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FIGURE 1.15 S-N curves of 95% survival probability under C-C loading. 0 = 0°, 45°, 90°.
FIGURE 1.16 S-N curves of 95% survival probability under C-C loading. 0 = 30°,60°.

Figs. 1.15-1.21, where experimental data are plotted along with theoretical predictions for comparison. It is clearly seen for all the cases treated, that is, 17 S-N curve series of tests at various R ratios and 0 values, that the statistical procedure implemented performs very well and thus allowables derived that way can be reliably used in design.

FIGURE 1.17 S-N curves of 95% survival probability under T-C loading. 0 = 0° 45°, 90°.
FIGURE 1.18 S-N curves of 95% survival probability under T-C loading. 0 = 30° 60°.

The effect of stress ratio R, on fatigue strength, depending also on off-axis loading orientation, was discussed in Section 1.10.2.1. The same trends are exhibited by 95% reliability S -N curves, and this can be shown by plotting in the same graph curves with different R values. The fatigue strength dependence on stress ratio for on-axis loaded coupons is illustrated in Fig. 1.22 while respective

FIGURE 1.19 S-N curves of 95% survival probability under T-T (R = 0.1) loading. e = 0°, 45°, 90°.
FIGURE 1.20 S-N curves of 95% survival probability under T-T (R = 0.1) loading. e = 15°, 75°.
FIGURE 1.21 S-N curves of 95% survival probability under T-T (R = 0.5) loading. e = 0° ,45°.
FIGURE 1.22 Effect of stress ratio R on 95% survival probability S-N curves. On-axis loading.

results from 45° off-axis tests are shown in Fig. 1.23. It is therefore verified that for on-axis loading the GRP laminate investigated is weaker to compressive stress ranges when N < 106, while for high-cycle fatigue it can withstand lower tensile stress ranges than compressive ones. In the contrary, for off-axis loading, compressive stress ranges withstood by material coupons were almost double the respective tensile ones; see Fig. 1.23.

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