TEST AND ANALYSIS OF COMPOSITE HAT STRINGER PULL-OFF TEST SPECIMENS

Hat stringer pull-off tests were performed to evaluate the delaminatio n failure mechanisms in the flange region for a rod-reinforced hat str inger section. A special test fixture was used to pull the hat off the stringer while reacting the pull-off load through roller supports at both stringer flanges. Microscopic examinations of the failed specimen s revealed that failure occurred at the ply termination in the flange area where the flange of the stiffener is built up by adding 451-45 ta pe plies on the top surface. Test results indicated that the as-manufa ctured microstructure in the flange region has a strong influence on t he delamination initiation and the associated pull-off loads. Finite e lement models were created for each specimen with a detailed mesh base d on micrographs of the critical location. A fracture mechanics approa ch and a mixed mode delamination criterion were used to predict the on set of delamination and the pull-off load. By modeling the critical lo cal details of each specimen from micrographs, the model was able to a ccurately predict the hat stringer pull-off loads and replicate the va riability in the test results.