Experimental study on post-buckled composite single-stringer specimens with initial delamination under fatigue loads

The fatigue damage tolerance of a composite stiffened structure in post-buckling conditions is experimentally investigated in this work. Single-stringer specimens with an initial delamination, artificially created during the manufacturing process, are tested under cyclic compressive load. Six nominally identical specimens are manufactured: two tested under quasi-static load to understand the compressive behavior of the structure and four under fatigue load cycling between pre- and post-buckling conditions at two different maximum loads. During the tests, digital image correlation system and ultrasonic C-scan are adopted to follow the evolution of the out-of-plane displacements and the propagation of the delamination. Depending on the load level, the delamination starts to grow already in the first cycle or after a few thousand cycles, but in both cases the propagation is fast at the beginning, then slows down gradually. The fatigue tests are interrupted after 150,000 cycles and the specimens are subjected to quasi-static compressive load to evaluate the residual strength of the structure.