Showing papers by "William H. Prosser published in 2005"

Calculation of the Response of a Composite Plate to Localized Dynamic Surface Loads Using a New Wave Number Integral Method

Abstract: This study is motivated by the need for an efficient and accurate tool to analyze the wavefield produced by localized dynamic sources on the surface or the interior of isotropicplates and anisotropic composite laminates. A semi-analytical method based on the wavenumber integral representation of the elastodynamic field is described that reduces theoverall computational effort significantly over other available methods. This method isused to calculate the guided wave field produced in a thin unidirectional graphite/epoxycomposite laminate by a dynamic surface point load. The results are compared with thoseobtained from a finite element analysis, showing excellent agreement, except for minordifferences at higher frequencies. A recently discovered feature of the calculated surfacemotion, namely, a spatially periodic ‘‘phase reversal’’of the main pulse with propagationdistance, is observed in both cases. The present work is expected to be helpful in devel-oping impact damage monitoring systems in defect-critical structural components throughreal time analysis of acoustic emission wave forms. @DOI: 10.1115/1.1828064#

Simulation of asymmetric Lamb waves for sensing and actuation in plates

Abstract: Two approaches used for monitoring the health of thin aerospace structures are active interrogation and passive monitoring. The active interrogation approach generates and receives diagnostic Lamb waves to detect damage, while the passive monitoring technique listens for acoustic waves caused by damage growth. For the application of both methods, it is necessary to understand how Lamb waves propagate through a structure. In this paper, a Physics-Based Model (PBM) using classical plate theory is developed to provide a basic understanding of the actual physical process of asymmetric Lamb mode wave generation and propagation in a plate. The closed-form model uses modal superposition to simulate waves generated by piezoceramic patches and by simulated acoustic emissions. The generation, propagation, reflection, interference, and the sensing of the waves are represented in the model, but damage is not explicitly modeled. The developed model is expected to be a useful tool for the Structural Health Monitoring (SHM) community, particularly for studying high frequency acoustic wave generation and propagation in lieu of Finite Element models and other numerical models that require significant computational resources. The PBM is capable of simulating many possible scenarios including a variety of test cases, whereas experimental measurements of all of the cases can be costly and time consuming. The model also incorporates the sensor measurement effect, which is an important aspect in damage detection. Continuous and array sensors are modeled, which are efficient for measuring waves because of their distributed nature.

Nondestructive Evaluation for the Space Shuttle's Wing Leading Edge

Abstract: The loss of the Space Shuttle Columbia highlighted concerns about the integrity of the Shuttle's thermal protection system, which includes Reinforced Carbon-Carbon (RCC) on the leading edge. This led NASA to investigate nondestructive evaluation (NDE) methods for certifying the integrity of the Shuttle's wing leading edge. That investigation was performed simultaneously with a large study conducted to understand the impact damage caused by errant debris. Among the many advanced NDE methods investigated for applicability to the RCC material, advanced digital radiography, high resolution computed tomography, thermography, ultrasound, acoustic emission and eddy current systems have demonstrated the maturity and success for application to the Shuttle RCC panels. For the purposes of evaluating the RCC panels while they are installed on the orbiters, thermographic detection incorporating principal component analysis (PCA) and eddy current array scanning systems demonstrated the ability to measure the RCC panels from one side only and to detect several flaw types of concern. These systems were field tested at Kennedy Space Center (KSC) and at several locations where impact testing was being conducted. Another advanced method that NASA has been investigating is an automated acoustic based detection system. Such a system would be based in part on methods developed over the years for acoustic emission testing. Impact sensing has been demonstrated through numerous impact tests on both reinforced carbon-carbon (RCC) leading edge materials as well as Shuttle tile materials on representative aluminum wing structures. A variety of impact materials and conditions have been evaluated including foam, ice, and ablator materials at ascent velocities as well as simulated hypervelocity micrometeoroid and orbital debris impacts. These tests have successfully demonstrated the capability to detect and localize impact events on Shuttle's wing structures. A first generation impact sensing system has been designed for the next Shuttle flight and is undergoing final evaluation for deployment on the Shuttle's first return to flight. This system will employ wireless accelerometer sensors that were qualified for other applications on previous Shuttle flights. These sensors will be deployed on the wing's leading edge to detect impacts on the RCC leading edge panels. The application of these methods will help to insure the continued integrity of the Shuttle wing's leading edge system as the Shuttle flights resume and until their retirement.

Detection of Impact Damage on Space Shuttle Structures Using Acoustic Emission

Abstract: Studies of the acoustic signals originating from impact damage on Space Shuttle components were undertaken Sprayed on foam insulation and small aluminum spheres were used as impactors Shuttle reinforced carbon‐carbon panels, panels with Shuttle thermal protection tiles, and Shuttle main landing gear doors with tiles were targets Ballistic speed and hypervelocity impacts over a wide range of impactor sizes, energies, and angles were tested Additional tests were conducted to correlate the acoustic response of the test articles to actual Shuttle structures