Simulating Failure for Success
Jointly developed SwRI-NASA computer program accurately simulates cracking in mechanical components allowing repair or replacement before costly failure occurs
When fracture mechanics makes the news, it is usually bad news. Structural failure caused by cracking is the primary threat to the integrity, safety and performance of many highly stressed mechanical components, from pipelines to bridges to vehicles. When failures occur, they often have major consequences: serious injury or loss of life, severe environmental damage or substantial economic loss.
The economic effects of fracture alone are striking. A 1983 National Bureau of Standards study, still considered an industry standard, estimated the total cost of fracture in the U.S. to be about $119 billion per year (in 1982 dollars). These costs arise from many factors. Large safety factors caused by design uncertainty require increased costs for materials and construction and also lead to reduced performance. Equipment failures incur direct costs for repair, maintenance, replacement, scrappage and performance or productivity losses. Inspections for material quality control and structural reliability introduce additional expenses. The NBS study further estimated the potential savings from application of available technology and additional fracture research to be $63 billion per year.
Calculating useful lifetime
Knowing how and when a structure might fail would allow for repair or replacement before a potential tragedy occurs. Engineers at Southwest Research Institute and the National Aeronautics and Space Administration have developed a computer program based on a code developed originally by NASA in the 1980s that accurately simulates crack growth and failure in structures. Named NASGRO®, the code calculates the crack growth rate and remaining structural life of components undergoing cyclic loading and also calculates the conditions, such as loads and crack sizes, that cause failure by fracture. NASGRO 4.0 is the commercially available version of the code.
Using NASGRO can increase safety and decrease costs by reducing the risk of fracture. A designer can use NASGRO to optimize a fracture-resistant structural design, determining safe stresses that permit a specified design lifetime. Alternatively, a designer can develop a fracture control plan that specifies the safe lifetime for a given design or the inspection intervals required to maintain safety. If damage is discovered in a component in the field, an operator or regulator can use NASGRO to determine the safe remaining life (if any) and the required inspection intervals (if any) to maintain safety.
The fracture mechanics challenge
The three fundamental challenges of fracture mechanics analysis are calculating the driving force for crack growth, known as the "stress intensity factor" (SIF); characterizing the material properties that describe the resistance to crack growth; and developing accurate crack growth algorithms that account for the effects of load history. Because the fracture problem is so significant, it should be no surprise that numerous other computer programs have been developed to address the problem. NASGRO stands alone, however, in its innovative approaches to these three fundamental challenges.
The NASGRO SIF library contains more than 50 standard solutions - twice as many as most competing codes - to permit more accurate modeling of the geometry and stress field. It is the only code, for example, that contains fully three-dimensional SIF solutions for stress fields that vary arbitrarily in both length and width of the crack plane. NASGRO is the only code that also contains an integrated boundary element module permitting exact calculation of new SIF solutions for any two-dimensional, user-defined geometry with any two-dimensional, user-defined load.
NASGRO has a massive material property database that comprises 476 different metallic materials, 3,000 sets of fatigue crack growth data, 6,000 fracture toughness data points and statistically derived crack growth equations for nearly all of the materials. The code contains all original fatigue crack growth test data and reference citations so that users can evaluate the equation fits and generate new properties as needed. NASGRO can fit the same crack growth equations to user-supplied test data.
The code uses an original crack growth equation that uniquely provides a proper physical basis for the effects of mean load level and small crack sizes on crack growth rate and the threshold for non-growing cracks. It uses a fully integrated, physically based crack closure model to provide significantly improved accuracy for load interaction effects, and it includes a practical engineering method for severe loading such as that experienced in rocket engines.
Despite sophisticated fracture mechanics technology and the sheer volume of the SIF and material property libraries, NASGRO is easy to use. An intuitive graphical user interface guides the user through a series of screens in which geometry, material properties and load history are specified before a final screen provides analysis output. Help is available online and in extensive reference manuals, and input and output are highly flexible. NASGRO executes on all Windows® and many Unix® platforms.
The NASGRO partnership
The NASGRO computer program (originally called NASA/FLAGRO) was first developed by NASA in the early 1980s to provide fracture control analysis for manned space programs. Public concern over "aging aircraft" in both the civilian and military fleets during the 1990s prompted NASA, the Federal Aviation Administration, and the U.S. Air Force to provide additional support for methods and software development, leading in 1999 to the public release on a NASA Johnson Space Center web site of a significantly improved code, NASGRO 3.0.
SwRI first became actively involved with developing the code in the mid-1990s. The Institute successfully completed several projects for the NASA Marshall Space Flight Center to develop new NASGRO methods and software modules to evaluate fracture problems in reusable space propulsion systems such as the space shuttle main engine. The operating conditions - stresses and temperatures - in the main engine are so severe that conventional fracture analysis methods are not valid. New methods had to be developed, validated and implemented.
Steady growth in the capabilities of NASGRO led to a large increase in the number of users. More than 1,700 users from 800 organizations downloaded NASGRO 3.0 from the NASA-JSC web site, and only about 25 percent of these users came from the space community. This increase exceeded NASA's ability to provide continued capability, growth and user support and prompted a new vision for the code's future.
This new vision was based on a partnership between NASA and SwRI that was formalized in a 2000 Space Act Agreement signed by the two organizations. Under this partnership, NASA and SwRI have become joint developers of NASGRO. SwRI has formed a consortium of 14 major companies from nine countries that includes major manufacturers of space structures, aircraft, rotorcraft and gas turbine engines that use NASGRO. The consortium provides additional financial support and guidance for methods and software development, and SwRI engineers are performing many of these enhancements.
Simultaneously, NASA is continuing to support NASGRO development by NASA engineers and contractors. These developments are coordinated, and SwRI and NASA are sharing the intellectual property that each develops to create a common version of the code. NASA has granted the copyright for NASGRO 4.0 to the Institute, which permits the Institute to license the code to consortium participants and others. At the same time, the government agencies that are actively supporting the code - NASA, FAA, and the European Space Agency - retain a perpetual royalty-free license for official use by their employees and contractors. NASA and SwRI work together to provide support to all users.
In use worldwide
The public domain version NASGRO 3.0 and the commercial version 4.0 are now used widely throughout the world. NASGRO is the standard software package used by all NASA centers and many NASA contractors and international partners for fracture control analysis of space hardware, including the space shuttle, the International Space Station and payloads, as well as safety-critical ground systems. Manufacturers of military and civilian aircraft and rotorcraft use the code extensively, and other users are found in the gas turbine engine, ground vehicle, petrochemical, pipeline, railroad tank car, ship structure and nuclear industries. Government users of NASGRO include national laboratories and military agencies including the U.S. Air Force, Army and Navy. The code is also used extensively in universities around the world for both teaching and research.
NASGRO continues to be improved through the coordinated efforts of NASA and SwRI. Version 4.1 is currently in test release, and Version 4.2 is scheduled for release in 2004. Major improvements completed or under way include modules to calculate crack formation life; additional SIF solutions; corrections for material yielding at highly stressed holes; effects of time, temperature, and environment on crack growth; analysis of cracks in complex, built-up airframe structures; probabilistic analysis modules; and further enhancements to the graphical user interface.
The NASGRO consortium recently completed the second year of an initial three-year term. Renewal for an additional three years is planned in 2004, and new consortium participants are welcome. SwRI also licenses NASGRO 4.0 for a fee to both individual users (single seat licenses) and organizational users (site licenses) outside the consortium.
For more information about NASGRO and the NASGRO Consortium, and links to information about other integrity and reliability software and research at SwRI, visit the NASGRO web site at nasgro.swri.org.
Published in the Fall 2003 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.