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

采用碳纤维增强聚醚醚酮(CF/PEEK)的双悬臂梁(DCB)试件，研究了高温蠕变和疲劳条件下的分层裂纹扩展行为。主要研究结果如下：(1)蠕变条件下，I型分层裂纹沿基体/纤维界面扩展。扩展速率与弹性能释放率有很好的相关性，这是由于纤维对蠕变变形的约束导致的小范围蠕变占主导地位。(2)高温疲劳裂纹扩展分为时间相关和周期相关两类。前者是由基体蠕变引起的，裂纹扩展速率随时间的变化由与静态蠕变相同的能量释放率决定。后者的行为类似于室温疲劳，裂纹扩展速率对应力周期的影响受能量释放率范围的控制。(3)疲劳周期内的应力保持在零水平，大大减缓了分层裂纹的扩展。讨论了碳化硅晶须增强6061铝合金的高温疲劳试验方法，考察了在最大应力和最小应力下保持应力的蠕变疲劳变形行为。保温期蠕变应变范围与应力呈幂函数关系。

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.