Composite materials make up an increasing portion of today’s aerospace structures (see, e.g. Boeing 787 and Airbus 380). These aircrafts’ fuselage, for example, is composed of a laminated composite skin connected to composite stringers and C-frames. Of primary importance is the detection of damage in these built-up structures, whether caused by the manufacturing process or in service (e.g. impacts). A related issue is the characterization of the composite elastic mechanical properties, that can also be related to the quantification of potential damage. Guided elastic waves propagating in the ~100s kHz regime lend themselves to provide the necessary sensitivity to these two conditions (damage and mechanical properties). This presentation will discuss the use of these waves to provide information on both damage and mechanical properties of composite structures that are typically used in modern commercial aircraft fuselages. In particular, a scanning system using air-coupled ultrasonic transducers and transfer function reconstruction will be presented for the detection and the quantification of impact-induced damage in laboratory test panels representative of fuselage construction. An optimization scheme that uses Simulated Annealing and the Semi-Analytical Finite Element (SAFE) technique as the forward model will be used to identify the layer-by-layer elastic properties of the composite skin laminate by observation of the guided wave phase velocity dispersive behavior.
Ultrasonic guided-wave testing can greatly benefit from (1) an ability to provide quantitative information on the damage that is being detected, and (2) an ability to select the best mode-frequency combination for maximum sensitivity to a given type of damage. Achieving these capabilities in complex structures (e.g. nonprismatic structures such as a stiffened panel in aerospace fuselages) is a nontrivial task. This paper will discuss an improved Global-Local (GL) method where the geometrical “local” discontinuity (e.g. the stiffener) is modelled by traditional FE discretization and the rest of the structure (“global” part) is modelled by Semi-Analytical Finite Element (SAFE) cross-sectional discretization. The boundaries of the “local” domain and the “global” domain are then matched in terms of wave displacement and stresses. GL models have been proposed in the past using theoretical (Lamb) wave solutions that only apply to isotropic plates. The authors have also previously studied GL methods using the SAFE approach for application to multi-layered anisotropic plates for which theoretical solutions are either not existent or hard to obtain. This work will extend recent research on these methods by optimizing the Matlab routine that is used to run the GL code, correcting some formulation errors that were present in the previous edition, and studying the specific case of a composite panel stiffened with cocured stringers that is representative of modern commercial aircraft construction (e.g. Boeing 787). The newlyformulated GL method will be shown to provide excellent results that can help designing a guided-wave test on these aircraft components for optimum detection of relevant damage that can be induced by impacts (including skin delaminations, stringer heel cracks, and stringer to skin disbonds). Other applications of the GL methods beyond stiffened aircraft panels will be discussed.