Development of multilayered woven panels with integrated stiffeners in the transverse and longitudinal directions for thermoplastic lightweight applications

Abstract

Weaving technique offers a large potential for the highly productive manufacture of woven 3D-preform structures in lightweight applications. This research work reports on the technological modification to the construction of a double-rapier weaving machine for the low damage production of double-walled shell structures made from glass/polypropylene hybrid yarn multilayered woven fabrics. On this basis, 3D-woven preform structures with integrated stiffeners in the longitudinal and transverse directions for high composite strength and rigidity within a single weaving process were developed and implemented.

Furthermore, textile engineering solutions were acquired for the required reinforcement of the gusset area between the stiffener and the skin. This broadens the range of uses for large-scale fiber-reinforced plastics with integrated stiffeners made from 3D-woven preform structures.

Keywords

multilayered structures woven panels 3D fiber-reinforced plastic integrated stiffeners in longitudinal and transverse directions gusset area

The development of composite materials has been established particularly well by the use of textile-based structures in lightweight engineering. Fiber-reinforced plastics (FRP) display reduced weight and great strength in comparison with metallic materials. They are increasingly used in technical areas where high material and energy efficiency are strived for. High-performance threads such as carbon (CF), glass (GF) and aramid (AR) are the most common reinforcement materials. The potential of the composite construction is partly founded on the strain-suited alignment and the arrangement of the thread material within the component and allows for the effective use of the positive material properties. The advantages of FRPs hold an extensive potential for innovation in the construction and manufacture of sandwich and honeycomb structures with load-oriented reinforcements. They have to fulfill substantial expectations, such as high composite strength and stability for aeronautics (such as the wing, empennage and fuselage covering), floor and wall elements in vehicle and container (tank) construction, as well as hulls in boatbuilding.¹

Sandwich structures are used in lightweight construction, mainly for their attainable high rigidity for a small specific mass. The established manufacturing of sandwich structures with honeycomb cores is extremely costly, characterized by many individual production steps, e.g. the separate manufacture and joining of the top surface areas with the honeycomb structure.² This causes higher production costs and a larger number of error sources, in particular the possibly inadequate bonding of surface layers with the honeycomb core, which can cause delamination and component damage even at normal stress loads.^3, ⁴

However, in the area of FRPs, woven textile spacer structures made from high-performance filament yarn are coming more and more into use. The danger of delamination of the outer layers from the core is low in combination with high-performance yarns. They have the potential to replace conventional sandwich components with a honeycomb core.

3D pile weaves consist of two plane outer layers connected by pile yarns. The outer layers contain warp threads in the x-direction and weft yarns in the y-direction. The pile yarns are oriented nearly vertically to the outer layers, and their length determines the height of the pile weave (Figure 1). The pile yarns oriented in the warp direction are alternately bound into the top and bottom outer layers, which achieves the high resistance against delamination of the pile weave.^5–7 Such pile weaves are manufactured as the basic technology for carpet production. Their use for sandwich structures laminated with a thermoset matrix is well known in boatbuilding panels, rail vehicle construction and aeronautics.^8, ⁹ By connecting the outer layers with threads instead of woven planes, the achievable compressive and shear strengths are lower, compared to sandwich components with a honeycomb core. These parameters are largely dependent on the individual pile yarn’s material properties. Also, no method is known for hot pressing, or impregnating and consolidating, like resin transfer molding (RTM), of pile weaves with pile yarns.
Figure 1.
3D pile weave structure.⁵

It is necessary to develop innovative spacer fabrics with “real” cross link fabrics between the outer layers for high strength and flexurally rigid thermoplastic FRPs. Methods for the production of such spacer fabrics are described in various patents,^10–12 whereas the greatest challenge is to realize such novel 3D spacer fabrics in a single process step by performing the required machine modifications and understanding the machine parameters. Based on pattern variations of the weaving technique it is possible to align high-performance threads stretched and continuous in the load direction of the component. This leads to low damage thread processing and high component rigidity, resulting in woven 3D structures with high thread density and preform rigidity, suitable for high strength FRPs.^13–15

To increase composite rigidity and, in particular, flexural rigidity, composite panels are being developed based on stiffeners, usually aligned vertically and along the structure. The connecting of such stiffeners with lightweight construction panels is usually done discontinuously either after preform manufacture by sewing, or after composite production by bonding via matrix joining with or without interior foam filling.^16–20 During sewing, however, the high-performance yarns run a risk of damage by pinholing, which causes reduced composite strength. Furthermore, there is only a limited range of technical needles and thread finenesses available, which cause material inhomogeneity at the joint and high additional costs. The connection of stiffeners and composite panels by matrix joining mostly entails composite degradation because in this spot the strength depends on the matrix properties. Such connections of stiffeners and composite panels are local weak spots and are near invariable origins of component failure.²¹

This study documents the development of technological construction and weaving solutions for the production of large-scale, plane preforms and weaving technically integrated orthogonal stiffeners in longitudinal and transverse direction. It also achieves the required component and pattern technical solutions of better and continuous high-performance thread reinforcement between stiffeners and plane preforms. The technical weaving challenge is to realize longitudinal and transverse stiffeners in an integrally constructed 3D component preform. These structures are to be manufactured reproducibly in a single process step.

