Showing papers in "Polymer Composites in 1996"

Thermoplastic nanocomposites filled with wheat straw cellulose whiskers. Part I: Processing and mechanical behavior

Abstract: Cellulose microcrystals with dimensions of ∼5 nm x 150-300 nm were obtained from wheat straw. To evaluate the reinforcing effect of these fillers within a thermoplastic matrix, composites with a weight fraction of cellulose ranging from 0 to 30 wt% were processed by freeze-drying and molding a mixture of aqueous suspensions of microcrystals and poly(styrene-co-butyl acrylate) latex. It was found that these microcrystals, or whiskers, bring a great reinforcing effect at temperatures higher than the glass transition temperature (T g ) of the matrix and improve the thermal stability of the composite. The relaxed modulus increased continuously with the filler content, and for a film containing 30 wt% of whiskers, it was more than a thousand times higher than that of the matrix. This effect is discussed with regard to theoretical calculations based on a mean field approach (Halpin-Kardos model). It is concluded that the great reinforcement observed seems to be due not only to the geometry and stiffness of the straw cellulose whiskers but also to the interactions of the microcrystals, their topological arrangement, and the probable formation of whisker clusters within the thermoplastic matrix, the cellulose fillers probably being linked through hydrogen bonds.

Phthalonitrile-carbon fiber composites

Abstract: Phthalonitrile polymers offer promise as matrix materials for advanced composite applications. The phthalonitrile monomer is readily converted to a highly crosslinked thermosetting polymer in the presence of thermally stable organic amine catalysts. Rheometric studies were conducted to elucidate the optimum amine concentration for composite formulations. High quality composite panels were processed in an autoclave using unsized IM7 carbon fibers. Mechanical properties of the phthalonitrile/carbon composite are either better than or comparable to the state-of-the-art PMR-15 composites. Dynamic mechanical analysis reveal that samples postcured at elevated temperatures (375°C) do not exhibit a glass transition temperature up to 450°C and also retain ∼90% of their initial modulus at 450°C. Flame resistance of phthalonitrile/carbon composites, evaluated by cone calorimetric studies, excels over that of other polymeric composites for marine applications. The composites also show low water uptake, <1% after exposure to water for 16 months.

Tensile behavior of nanocomposites from latex and cellulose whiskers

Abstract: Cellulose whiskers have been used as reinforcement in a copolymer matrix prepared from a latex phase. If a water suspension-mixing procedure is adopted, the fibril breakage that usually occurs during the mixing with a molten polymer can be avoided, and an enhanced filler dispersion can be expected. In this study, different processing methods have been used to prepare composite films, either by film casting (water evaporation) or by freeze drying, followed by classical compression or extrusion processes. The thermomechanical properties of these nanocomposites have been investigated, and the influence of processing conditions and the effect of whisker content have been considered. Processing conditions have a large influence on the mechanical behavior and can be classified in ascending order of their reinforcement efficiency : It can be attributed to a decrease of the apparent whisker aspect ratio, due to gradual breakage and/or orientation of the whiskers when hot pressing or extrusion is used. Below T g , good agreement is found between experimental moduli and the theoretical predictions of the Halpin-Kardos equation. On the other hand, above T g , a spectacular reinforcing effect is observed, which is widely underestimated by this short fiber composite model. This is related to the presence of a rigid cellulose network, linked by hydrogen bonds, when the whisker content is above its percolation threshold. The quality of this network (i.e., density and homogeneity) and thus, the magnitude of the reinforcing effect, depend on processing conditions.

Influence of fiber surface treatment on the properties of sisal-polyester composites

Abstract: The effect of several chemical treatments, viz. organotitanate, zirconate, silane, and N-substituted methacrylamide, on the properties of sisal fibers used as reinforcement in unsaturated polyester resin (∼50 vol%) was investigated. An improvement in the properties was observed when sisal fibers were modified with surface treatments. Under humid conditions, a decrease of 30 to 44% in tensile and 50 to 70% in flexural strength has been noted. The strength retention of surface-treated composites (except silane) is high compared with untreated composites. It is observed that N-substituted methacrylamide-treated sisal composites exhibited better properties under dry as well as wet conditions. Fractographic evidence such as fiber breakage/splitting and matrix adherence on the pulled-out fiber surface explains such behavior.

