Southwest Research Institute 1997 Annual Report

Materials research, development, and testing activities at the Institute include formulating and synthesizing new materials, evaluating their performance under service conditions, and assessing the remaining useful life of critical components and structures. Performance can be improved with surface treatments and coatings. Composites, metals, polymers, ceramics, cements, and adhesives are some of the materials being evaluated. An integrated approach to structural mechanics embraces the analysis, design, and testing of a variety of aerospace, land, and underwater structures.

A large print roller was strengthened by implanting nitrogen ions onto its surface using a high-energy plasma bucket ion source, a recent addition to SwRI's ion beam surface modification facility. Modification of materials by direct ion implantation strengthens their surface structures. In related work, ion beam-assisted coatings of metals such as platinum and tantalum were added to the surfaces of industrial valves and heaters to reduce corrosion. Additionally, ion-beam technology allows diamond-like carbon coatings to be applied to aircraft hydraulic actuators as a replacement coating for environmentally hazardous chromium.

Under internal research, SwRI engineers are developing blood-compatible coatings for metallic components used in cardiovascular applications such as substitute heart valves and coronary stents. Current materials, although biocompatible, are not usually blood compatible. As a result, interaction between the material surface and red blood cells can cause thrombosis, or blood clotting. Anticoagulant drug therapy is used to reduce the risk of blood clot formation, but is not always effective. Materials scientists and biomedical engineers are collaborating to evaluate hemocompatible coatings. These surface coatings are deposited using ion-beam coating technology, producing a nonequilibrium structure that is expected to provide hemocompatibility.

Debilitating bone diseases such as osteoporosis, Paget's disease, and osteogenesis imperfecta affect millions of people each year. Mechanical stresses in bone caused by everyday activities may result in the eventual weakening of the bone. Bone cells are thought to be the natural "sensors" that interpret skeletal forces and control the natural bone remodeling process. Using Institute-developed micromechanics techniques, researchers are investigating the relationship between biomechanical skeletal loading and micromechanical strains acting on bone microstructure and bone cells to better understand the fundamental mechanisms of bone damage accumulation, fracture behavior, and remodeling. This research will aid in developing treatments that eventually may be used to prevent osteoporosis and other bone diseases.

The Institute is exploring repair methods for the space shuttle's windows. During shuttle liftoff, low-velocity impacts cause pitting of the window surface, a problem termed "window haze." Institute scientists have developed a chemical formulation based on sol-gel chemistry that repairs the pits and increases the clarity of the hazed window. A material containing silicon is painted on the surface and cured to produce an 800°F stable composite containing nano-sized particles of silica that cover the pits on the shuttle window. Further investigations are in progress to develop a material that can repair the haze while withstanding a temperature of 1,100°F, thus ensuring window survivability during space shuttle reentry.

SwRI is leading a new research and development effort in partnership with a major automaker, an automotive specialty engineering firm, and the National Highway Traffic Safety Administration to evaluate the degradation in mechanical strength properties of the metallic, plastic, and rubber materials used in fuel systems of eight- to 12-year-old automobiles. SwRI's experience and capabilities in material mechanics and structural characterization will enable this partnership to better understand the extent of degradation of fuel system component materials and how this degradation might affect the risk of a collision fire in older automobiles. This research should provide insights for future designs of fuel system components and aid in materials selection, thus improving long-term safety margins.

High cycle fatigue (HCF) of engine components in military aircraft has become a significant problem in recent years. The Institute is addressing this problem as part of a national team effort sponsored by the U.S. Air Force Wright Laboratory and the Air Force Office of Scientific Research. The team includes major domestic engine manufacturers, the University of Dayton Research Institute, and other U.S. universities. Advanced experimental and analytical techniques are being used by SwRI to clarify the underlying processes responsible for the initiation and early growth of microcracks associated with complex cyclic loading, foreign object damage, and fatigue. This information is being used to guide the development of engineering models to predict such behavior. This multi-year effort will result in a new design methodology to alleviate HCF in current and future gas turbine engines.

Institute engineers are addressing the durability and safety of onboard compressed natural gas (CNG) fuel-storage cylinders in natural gas vehicles by applying technology developed in several long-term programs sponsored by the Gas Research Institute (GRI). Recommendations and guidance on proper installation, maintenance, and inspection of CNG fuel cylinders are being provided to fleet operators to resolve short-term difficulties, while long-term durability testing of CNG fuel-storage cylinders continues.

For the past 10 years under contract to NASA, the Institute has been developing advanced elastic-plastic fracture mechanics technology to improve component reliability for reusable aerospace propulsion systems. During the past year, SwRI researchers delivered two handbooks that document practical engineering methods and guidelines for design and interpretation of structural proof testing of fracture-critical components. The recommended methods were validated through testing of laboratory coupons and full-scale components, as well as through elastic-plastic finite element analysis. The analytical methods are being incorporated into software tools for NASA and its contractors.

