Issue34

S. Keck et alii, Frattura ed Integrità Strutturale, 34 (2015) 371-378; DOI: 10.3221/IGF-ESIS.34.41 377 G EOMETRY C ORRECTION F ACTOR F UNCTIONS he evolution of the cyclic equivalent stress intensity factors with the growing crack does not follow the well- known stress intensity factor solution for compact tension specimens proposed by Srawley [6] since the material is no longer homogeneous and isotropic, in consequence the crack is not only governed by the stress state. It is necessary to carry out extensive numerical simulations to evaluate the stress intensity factor evolution along the growing crack in order to be able to determine fatigue crack growth rate curves. For numerical simulations experimentally determined crack lengths are required to calculate the corresponding stress intensity factors. Hence, special functions for calculating the geometry correction factors for each fibre direction and fibre volume fraction are needed. Here, the geometry correction factor Y is computed with the equivalent stress intensity factor K eq, the specimen thickness B , the length of the ligament (specimen width) X , and the applied force P to eq K B X Y P    (3) The single geometry correction factors can be expressed by a mathematical function. Commonly, the function depends on the crack length a and the length of the ligament, mostly named with the character W . Here, those lengths are denoted with the character X to point out that X is a directional parameter. For example, if the fibre direction is perpendicular to the loading direction (90°), X is assumed as the originally length of the ligament W . The function is described by 2 3 4 5.5 2 20 8 70 a a a a a a Y Y X W W W W W                                           (4) By means of the geometry correction factor functions it is possible to calculate values of the stress intensity factor and obtain fatigue crack growth rate curves for different fibre directions. The geometry correction function data for a compact tension specimen with 90° fibre direction are plotted in Fig. 10. For further fibre directions functions need to be developed. Therefore, it is suitable to only modify the parameter X in Eq. (4). Concerning this matter the length of the ligament X could be changed directional. Figure 10 : Geometry correction factor data for compact tension specimen (90° fibre direction). C ONCLUSIONS AND O UTLOOK n this paper mechanical and fracture mechanical behaviour of natural fibre-reinforced composites were presented. Thereby, crack paths for compact tension specimens with different fibre directions and fibre volume fraction were under consideration. Both fibre direction and fibre volume fraction influence the crack path. The more fibres are T I