Development and characterization of high-performance polybenzoxazine composites

Abstract: To develop high-performance composites with high temperature resistance, two newly synthesized polybenzoxazines were successfully cured with carbon fiber to obtain composites with 60 vol% fiber. Results from differential scanning calorimetry studies were used to modify the benzoxazine monomers to improve processability in terms of melting point and solubility. The density and void content of these composites were measured. Dynamic mechanical tests were performed to determine the glass transition temperature (T g ) and the activation enthalpy of the glass transition process for these two composites. The effect of cure temperature on the T g of the composites was investigated. Thermal characteristics were studied by means of dynamic mechanical analysis in terms of isothermal aging, decomposition temperature from thermogravimetric analysis, and storage moduli change at high temperatures. Mechanical evaluations of these composites were conducted by flexural and interlaminar shear tests. The mechanical and thermal properties of these two composites exceed bismaleimide composites and compete with polyimide composites, while exhibiting easier processability than polyimides.

Permeability measurement and flow simulation through fiber reinforcement

Abstract: Continuous fiber reinforced polymer composites can be produced by the injection of a reactive fluid into a mold with preplaced reinforcement. Mold filling modeling softwares such as our RTMFLOT software are being developed to help design and production engineers meet their requirements for part production. The utilization of accurate permeability values is an absolute necessity for relevant mold filling simulation. This paper presents permeability results for several reinforcements. Experimental techniques being used to measure the permeability are also discussed. Several points are highlighted and suggestions are made to help the investigator in the search for reliable permeability data.

Concurrent methods for permeability measurement in resin transfer molding

Abstract: The numerical simulation of the resin transfer molding process (RTM) requires knowledge of the physical properties of the fibrous material. In particular, the resistance to the resin flow is measured by the permeability of the preform in the mathematical model of Darcy's law. A concurrent method for low-cost permeability estimation is proposed. The method uses a rectangular mold for the numerical determination of the principal permeabilities. The experimental data include a built-in correlation with Darcy's law and allow an estimation of both experimental and numerical errors. Since the experimental procedure can introduce a significant uncertainty on the estimated permeability, practical considerations are pointed out and some relevant parameters such as the minimum injected length and maximum injection pressure are introduced to increase the reliability of permeability measurements.

Experimental investigation of flow-induced microvoids during impregnation of unidirectional stitched fiberglass mat

Abstract: The complexity of the resin injection step in liquid composite molding (LCM) processes, such as resin transfer molding (RTM) and structural reaction injection molding (SRIM), often results in flow-induced defects such as poor fiber wetting and void formation. These defects have a deleterious effect on the mechanical properties of composites. In this work, high resolution video-assisted microscopy was used for in situ observation of flow-induced interstitial voids or microvoids formed inside the fiber tows during mold filling. Flow visualization experiments were carried out with different liquids to better understand the microscale flow behavior that led to the formation of microvoids for flow both along and normal to the fiber tows. Microvoid formation was correlated to the modified capillary number, Ca#* = μυ/γ cos(θ). The study revealed that for axial flow, microvoids were formed at Ca#* > 10 -3 . For transverse flow, microvoids were formed at an even lower capillary number, ∼10 -4 . Once formed, microvoids were difficult to purge and remained trapped even after bleeding the liquid at much higher flow rates than those at which they were formed.

Measurement of in-plane permeability of anisotropic fiber reinforcements

Abstract: Three of the most common methods (two with parallel flow and one with radial flow) for determination of the in-plane permeability tensor are studied both theoretically and experimentally. An error analysis shows that the difference between the methods is negligible if the error levels are equal. However, the radial flow method is found to be susceptible to large errors from mold deflection in an experimental comparison between the methods. Additional experiments with the radial flow method in a stiffer mold show that the method gives the same values for the permeability tensor as the other two methods. A new method with multiple cavities in parallel is proposed that combines the simplicity of the radial flow method with the stiff mold of the parallel flow method. Only mass and time need to be measured in one experiment and it eliminates the need to measure fluid viscosity, temperature, and injection pressure. The method depends on the availability of a reference material with known permeability.

