Issue34

S. Keck et alii, Frattura ed Integrità Strutturale, 34 (2015) 371-378; DOI: 10.3221/IGF-ESIS.34.41 373 E XPERIMENTAL I NVESTIGATIONS Mechanical properties he Young’s moduli are measured by a universal testing machine (Zwick Roell Z250) under ambient temperature. Tensile tests were conducted to characterize the mechanical properties, which are required to execute the numerical simulations with real values. The mean values and the standard deviations of the Young’s moduli of the unnotched specimens with different fibre directions and fibre volume fractions are plotted in Fig. 3. Thereby, character ’F’ stands for flax. The number left to the slash describes the yarn count (unit: meters per gram). A ’1’ behind the slash marks a yarn and a ’2’ would mean a twisted yarn. Subsequently, fibre volume fraction and fibre direction are named. The fibre directions are illustrated schematically in Fig. 4. Both constituents (fibres and polymer matrices) and composites have brittle fracture behaviour. Additionally, tensile test results show that there is a non-linear behaviour of the composites. Figure 3 : Young’s moduli of unnotched specimens. Figure 4 : Schematic illustration of compact tension specimens with different fibre directions (0°, 45°, 90°). Crack paths The crack paths of the compact tension specimens were determined under static as well as cyclic loading conditions. In fatigue tests the stress ratio R is set to 0.1. Different values of remote tensile forces were applied, thus, the initial crack is in Mode I. Fig. 5 shows the crack paths of the specimens (fibre volume fraction 35%) under constant amplitude cyclic loading. After executing fatigue tests those were loaded until failure. If the fibre direction is perpendicular to the loading direction, the fatigue crack grows along the fibres (Fig. 5, right hand side). This particular crack path is identical with a path in homogeneous isotropic materials. If the angle between fibre direction and loading direction is about 45°, the crack path kinks at the crack tip and grows along the fibre direction (Fig. 5, middle). If the reinforced direction is parallel to the loading direction, the fatigue crack kinks at the crack tip in fibre direction and grows along the fibres (Fig. 5, left hand T