Experimental Investigation of the Postbuckling Response and Collapse of a Single-Stringer Specimen

The postbuckling response and the collapse of composite specimens with a co-cured hat stringer are investigated experimentally. The specimens are designed to evaluate the postbuckling response and the effect of an embedded defect on the collapse load and the mode of failure. Tests were performed using controlled conditions and instrumentation that included pre-test ultrasonic inspections and measurement of initial geometric imperfections, strain and displacement measurement through strain gauges, LVDTs and three-dimensional digital image correlation system, as well as high-speed video cameras. The test results reveal that minor imperfections due to manufacturing and residual thermal strains can result in large differences in the postbuckling responses. In addition, an embedded delamination can cause a reduction of the collapse load of about 17% for a 20-mm Teflon insert and about 28% for a 40-mm Teflon insert. Using a high speed camera, it was also observed that the collapse initiates as a skin/stringer delamination, which induces an immediate crippling of the stringer. The results obtained with these inexpensive-to-manufacture panels indicate that these test specimens can be useful for the evaluation of damage tolerance of postbuckled structures and could therefore fill the gap between test coupons and multi-stringer panels in the building block approach to the design and certification of aerospace structures.