Experimental details

The experimental developments of woven 3D preform structures were made on a laboratory double-rapier weaving machine VTR-23 (NV Michel Van de Wiele, Belgium) of a one meter working width and a warp thread density of 20 threads per centimeter per layer (a total of 40 threads per cm). The use of this machine size served to develop and implement the technology for production, and to analyze the properties of large size preform structures made from hybrid yarns of 410 tex fineness consisting of reinforcing GF and polypropylene (PP) as technical thermoplastics.

Construction and technological machine modifications

To manufacture integral 3D preform structures with woven cross links between the cover layers and with out of plane stiffeners, a terry weaving mechanism with additional fabric storage and a warp pull-back mechanism were developed and integrated into the double-rapier weaving machine. This technology is necessary for the development and manufacture of 3D woven preform structures and used for further structural developments.^22–25 The terry weaving mechanism contains three deflecting rollers, of which the central roller (red) serves to store fabric and the two deflecting rollers (blue) are used for fabric transport (Figure 2). The length of fabric stored temporarily by the central roller is equivalent to the length of a single cross link or the double length of a single stiffener. The terry weaving mechanism was developed further, permitting longer cross link and stiffener fabrics of up to 80 mm. To realize the pleat as a woven cross link or stiffener, the store fabric is released, and the ground threads’ float is pulled back into the machine. The back rest roll system of the double-rapier weaving machine, creating warp tension with a lever arm and a weight, allows for the warp pull-back without an additional machine element. The terry weaving mechanism and warp pull-back are synchronized with the weaving machine controls, which enables the user to perform the storage and release of fabric and the warp pull-back function for the production of 3D woven preforms without interruptions to the weaving process. This makes a flexible formation of integrated woven cross links and stiffeners at different distances feasible.²⁶
Figure 2.
Modified double-rapier weaving machine.²⁶

Every weaving process requires a fabric take-up. Usually, the take-up force and speed are determined by the wrapping angle and the pressing of the textile preform onto the take-up rollers. This conventional take-up technology causes varying fabric lengths in the top and bottom layers and the buckling of the cross links and stiffeners, damaging the composite component’s load bearing glass fibers. Only if the fabric remains stretched in the take-up area and the take-up forces are brought forward without the buckling of the cross links and stiffeners, then the fiber sparing required for later load bearing and the form retention necessary for use as a preform can be guaranteed. Therefore, a stretched roller take-up system with automatic supporting bars to bring forward the take-up force was devised and technically added to the double-rapier weaving machine (Figure 2). As the supporting bars were at first used exclusively as inner counter bearings, they can be removed at the end of the take-up area. They are indispensable for the damage free take-up of 3D component preforms.^26–27

In order to optimize the reproducible production, the regulation and control system of the double-rapier weaving machine was extended so as to ensure the continuous detection of thread tensile forces, fabric take-up forces and fabric motion. The established readings, which depend on weave pattern repeat, are used for significant adjustments to the machine parameters (such as warp let-off, shed opening, weft density and take-up) and to guarantee a stable and undisturbed weaving process. This results in a gentler thread material processing, as well as allowing the production of scalable, dimensionally stable preforms. Figure 3 shows the arrangement of thread tensile force sensors on the weaving machine, and an example of the diagrams of the determined readings. Waweon (Vúts Liberec, Czech Republic) was used as measuring sensor.
Figure 3.
Thread system measuring points with exemplary diagrams of the determined readings.

The diagrams in Figure 3 show a section of the complete pattern repeat measurement. In the top diagram, the force peaks in the two warp systems’ course, resulting from the reed beat-up, are clearly visible. The medium thread force of warp system 1 is near constant at approximately 120 cN. The medium thread force as well as the force peaks of warp system 2, however, are much smaller (50 cN) than those of warp system 1. This difference depends largely on the weaving machines warp system equipment and the fabric weave. Due to lower medium thread force, warp system 2 has a smaller influence on the total take-up force of the fabric. The bottom diagram in Figure 3 shows an increase of take-up force over time. This is caused by the weave pattern repeat, which consists of different weaves. It can be concluded that the fabric take-up force fluctuates periodically, depending on the structural pattern repeat of the fabric (outer layers and cross link from 3D spacer fabrics).

