Volume 4, Issue 4 (April 2007)
Strength of Quasi-Isotropic Co-Cured Composite Joints Under Quasi-static and Fatigue Loading
Three composite joint configurations, namely, the straight laminate (SL), the single lap, and the single nested overlap (SNO) joints are investigated to ascertain the improvement of the SNO joint over the single lap joint under quasi-static and cyclic loading. The SNO joint is a co-cured composite joint design proposed previously, while the SL represents a perfect joint and serves as an upper bound for static strength and fatigue life comparisons. Stress-number curves are generated based on constant amplitude tension-tension cyclic loading at a frequency of 5 Hz and stress ratio (R) of 0.1 for quasi-isotropic graphite/epoxy (IM7/8551-7) joints. The maximum fatigue load is varied between an average of 70-90 % UTS for SL, 20–80 % ultimate tensile strength (UTS) for single lap joints and 40–95 % UTS for SNO joints. Fatigue run-out is defined at 1×106 cycles. The fatigue strength improvement of SNO joint is ascertained quantitatively through two indicators. Three quasi-static acoustic emission (AE) count peaks were identified during monotonic loading before eventually reaching a maximum at ultimate failure. The cumulative AE count peaks are used as a collective measurement of the significant damage event incurred under specific loading and subsequently correlated to the fatigue performance of each of these three joint configurations.