Issue 38

S. Hörrmann et alii, Frattura ed Integrità Strutturale, 38 (2016) 76-81; DOI: 10.3221/IGF-ESIS.38.10 80 For the fatigue results, the moment of damage initiation was found to be dependent on the distance c (Fig. 2c). Thus, three ranges of the distance c were defined in order to study its influence on the test results; for folds with a smaller distance c damage initiated earlier for all considered load cases and load ratios. However, for T-T as well as T-C at small c values, failure occurred within the scatter band of the tests without defect. For T-C with c > 3 mm the fatigue life is increased, compared to the material without defect. Transverse to Fibers Direction In the transverse to fibers direction the damage mechanism is inter fiber fracture. In the unidirectional laminate the specimens fail within one load cycle due to fracture through the whole thickness of the laminate; no progressive damage takes place. Two different damage mechanisms were observed for the transverse load cases. One is tension failure, corresponding to static tension and fatigue T-T and T-C (the transverse static tension strength is only 30 % of the static compression strength). The transverse tension failure is due to matrix cracking or interfacial debonding between matrix and fibers with a fracture plane perpendicular to the load direction. This fracture plane in most of the cases was located at the turning point of the fold in the resin rich area. Figure 4 : Transverse fracture: (a) at the fold turning point, T-T loading (before and after test); (b) at resin rich area away from fold, T-T loading; (c) at both outer turning points, C-C loading (before and after test). Fig. 4a shows the edge of a T-T specimen before and after fracture at the inner turning point of a fold. The two additional plies in the folded area are displayed darker and the crack is indicated by the dashed line. However, for two of the T-T specimens fracture was observed not at the fold area, but within the gauge section away from the fold, where at least three gaps between tows occurred aligned along the same vertical line, see Fig. 4b. It follows that in transverse tension additional weak resin rich areas between additional tows at the fold have a higher influence on the static and fatigue strength than the local increase in fiber volume fraction. (a) (b) (c) Figure 5 : Transverse to fiber direction SN curves, normalized on the static strength.