Modeling of void formation and removal in liquid composite molding. Part II: Model development and implementation

Abstract: The objective of this Part II is to model void formation and their removal in LCM From the micro-scale flow pattern and void formation mechanisms observed by flow visualization, a phenomenological model based on the multi-phase Darcy's law was developed to simulate the void formation during LCM mold filling Results from Part I were used to determine several important parameters of the model The model was able to predict macro-void fractions with flow rates and liquid properties A criterion for the movement of trapped voids was also developed

Permeability model for a woven fabric

Abstract: The resin transfer molding (RTM) method is used to manufacture composite parts. The reinforcing fibers are placed in a mold cavity and the resin is injected to fill up the empty spaces. After the resin cures, the mold is opened and the part ejected. To predict necessary pressures and filling times and the proper locations for the inlet ports for resin injection and vents for air ejection it is necessary to model the resin infiltration process. A key to this modeling is permeability which characterizes the resistance of fibers to the flow of infiltrating resin. A simplified model for in-plane permeability of fabric reinforcement (preform) is developed here. This model uses lubrication theory for modeling the flow through open pores and Darcy's law for the transverse flow through the reinforcement. Scaling analysis is provided to justify the simplification and to estimate the range of validity for resulting expressions. Extension of the model to cover multi-layered preforms is derived. Boundary conditions and the data necessary to specify the problem geometry are discussed. A numerical experiment is conducted to estimate the influence of the transverse permeability of the preform on the solution. A calculation is provided for the permeability of a plain weave fabric.

Modeling of void formation and removal in liquid composite molding. Part I: Wettability analysis

Abstract: This two-part paper focuses on the characterization and simulation of two important molding defects in liquid composite molding-poor wetting and void formation. Part I analyzes resin-fiber wettability. This involved characterization of various liquids/resins and fiber filaments/fiber mats by using wicking test and capillary pressure measurement. Methodology to quantify capillary pressure-wettability relationships was developed. It was found that the Leverett J function can correlate capillary pressure-saturation relationships for fiber reinforcements with various porosities and fiber architecture.

Review of failure criteria of fibrous composite materials

Abstract: Composite materials exhibit various and complex failure behavior. Different formalisms have been used to predict failure. Improvement of old theories and new ones continue to be published. In this paper, the most recent and widely used models are presented. Failure criteria such as Tsai-Wu, parametric formulations, maximal stress and strain, Hashin criterion, Hart-Smith criterion, and the method based on kriging are presented. These failure theories may be classified in two categories, depending whether they integrate failure modes or not. The formalism of each theory is briefly described and their application to model failure of composite laminates is discussed by comparing the advantages and limitations of each method. The diversity of experimental failure envelopes, as reported in the literature on composites, is outlined and it is shown that most criteria permit modeling only particular failure properties of composite laminates.

A generalized model for the transverse fluid permeability in unidirectional fibrous media

Abstract: In liquid molding processes such as resin transfer molding (RTM), fluid is injected into a mold filled with fiber reinforcement. The microstructure of the reinforcement strongly influences the resistance it offers to fluid flow. This resistance is characterized by the permeability that determines the ratio between the superficial velocity and the pressure drop in the porous medium. Currently values of the permeability have to be determined experimentally. Therefore, each type of reinforcement has to be characterized before a computer simulation can be used to predict the overall mold filling pattern. A model for predicting the permeability as a function of structure would help reduce the number of experiments needed to determine the input parameters for mold filling simulations. Also, by understanding the physics of the flow through such materials, one may tailor the microstructure such that it has both the desired reinforcing capability and the necessary permeability to fill efficiently. In response to this need, we have developed a predictive semi-analytical solution for flow across arrays of aligned cylinders with elliptical cross sections modeling the fiber mats. The shape of the tow, its porosity, and the packing configuration are found to influence the transverse permeability of such an array significantly. Predicted results of the permeability from this model compare very well with numerical results obtained from finite element calculations over a range of volume fractions, cross-sectional shapes, and tow permeabilities.

Compressibility and flow permeability of two-dimensional woven reinforcements in the processing of composites

Abstract: The processing of composites under pressure is described by a viscoelastic model in which the pressure acts on the fibers in an elastic manner and on the liquid matrix in a viscous manner. Following a micro-mechanics approach, an analytical model has been derived for the nonlinear elastic compression of two-dimensional, orthotropic, plain woven fiber cloths, containing two empirical constants. The model is validated by experimental data and the empirical constants determined in the compression of a wet, plain woven, glass fiber reinforcement. The effects of the compression of woven cloths during processing are demonstrated by the change of their permeability.