As part of a major, multiyear project for the Federal Aviation Administration (FAA), SwRI engineers are developing a probabilistic damage tolerance design code to improve the structural integrity of future gas turbine rotor disks used in commercial aircraft. The probabilistic code, known as DARWIN (Design Assessment of Rotors With Inspection), incorporates sophisticated risk assessment methods into design procedures so that risk of failure is minimized and inspections can be scheduled realistically. Code development is being managed by SwRI in collaboration with four aircraft gas turbine manufacturers. When completed, the code will be the basis for an approved FAA certification standard that engine companies can incorporate into their design systems.

SwRI operates the Materials Center for Combustion Turbines (MCCT) for the Electric Power Research Institute (EPRI). The MCCT provides cost-saving technical advice and develops new technology for EPRI member utilities. Since 1993, EPRI reports more than $10 million in documented savings as a result of these efforts. Current developments include a method for predicting the life of protective coatings on hot section components and a procedure for determining component operating temperature from microstructural analysis. The Center also has developed an automated thermal cycling facility to simulate turbine hot section conditions. In a separate project for EPRI, Institute engineers are developing a diffusion barrier that can be applied to slow the degradation of turbine blade and vane coatings.

A major manufacturer of land-based gas turbines is supporting the work of SwRI scientists to develop a life model for thermal barrier coatings. These ceramic/metal combination coatings are being applied to advanced engine components and are critically important to maintain lower operating temperatures. The model will permit the calculation of the number of hours and start/stop cycles that are allowable for specific coatings and turbine operating conditions so that components can be refurbished before significant damage develops.

Horizontal directional drilling (HDD), or guided boring, is used to install polyethylene (PE) natural gas pipes in heavily developed urban areas, beneath obstacles, and under environmentally sensitive areas where traditional trenching methods are costly or prohibited. In response to the growing use of directional drilling, SwRI developed HDD installation procedures for the Gas Research Institute and PE LIFESPAN FORECASTINGª, an interactive software system for assessing the life expectancy of polyethylene gas pipe installed using HDD.

Polyethylene geomembrane liners, used to line evaporation and ash ponds of electric power generating stations and solid waste landfills, often represent the primary barrier to intrusion of contaminants into soil and groundwater. Long-term durability of these barriers is essential. SwRI engineers are developing for EPRI a fracture mechanics methodology for predicting the life expectancy of these liners under service conditions. This program will lead to the establishment of an allowable initial defect size for a prescribed loading and service life, and to the specification of inspection requirements as well as quality control and assurance standards.

Under the Tank Car Safety Research and Test Project of the Railway Progress Institute and the Association of American Railroads (AAR), SwRI engineers are adapting damage tolerance analysis (DTA) methods, traditionally used for aircraft, for the railroad industry. This work focuses on analysis of fatigue crack growth in the underframes (stub-sills) of tank cars. SwRI personnel are assisting tank car engineers in the application of DTA to evaluate the lifetime of postulated cracks in welds that attach the tank to the underframe, and to establish safe periodic inspection intervals. In an associated effort sponsored separately by the Federal Railroad Administration, SwRI is collaborating with the AAR Transportation Technology Center (AAR-TTC) in Pueblo, Colorado, in a full-scale damage tolerance test of a stub-sill tank car. In this program, Institute engineers are providing technical guidance in the areas of spectrum loading, pre-cracking methods and locations, crack length measurement, and results interpretation. The SwRI/AAR-TTC team will assist the tank car industry in establishing a DTA methodology that is correlated with data obtained from an actual tank car stub-sill under simulated loading conditions.

Operators of the F-5 aircraft in four foreign countries and the T-37 aircraft in three foreign countries have received SwRI reports that document the results of durability and damage tolerance analyses performed for their fleets. Using flight data collected from aircraft flown in the respective countries, stress spectra for known fatigue-critical locations were developed for use in laboratory coupon testing and analytical crack growth analyses. These analyses were the basis for revising structural inspection intervals and assessing long-term durability of the airframe to ensure flight safety and to determine the need for structural modifications and spare parts.

The Institute's involvement in smart structures research for the U.S. Air Force includes an innovative program to provide weight-competitive adaptive wing structural concepts that do not have large power requirements. A primary goal of the U.S. Air Force Wright Laboratory is an adaptive wing structure that provides more "bird-like" shape control for the next generation of combat aircraft. The difference in pressure distribution between the upper and lower surfaces of the wing provides a twisting force that can be exploited to shape the wing and thus provide aircraft control. Institute engineers and technicians have developed a concept under Air Force sponsorship that provides a significant part of this shape control through variable-stiffness wing structural components that tap wing aeroelasticity as the primary power source. The Institute has built a generic wing structure test fixture to explore the mechanical concepts needed to provide this variable stiffness, and has designed and built a prototype wing.

Copyright© 1997 by Southwest Research Institute. All rights reserved under U.S. Copyright Law and International Conventions. No part of this publication may be reproduced in any form or by any means, electronic or mechanical, including photocopying, without permission in writing from the publisher. All inquiries should be addressed to Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510.

Materials Research and Structural Mechanics