Fatigue behaviour of composite tubes under multiaxial loading

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3.3 Fatigue behavior
Fifth International Conference on Fatigue of Composites
Fig. 4. Temperature dependence of tensile and compressive strengths.
The fatigue data for the stress ratios R = 0.1, 0.5, 10, -1.0, -0.553 (= ) that were obtained from
constant amplitude fatigue tests at RT are shown Fig. 5, as plots of maximum fatigue stress level against
the logarithm of number of reversals to failure log(2 N f ) . Note that while the values of max are
plotted for T-T loading and T-C loading at R = , the absolute values of minimum fatigue stress min
are plotted for C-C loading and T-C loading at R =- 1.
The static tensile strength T and compressive
strength C that were obtained at the same test temperature are plotted at 2N f = 1 as points of the
ordinate in the S-N diagram, respectively. The dashed lines in the figure indicate the S-N curves fitted to
the fatigue data. A nonlinear function of the same form as Eq. (8), which is presented later, was used to
describe these S-N curves. The S-N curves fitted to the fatigue data help us not only to observe the
similarity and difference in shape between the S-N relationships for different stress ratios, but also to
evaluate the maximum fatigue stress levels for different constant values of life that are required to
identify the experimental CFL diagram for the woven CFRP laminate; the construction of CFL diagram
is discussed later.
In Fig. 5, it is seen that the S-N relationships at RT greatly depend on stress ratio. The overall
features in the sensitivity to mean stress are similar to those reported so far, e.g. [17-27]. The
sensitivity to fatigue becomes highest under T-C loading at R = , suggesting that a larger value of
alternating stress amplitude has a more degrading effect on the fatigue of the composite. It is also seen
in Fig. 5 that the S-N data for the stress ratios in the range R 1 can approximately be described
by means of the smooth dashed curves that are connected with the point indicating the tensile strength.
By contrast, the S-N curves fitted to the fatigue data for T-C ( R = -1 ) and C-C ( R = 10 ) fatigue
loading can smoothly be connected to the compressive strength. The specimens are apt to fail in a
compressive mode under the completely reversed loading condition at R = -1.0 since = -0.55 -
1