ABSTRACT

Advanced composites are being increasingly used in state-of-the-art aircraft and aerospace structures. In spite of their many advantages composite materials are highly susceptible to hidden flaws that may occur at any time during the life of a structure and if undetected, may cause sudden and catastrophic failure of the entire structure. An example of such a structural component is the "honeycomb composite" in which thin composite skins are bonded with adhesives to the two faces of extremely lightweight and relatively thick metallic honeycombs. These components are often used in aircraft and aerospace structures due to their high strength to weight ratio. The degradation of composite materials and composite structures can be assessed using ultrasonic nondestructive evaluation (NDE) techniques. For successful application of these techniques to locate and estimate the severity of the damage, it is extremely important to understand the propagation characteristics of ultrasonic waves in these structures. Wave propagation in composites is extremely complex due to its material inhomogeneity and anisotropy, where characteristics of the waves depend on the laminate layup, direction of wave propagation, frequency, and interface conditions. In this paper a significant effort is applied to characterize these structural components using guided wave propagation. The bond between the honeycomb and the skin may degrade with age and service loads can lead to separation of the load-bearing skin from the honeycomb (called "disbonds") and compromise the safety of the structure. The need for model-based studies is widely recognized in the NDE community and a great deal of work has indeed been carried out for simple, metallic structures. However the literature on composite structures is rather limited due to the enormous mathematical complexity involved in dealing with them. In this dissertation a comprehensive approach including numerical (finite element method) and analytical method is used for calculating the ultrasonic wavefield in composite structural components with and without defects. Laboratory experiments are carried out on a composite honeycomb specimen containing damage to the skin or a localized disbond at the skin-core interfaces. The skin and the honeycomb composite are considered separately in order to understand the interaction of ultrasonic waves with damage in the two structures. The waves are launched into the specimen using a broadband PZT transducer and are detected by a distributed array of identical transducers located on the surface of the specimen. The guided wave components of the signals are shown to be strongly influenced by the presence of a defect in the skin or the honeycomb composite structure. The experimentally observed results are used to develop an autonomous scheme to locate the disbonds. The calculated results from the simulations are compared with existing and new experiments to validate and improve the models. The results should be very useful in model-based understanding of ultrasonic data collected during nondestructive inspection and evaluation (NDI/NDE) of advanced aircraft and aerospace structure and in the development of reliable health monitoring systems in the structures.

BIO

Dr. Harsh Kumar Baid - graduated with a Ph.D. in Mechanical and Aerospace Engineering at UCLA in June 2012 (Outstanding PhD Student Award). His expertise is in the field of Structural Health Monitoring using ultrasonic wave propagation technology. Dr. Baid has extensive experience and knowledge in the field of Structural Health Monitoring, having performed research at the UCLA Material Degradation and Characterization Laboratory since March 2005. Dr. Baid's Ph.D. work includes development and implementation of more accurate and efficient nondestructive technique then currently available to detect damages in advanced composite materials and structures used in aircraft, aerospace, marine, automotive and other structures. His approach included experimental, theoretical, and computational analysis in understanding guided wave propagation in advanced composite structures with and without defects. He has over 10 conference publication in structural health monitoring and has participated in numerous oral and poster presentation in the past 6 years. Currently Dr. Baid is working at AlphaStar Corporation in Long Beach for over 2 years and is involved in numerous projects.

Location: