Guided wave interaction with aerospace aluminium stringer feet

Abstract

Acoustic Emission has shown itself to be a valuable technology for reliably detecting damage initiation and growth in large structures. Monitoring of a structure, throughout its life, is possible with sparse sensor arrays. However aerospace structures can be geometrically complex and contain many structural features, the most common being stringers. Stringers are arranged in a way that they can span the length of the wings or fuselage, separated by less than 200mm in certain cases. Therefore it is almost inevitable that, for any reasonable sensor spacing, acoustic emission events propagating guided waves will interact with multiple stringers. A large aerospace aluminium panel is used to minimise the effects of edge reflections and to allow the two fundamental guided wave modes to separate before reception. It is shown that stringer foot height has a noticeable impact on guided wave propagation, for typical aerospace arrangements. A reduction in transmitted signal amplitude was noted as the stringer thickness was increased. However a local maximum was seen when the stringer foot thickness was equal to that of the plate thickness. This paper discusses quantitative analysis of stringer interaction with the fundamental guided wave modes. The effect of the stringer as a feature has been divided into three main interactions; stringer dimensions, coupling media and riveting. Stringer dimensions and coupling media interactions has been investigated here to quantify their effect on transmission and reflection of the fundamental guided wave modes.