Leibniz-Institut für Verbundwerkstoffe (IVW) has been active in the research field of fiber composites for more than 30 years, from the concept to the finished part. During this time, it has also been able to acquire profound knowledge in the field of filament winding technology. Fiber winding is an established process that is widely used to manufacture rotationally symmetrical fiber composite components such as pipes, pressure vessels and shafts. Over the years, it has proven to be an extremely efficient method for producing complex structures. Due to the often highly curved surfaces that need to be wound on with fiber material, fiber adhesion on the surface is essential. Too little fiber adhesion can lead to a displacement of the rovings, which are then no longer on the predetermined path, but in an undefined area of the component, without contributing to the overall strength of the component to the calculated amount. Among other things, precise knowledge of the friction coefficient between the roving and the surface is therefore crucial for its planning and control, irrespective of the specific winding process. In the worst case, an incorrect estimation of the frictional properties can significantly change the quality of the wound component. In addition, unwanted slippage of the fibers on the component can lead to disruptions in the process flow.
Fiber displacement on curved surfaces always occurs when the geodesic path is left and the adhesion force of the roving is not sufficient to hold the fiber in position. The geodesic path describes the shortest connection between two points on a surface. Uniaxial stress state prevails here, which means that forces only occur along the longitudinal axis of the fiber. Outside of the geodesic path, restoring forces act on the fiber, which favors slippage. A simplified sketch of the equilibrium of forces acting on the roving as well as the effects of insufficient adhesion force is shown in Figure 1.
The exact determination of the coefficient of friction between fiber and surface is crucial for the entire winding process, however, it is a complex task influenced by many factors (surface condition, temperature, fiber type, etc.). Therefore, various methods have been developed in the past to determine the coefficient of friction. A common determination technique is the capstan method, in which a bundle of fibers is placed over a roller and loaded with a weight. The resulting forces allow conclusions to be drawn about the friction properties. However, due to the specific design of the measurement system, the coefficient of friction can only be determined along the fiber. As a result, the findings of the measurement can only be transferred to the prevailing stress state during the winding process to a limited extent. Another common method uses specially shaped winding mandrels to visually record the slippage behavior of the fibers. This allows to determine the behavior very close to the real production process, but the visual evaluation offers only limited possibilities for automating the measurement process. In practice, other methods are used to determine the coefficient of friction, but they all have different limitations and disadvantages.
In order to obtain repeatable and automatically evaluable results for the coefficients of friction of fibers, a system for determining the coefficient of friction transverse to the longitudinal direction of the fibers was developed at IVW. This makes it possible to circumvent some of the major disadvantages of current measurement methods and thus to perform repeatable and automated measurements for determining the coefficient of friction. Both, material parameters such as the fiber type and process parameters such as the surface property for deposition, can be varied and adapted to the desired conditions (fiber-fiber contact, different take-off angles, temperature, aging, etc.). The highly variable setup thus offers extensive potential for investigating the coefficient of friction and its influencing factors. Especially in the manufacture of hydrogen pressure tanks and their specially shaped dome area, knowledge of the exact coefficient of friction is of crucial importance as this has a direct influence on the quality and performance of the composite shell produced. To optimize the layer structure of the pressure tank, it may be necessary to leave the geodesic path. Using an incorrect value for the coefficient of friction can lead to irregular fiber placement, which in turn affects the mechanical properties of the final product. In addition, a correct coefficient of friction can reduce the necessary transfer paths between layers of high and low winding angles to the absolute minimum. Therefore, accurate characterization and control of the coefficient of friction is a critical factor in ensuring that manufactured pressure tanks have the required strength, reliability and safety. In addition, the resulting optimized layer structure and reduction of transfer paths can save cost-intensive fiber material, resulting in a more economically efficient process.
Figure 2 shows the setup of the test rig in a test-ready state with clamped fiber and a characteristic measurement result for a friction value measurement of a carbon fiber towpreg. During the buildup of the fiber adhesion force, a force increase can clearly be seen in the course of measurement. After exceeding the adhesion force, a sharp drop in the measured force follows. This then changes to a sliding friction curve.
The results of the tests can be used to determine the influence of temperature, aging, surface properties, etc. on the adhesion force of the roving on a defined surface. Transferring the results to the practical winding process supports to further optimize with regard to the economy of the overall process and component performance. As a result, the load-path-compliant design of struts, tubes, or pressure vessels such as hydrogen storage tanks, can be precisely adapted to the specific adhesive properties of the fiber material. Slippage of the fibers and the associated reduction in component performance is thus avoided. In addition, precise knowledge of the coefficient of friction means that transfer paths within wound components, which are unnecessary for component strength, can be reduced to the absolute minimum. This saves both fiber material and weight.
Dipl.-Ing. Benedikt Bergmann
Roving- & Taper Processing
Leibniz-Institut für Verbundwerkstoffe GmbH
Telephone: +49 631 2017 304