In the March issue of HPC, I began a discussion of the U.S. Department of Defense (DoD) Composites Affordability Initiative (CAI), which was undertaken in the 1990s to address issues that presented roadblocks to widespread use of advanced composite structures on aircraft, including their high cost, compared with legacy aircraft materials. CAI research was conducted by a team from the Air Force Research Laboratory’s Materials and Manufacturing Directorate (AFRL/ML) and Air Vehicles Directorate, as well as the Office of Naval Research, Bell Helicopter, Boeing, Lockheed Martin and Northrop Grumman. The CAI program tackled the problems of making composites more affordable and more widely used on aircraft. Ultimately, a $152 million (USD), 11-year effort was launched to meet these goals.
Previously, I noted that CAI’s preliminary investigation revealed that the primary roadblock to composites use was assembly costs. I outlined CAI efforts to replace multiple part assemblies with larger integrated structures, produced by the vacuum-assisted resin transfer molding (VARTM) process and assembled by means of adhesive bonding rather than conventional fasteners at part joints. The following focuses on enabling tools for bonding. The team developed and validated the performance of advanced structural analysis tools and composite failure criteria to predict the structural behavior of bonded joints in three-dimensional loading.
Reusable finite element models of bonded joints: Conventional analysis methods for bonded joints are limited in their capabilities and accuracy. To date, the only approach available has been to develop detailed finite element models of a joint. This approach is time-consuming and requires great skill and care by the analyst to ensure stresses and strains in critical locations of the joint are properly quantified. Small errors in modeling can lead to substantial errors in joint performance prediction. To alleviate this problem, the CAI used the “handbook” functionality of ESRD Inc.’s (St. Louis, Mo.) trademarked StressCheck P-version finite element software to develop reusable models of typical joints. These joints include single-lap shear, double-lap shear, scarfed-lap shear and step-lap joints for inplane loading as well as pi joints and back-to-back angle joints for out-of-plane loading. The handbook for each joint type is parameterized, enabling engineers to model similar joints in the future simply by updating the geometric parameters of the existing model. StressCheck then remeshes the model automatically, calculates results, checks for convergence problems in the new joint configuration and even post-processes the results.
Durability and damage-tolerance analysis methods: Stress analysts are concerned about how a structure will endure small, undetected flaws and how much residual strength a structure possesses after sustaining larger service-induced damage. Finite element analysis (FEA) methods are being used to predict where and under what conditions a structure will fail. In order to increase user confidence in the advanced composite structural designs that are maturing under its oversight, CAI has successfully pursued improvements to FEA predictive capability.
Over the past several years, CAI has developed, refined and proven a new approach for predicting the reliability of major composite structural components. The result is new software that is being applied to the evaluation of delaminations and disbonds in composite structures. Based on a novel implementation of the Virtual Crack Closure Technique (VCCT) developed by The Boeing Co. (Seattle, Wash.) to predict fracture and failure in composite laminates, CAI’s VCCT approach will play an important role by bringing unprecedented capability to the design of aerospace structures involving composites. Boeing has filed a patent application for this interface fracture analysis software and ABAQUS Inc. (Pawtucket, R.I.) will commercialize an enhanced version of the technology.
Quality assurance via non-destructive inspection: Among the barriers that have inhibited the use of efficient bonded primary structures, a major hurdle has been the lack of a nondestructive system that could assess the strength of a bonded joint. Boeing has developed a patent-pending, laser-based bond inspection technique that uses high-peak-power, short-pulse-length laser excitation to generate stress waves that can be used to discriminate between weak and strong bonds in carbon fiber/epoxy composite-to-composite structures. The technique identifies variations in surface preparation techniques, levels of surface contamination and/or changes in paste adhesive mixing. In numerous (more than 3,000) laser stress wave experiments, this approach has proven to be repeatable and reliable in the detection of weak joints. Such an approach offers a potentially cost-effective method for determining with certainty, after manufacture or in service, that a bonded joint possesses the minimum predetermined load-carrying capability. A production-floor version of the Boeing technology, called Laser Bond Inspection, is being developed and optimized by LSP Technologies Inc. (Dublin, Ohio) with funding from two Small Business Innovative Research (SBIR) programs sponsored by AFRL/ML. The first SBIR to develop a production floor hardened compact laser system should have the system available in 2008. Under the second SBIR, a production floor hardened beam delivery arm should be available for in 2009.
Certification: The CAI team worked with airframe certification authorities from the U.S. Air Force, Navy and Federal Aviation Admin. (FAA) to understand and then eliminate the barriers to airframe certification of large integrated and bonded structures. The CAI team prepared certification plans for three structures, each with increasing levels of innovation. These plans included the use and validation of CAI-developed analysis tools as well as the use of CAI-developed bonding process controls, guidelines for advanced processes and advanced bondline inspection tools. When these tools and technologies and a sound certification plan (analysis supported by testing) were reviewed by the certification authorities, they were able to say with confidence that they believe the methods are sound enough to win certification for an actual structure. This was a major breakthrough in the CAI team’s effort to realize the cost, cycle time and durability benefits of advanced bonded structures.
The CAI tools and technologies discussed here and last issue have transitioned to the industrial base. AFRL is currently aware of 22 companies/organizations benefiting from CAI-derived technologies. These include VARTM, pi-joints, Laser Bond Inspection, StressCheck and crack propagation analysis tools, and certification plans. Bonded structures are flying on the F-35 AA-1. StressCheck and crack propagation analysis tools have become standard industry practices and are being used to design and analyze DoD and commercial aircraft. The C-17 landing gear door (photo at left) will be fabricated by a first-tier supplier for future C-17s and as a preferred spare.
It’s important to note that this two-part article covers only a portion of the technologies developed under CAI. Other tools include an improved cost model for composites, recently commercialized as SEER-DFM with Composite Plug-in by Galorath Inc. (El Segundo, Calif.) and already in use by more than 10 organizations worldwide. Additionally, a process maturation database, which captures the entire CAI database with a complete pedigree of processing data, environmental exposures, etc., is hosted on the Advanced Materials, Manufacturing and Testing Information Analysis Center (AMMTIAC) National Materials Information Systems (NAMIS) Web site (https://namis.alionscience.com/CAI/). And an exhaustive set of guidelines has been prepared to provide potential users with a clear understanding of advanced materials, designs, analysis tools, process controls, fabrication and assembly processes, quality assurance and repair. All of the CAI technologies, reports and data are open to the DoD and DoD contractors.
The Composites Affordability Initiative was a huge success and has fostered the use of large integrated and bonded composite structures across the fixed- and rotary-wing industrial base. CAI’s multidisciplinary approach was key to enabling technologies for the next generation of airframes. CAI research accelerated the maturation of materials and processes, increased our understanding of structural behavior in bonded joints, encouraged development of new quality assurance methods to ensure bonded joints remain bonded throughout an aircraft’s service life, and — critically important — ultimately gained large integrated and bonded structures the essential buy-off of DoD aircraft certification authorities. The structural performance and cost-effectiveness of composite structures developed as a result of the CAI exceed that of state-of-the-art assemblies produced by other means, and applications continue to increase. As the use of CAI technology increases, we envision a day when composites become the default material in DoD airframes, exceeding 50 percent by weight of the structure.
www.globalsecurity.org/military/systems/aircraft/f-22-mp.htm. (July 5, 2006)