Experimental study of post-buckled single-stringer composite specimens under fatigue loads with different load levels and load ratios

This study aims at better understanding the damage tolerance of stiffened composite panels subjected to fatigue loads in post-buckling regime. Ten single-stringer hat-stiffened specimens with an initial delamination between skin and stringer foot were manufactured, then tested under quasi-static conditions, different load levels ratios. The tests monitored digital image correlation ultrasonic system, providing data on displacements, strains, extension length. results showed that onset, when begins propagating, occurred over twice buckling load, while collapse for values almost 20% higher than onset. During testing below able sustain 150000 cycles then, statically after fatigue, average was reduced by less 10% respect benchmark. When maximum during increased 5% onset still withstood 8000 16500 before collapse, depending ratio. It also seen same level, high ratio had a slower propagation.