Lamb Wave Interaction at Delamination and Debondings Due to Impact Damage in Complex Stiffened CFRP Structures

The increased usage of carbon fiber reinforced plastics (CFRP) for primary aerospace structures involves dealing with the susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among impact sources is the so-called blunt impact, which may cause damage primarily to the internal structure. Therefore, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for structural health monitoring by guided ultrasonic waves. Wave propagation phenomena at impact damages as well as the signal processing utilized to extract a damage related feature (i.e., damage index (DI)) contribute to the sensitivity, and thus, to the reliability of structural health monitoring (SHM) systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of delaminations and stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for the use with imaging algorithms, such as the reconstruction algorithm for the probabilistic inspection of damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of delaminations and debondings as well as the implications on imaging algorithms is examined.