Southwest Research Institute 2000 Annual Report, Materials Research and Structural Mechanics

Materials and structural research, development, and testing activities at the Institute include formulating new materials, evaluating their performance and failure mechanisms under actual and simulated service conditions, and assessing the remaining life of critical components. Probabilistic and reliability methods are being developed and applied to evaluate and improve the safety, efficiency, and lives of materials and structures in a variety of aerospace, land, and underwater systems.

In a project for the U.S. Air Force, the Institute is collaborating on the development of novel electrochemical sensors for detecting the early stages of fatigue damage in military aircraft. These sensors can detect fatigue microcracks as small as 12 micrometers on uncoated airframe materials. For more complex cases involving coatings, complicated geometries, or buried flaws, cracks can be detected with the new sensors in the range of 1-4 mm. With further developments, this technology could be used to locate fatigue cracks during flight, thereby providing a means of assessing the durability of aging aircraft.

The Institute has added both plasma immersion ion processing (PIIP) and plasma immersion ion implantation (PIII) to its Surface Modification Facility. PIIP is a relatively new vacuum technology for the application of hard, wear-resistant coatings. Like conventional physical vapor deposition methods, PIIP is used to deposit various coatings, but the non-line-of-sight PIIP approach allows large components and complex shapes to be simultaneously treated. Moreover, the recent availability of less expensive, high-power pulsed modulator equipment and the inherent scalability of PIIP lend an economy to this process that is difficult to match using other methods. PIII is used primarily for nitrogen implantation to improve the wear resistance of metals and for other implantation processes for the semiconductor industry. SwRI's Surface Modification Facility is the largest and most versatile commercial facility of its kind in North America.

The NASA Johnson Space Center and SwRI have signed a Space Act Agreement to enhance and commercialize the NASGRO computer code for fracture mechanics and fatigue crack growth analysis. Originally developed for assessing structural parts of the space shuttle, the code is widely used by the aerospace industry, but it has also been applied to pressure and piping systems and can be used for other applications. Under the agreement, SwRI is forming a consortium of industrial NASGRO users to further develop and provide continuing support of this important code.

Using a unique microscopic displacement and strain measurement system, SwRI staff are examining the mechanical factors that affect bone. Bone structure is thought to change in response to bone matrix deformation or stress-generated fluid flow forces acting on the cells. However, SwRI scientists believe the resulting deformation within the osteocyte cell - the cells thought to sense mechanical stress - triggers other cells involved with bone function to respond. Preliminary laboratory results suggest that strains immediately surrounding osteocytes can be up to three times greater than globally applied strains, greatly magnifying the deformation sensed near the osteocytes. This has important implications for the bone remodeling process and for such bone-weakening diseases as osteoporosis.

Manufacturing state-of-the-art integrated circuits requires that a semiconductor wafer be planarized using chemical-mechanical polishing. During this process, material on elevated features (currently 0.5 m and smaller) is abraded away by a polyurethane polish pad. The manufacturing process is hindered when unwanted wafer-pad contact occurs in open areas of the circuit pattern. To overcome this difficulty, SwRI has developed software that adds elevated features, called "dummy fill," to existing patterns. The dummy fill software reads circuit layout files produced by circuit designers, places the dummy fill in the patterns, and writes a new circuit layout file in a format ready for fabrication.

In a new program area at SwRI, researchers are developing sensor materials that can detect minute quantities of a variety of compounds, ranging from insecticides to biologically relevant compounds such as drug metabolites and steroids. A microelectromechanical device incorporating these materials will mimic the most basic biochemical function, namely the binding of a molecule to a receptor site such as in the lock-and-key interactions that occur in the human immune system. To achieve this function, polymers are being developed to function at the molecular level.

Large transport aircraft are operated over a wide range of flight conditions. The fuselage skins of these aircraft are subjected to continual changes in structural loads. The pressure changes caused by these changing flight conditions are caused primarily by variations in atmospheric pressure with the altitude of the aircraft and are the dominant fatigue loading for most transport fuselages. Under funding from the Warner-Robins Air Logistics Center (ALC), SwRI has created a test facility that reproduces the airloads, mechanical loads, and air temperatures that aircraft fuselage skins are subjected to during flight. This facility is designed to test large specimens (10 feet by 10 feet) from salvaged aircraft fuselages to achieve the highest fidelity in fuselage structure simulation.