Photodegradation of talc-filled polypropylene

Abstract: Injection-molded talc-filled polypropylene (PP) composites have been exposed to ultraviolet (UV) radiation in the laboratory for periods up to 26 weeks. The extent of chemical degradation has been assessed by means of Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC), and the results correlated with the mechanical properties. In the early stages of exposure, the photo-oxidation is faster in the talc-filled composites than in the unfilled polymer, but this trend is reversed for exposures longer than ∼12 weeks. Scanning electron microscopy (SEM) inspection has revealed that surface cracks caused by photodegradation in the filled PP occur in the surface exposed to the UV source only, resulting in much less deterioration in tensile properties when compared with the unfilled polymer which degrades significantly at the unexposed surface also. Measurements of melting temperatures by differential scanning calorimetry (DSC) gave a consistent picture of degradation with that obtained by FTIR and GPC studies. DSC analyses have also shown that an increase in the melting enthalpy for both the unfilled and filled grades occurs during exposure.

Numerical simulation of the resin transfer mold filling process using the boundary element method

Abstract: A numerical simulation of the mold filling process during resin transfer molding with a heated die was performed using the boundary element method. The governing differential equation with a variable coefficient was rearranged into a system of Poisson equations using the perturbation technique. The boundary element method was employed to solve the resulting equations. The resin viscosity was calculated by introducing markers at the resin inlet and tracing them. As the calculation domain changes because of the proceeding resin front, numerical calculation nodes on the boundary were rearranged for each time step and integration was performed only for the meshes in the calculation domain among the fixed meshes over the mold. Sample calculations were performed for two molds with different shapes. To check the validity of the numerical scheme, the calculated mass flow rate at the resin front was compared with the mass flux at the inlet. Close agreement was observed.

Stress distribution around inclusions, interaction, and mechanical properties of particulate‐filled composites

Abstract: Stress analysis was carried out to determine the stress distribution around particles in particulate-filled composites. The effect of interacting stress fields was also taken into account. At large filler contents, interacting stress fields compensate for the effect of stress concentration. The solutions were introduced into the Von Mises criterion for yielding. The composition dependence of tensile yield stress was determined by using different boundary conditions and averaging techniques. An analytical expression was derived that predicts particle size and adhesion dependence of the yield stress. The analysis shows that large particles and weak interaction lead to debonding. In the case of strong adhesion, the dominating micromechanical deformation process is the shear yielding of the matrix. In such cases, particle size dependence can be explained with the effect of interfacial interactions, which lead to the formation of an interphase. The dominating deformation mechanism is determined by particle characteristics and adhesion of the components. The predictions of the analysis are in good agreement with experimental observations.

Improvement of adhesion between polyethylene and regenerated cellulose fibers by surface fibrillation

Abstract: Regenerated cellulose fibers spun from straw pulp using the N-methylmorpholine N-oxide (NMMO) process were evaluated as a reinforcement for low-density polyethylene (LDPE). Surface fibrillation was carried out by a mechanical treatment to improve interfacial adhesion. Surface fibrillation resulted in a gradual change in surface topography, as detected by SEM. Long and numerous twisted fibrils were observed on the surface of the treated fibers. The fiber perimeters, determined by the Wilhelmy plate method, increased with an extended degree of fibrillation, while the strength of the fiber was not affected by the surface treatment. Model composites were prepared by embedding untreated and surface-fibrillated single fibers into an LDPE matrix, and the single fiber fragmentation (SEF) test was carried out to determine the critical fiber length. The interfacial shear strength (τ) was then calculated by applying a modified form of the Kelly-Tyson equation. It was found that the interfacial shear strength increased significantly as a result of surface fibrillation. The proposed mechanism for the improvement of interfacial adhesion is a mechanical anchoring between the matrix and the fiber.

Needle‐Punched hybrid nonwovens of flax and ppfibers—textile semiproducts for manufacturing of fiber composites

Abstract: Flax-nonwoven reinforced polypropylene has become a competitor to textile glass fabric reinforced polypropylene because of its economic and ecological advantages. Suitable application forms are needle-punched 100% flax or hybrid fabrics of flax/polypropylene manufactured of rough and fine decorticated flax. The construction of the nonwoven influences the strength, handle, matrix compatibility, and flow of the fabric. In this way, composite properties may be tailored to each end-use. The main application fields are subassemblies exposed to a medium range of stress. The method of hybrid-nonwoven manufacturing represents a technological alternative to the existing film-stacking method. Produced with lower technology, the parameters of composites of hybrid-nonwovens are comparable to properties of composites manufactured by the film-stacking method. Adhesion characteristics may be improved by addition of compatibilizers. The compatibilizers may be added to the flax-fiber surface or may be inserted into the polypropylene. All these methods lead to comparable mechanical parameters for the fiber composites.

