Robust Identification of Strain Waves due to Low-Velocity Impact with Different Impactor Stiffness

Abstract

Low-velocity impacts represent a major concern for aeronautical structures, sometimes producing barely detectable damage that could severely hamper the aircraft safety, even with regards to metallic structures. For this reason, the development of an automated impact monitoring system is desired. From a passive monitoring perspective, any impact generates a strain wave that can be acquired using sensor networks; signal processing techniques allow for extracting features useful for impact identification, possibly in an automatic way. However, impact wave characteristics are related to the impactor stiffness; this presents a problem for the evaluation of an impact-related feature and for the development of an automatic approach to impact identification. This work discusses the problem of reducing the influence of the impactor stiffness on one of the features typically characterizing the impact event, i.e., the time of arrival (TOA). Two passive sensor networks composed of accelerometers and piezoelectric sensors are installed on two metallic specimens, consisting of an aluminum skin and a sandwich panel, with aluminum skins and NOMEXTM honeycomb core. The effect of different impactor stiffnesses is investigated by resorting to an impact hammer, equipped with different tips. Subsequently, a method for data processing is defined to obtain a feature insensitive to the impactor stiffness, and this method is applied to multiple impact signals for feature uncertainty evaluation.

Impact Localisation in Composite Plates of Different Stiffness Impactors under Simulated Environmental and Operational Conditions

“…Impacts generate lamb waves that propagate outwards from the impact location as the source [8,9,10]. These waves have been used to identify the location of impacts using multiple methods available [8,9,10,11,12,13,14,15,16,17]. Methods that are based on the physical behaviour of wave propagation in the observed structure may require complex modelling/solving [8,15,18] or may not be suited to complex structures (cut outs, etc.)…”

“…However, besides the parameters assessed in the study [23], variation of impacts can also come from differences in impactor stiffness caused by different impactors (debris, bird strike, tools, etc.) [10]. Out of the few studies that have investigated the effect of impactor stiffness on the generated lamb waves, it was found that lower impactor stiffness generates significantly lower frequency lab waves [10,15,18,25].…”

“…[10]. Out of the few studies that have investigated the effect of impactor stiffness on the generated lamb waves, it was found that lower impactor stiffness generates significantly lower frequency lab waves [10,15,18,25]. These differences can significantly affect feature extraction [10,18], which in turn affects the accuracy of data driven localisation, as the difference in stiffness of a test case compared to the initial training case becomes larger [25].…”