Prebuckling, buckling, and postbuckling response of segmented circular composite cylinders

Discussed is a numerical and experimental characterization of the response of small-scale fiber-reinforced composite cylinders constructed to represent a fuselage design whereby the crown and keel consist of one laminate stacking sequence and the two sides consist of another laminate stacking sequence. This construction is referred to as a segmented cylinder. The response to uniform axial endshortening is discussed. Numerical solutions for the nonlinear prebuckling, buckling, and postbuckling responses are compared to experimental results. Focus is directed at the investigation of two specific cylinder configurations, referred to as axially-stiff and circumferentially-stiff cylinders. The eight-layer stacking sequence for the crown and keel segments is [+/-45/02] S for the axially-stiff cylinder and [+/-45/902] S for the circumferentially-stiff cylinder. The eight-layer side laminates are [+/-45]2S in both cylinders. Four methods are used to predict the buckling values of endshortening and load for both cylinders, and the four values are in good agreement. The experimentally-measured buckling conditions, however, show that the models overpredict buckling values. For the axially-stiff cylinder, the difference could be due to the fact material failure not included in the model plays a role in the cylinder response. For the circumferentially-stiff cylinder, the difference is definitely due to material failure characteristics not included in the model. The predicted postbuckling response of the segmented cylinders is shown to be dominated by the existence of inward dimples in some or all of the segments. For the axially-stiff cylinder, the postbuckled shape is predicted to consist of a single inward dimple in the crown region and one in the keel region. The predicted postbuckled shape of the circumferentially-stiff cylinder is dominated by two circumferential rings of six inward dimples having the cylinder midlength as a line of anti-symmetry. For the axially-stiff cylinder, the dimpled crown and keel configuration is observed in the experiment but at a load 12% below predicted values. Material failure is present in the postbuckled configuration and it is felt that it occurred just prior to or just after the buckling event. For the circumferentially-stiff cylinder material failure in the linear prebuckling range of response triggered buckling that resembled circumferential rings of dimples, but at a load 31% below predictions. It is felt that the lack of any fibers in the axial direction in any portion of the cylinder was responsible for the behavior of the circumferentially-stiff cylinder. Finally, it is shown that the effect of including the measured imperfections in the model has little observable effect on the circumferentially-stiff cylinder. For the axially-stiff cylinder the inclusion of the imperfections is found to effect the transition from buckling to postbuckling, but ultimately has little effect on postbuckling deformations. The study concludes with recommendations for future activities on this topic.