same settings, two different weave types, and three different stitching arrangements were designed and manufactured. Then, the tensile properties of these double fabrics were tested by applying tensile test at warp and weft directions, and bias-extension test at 45° bias direction. The effect of structural properties on tensile and air permeability results was discussed statistically. The tensile properties of self-stitched double fabrics having plain weave types are mostly better than 2/2 twill ones. The differences between tensile properties of self-stitched double fabrics generally were not found statistically significant according to stitching arrangement except the double fabric having plain weave type and higher stitching points. On the other hand, the differences between air permeability properties of all self-stitched double fabrics were found statistically significant at 95 % confidence level in terms of both stitching arrangement and weave type.

Mesoscale draping simulation of carbon fiber mat for arbitrary weave architecture

We perform a mesoscale draping simulation using a woven carbon fiber mat model consisting of fiber bundles. The deformation of various types of fabrics during draping is predicted using fiber bundle data; no fabric experiments are necessary. First, bending experiments and simulations are performed to determine the material constants of the fiber bundle. To confirm the validity of the mesoscale model, we compare the experimental and simulation results of bias-extension tests. The simulated test shows that the simulation reproduces the deformation behavior of the carbon fiber mat under tensile force. The draping of a plain-weave or twill-weave carbon fiber mat on a hemispheric mold is simulated using the proposed mesoscale model and investigated experimentally. The results confirm that the simulation reproduces the deformation of the fabric using only the fiber bundle data.

Textile Composite Damage Analysis Taking into Account the Forming Process

The internal structure of composite materials is modified during manufacturing. The formation of woven prepregs or dry preforms changes the angle between the warp and weft yarns. The damage behaviour of the consolidated composite is modified by these changes of angle. It is important when designing a composite part to consider this modification when calculating the damage in order to achieve a correct dimensioning. In this paper, a damage calculation approach of the consolidated textile composite that takes into account the change in orientation of the yarns due to forming is proposed. The angles after forming are determined by a simulation of the draping based on a hypoelastic behaviour of the woven fabric reinforcement. Two orthogonal frames based on the warp and weft directions of the textile reinforcement are used for the objective integration of stresses. Damage analysis of the cured woven composite with non-perpendicular warp and weft directions is achieved by replacing it with two equivalent Unidirectional (UD) plies representing the yarn directions. For each ply, a model based on Continuum Damage Mechanics (CDM) describes the progressive damage. Two examples are presented, a bias extension specimen and the hemispherical forming coupon. In both cases, the angles between the warp and weft yarns are changed. It is shown that the damage calculated by taking into account these angle changes is greatly modified.

Discrete versus homogenized continuum modeling in finite deformation bias extension test of bi-pantographic fabrics

A 2D-continuum model describing finite deformations in plane of discrete bi-pantographic fabrics has been recently obtained by applying an asymptotic procedure based on a set of local generalized coordinates. Rectangular bi-pantographic prototypes were additively manufactured by selective laser sintering using polyamide as raw material. Displacement-controlled bias extension tests were performed on such specimens for total elastic deformations up to ca. 25%. Experimental force measurements, complemented by discrete displacement measurements obtained by local digital image correlation, were used to fit the continuum model. In the present paper, a global and minimal set of generalized coordinates, alternative to the one used for the homogenization, is introduced for the discrete model. The mechanical constitutive parameters appearing in the discrete model are then found by means of collected experimental data. Finally, a comparison between experiments, the discrete and the continuum model is presented. It is concluded that (a) the discrete model and the experimental data are in excellent agreement, and that (b) the continuum retains the relevant phenomenology of the discrete system even for a rather low number of cells.