Issue34

S. Keck et alii, Frattura ed Integrità Strutturale, 34 (2015) 371-378; DOI: 10.3221/IGF-ESIS.34.41 374 side). In either case the crack paths are determined by the fibre direction and there are inter-fibre failures. The same crack paths were obtained by testing compact tension specimens under static loading. Figure 5 : Compact tension specimens with different directions after cyclic loading (fibre volume fraction 35%). Further investigations showed that there is a relation between fibre volume fractions and crack paths. The dependence on the fibre volume fraction using compact tension specimens (0° fibre direction, constant amplitude cyclic loading, R = 0.1) is illustrated in Fig. 6. The more fibres are contained, the faster the crack path tends to grow in fibre direction. Figure 6 : Compact tension specimens with different fibre volume fractions (0° fibre direction). F ATIGUE CRACK GROWTH RATE CURVES he fatigue tests were conducted on a resonance testing machine (Roell Amsler 100 HFP 5100) under constant amplitude cyclic loading and ambient temperature. The stress ratio R is set to 0.1 and the frequencies are about 30 Hz. The fatigue crack growth rate curves were determined by experimental investigations and application of numerical simulations. The crack length a is detected by an optical measuring system. In conjunction with the recorded load cycles N the crack growth rates da / dN can be calculated. The equivalent stress intensity factors K eq and the cyclic equivalent stress intensity factor Δ K eq are calculated according to Richard [3] as 2 2 2 1 2 1 4( ) 4( ) 2 2 I eq I II III K K K K K       (1) where α 1 = K Ic / K IIc and α 2 = K Ic / K IIIc . There is no formula available to calculate the stress intensity factors for such anisotropic material. Hence, those are determined by the three-dimensional crack simulation program ADAPCRACK3D. The program uses the modified virtual crack closure method to determine the energy release rates and corresponding values of stress intensity factors [4]. The correlation function according to the ASTM standard E647-13 [2] to examine the crack length in order to the crack tip opening displacement is not applicable. Therefore, it is necessary to calculate the stress intensity factors corresponding to the determined crack lengths. A three-dimensional finite elements model of the specimen is constructed in the T