Xinyuan Tan

Anticlastic Stability Modeling for Cross-ply Composites

A viable aspect of fiber-reinforced composites is their elastic tailoring ability. Anticlastic curvature is one example of elastic tailoring that occurs with an unsymmetric cross-ply layup. Residual stresses resulting from differences in coefficients of thermal expansion and elastic properties in each lamina can cause large out-of-plane deformations. In the case of thin unsymmetric cross-ply laminates under thermal curing load, a cylindrical shape is observed because of this inherent geometrical nonlinearity as opposed to the saddle shape that classical lamination theory predicts. In this article, a finite element approach, using ABAQUS™, is implemented in order to predict the unsymmetric cross-ply laminate shapes under thermal curing stresses and understand the underlying limit point instability. Numerical results for curvatures of the predicted shapes are in agreement with published experimental and analytical data. The stability of the cylindrical laminates is also investigated. Depending on the aspect ratio of the rectangular laminate, a cylindrical shape may snap-through from its current stable configuration to another stable cylindrical shape with a different curvature. In particular, both the critical aspect ratio where snap-through will cease to occur and the buckling load are reported.

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.

Influence of Free-Edge Delamination on the Strength of Composite Joints

A study was conducted to investigate and validate the effect of free-edge delamination in the midplane of IM7/8551-7 eight-plied symmetric quasi-isotropic laminates and joints under both monotonic and cyclic loading. For this purpose, a novel approach of using Infrared Thermoelastic Stress Analysis (IR-TSA) technique to qualitatively monitor damage progression under fatigue was successful in identifying the cohesive fracture mode and the extent of free-edge delamination in co-cured composite laminates and joints. Free-edge delamination was suppressed through re-ordering of the stacking sequence to generate a closing mode in the midplane, resulting in two lay-ups, [45/0/-45/90]s and [45/90/-45/0]s, which enhanced and suppressed free-edge delamination in the midplane, respectively. These two lay-ups were compared with the original [0/±45/90]s lay-up. While the free-edge delamination suppressed lay-up resulted in significantly improved quasi-static strength for the laminates, there was no improvement in fatigue endurance. There was also no significant improvement in quasi-static strength or fatigue endurance for the two types of co-cured composite joints investigated, namely Single Lap (SL) joint and Single Nested Overlap joint (SNO), indicating that free-edge delamination was a critical failure mechanism for the IM7/8551-7 graphite/epoxy laminates but not for the SL and SNO joints. This experimental study shows the effect of stacking sequence and specimen width on free-edge delamination under quasi-static and fatigue loading in IM7/8551-7 eight-plied symmetric quasi-isotropic laminates, SL joints, and SNO joints.