In the field of engineering design, "factors of safety" are derivatives of inadequate knowledge and therefore are a necessary, but costly, element of engineering design. Designing components with excessively high factors of safety is needless over design that results in partial loss of component functionality and increased costs to produce and use the component. To design components that incorporate rational factors of safety, engineers must have precise knowledge of both a component's performance requirements and the properties of its constituent materials during fabrication and while in service.

Figure 1. Universal curve for creep versus time generated by the time-temperature superposition principle for composites tested in wet and dry conditions

Considerations of design and safety are central to aircraft and space structures, for which composites have supplanted metals in many structural components. The specific (i.e., on a constant-mass basis) strength and stiffness characteristics of polymer matrix composites are generally superior to those of rival materials, and the compositions of modern aircraft reflect their growing acceptance. Engineers estimate that the structures of both the Joint Strike Fighter and the Boeing 787 will consist of approximately 50% composites by weight. Composite materials now account for approximately 50% of the weight of the multinational Eurofighter and India's Light Combat Aircraft. For this ascendance of composite usage in aircraft to hold, and perhaps expand, researchers must convincingly demonstrate the associated long-term performance and economic advantages. Scientists worldwide are devoting an enormous amount of effort to considerations related to composite durability, including design methodology, quantification of both load-stress-strain relations and time-dependent responses, assessment of the effects of exposure to atmospheric agents and damage on performance, detection and quantification of damage, and repair techniques. The US Air Force (USAF) contributes significantly to this research effort and is a primary beneficiary of the advances that result.

AFRL, through a series of foreign research contracts at the Asian Office of Aerospace Research and Development (AOARD), has addressed several compelling questions in the design and failure of composites. Professor Yasushi Miyano (Kanazawa Institute of Technology, Japan) developed a model for long-term use of aircraft composites based on the idea that failure mechanisms remain constant irrespective of temperature or loading history. Based on this model, researchers can reduce data from various tests to a single master plot and predict failure for a given temperature and stress or for temperature and fatigue loading history (see Figure 1 on previous page). The primary result of Prof Miyano's work is that researchers can reduce the testing required to certify a composite for use by approximately an order of magnitude. In other words, manufacturers can cut certification costs by roughly 90% and spend 90% less time performing the tests.

Prof Sung Ha (Hanyang University, Korea) worked for 1 year on modeling stress levels in real composites (see Figure 2 on previous page). Currently accepted models assume either hexagonal or square cross-sectional distributions of fibers in composite materials. Prof Ha's approach incorporates random fiber distributions, which more realistically reflects the fiber distribution in actual materials and thus enables researchers to predict stresses more accurately. His results have improved the accuracy of stress calculations by approximately 10%-15%.

Figure 2. Finite element prediction of strains in actual fiber composite

Prof Tong Earn Tay (National University of Singapore) developed a numerical method for examining stress states and damage in fiber composites (see Figures 3 and 4). Conventional, fracture-mechanics based, finite element methods fail to capture key attributes of cracks and their propagation. Prof Tay's method exhibits high fidelity, and it is also computationally efficient and robust. He refined and validated the methodology in a 2.5- year study of damage in aerospace composites, and these study results now enable researchers to more accurately assess the effects of various types of damage on the composite structure performance.

Prof Steven Tsai (Stanford University, California, and formerly of AFRL), in collaboration with engineers at Boeing, has coordinated and supplemented this work. Prof Tsai and his colleagues incorporated the respective contributions of these researchers, along with input from researchers at AFRL and Stanford, into a computer code called Super Mic Mac. AFRL developed the initial version of the code, Mic Mac, in the 1980s. The new code expands substantially on the original and is now available for licensing at no fee. Super Mic Mac enables designers to create more efficient composite structures and to reduce unnecessarily forgiving factors of safety. For example, engineers typically design composite tail sections of commercial airliners to withstand a stress 2.5 times greater than the maximum stress anticipated during their service life—a factor of safety of 250%. With a better understanding of the stress states and long-term performance of composites, aerospace designers should be able to significantly reduce this factor of safety.

Much work remains to be done in fully understanding composite durability. The AOARD continues to fund approaches to acquiring this understanding, and one of the organization's recent contracts is with Prof Woo Il Lee (Seoul National University, Korea). Prof Lee is modeling the flow of resin that occurs during a composite's fabrication, because resin flow is directly related to the development of residual stresses during fabrication processes. Composite performance depends on the sum of applied and residual stresses. The results of his work should add another piece to the complex puzzle of intelligent use of composites. The propensity for scientists to collaborate across international borders, and the capability of AFRL to support such collaborations, has been integral to advancing the understanding of composite materials performance in aerospace applications.

Industry demands for increased performance and decreased weight dictate that aircraft engineers continue to incorporate composite materials in their designs. Employing Super Mic Mac or similar tools should enable engineers to achieve more efficient designs and users to extract full value from their composite enhanced creations. Maintenance and repair strategies can evolve based on a thorough understanding of the long-term performance of composites. The overall benefit to the USAF derived from a better understanding of composite properties will entail enhanced safety, increased payloads, and improved performance—at a reduced total cost.

Dr. Ken Goretta, of the Air Force Research Laboratory's Air Force Office of Scientific Research, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at . Reference document OSR-H-05-09.