How do lamb waves propagate in complex C-FRP components?

The use of Lamb waves (LW) for non-destructive testing of C-FRP components has become established in the aerospace sector due to their advantageous propagation characteristics in plate-like structures. However, the propagation of LW modes is very complex with multifarious factors such as utilised ultrasonic signal, the mechanical behaviour of the materials involved as well as type and location of the defect to be found influencing the developing wave field–just to name a few aspects. In addition, the geometry of the component to be inspected plays a vital role when LW are used to find anomalies as it alters many of the boundary conditions mentioned above. Although different C-FRP components have already been studied in literature, there is a lack of a comprehensive study that clearly and concisely illustrates geometric parameters on Lamb wave propagation in these lightweight components. The target of the present study was thus to identify and discuss similarities and differences for a congruent and geometrically consistent testing program involving various C-FRP components of increasing geometric complexity and affected with different types of damages. To achieve this goal, five numerical models were developed, ranging from comparatively simple monolithic laminates with different thicknesses to more complex shapes such as sandwich components with doublers attached to the facesheets. First, the numerical results were validated using an ultrasonic inspection system that allows for reuse of the tested components by combining a temporarily adhesively bonded actuator with an air-coupled ultrasonic receiver for the ultrasonic measurement. The validated numerical models were then used to highlight technical relationships as well as possibilities and challenges of LW-testing for C-FRP components–important aspects that were finally brought into an overarching context.