PREDICTION OF POSTBUCKLING STRENGTH OF STIFFENED LAMINATED COMPOSITE PANELS BASED ON THE CRITERION OF MIXED-MODE STRESS INTENSITY FACTORS

An analytical investigation is conducted to predict the post-buckling strength of laminated composite stiffened panels under compressive loa ds. When a stiffened composite panel buckles, the skin would deform in to a sinusoidal mode shape, and hence induces additional moments and f orces near the skin-stiffener interface region. These induced loads wo uld cause the existing small edge delamination cracks to propagate alo ng the skin-stiffener interface, and this in turn would lead to the gl obal failure of the stiffened panel. To reduce the cost of the analyti cal investigation, the failure of the stiffened panel under post-buckl ing loads is modeled in two stages: a global analysis to model the pos t-buckling behavior of the stiffened panel; and a local analysis to mo del the onset of propagation of the edge delamination crack at the ski n-stiffener interface. The results from this study are compared with a n experimental investigation conducted by Starnes, Knight, and Rouse ( 1987). It is found that for the eight different specimens that are con sidered in this study, the calculated critical energy release rate for the propagation of the edge delamination crack in each specimen diffe rs substantially from those for the others; hence it may be concluded that the total energy release rate would not be a suitable fracture pa rameter for predicting the post-buckling strength of the stiffened pan els. On the other hand, using the fracture criterion based on the crit ical mixed-mode stress intensity factors, the predicted post-buckling strength of the stiffened panels compares quite favorably with the exp erimental results and the standard deviation of the error of predictio n is less than 10%. Furthermore, by applying the criterion of critical mixed-mode stress intensity factors on a simple damage model, the pre sent analysis is able to predict the significant reduction in the post -buckling strength of stiffened panels with a damage due to a low-spee d impact at the skin-stiffener interface region.