D. Koch et al. Composites Science and Technology 68(2008)1165-1172 2W= 30 mm Fig 9. DEN specimen geometry(left) and fracture surfaces of 0/900 and +45/-45 specimens(center and right specimens and is not yet evaluated sufficiently. FE-model ing shows that locally induced stress concentrations at the notch tip and multiaxial loading conditions lead to a600 damage and succeeding stress redistributions. The dam FE- Calculation aged zone evolves in large areas above and below the liga 50 ment (Fig. 9, center) resulting in strength values comparable to unnotched specimens. The FE calculation is in good agreement with the measured values(Figs. 10 and 11 If DEN specimens are loaded in ofi-axis orientation +45/-45) the strength values decrease strongly with 苏 increasing ligament width and decreasing notch length and this can be verified by calculating the strength values +45°-45° using ligament width as reference cross section. The results suggest a notch induced improvement of material proper ties. however if the failure behavior is evaluated Fig. 9, right)it becomes obvious that the load carry 8101214161820 cross section is not the ligament width only but has to be Ligament Width(mm defined as ligament width plus the length of one notch b300 With this calculation the den results are almost identical with respect to the tensile tests performed in +45/-45 considering orientation as it is shown in Fig. 10b real loaded cross section ▲FE· Calculation The elongation close to the ligament was measured using laser extensometer with a gauge length of 25 mm This results in an integral measurement of inhomogenous strain distribution in the damaged area above and below the load carrying cross section. The experimental curves 100 are shown in Fig. lla for 0/90 orientation with ligament tensile strength widths of 3 mm and 12 mm, respectively. As the failure is concentrated in the area where the strain is measured the experiments can be calculated accurately with the FE- model. In case of testing DEN specimens with +45/-44 orientation(Fig. Ilb)the measured strain values do not Ligament Width [mm] fit the calculated curves in the same perfect manner as Fig 10. DEN tests and respective modeling of strength(a) dependent on the occurring failure is not limited to the gauge length of ligament width and fiber orientation and (b)evaluation of data taking into 25 mm ed cross section(o-axis+45°/-45°specimens and is not yet evaluated sufficiently. FE-model￾ing shows that locally induced stress concentrations at the notch tip and multiaxial loading conditions lead to damage and succeeding stress redistributions. The dam￾aged zone evolves in large areas above and below the liga￾ment (Fig. 9, center) resulting in strength values comparable to unnotched specimens. The FE calculation is in good agreement with the measured values (Figs. 10 and 11). If DEN specimens are loaded in off-axis orientation (+45/45) the strength values decrease strongly with increasing ligament width and decreasing notch length and this can be verified by calculating the strength values using ligament width as reference cross section. The results suggest a notch induced improvement of material proper￾ties, however, if the failure behavior is evaluated (see Fig. 9, right) it becomes obvious that the load carrying cross section is not the ligament width only but has to be defined as ligament width plus the length of one notch. With this calculation the DEN results are almost identical with respect to the tensile tests performed in +45/45 orientation as it is shown in Fig. 10b. The elongation close to the ligament was measured using laser extensometer with a gauge length of 25 mm. This results in an integral measurement of inhomogenous strain distribution in the damaged area above and below the load carrying cross section. The experimental curves are shown in Fig. 11a for 0/90 orientation with ligament widths of 3 mm and 12 mm, respectively. As the failure is concentrated in the area where the strain is measured the experiments can be calculated accurately with the FE￾model. In case of testing DEN specimens with +45/45 orientation (Fig. 11b) the measured strain values do not fit the calculated curves in the same perfect manner as the occurring failure is not limited to the gauge length of 25 mm. 2 4 6 8 10 12 14 16 18 20 0 100 200 300 400 500 600 Experiment +45°/-45° 0°/90° Strength [MPa] Ligament Width [mm] 0 5 10 15 20 0 50 100 150 200 250 300 tensile strength DEN - results from ligament width DEN - results considering real loaded cross section FE - Calculation Strength [MPa] Ligament Width [mm] FE - Calculation Fig. 10. DEN tests and respective modeling of strength (a) dependent on ligament width and fiber orientation and (b) evaluation of data taking into consideration real loaded cross section (off-axis + 45/45). Fig. 9. DEN specimen geometry (left) and fracture surfaces of 0/90 and +45/45 specimens (center and right). D. Koch et al. / Composites Science and Technology 68 (2008) 1165–1172 1171