Using the facility's fatigue testing capability, cracks were successfully grown from flaws introduced into a skin specimen taken from a surplus C-141 Starlifter military transport. Some of the cracks were then successfully repaired using composite doubler repair technology that SwRI is helping to develop for Warner-Robins ALC. The success of the repair doublers in load and temperature simulations suggests significant repair cost savings for the aging fleets of U.S. military transports. Future work includes testing specimens taken from a variety of transport types to explore the potential of composite repair doublers for a wide range of structural fatigue problems.

Institute engineers are examining a novel approach that allows aircraft structural designers to directly design for a desired probability of failure. Under U.S. Air Force Research Laboratory (AFRL) sponsorship, SwRI and Boeing-St. Louis successfully applied these new techniques to an advanced fighter wing design, based on F/A-18 E/F Hornet requirements. Using this new approach, a critical joint was redesigned to have a many orders-of-magnitude reduction in the probability of failure without adding more weight to the aircraft. Results of this study have helped create a new initiative at AFRL to rethink how future Air Force aircraft will be designed.

DARWIN^TM (Design Assessment of Reliability With INspection^TM), a software code for commercial jet engine design developed by SwRI under funding from the Federal Aviation Administration (FAA), has been named one of the 100 most significant technical accomplishments of 2000 by R&D Magazine. DARWIN is designed to quantify the risk of gas turbine engine rotor failure caused by fatigue cracks that initiate at hard alpha anomalies in titanium. The software integrates a graphical user interface, finite element analysis results, fracture-mechanics-based life assessment for low-cycle fatigue, material anomaly data, probability of anomaly detection, and inspection schedules to determine the probability of fracture of a rotor disk. The program also indicates the region of the disk most likely to fail. Work is under way to enhance the software to handle anomalies in cast wrought and powder nickel disks, and manufacturing and maintenance-induced surface anomalies in all disk materials. The code is part of the FAA's plan to reduce the U.S. commercial aviation fatal accident rate by 80 percent by 2007.

Challenges posed by the deep operating environment of the Gulf of Mexico have resulted in a number of research projects for the oil and gas industry concerned with drilling, production, and transportation. Under funding from the oil company consortium DeepStar, SwRI staff are testing subsea large-bore gate and ball valves under external pressure at maximum operating pressures. Evaluations include the life cycle of internal valve seals and components and structural integrity of the valves at depths to 10,000 feet.

Because of increased concerns over deep-water pipeline blockages caused by paraffin or other chemicals that build up inside the pipeline, thermal insulation of pipelines and subsea production trees is critical to production efforts. SwRI engineers are developing methods for testing insulation systems from vendors worldwide. The systems are being tested for flexibility, bending, thermal shock, cathodic disbondment, water absorption at depth, adhesion properties, and wet specific heat properties.

The Institute conducted a teardown inspection of the fuselage of a U.S. Air Force T-38 Talon, an advanced fighter training aircraft scheduled to remain in service until 2040. This destructive inspection of a fuselage taken from service will help assess aging of the T-38 fleet and determine locations that are experiencing structural damage from metal fatigue and stress-corrosion cracking. Bulkheads, fuselage skins, and longerons, the fuselage's primary load-carrying members, are being examined using several nondestructive inspection methods. Results of this program will help assess the long-term structural life of the T-38.

An automatic tire inflation system, developed by SwRI for Pressure Systems International, Inc., was named one of the 100 most significant technical accomplishments of 2000 by R&D Magazine. The system uses compressed air from the air-brake reservoir of a long-haul truck to inflate any trailer tire that falls below the system's pre-set air pressure setting. Air is delivered to a leaking tire even as the truck is pulling a trailer down the highway, helping to eliminate low tire pressures during operation. Since becoming commercially available in 1999, the system has been installed on more than 100,000 trailers nationwide. Engineers are currently designing a system for Class 8 tractors that will inflate every tire on a long-haul truck. The steering axle design is undergoing field testing, and detail design is under way for Class 8 drive axles.

Copyright© 2001 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, phone (210) 522-2257, fax (210) 522-3547.