More orientation in property description


Compared to their continuous fiber-reinforced relatives, short fiber-reinforced thermoplastics are not only considerably less expensive, but can also be processed efficiently in automated processes and in very large quantities to produce components with complex geometries. The mechanical properties have the potential to substitute metallic components in vehicle construction, for example. In order to fully utilize the material for higher-value applications, knowledge of the mechanical properties is essential. Despite the short fiber reinforcement, the properties are distinctly anisotropic. In contrast to continuously fiber-reinforced plastics, however, the fibers do not all lie in directions specified by the design, but exhibit a statistical orientation distribution that is essentially dependent on the flow conditions of the plastic melt in the manufacturing process. This in turn means that, depending on the location, there are both different preferred orientations and different degrees of this preferred orientation in the component. Figure 1 shows an example of a tomographic section through an injection-molded sheet, in which a preferred orientation of the fibers in the injection molding direction can be seen in the shell layers and perpendicularly thereto in the core layer. The direction and characteristic of the fiber orientation in 3D space can be described by orientation tensors.

When dimensioning components made of short fiber-reinforced thermoplastics for structurally loaded components, it is necessary to know the anisotropic properties of the material. Since the mechanical properties are determined by both the preferred fiber orientation and the statistical characteristics of the fiber orientation, a purely experimental determination of characteristic values in different directions does not go far enough. In addition, the tested specimen itself has a given microstructure that determines the properties. Therefore, it is necessary to determine analytical relationships that describe the properties as a function of fiber orientation, fiber orientation characteristics and possibly fiber length distribution. Further dependencies arise from the influence of temperature and moisture.

In a recent study, a method was developed to first determine the material stiffness (Young's modulus) based on a few tensile tests and local fiber orientation distributions. [1] Experimentally, the fiber orientation tensor can be determined by computed tomography. In component development, on the other hand, a determination of the local fiber orientation tensor by means of an upstream process simulation is a suitable approach. By mapping the material stiffnesses as a function of the local eigenvalues of the orientation tensors, an accurate stress and deformation analysis of the component can be performed. As an example, Figure 2 shows the dependence of the Young's modulus on the directionally relevant eigenvalue of the orientation tensor for 30 wt% glass fiber-reinforced polyamide 46 at -20°C.


[1] Hausmann, J., Schmidt, S. and Esha, (2023), Improved Mean Value-Amplitude Method for Determination of Orientation-Dependent Modulus of Short Fiber-Reinforced Thermoplastics. Adv. Eng. Mater. 2300221.

Esha, M.Sc.
Scientific Employee
Fatique & Life Time Prediction
Phone: +49 (0) 631/2017-139

Stefan Schmidt, M.Eng.
Scientific Employee
Mechanical Characterization & Modeling
Phone: +49 (0) 631/2017-274

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