Lamb wave based in-situ structural health monitoring approach for future metallic structures

Open Access

Abstract

All areas of structural design are moving towards high performance structures which minimise material usage leading to desirable savings in cost and weight. However, doing so leads to proportionally lower safety margins. These margins would previously absorb weaknesses which developed through mechanisms such as impact, corrosion and fatigue. This shift is highlighted in the aerospace industry, especially in military applications. Consequently military aerospace structures will be the focus of this thesis. Currently, the lowered safety margins are combated by regular inspections with skilled staff specialised in non-destructive testing (NDT). This is a manual, time consuming job which may require disassembly of components. As a result the frequency of inspections is limited and many hard to reach areas cannot be inspected. An emerging solution to this problem is structural health monitoring (SHM). This approach relies on permanently attached sensors to the structures critical areas. By doing so different approaches (such as baseline comparison) can be utilised and the system can be automated. This increases the frequency of inspection (potentially as frequent as real time) as well as the confidence in the results. Out of the competing technologies ultrasonic Lamb waves have stood out. Their low weight, profile and cost combined with versatility (many capabilities) make them the most promising technology at this time. As such the thesis shall propose a strategy for the implementation of Lamb wave based SHM into future structures during the design stage. Therefore it will contribute validated modeling techniques, signal processing techniques and a design philosophy. In order to formulate a strategy for implementing Lamb waves a variety of methods were developed. A customised automated laser vibrometry (ALV) rig has been built to capture Lamb wave induced displacements across entire plate specimens. The optimisations detailed in this thesis enable the ALV to capture high resolutions at practical speeds making it one of the best in the world. A finite element (FE) model has been developed alongside. Using custom scripting displacements across entire plate has been extracted for direct comparison with high resolution ALV data. This has enabled validation of FE methodology. ALV and FE were used extensively to characterise Lamb waves and establish fundamental parameters such as excitation frequency and modal content. It also revealed subtle features such as modal interference, mode conversion and refraction. These features form the basis for ray trace modelling of Lamb waves, a concept inspired by the field of optics. The development of the ray tracing tool provided an alternative method for modelling Lamb waves with low solution times. Following the development of an array of tools, the calculation of scattered Lamb waves was pursued to enable damage detection and possible quantification. This method was applied to an idealised flat plate which has been widely reported on, followed by a realistic specimen based on a realistic aircraft lower wing skin. This revealed that variations in plate geometry have a substantial influence on scattered waves used in damage detection. To understand this influence, the ray tracing tool was applied to observe the effects of plate geometry. It was realised that this influence could be controlled to direct Lamb waves at the desired inspection areas thereby improving sensitivity and reliability. Both these traits are critical to the success of Lamb wave based SHM. Finally, two case studies were performed where strategies were developed to improve sensitivity and reliability as previously mentioned. The strategies incorporate all the tools and techniques developed here and demonstrate their intended application. In each case existing realistic structures were redesigned to enhance their SHM prospects. The design philosophy focused on directing Lamb wave energy towards the desired detection area. This design change was facilitated by the ray tracing tool. Its ability to solve the effects of plate geometry on Lamb wave propagation in seconds results in near instant feedback enabling iterative design. The proposed optimisations resulted in substantial (50-90%) increase to damage sensitivity of the Lamb wave based SHM system.