Showing papers by "William H. Prosser published in 2004"

Structural Health Management for Future Aerospace Vehicles

Abstract: Structural Health Management (SHM) will be of critical importance to provide the safety, reliability and affordability necessary for the future long duration space missions described in America's Vision for Space Exploration. Long duration missions to the Moon, Mars and beyond cannot be accomplished with the current paradigm of periodic, ground based structural integrity inspections. As evidenced by the Columbia tragedy, this approach is also inadequate for the current Shuttle fleet, thus leading to its initial implementation of on-board SHM sensing for impact detection as part of the return to flight effort. However, future space systems, to include both vehicles as well as structures such as habitation modules, will require an integrated array of onboard in-situ sensing systems. In addition, advanced data systems architectures will be necessary to communicate, store and process massive amounts of SHM data from large numbers of diverse sensors. Further, improved structural analysis and design algorithms will be necessary to incorporate SHM sensing into the design and construction of aerospace structures, as well as to fully utilize these sensing systems to provide both diagnosis and prognosis of structural integrity. Ultimately, structural integrity information will feed into an Integrated Vehicle Health Management (IVHM) system that will provide real-time knowledge of structural, propulsion, thermal protection and other critical systems for optimal vehicle management and mission control. This paper will provide an overview of NASA research and development in the area of SHM as well as to highlight areas of technology improvement necessary to meet these future mission requirements.

Development of embedded sensor models in composite laminates for structural health monitoring

Abstract: A framework is developed to study the transient analysis of composite laminated plates with embedded discrete and continuous sensors in the presence of delaminations. The computational modeling involves development of a finite element scheme using an improved layerwise laminate theory to model laminates of arbitrary thickness. Parametric studies are conducted using laminated plates with both embedded sensors and continuous sensor architecture. The response of the plates under both low-frequency vibration and high-frequency acoustic emission are investigated. The effects on plate displacement and sensor outputs due to delaminations are studied. The scattering of the acoustic emission caused by the presence of delaminations is also investigated. It is expected that the developed model would be a useful tool in simulation studies aimed at characterizing the presence of delaminations in composite laminated structures.

Analysis of transient Lamb waves generated by dynamic surface sources in thin composite plates

Abstract: 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

Acoustic emission detection of impact damage on space shuttle structures

Abstract: The loss of the Space Shuttle Columbia as a result of impact damage from foam debris during ascent has led NASA to investigate the feasibility of on-board impact detection technologies. AE sensing has been utilized to monitor a wide variety of impact conditions on Space Shuttle components ranging from insulating foam and ablator materials, and ice at ascent velocities to simulated hypervelocity micrometeoroid and orbital debris impacts. Impact testing has been performed on both reinforced carbon composite leading edge materials as well as Shuttle tile materials on representative aluminum wing structures. Results of these impact tests will be presented with a focus on the acoustic emission sensor responses to these impact conditions. These tests have demonstrated the potential of employing an on-board Shuttle impact detection system. We will describe the present plans for implementation of an initial, very low frequency acoustic impact sensing system using pre-existing flight qualified hardware. The details of an accompanying flight measurement system to assess the Shuttle s acoustic background noise environment as a function of frequency will be described. The background noise assessment is being performed to optimize the frequency range of sensing for a planned future upgrade to the initial impact sensing system.

Development of piezoelectric acoustic sensors and bio-inspired embedded sensor array architecture for structural health monitoring

Abstract: This paper looks at developing novel piezoelectric acoustic sensors, which are capable of sensing high frequency acoustic emissions and impact in a composite/metallic plate. The fabrication of the piezoelectric acoustic sensors made from piezoceramic ribbons would be described in details. An attempt was made to build directionality into the sensing system itself. Continuous sensors placed at right angles on a plate are being discussed as a new approach to measure and locate the source of acoustic waves. Novel signal processing algorithms based on bio - inspired neural systems for spatial filtering of large numbers of embedded sensor arrays in laminated composite media is presented. It is expected that this present work would help in development of sensing techniques for highly efficient health monitoring of integrated aerospace vehicles and structures.

Development of Piezoelectric Acoustic Sensors and Novel Bio‐Inspired Sensor Array Architecture

Abstract: This paper looks at developing novel piezoelectric acoustic sensors, which are capable of sensing high frequency acoustic emissions and impact in a composite/metallic plate. The fabrication of the piezoelectric acoustic sensors made from piezoceramic ribbons would be described in details. An attempt was made to build directionality into the sensing system itself. Continuous sensors placed at right angles on a plate are being discussed as a new approach to measure and locate the source of acoustic waves. Novel signal processing algorithms based on bio‐inspired neural systems for spatial filtering of large numbers of embedded sensor arrays in laminated composite media is presented. It is expected that this present work would help in development of sensing techniques for highly efficient health monitoring of integrated aerospace vehicles and structures.

Modeling delamination in composite structures by incorporating the Fermi-Dirac distribution function and hybrid damage indicators

Abstract: Conventional finite element approaches for modeling delaminations in laminated composite structures use the Heaviside unit step function at the interfacial nodes in the delaminated zone of the structure to model the possible jumps in the displacement field during “breathing” of the delaminated layers. In quantum mechanics, the Fermi-Dirac distribution applies to Fermion particles whose characteristics are half-integer spins. The present paper uses the Fermi-Dirac distribution function to model a smoother transition in the displacement and the strain fields of the delaminated interfaces during the opening and closing of the delaminated layers under vibratory loads. This paper successfully shows that the Fermi-Dirac distribution function can be used to more accurately model the dynamic effects of delaminations in laminated composite structures. Optimizing the parameters in the Fermi-Dirac distribution function can lead to more accurate modeling of the dynamic and transient behavior of the delaminated zones in laminated composite structures. Further applications of the Fermi-Dirac distribution function in other physics based dynamic models are suggested. This paper also effectively demonstrates how hybrid sensors comprising of out of plane displacement sensors and in plane strain sensors can effectively map a composite structure to detect and locate the delaminated zones. It also shows how simple mode shapes can be used to determine the locations of single and multiple delaminations in laminated composite structures.