A failure-mode based anisomorphic constant life diagram for a unidirectional carbon/epoxy laminate under off-axis fatigue loading at room temperature

Abstract

The off-axis constant fatigue life diagrams for a unidirectional carbon/epoxy laminate in different fiber orientations are identified over the whole range of stress ratio. The experimental results show that the off-axis constant fatigue life diagram plotted in the plane of alternating and mean stresses tends to shrink and incline to the left of the alternating stress axis more significantly as the off-axis angle of a specimen increases. The overall shapes of the off-axis constant fatigue life envelopes for different constant values of life are highly non-linear and asymmetric about the alternating stress axis, regardless of fiber orientation. These observations suggest that the sensitivity to mean stress in off-axis fatigue differs depending on the mode of fatigue loading, i.e. tension-tension, tension-compression, and compression-compression loading, and the difference is associated with the different modes of failure under different modes of fatigue loading. To deal with the off-axis fatigue strength of a unidirectional composite for any fiber orientation over the whole range of stress ratio, a most general form of the anisomorphic constant fatigue life diagram is developed that can take into account different sensitivities to mean stress in fatigue and distinguish between the tensile-and compressivedominated failure modes. It is demonstrated that the generalized anisomorphic constant fatigue life diagram allows accommodating itself to a significant change in shape of a constant fatigue life envelope that depends on the range of stress ratio, and thus it can successfully be applied to accurate description of the off-axis constant fatigue life diagram for the unidirectional carbon fiber reinforced plastic laminate, regardless of fiber orientation.

2D woven fabric composites under fatigue loading of different types and in different environmental conditions

“…This reveals that the shape of CFL diagram for off-axis fatigue loading turns out to be more complicated (Kawai & Itoh, 2014). Obviously, such an off-axis fatigue behavior depends on loading direction relative to fiber direction.…”

Effect of alternating R-ratios loading on fatigue life of woven fabric carbon/epoxy laminates

“…The sensitivity to mean stress in fatigue of the woven CFRP laminate is similar to that of different kinds of composite laminates reported so far. [18][19][20][21][22][23][24][25][26][27][28] The sensitivity to fatigue in the woven CFRP laminate becomes highest under TC loading at R ¼ , suggesting that a larger value of alternating stress amplitude in cyclic loading has a more significant effect on fatigue degradation in the composite. It is also seen in Figure 5 that the S-N data for stress ratios in the range 5 0 R 5 1 can approximately be described by means of the smooth dashed curves that can be extrapolated back to the level of the tensile strength.…”

“…This suggests that the fatigue damage in the woven CFRP laminate develops most rapidly under constant amplitude loading at the critical stress ratio, which is in line with the feature of fatigue degradation that was observed in nonwoven CFRP laminates. [17][18][19][20][21][22] The solid lines in Figures 5 and 6 indicate the S-N curves predicted using the anisomorphic constant fatigue life diagram that is described later.…”

“…The anisomorphic CFL diagram method has been generalized so that it can cope with highly nonlinear shapes of CFL envelopes that are caused by a significant change in sensitivity to stress ratio. 19,20 Moreover, it has been developed into a unified methodology that allows predicting the nonlinear CFL diagrams for CFRP laminates at different temperatures. 21,22 For fatigue life analysis of structures under service loading conditions, it is further required to identify dominant factors that contribute to fatigue failure under variable fatigue loading, and to formulate a criterion for fatigue failure during variable cyclic loading.…”

“…To the best of the present authors' knowledge, however, the effect of variation in mean stress, in other words, the effect of variation in stress ratio, which is independent of the effect of variation in alternating stress amplitude, has not systematically been examined, in contrast to comprehensive investigation into the effect of stress ratio on constant amplitude loading fatigue behavior of composites. [17][18][19][20][21][22] The systematic studies on block loading fatigue of composites by Adam et al 30 and Harris et al 31 have focused on the effect of combination of tension-tension (TT), tension-compression (TC), and compression-compression (CC) cycles in a block on block-loading fatigue life, but the effect of change in waveform, or the change in stress ratio, in a unit level on variable loading fatigue life has not been discussed in detail. In other words, the influence of change in stress ratio on fatigue life of carbon fiber composites has not sufficiently been understood.…”