Fatigue life and damage tolerance of postbuckled composite stiffened structures with initial delamination

The durability and damage tolerance of postbuckled composite structures are issues that are not completely understood and remain difficult to predict due to the nonlinearity of the geometric response and its interaction with local damage modes. A research effort was undertaken to investigate experimentally the quasi-static and fatigue damage progression in single-stringer compression specimens. Three specimens were manufactured with a co-cured hat stringer, and an initial defect was introduced with a Teflon film inserted between one flange of the stringer and the skin. Pre-test finite element analyses were conducted using the virtual crack closure technique to select the range of defect sizes to be considered and the load levels to be applied during the fatigue tests. The tests were monitored with digital image correlation, passive thermography, and ultrasound systems. After an initial opening and extension of the Teflon-induced embedded defect, the specimens sustained a high number of cycles. It was observed that when the skin/stringer separation develops in the opposite flange, it propagates rapidly within a small number of cycles and causes the collapse of the specimen. These test results contribute to a better understanding of the complex response phenomena exhibited by postbuckled stiffened structures subjected to fatigue loads in the postbuckling range.