Coupled analysis of injection molding filling and fiber orientation, including in‐plane velocity gradient effect

Abstract: On injection molding of short fiber reinforced plastics, fiber orientation during mold filling is determined by the flow field and the interactions between the fibers. The flow field is, in turn, affected by the orientation of fibers. The Dinh and Armstrong rheological equation of state for semiconcentrated fiber suspensions was incorporated into the coupled analysis of mold filling flow and fiber orientation. The viscous shear stress and extra shear stress due to fibers dominate the momentum balance in the coupled Hele-Shaw flow approximation, but the extra in-plane stretching stress terms could be of the same order as those shear stress terms, for large in-plane stretching of suspensions of large particle number. Therefore, a new pressure equation, governing the mold filling process, was derived, including the stresses due to the in-plane velocity gradients. The mold filling simulation was then performed by solving the new pressure equation and the energy equation via a finite element/finite difference method, as well as evolution equations for the second-order orientation tensor via the fourth-order Runge-Kutta method. The effects of stresses due to the in-plane velocity gradient on pressure, velocity, and fiber orientation fields were investigated in the center-gated radial diverging flow in the cases of both an isothermal Newtonian fluid matrix and a nonisothermal polymeric matrix. In particular, the in-plane velocity gradient effect on the fiber orientation was found to be significant near the gate, and more notably for the case of a nonisothermal polymer matrix.

Bubble transport through constricted capillary tubes with application to resin transfer molding

Abstract: This paper describes and expounds a theoretical and experimental study of bubble motion through constricted capillary tubes. In the experiment, two liquidfilled capillaries are used. They have unequal radii and are glued together. Gas bubbles are injected into the larger capillary. Then the pressure required to force the bubbles through the constriction is measured for various liquids, bubble lengths, capillary radii and constriction geometry. It appears that the pressure directly follows Young's-Laplace law for capillary pressure. The results of the study are used to understand the bubble transport through fiber reinforcements, which generally takes place during the manufacturing of composites. The bubbles are carried if: (i) the pressure gradient is high enough, (ii) the surface tension of the liquid is low enough, (iii) the cross-sectional area of the channels in the reinforcement is sufficiently uniform. The theory reveals that the bubbles are more likely to be trapped on a small scale, i.e. within fiber bundles rather than on a large scale, i.e. between the bundles. It is also concluded that, if the bubbles are trapped at the resin flow front, a converging flow is better for the transport of the voids than a diverging flow.

Consolidation and cure simulations for laminated composites

Abstract: Fiber reinforced polymer composites have been increasingly used in various structural components. One of the important processes for fabricating high performance laminated composites is autoclave assisted prepreg lay-up. Since the quality of laminated composites is largely affected by the cure cycle, selection of the cure cycle for each particular application is important and must be optimized. Thus, some fundamental model of the consolidation and cure processes is necessary to properly select the suitable parameters for each application. This study used the viscoelastic solid model for the consolidation of the laminate. In addition, variations of permeability and thermal properties caused by the change of the fiber volume content during the consolidation process were also included. Simulated thickness variations of epoxy continuous carbon fiber prepreg (AS4/3501-6 from Hercules) laminate under consolidation were compared to the experimental results to test the model. Based on the model analysis, one can predict the pressure, velocity, and laminate thickness during the consolidation process, which can be used to properly select the cure cycle for applications of laminated composites.

Sensor system for monitoring impregnation and cure during resin transfer molding

Abstract: In the resin transfer molding (RTM) fabrication of composites, knowledge of the position of a moving resin front during impregnation is important for process optimization. We describe here a simple, inexpensive, multi-point sensor system based on DC conductometry for determination of resin position in an RTM mold. This Resin Position Sensor (RPS) system consists of a matrix of small sensors embedded in the RTM tool, whose combined output can be used to produce a resin flow pattern at any given time after the start of impregnation. As it cures, the resin resistance increases and the sensor can then function as a cure monitor. A large, 24-sensor RTM tool was fabricated for demonstration of the RPS. Flow contour maps generated from sensor data during impregnation of both E-glass and carbon fiber preforms are shown.

