## Info

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.

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

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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