Elastic Waves from Localized Sources with Applications to Nondestructive Evaluation (NDE) of Composite Aerospace Structures
In the aerospace industry, the non-destructive evaluation (NDE) of reusable composite structural components in launch vehicles requires the development and implementation of efficient and reliable techniques. Most conventional damage detection methods do not meet these requirements. Recently develop methods using guided ultrasonic waves have the potential to improve the efficiency and reliability of damage detection in aerospace structures. However, they rely on previously collected baseline data that can be different in similar parts in composite structures due to their manufacturing tolerances. To overcome this difficulty, databases from analytical and numerical models may be employed to be correlated with data collected during the NDE of the structures. Nonetheless, the lack of knowledge of the exact material properties -- together with manufacturing tolerances -- could lead to erroneous conclusions. Evaluation of these uncertainties can be difficult and occasionally infeasible. Rapid and reliable guided ultrasonic wave-based techniques, which include uncertainty in models and material parameters and can localize and characterize flaws in large composite structures, could gain acceptance for the NDE of reusable structures.In this work, the first objective is to study guided waves formed by the initiation of microcracks in composites, aiming to improve the current passive NDE techniques. Based on an approach developed by seismologists to study earthquake sources, an efficient model has been developed based on a new form of the Green's function to identify the ultrasonic waves produced during the crack formation. The second objective in this work is to develop a methodology to implement the ultrasonic guided wave based technique as one of the qualifications criteria in composite manufacturing. A baseline-free technique to localize delamination in fiber-reinforced composites is proposed. The technique compares data from waves propagating in the healthy regions of the structure against data from waves crossing defective areas. Therefore, the percentage of the damaged region in the inspection area needs to be small. A damage index approach based on the cross-correlation and energy carried by the signals is used for path identification, with imaging techniques to visualize the damage. The baseline-free guided ultrasonic wave-based technique has been used to identify an artificial delamination in a quasi-isotropic fiber-reinforced composite plate. The technique has shown acceptable results when wave reflectors are not present in the vicinity of the delamination. A technique based on guided ultrasonic waves, coupled with an uncertainty analysis, is developed to quantify the deviations from the assumed nominal values of the material constants of quasi-isotropic fiber-reinforced composites. It is shown using a fuzzy arithmetical assessment that the measured group velocities vary depending on the location within the laminate, opening the possibility of questioning if the assumed nominal values of the material properties could accurately represent the entire material system in any region. Furthermore, after the identified material parameters are defuzzyfied, a new set of nominal values for the material properties is determined.