Hydrostatically extruded glass-fiber-reinforced polyoxymethylene. I: The development of fiber and matrix orientation

Abstract: This paper describes the determination of fiber and matrix orientation in oriented short-glass-flber-reinforced polyoxymethylene (POM) composites produced by hydrostatic extrusion. The starting material was random glass fibers (25 wt% and average length 150 μm) in an isotropic POM matrix, and the oriented composites were produced by extrusion through a reducing conical die at 15°C below the polymer melting point : after extrusion the average fiber length was reduced slightly to 133 μm. Fiber orientations were measured using an image analysis method developed at Leeds University, and the matrix orientation was determined using wide angle X-ray diffraction. The development of fiber orientation with extrusion ratio was found to be close to that predicted by the pseudo-affine deformation scheme although the fiber orientation was greater than that predicted by the model at low draw ratios and slightly less at the highest draw ratio. The development of the orientation of the crystalline portion of the matrix was found to be always significantly greater than that predicted by the pseudo-affine scheme.

Stamp forming of fabric-reinforced thermoplastic composites

Abstract: A stamp forming technique has been used to process a fabric woven composite made of glass fibers (GF) and polyetherimide (PEI). A hemispherical mold with a built-in hold-down arrangement was designed and used at room temperature to stamp parts from preheated flat preconsolidated laminates. Tensile properties of the material were measured under similar heating conditions as in the relevant stamp-forming process. Stretch in the fiber direction was found to be smaller than the maximum elastic extension of the glass fibers. Reduction of the angle between the crossing fibers was quite large when the satin woven fabric composite was pulled in the 45° direction. The effect of die geometries and original laminate dimensions on the “shear-buckling” were studied. The results described the correlations between processing parameters and fiber buckling. Finally, the local strain of fiber bundles was investigated in relation to different directions of fiber orientation.

Prediction of mechanical properties of recycled fiberglass reinforced polyamide 66

Abstract: Reference LTC-ARTICLE-1996-002View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/107639242 Record created on 2006-06-26, modified on 2016-08-08

Viscoelasticity of polymers filled by rigid or soft particles: Theory and experiment

Abstract: An improved modeling for the viscoelasticity of polymers filled by rigid or soft inclusions is proposed. Such a self-consistent scheme can predict the strong increase in the reinforcement effect of the polymer matrix observed for large volume fractions of fillers. Then, it is based on both (i) the percolation concept and (ii) the definition of an original “representative morphological motif” accounting for local phase inversions due to the presence of clusters of particles. To illustrate the validity of such an approach, the predicted viscoelasticity of polystyrene filled either by glass beads or by rubbery inclusions is compared with experimental data and/or theoretical results that issue from other modelings.

In‐plane permeability determination for simulation of liquid composite molding of complex shapes

Abstract: Process simulation forms an important part of the design loop for parts manufactured by liquid composite molding. Critical to this is the determination of accurate permeability data. While constitutive models and test methods for determination of in-plane permeability of mats and fabrics exist, these do not take account of the effects of changes in fiber architecture during preform manufacture. The deformation behavior of mats and fabrics is reviewed, leading to fiber architecture predictions based upon deformed geometry. Existing models are applied to estimate the distribution of in-plane permeabilities within the preform based upon the predicted fiber architecture. The predictions are compared with test results for pre-stretched and pre-sheared reinforcements, and the method is demonstrated for two prototype automotive parts.

Deep drawing of fabric‐reinforced thermoplastics: Wrinkle formation and their reduction

Abstract: Producing highly stiff and strong thin-walled fabric-reinforced parts for mass application can be economic using a double-belt press to consolidate thermoplastic sheet and a hydraulic press for non-isothermal stamp forming in a second step. During stamp forming, undesired wrinkle formation can occur at sites of three dimensional forming. To understand the mechanisms of wrinkle formation, shear tests were performed with different types of reinforcing glass and carbon fabric, both dry and impregnated with a polyamide (PA12) matrix. Evidence has been found that wrinkle formation during fabric shear is strongly dependent on membrane stresses within the fabric. High membrane stresses of ∼20 MPa can prevent wrinkling even at high shear angles of ∼60°. These stresses can be generated during stamp forming by means of blank holders. A new flexible roller tracking device has successfully been installed on a stamp forming pilot plant. It was possible to form chests sized 450 mm. 300 mm. 40 mm (w. l. d) free of wrinkles with a 4-ply glass plain weave fabric preimpregnated with polyamide matrix (PA12) and a fiber content of 45 vol%.