Experimental investigation of frictional behavior in a filament winding process for joining fiber-reinforced profiles
Introduction
Due to people's changing view on resource conservation and sustainability and due to changes in industry and economy, fiber-reinforced plastics (FRP) are increasingly being used in areas where steel or other metals have predominantly been used before. Some of the conventional joining processes, such as welding and bolting, can, however, not be applied for FRPs because of their material properties or because the strength of the component is permanently impaired by damage.
A new way of joining two fiber-reinforced lightweight profiles is to wrap them with carbon fiber tows and thus produce a very light and highly rigid joint [1]. A device with an open, c-shaped stator, which entails a rotating c-shaped rotor, constitutes the central element of the winding process for joining. The open design allows to join closed hollow profile structures without collision. The rotor is equipped with a fiber bobbin, a pretensioning unit and a thread eye for guidance (see Fig. 1). The device is guided by a vertically articulated arm robot. The winding process can be carried out with every kind of fiber tows.
As in the conventional fiber winding process with a rotating mandrel, the winding conditions must likewise be fulfilled for ring winding in order to ensure precise placement and high reproducibility. In addition to the lift-off safety, which is ensured by the geometry of the winding object, the slippage resistance is of paramount importance. Slippage of the tow leads to undefined fiber paths and poor reproducibility of the process. One way to avoid this is by using geodesic winding paths. However, this considerably limits the design space of possible winding paths and makes it impossible to use winding as a joining technique for complex structures because of the complex surfaces (see Fig. 2). Consequently, an adaptation of the local fiber orientation to the mechanical loads is only possible to a limited extent. The optimization potential was shown in [2], [3], [4] by using non-geodesic winding paths and under the consideration of the coefficient of friction.
Section snippets
State of the art in the characterization of frictional behavior of dry carbon fibers
First experiments on friction between dry carbon fibers and a metal surface were carried out in [5]. Roselman and Tabor investigated the influence of surface roughness and normal force utilizing a capstan test setup. Mulvihill et al. were able to gain new insights into the frictional behavior of carbon fiber tows in contact with a flat surface by means of pull-through experiments in combination with an optical measurement of the real contact length of the filaments [6]. Their investigations
Objective of investigations
The main advantage of winding experiments constitutes in the possibility to investigate the friction in the winding process under realistic test conditions. This allows to determine the influence of process parameters such as winding speed or tow tension. However, previous publications have shown that these process parameters do not have any significant effect on the friction coefficient during filament winding. Instead, the material parameters, i.e. the used tows and the properties of the
Materials and experimental setup
In order to analyze the influence of material pairing on the frictional behavior during fiber winding in more detail, tests were carried out with various dry tows. The used fibers and tows are listed in Table 1.
Previous investigations in literature showed that the mandrel surface and the type of fibers used have a large influence on the friction coefficient during the filament winding process. Since the winding process can be used for joining metal profiles as well as plastic or FRP profiles,
Aluminum plates
First, the results of the sled tests in longitudinal and transverse direction are discussed for four aluminum plates of different roughness using the Carbon-Werke NF-24 fibers. Fig. 8 shows the averaged friction coefficients as a function of the surface roughness measured in the direction of motion. The experiments were performed applying five different contact pressures ranging between 1 and 7 kPa. Each run was repeated three times. Consequently, each point in the diagram represents the
Discussion
The predominant slipping mechanism has proven to be central to the frictional behavior of dry fibers in transverse direction, but the influence of surface roughness and the correlation between the test principles are equally crucial aspects. Two different modes of the transverse frictional behavior of carbon fiber tows have been identified. They have been contributed to an interaction between friction and tow-mechanics. The results of the winding tests and sled tests in transverse direction
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been supported by the German Research Foundation (DFG: Deutsche Forschungsgemeinschaft, Project-Nr.: 397377132).
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