Cutting and storing system for a continuous process chain

Apart from the structural geometry created by weaving, the manufactured 3D preform structures are required to have the dimensions suitable for the hot pressing machinery used for preform consolidation. Therefore, a combined cutting and storing system was developed parallel to the take-up system (Figure 4). It is used for trimming the fabric from the continuous weaving process to the preform length needed for the pressing tool, and for the defined, easy to handle storing of the produced preforms.
Figure 4.
The cutting design (left) and storing system (right).²⁶ CAD: Computer-aided design.

Ultrasonic (US) cutting was chosen as the sectioning principle. This shatters the GF fibers, while the PP fibers are locally and briefly fused at the cutting edge. The latter causes local consolidation and thus prevents unwanted fabric disintegration at the cutting edge. The cutting and storing of woven 3D preforms are realized within a step, without interruption to the weaving process. This goes to show the advantages of technologically modifying the construction of the machine (supporting bars, stretched take-up, cutting and storing) for a continuous, high quality and reproducible 3D preform manufacture.²⁶

The redesigned and implemented technology is based on technologically modifying the construction of the weaving machine, allowing for multilayered and double-walled woven 3D spacer fabric preforms. Figure 5 shows 17-layered fabric structures (preform thickness 7 mm) making greater component wall thicknesses (composite sheet thickness 3 mm) and component strengths possible for better adjustable FRP properties.^14, ²⁶
Figure 5.
The produced preforms and composites.

Tensile, flexural and compression tests on the 2D composite panels have already been performed in accordance with DIN EN ISO 527-4, DIN EN ISO 14125 (four point flexural testing) and DIN EN ISO 14126 (ASTM D 3410/A Celanese-type) on a Zwick Z100 (Zwick GmbH & Co. KG, Germany) tensile tester at a traverse speed of 1 mm per minute.¹⁴ The determined readings of the mechanical properties of the 2D composite panels show the promising potential for their use in 3D composite structures, since the mechanical properties of the 3D composite structure are a combination of the parameters of the 2D composite sections and the global component structure.

Results and discussion

Production principle of 3D woven preforms with stiffeners in the transverse direction

The following innovative structure developments with stiffeners were prepared based on 3D spacer fabric production technology. For the manufacture of 3D preforms with stiffeners, two warp yarn systems from GF-PP hybrid yarns and a binding system from PP are necessary. Both warp systems’ warp threads, together with the weft threads and the binding warps, form the skin. When forming the stiffener fabric, only the warp threads of the second warp system and the binding warps are used, while the remaining threads from the skin form floats. When the desired length of the stiffener has been woven, the two warp thread systems and the binding warps with the weft threads weave together the skin. Afterward, half of the temporarily stored fabric length, which matches exactly the length of the floating warp yarns and twice the height of the stiffener, is released by the terry weaving mechanism. The floating warp threads are pulled back in warp beam direction. At the stiffener fabric’s last weft thread’s beat-up, the reed pushes the formed stiffener together as a pleat, which allows for the creation of woven 3D preform structures with transverse stiffeners. Figure 6 shows the successfully implemented production principle of transverse stiffeners before (left) and after (right) warp pull-back, and a microscopic cross-section clarifying the alignment of the high-performance yarns in the manufactured preform structure (multilayered woven fabric).
Figure 6.
Structural 3D preform with an integrated stiffener in the transverse direction.

The gusset area between the skin and the stiffener is a significant influence on the stiffener’s strength. After pleat formation or, respectively, the set-up of the stiffener, the pleat opens up again, due to the lack of joints of the warp thread systems at the beginning and the end of the stiffener fabric and the low friction force caused by the binding warps. This disintegration of the pleat causes a gap in the gusset area, leading to structural degradation and irregular geometrical deviations. Pleat formation in the top fabric layer also carries the danger of delamination of the skin, risking composite failure, similar to sewing and welding. It is therefore vital to develop a new weave pattern in order to eliminate the aforementioned disadvantages. In order to achieve this, the z-direction reinforcement of the skin and the gusset area is strengthened by using high-performance fibers instead of PP threads (binding warps) and/or the stiffener fabric is made from the bottom skin layer, ensuring greater strength in the gusset area.

An effective textile engineering solution for the improvement of the pleat formation is the linking of the warp threads from the skin and stiffener fabrics immediately in front of and behind the stiffener, ensuring the friction force between the warp threads necessary to prevent the disintegration of the pleat. Figure 7 depicts the 3D preform structure devised over numerous iterative steps, including a transverse stiffener made from multilayered woven fabric, satisfying the textile engineering demands and fulfilling the composite material engineering requirements for the high quality formation of the gusset area.
Figure 7.
Transverse stiffener and a polished cross-section of the developed gusset area.

