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The most common studies in the literature are those analyzing fatigue life under cyclic loading for flax fiber-reinforced composites. A novel type of staple fiber yarn made from flax tow with almost unidirectional fiber orientation and a quasi-unidirectional fabric was developed for composite applications. Additionally, a hybrid material made of flax and glass was produced for a demonstrator component (an axle tie of a narrow-gauge railway). For such an application, the investigation of fatigue strength is of particular importance. Therefore, the fatigue behavior of flax, glass, and hybrid flax/glass composites was investigated in the high cycle fatigue range. A total of 106 load cycles were carried out. From about 7³ to 8³ loading cycles, the flax laminate was found to have higher fatigue strength than the glass fiber-reinforced composite. The hybrid materials tend to show a higher fatigue strength than the glass type from approximately 2 × 105 load cycles. Results based on a finite element method also demonstrate better fatigue properties at an increased number of load cycles for flax-based composites than the glass fiber-reinforced component. The flax/glass component’s fatigue strength ranged between the flax values and the glass fiber-reinforced composites. Overall, the hybrid material shows significantly better static bending and impact characteristics than flax and considerably better fatigue properties than the glass fiber-reinforced composite making the hybrid material attractive for an application in an axle tie.

In lightweight constructions fiber reinforced polymers are an important material group. They combine low density with high strength and stiffness. The characterization of fiber reinforced polymers includes the characterization of the fibers, the matrix and the intermediate interface. The fibers and the matrix can be characterized on the neat materials. Single fiber tests are typically used to characterize the interface. However, the interaction between different fibers inside a composite is not taken into account in a single fiber test. The investigated sheet molding compound (SMC) has a bundle arrangement of the fibers, where the contact of different fibers is very close and therefore also the fiber-fiber interaction has a high influence. Here we investigate the micromechanical behavior of hourglass shaped micro specimens with several hundreds of fibers with quasistatic tensile tests. For the interface characterization the microstructure of the specimens is rebuilt in a simulation. The glass fibers are modeled as linear elastic, the matrix as hyperelastic and the interface by means of a cohesive zone model.