As fiber reinforced composites continue to gain popularity as primary structures in aerospace, automotive, and powersports industries, quality control becomes an extremely important aspect of materials and mechanical engineering. The ability to recognize and control manufacturing induced defects can greatly reduce the likelihood of unexpected catastrophic failure. Porosity is the result of trapped volatiles or air bubbles during the layup process and can significantly compromise the strength of fiber reinforced composites. A comprehensive study was performed on an AS4C-UF3352 TCR carbon fiber-epoxy prepreg system to determine the effect of porosity on flexural, shear, low-velocity impact, and damage residual strength properties. Autoclave cure pressure was controlled to induce varying levels of porosity to construct six laminates with porosity concentrations between 0-40%. Porosity concentrations were measured using several destructive and nondestructive techniques including resin burnoff, sectioning and optical analysis, and X-ray computed tomography (CT) scanning. Ultrasonic transmission, thermography, and CT scanning provided nondestructive imaging to evaluate impact damage. A bilinear relationship accurately characterizes the change in mechanical properties with increasing porosity. Strength properties are relatively unaffected when porosity concentrations are below approximately 2.25% and decrease linearly by up to 40% in high porosity specimens.
Nondestructive evaluation methods often require high accuracy of transducer placement and repetition of multiple experimental steps. This study proposes the design of a Cartesian manipulator robot to accurately move and place the transducers on the test specimen. The electrical and software systems are also developed to allow the automation of the experiment by creating a program. The robot is capable of linear speeds of 250 mm/s and the ranges of motion from 50 to 1000 mm with 0.05-mm position accuracy. The allowable workspace is 900 x 930 x 18 mm in dimensions (length x width x height) and the maximum load capacity is 25 kg. The software allows for 13 commands that can be combined freely in a variety of experiment settings. The manipulator will be used in the nondestructive evaluation of composite aircraft and aerospace structural components in Professor Mal’s laboratory.
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. Unfortunately, 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.
A theoretical analysis is carried out in an effort to understand certain unusual properties of transient guided waves produced in a thin unidirectional graphite/epoxy composite plate by a localized dynamic surface load. The surface motion is calculated using an approximate plate theory, called the shear deformation plate theory (SDPT), as well as a recently developed finite element analysis (FEA), for their mutual verification. The results obtained by the two methods are shown to have excellent agreement. An interesting, nearly periodic "phase reversal" of the signal with propagation distance is observed for each propagation direction relative to the fiber direction. For clarification, a closed form analytical expression for the vertical surface displacement in an aluminum plate to an impulsive point force is obtained using the steepest descent method. It is found that the strong dispersion of the first antisymmetric waves at low frequencies is the main reason behind the phase reversal. This is verified further by measuring the surface response of a relatively thick aluminum plate to a pencil lead break source. The understanding developed in the paper is expected to be helpful in detecting and characterizing the occurrence of damage in composite structures. (C) 2004 Acoustical Society of America.
Ultrasonic guided waves offer an attractive cost effective tool for inspecting large structures. This is due to the fact that guided waves can propagate a large distance in plate like structural components and are strongly affected by the presence of defects in their propagation path. Propagation characteristics of Lamb waves have been studied extensively over many years, where the waves have been studied at a large distance away from the source, independent of the source influence. The literature on propagation of Lamb waves including the influence of the particular source responsible for the wave motion in the plate is comparatively much more sparse. In this dissertation the propagation of waves in a plate due to different kinds of applied loads is studied. Two dimensional and axisymmetric models are analyzed and the waves are studied both near the source and far away from it. The energy carried by the waves and how they vary depending on the type of applied load are studied. This provides the knowledge base to characterize and ultimately minimize the energy loss in various engineered structures and devices, including in the design of resonators by minimizing the ``anchor loss" associated with them.
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