Materials Research and Structural Mechanics

Materials, and the structures and systems that incorporate them, are closely integrated. Materials research, development, and testing at the Institute include the formulation and synthesis of new materials such as composites, polymers, ceramics, cements, and adhesives. Their performance under service conditions is evaluated in mechanical and corrosion tests and, where appropriate, improvements are made either by surface treatment or the addition of coatings. Structural mechanics includes analysis, design, and full-scale testing of a variety of aerospace, land-based, and underwater structures.

Research involving polymeric materials that have been engineered to effect a desired function at the molecular level has gained importance in recent years. This work is motivated by a need to develop "smart" sensors capable of detecting minute quantities of relatively small airborne chemicals in the environment or larger molecules of biological and military importance. Through computational modeling and novel chemical synthesis techniques, SwRI material chemists continue to investigate prospective building blocks of polymeric materials that, once assembled, may be used as the essential element of a sensor device.

SwRI has been working with clients in the plastic composites industry to demonstrate nitrogen ion implantation to improve wear resistance and release characteristics of chrome-plated pultrusion dies. Pultrusion is a process during which glass fibers are impregnated with a resin and "pulled" through a heated die that cures and forms the product. Ladders, gratings, window valances, and even bridge supports can be formed using this process. Ion implantation has demonstrated the potential to extend the life of the die by a factor of two, for less expense than repairing and replating the tool.

It is well known that bone adapts its architecture to the demands of skeletal loads; however, the mechanisms underlying this process are not well understood. In an ongoing National Institutes of Health research project, Institute scientists are investigating the effects of mechanical stimulus on bone tissue and bone cells. A unique aspect of the project is that extremely localized strains in bone are being measured, facilitated by technology transferred from earlier Air Force-funded work involving the measurement of local deformation in high-strength aerospace alloys. In addition, an atomic force microscope is being used to characterize nanoscale damage in bone caused by the local strain fields, which are much more intense than the average strains within a macroscopic bone sample. A better understanding of the relationship between skeletal loads, cellular stimulus, and biological response may lead to advances in the treatment of osteoporosis, in long-term orthopedic implant performance, and in developing countermeasures against bone loss as a result of long-term space flight.

With the U.S. Air Force Office of Scientific Research as sponsor, SwRI has developed new in situ composites based on niobium, chromium, and titanium. To improve fatigue and fracture resistance, the composite microstructures were manipulated using innovative processing and heat-treatment techniques. Unique, Institute-developed micromechanical testing techniques were used to characterize the effects of alloying addition and microstructure on the fatigue and fracture response of the in situ composites. The experimental effort was coupled with theoretical modeling at the atomic level to develop fundamental understanding of the influence of alloying on fracture behavior. This understanding will help lead to damage-tolerant, in situ composites for high-temperature applications.

Operators of large, land-based gas turbines for power generation are responding to changing markets by increasing the thermal efficiency of these engines. This is generally achieved by increasing the turbine inlet temperature, which results in higher blade and vane metal temperatures. Thermal barrier coatings are applied to the surfaces of these high-performance components to help them withstand the increased temperatures. SwRI has developed and delivered to a major gas turbine manufacturer a code to predict the life of these ceramic and metal combination coatings. This code will be used in the design process to assure adequate coating life and to set economical refurbishment intervals.

A direct current magnetron sputter source was recently added to SwRI's Surface Modification Facility. This equipment was used as part of an EPRI-funded project to develop and explore nickel-rhenium alloy interlayers for improving the performance and service life of coatings on nickel superalloy turbine blades in land-based power generation applications. A variety of novel metal alloys and composites can be deposited with high efficiency using this device.

The reliability and performance of moving parts within gas turbine engines is ultimately constrained by the materials used and the lubrication methodology applied. For the next generation of military and commercial propulsion systems, the development of advanced solid lubricant coatings, which can withstand a severe range of operating conditions, is a viable technology. SwRI recently took part in a small-business technology transfer program, sponsored by the U.S. Air Force Office of Scientific Research, for the development of high-temperature, adaptive solid lubricants. SwRI's ion beam facility was used to deposit novel multilayer and quasi-crystalline coatings on a variety of representative substrate materials and to demonstrate the potential for scaling up these processes. Friction and wear performance of these coatings was evaluated in ambient temperatures up to 650°C using an SwRI-designed, high-temperature wear test rig.

Institute engineers created Design Assessment of Rotors with Inspection, a probabilistic damage tolerance design code developed to improve the structural integrity of gas turbine rotor disks used in commercial aircraft. Shown here is a verification test of a radial compressor using a two-dimensional, axisymmetric model of the component. The work was supported by the Federal Aviation Administration in collaboration with four major aircraft gas turbine manufacturers.

Institute scientists and engineers are part of a national team, sponsored by the U.S. Air Force Research Laboratory, working to develop an improved design methodology to alleviate high-cycle fatigue (HCF) problems in military turbine engines. Other team members include the four major U.S. engine manufacturers, the University of Dayton Research Institute, and other major universities. SwRI is applying its unique capabilities in high-resolution micromechanics experimentation, coupled with analytical modeling, to formulate engineering methods to assess various HCF damage mechanisms - including foreign object impact damage to turbine blades, fretting-fatigue damage to blade-to-disc attachments, and damage due to complex load interactions.

