**1. Introduction**

The importance of energy and resource efficiency is constantly growing, consequently, the significance of innovative and interdisciplinary lightweight technologies rises as well. This applies, in particular, to passenger and freight transport, and mechanical and plant engineering [1]. For this reason, high-strength fiber-reinforced plastics (FRPs) are increasingly used, in addition to traditional construction materials, such as aluminum or steel. Due to their fibrous structure, high formability and adjustable directional properties, textile reinforcements (e.g., woven, knitted, braided or laid fabrics) are commonly used in the manufacturing of FRPs [2].

Such FRPs are lightweight, but expensive and often require a high effort of manual labor. Nevertheless, a double-digit growth of carbon fiber-reinforced plastics is expected by 2020 [3]. Therefore, it is crucial to significantly reduce costs by shortening product development cycles and making manufacturing resource-efficient. Computer-aided methods can greatly help with this as they are particularly suitable for complex and even double-curved FRP products. Especially in the early stages of designing an FRP, when various models are considered, costs can be avoided with kinematic draping simulations. In contrast to FE-based simulations, which are suitable for the subsequent planning of automated processes, kinematic draping simulations are faster, easier to handle and less sophisticated with regard to material characterization. Nevertheless, the manufacturing process is taken into account by predefined handling methods. Two important aspects in evaluating kinematic draping results and FE-based forming simulations are the fold-free forming of reinforcing textiles and a load-adapted orientation of reinforcing fibers. In particular, folds occurring during manufacturing must be strictly avoided as they lead to a significant reduction in strength or impaired damage behavior of the final FRP product [4,5].

While numerous investigations focus on FE-based simulation methods and the related characterization of materials [6–10], kinematic draping simulations and their requirements are investigated sparsely. However, only one parameter that limits the formability is needed for the assessment of a kinematic draping simulation. In available software solutions and in the industry, this material-specific parameter is often named "critical shear angle", which is not the same as the "locking angle" as used in the FE-based simulations literature. The "critical shear angle" can be determined by shear behavior testing, although there is no standardized method yet. The investigations and the developed test stand presented in this paper aim to determine the critical shear angle reproducibly and comprehensibly for reliable kinematic draping simulations, and to give an assessment in the product development cycle.

### *1.1. Determination of Shear Deformation Behavior*

Besides tensile and flexural behavior, shear behavior is essential for assessing the overall deformation behavior of reinforcing textiles [11–13]. While the former two are evaluated according to standardized test methods [1], no standard for the determination of shear behavior exists yet, in spite of numerous investigations regarding 2D and 3D characterization [14]. This obviously shows the difficulty of determining textile shear behavior. In developing a shear test method, three assumptions are made for textiles based on less extensible fibers, e.g., glass or carbon: The fibers are inextensible, no slippage occurs between fibers and the flexural stiffness of fibers is negligible [15]. These assumptions are the same as the assumptions in kinematic draping simulations [16–18]. Therefore, shear behavior tests with these assumptions are suitable for determining the "critical shear angle". This angle is defined here as the shear angle at which a specific fold height occurs during shear testing.

The shear process of textiles is divided into two main shear principles: pure shear and simple shear [19]. For simple shear, the distance between the two clamping lines remains constant during the shear process. This leads to an elongation of the unclamped sample edges. It not only changes the angle between the yarns, but also causes tensile strain due to yarn torsion at the crossing points. In contrast, pure shear only changes the angle between the yarns without yarn elongation, because the distance between the clamping lines does not remain constant. Based on these assumptions, shear processes inducing both pure and constant shear on textiles are preferred, e.g., the bias extension test or picture frame test [12].
