Experimental evaluation of surface damage relaxation effect in carbon-fiber reinforced epoxy panels impacted into stringer

Abstract Evaluation of the impact damage visual detectability threshold in composite structures requires not only determination of damage state right after the impact, but also at the actual moment of the visual inspection. Due to the specific features of aircraft maintenance programs the time between those two events may be considerable. During that time the damage size in the structure, subjected to operational and environmental factors, can be reduced (relaxed). To characterize the relaxation effect in typical aircraft stiffened panels, the long-term behavior of surface impact dents was investigated. It was demonstrated that among all the factors potentially reducing the dent depth, the combination of moisture and elevated temperature appears to be the critical case. Based on the results of the study the input data for damage tolerance analysis of commercial aircraft composite structures was obtained and recommendations on its applicability were formulated.