SwRI is collaborating with researchers from the University of Pennsylvania and Tensiodyne Scientific Corporation in U.S. Air Force-sponsored work aimed at developing electrochemical sensors to detect the early stages of fatigue damage in military aircraft. These sensors have the potential for detecting fatigue micro-cracking, as well as fatigue damage prior to the formation of cracking, thus significantly enhancing the Air Force's ability to assess the durability of aircraft and more effectively maintain the readiness of its aging fleets.

The Institute has developed new elastic-plastic fracture mechanics technology for NASA to predict the growth of fatigue cracks under severe cyclic loading conditions in advanced, reusable space propulsion systems. The new approach represents a substantial advance over existing methods based on linear elastic fracture mechanics. The technology has also been implemented in engineering software tools for improving component reliability and safety. Technology and software both have been validated by experimentation and advanced numerical analysis.

Space shuttle windows are subjected to micrometeorite and space debris hypervelocity impacts, which degrade the residual strength of the windows. Shown here is a hypervelocity impact crater and associated microfracture in an actual shuttle window. Under contract to NASA, Institute scientists have characterized the damage associated with these impact events and are developing analytical criteria based on impact-crater size to help NASA determine when these windows should be removed from shuttle service.

Under the Tank Car Safety Project of the Railway Progress Institute (RPI) and the Association of American Railroads (AAR), SwRI engineers have transferred damage tolerance analysis (DTA) methods, a proven structural integrity assessment technology used for many years in the military and commercial aircraft industry, to railroad tank car manufacturers and owners. This four-year effort involved analysis of fatigue crack growth in the underframes (stub-sills) of tank cars. Institute staff members have instructed tank car engineers in the use of DTA methods to evaluate the lifetime of postulated cracks in the welds that attach the tank to the underframe and to establish safe periodic inspection intervals.

In a separate, parallel effort sponsored by the Federal Railroad Administration, SwRI collaborated with the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado, in conducting a full-scale damage tolerance test of a tank car stub-sill on the TTCI Simuloader. SwRI engineers provided TTCI with technical guidance in spectrum loading, pre-cracking methods and locations, crack length measurement, and results interpretation. The results of this DTA program, the full-scale tank car testing performed at TTCI, and the tank car stub-sill DTAs being performed by tank car manufacturers and owners, will ensure the integrity of railroad tank cars through the establishment of safe inspection intervals.

The Institute has started a multi-year program to evaluate the fatigue life of A-4 Skyhawk aircraft flown by the Royal New Zealand Air Force. The first part of the project consisted of an assessment of the aircraft and associated engineering data. This assessment is to be followed by installation of flight data recorders in two Skyhawks and collection of aircraft usage data over a 15-month period. Institute engineers will perform structural fatigue analyses using the measured flight data, the results of which will assist the Royal New Zealand Air Force in making decisions concerning the remaining fatigue life and future management of the Skyhawk fleet.

For more than nine years, under contract to the Alyeska Pipeline Service Company, the Institute has been developing failure prediction tools to assist operators in maintaining aging and corroded transmission pipelines. While in service, pipelines may be subjected to added stresses arising from the transport of hot oil and from settlement of the pipeline in permafrost, which act in concert with normal stresses from pressurization of the transported oil. SwRI researchers during the past year delivered a unique, elastic-plastic, strain-based rupture prediction model that can accurately assess the margin of safety between operating conditions and those at burst of the corroded pipeline. The failure prediction protocol, which was validated at SwRI by full-scale tests and finite element analyses of mechanically corroded, 48-inch-diameter pipes, surpasses the abilities of existing failure prediction protocols that neglect the effects of combined loading and the nonlinear behavior of pipeline steels at burst. Further validation of the procedure, which has been incorporated into a software tool for Alyeska, continued in 1998 as an internal research program, and is planned for use in upcoming work for the petroleum industry.

Major oil companies, subsea equipment suppliers, and industry consortia have employed the ocean simulation facilities at SwRI to test and qualify equipment in environments similar to those of the Gulf of Mexico. Here, engineers perform qualification tests, in accordance with American Petroleum Institute standards, on a subsea valve used in offshore oil production. The valve was tested at simulated ocean depths of 7,500 feet.

SwRI has developed and published installation and operating guidelines for the insertion of plastic liners into leaking metal pipelines. This work, sponsored by the Gas Research Institute, is aimed at demonstrating the suitability of commercialized liner technologies to rehabilitate pressurized natural gas pipes that are damaged. The guidelines are based on results of materials testing and analyses, full-scale testing of lined pipes, fracture mechanics modeling of service histories, and extensive discussions with field personnel from gas companies.

SwRI, as part of a high-technology team, provided the cornerstone concept and optimization software for the EPRI Turbine/Generator Health Management software, TURBOX. This software will aid personnel in the electronic power generation industry by providing a decision analysis tool that combines engineering, economic, and risk analysis for guidance in planning and potentially extending machine overhauls in the new, deregulated environment. Because of the financial significance of overhauls to electric utilities, this has been the highest priority project of the EPRI steam turbine/generator business focus group for the past two years.

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