Woven stiffeners in the longitudinal direction

The stiffeners in the longitudinal direction are developed on the basis of tubular fabric. Use of the terry weaving technique, in the weft direction (transversely to the take-up direction) is unsuitable in this case. Therefore, a technical solution has been designed for stiffeners in the longitudinal direction. Here, a double woven fabric was produced, containing pattern modifications, located across the fabric width, for the tubular fabric. To make better use of the glass fiber substance strength for the whole component structure, the GF-PP warp and weft threads tied by the PP binding threads are aligned by stretching within the fabric. Afterward, the tubular fabrics are centrally cut (x-direction) and ranged upright in the z-direction (Figure 8).
Figure 8.
Tubular canals (left) and longitudinal stiffeners (right) from multilayered woven fabric.

The length of the stiffeners depends on the tubes’ width and can be designed as desired within the fabric width. Because of the stretched thread alignment, within the whole preform, the gusset area is only weakly formed and does not possess the necessary strength. The toughening of the gusset area can be supplementary improved by a binding modification, such as the redirection of the warp threads in the z-direction right in front of the stiffener.

Technological concept of stiffener integration in the transverse and longitudinal direction

Often, practical applications require stiffener-reinforced shells in the longitudinal as well as the transverse direction. The weaving technology has the potential to realize such preforms. The experience from parts production principles of 3D woven preform with stiffeners in the transverse direction and woven stiffeners in the longitudinal direction are used to manufacture stiffeners in the longitudinal and transverse direction (Figure 9b). Both stiffeners have to be connected continuously in order to achieve the strength expected of high-performance fibers and the rigidity desired in integral construction preforms. The technological step lies in pattern technically realizing double layered fabric from stretched warp and weft threads in certain sections, while the stiffener is still being manufactured and before the warp pull-back. The bottom fabric layer acts as transverse stiffener, while the top fabric layer creates the continuous thread connection of the transverse and longitudinal stiffener (Figure 9). While the transverse stiffener is woven, a cutting head simultaneously slices the weft yarns of the top fabric layer in the warp direction. After the warp pull-back, the top layer is still cut, and arranged upright after the take-up device. Thus, a single weaving process creates a component structure with the transverse and longitudinal stiffeners of different distances and lengths. The gusset area in this case is made from continuous GF-PP fiber reinforcement material along the length of the stiffener.
Figure 9.
Technological concept to manufacture stiffeners in the transverse and longitudinal direction: CAD models of (a) woven stiffeners in the transverse direction and (b) transverse as well as longitudinal direction.

Figure 10 shows the developed and manufactured composite structures made from GF-PP hybrid yarns with a reinforced gusset area. According to the application, GF-PET (polyethylene terephthalate), CF-PEEK (polyether ether ketone) or AR-PEEK can be used as the reinforcement or matrix material and easily processed into the component structures, using the technology described above.
Figure 10.
Composites with integrated stiffeners from multilayered woven preforms.

Conclusion and outlook

This research work was dedicated to the development and production of woven multilayered 3D preform structures with stiffeners made from GF-PP hybrid yarns for FRP applications. By technologically modifying the construction of a double-rapier weaving machine, stiffeners could be formed in the transverse as well as in the longitudinal direction. For a significant increase of composite strength, particularly in large-scale components, the preform structure was extended by weaving, technically integrating transverse and longitudinal stiffeners in a single process step without additional stages (like sewing, bonding or welding). Thanks to the continuous course of the high-performance fibers, the developed 3D preform structure displays a load suitable gusset area from the skin to the stiffener. In the future, the gusset area’s binding technology will be further optimized to increase the quality of the composite rigidity and reproducibility. The challenge lies in combining the demands of a suitable fabric structure, technological implementation and the composite requirements. After validation of the developed technology, further investigations of the mechanical properties and their comparison with reference structures are planned.

The results are an important experiential basis for future activities in the frame of this research work, considering the flexible and economic formation, such as curved 3D preforms and shell–rib structures.

Footnotes

Acknowledgment

This article presents part of the results gained within the A3 and A4 subprojects within the Collaborative Research Center SFB 639 “Textile-reinforced composite components for function-integrating multi-material design in complex lightweight applications” at the Technische Universität Dresden. The fruitful cooperation with NV Michel Van de Wiele Carpet and Velvet Weaving Machines is highly appreciated.

Funding

The Collaborative Research Center SFB 639 is funded by the Deutsche Forschungsgemeinschaft (DFG) at the Technische Universität Dresden. The authors are grateful for their kind financial support.

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