Basic Research on Strength of Composite Materials under Creep-Fatigue Interaction at Elevated Temperatures

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Behavior of delamination crack propagation was examined under creep and fatigue conditions at elevated temperatures using double cantilever beam (DCB) specimens of carbon-fiber reinforced poly-ether-ether-kethon (CF/PEEK). The results are summarized as follows.(1) Mode I delamination crack propagated along the boundaries between matrix and fibers in creep conditions. The propagation rate was correlated well with the elastic energy release rate, as the small scale creep, which was due to constraint of creep deformation in the matrix, by the fibers, dominated the propagation.(2) The crack propagation in elevated temperature fatigue was classified into two categories, time-dependent and cycle dependent ones. The former was caused by the matrix creep and the crack propagation rate against the time was governed by the energy release rate being the same as that in static creep. The behavior in the latter was similar to that in room temperature fatigue and the crack propagation rate against the stress cycles ws controlled by the range of energy release rate.(3) The stress hold at zero level during the fatigue cycle drastically decelerated the delamination crack propagation. This was caused by the creep recovery of matrix.The method of fatigue test at elevated temperatures for a SiC whisker reinforced 6061 alminum alloy was discussed and the deformation behavior was examined in creep-fatigue with stress hold at the maximum and minimum stresses. The creep strain range during the hold period had a power relation with the stress.