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Compiled by Don Rosato
Keyword Abstract: creativity, composites, matrix, reinforcement, modeling, U.S. Air Force Research Laboratory, Asian Office of Aerospace Research and Development, Kanazawa Institute of Technology, Stanford University, Hanyang University, National University of Singapore, element failure method, nodal forces, crack region
Advanced Composite Materials Introduction
Composites are engineering materials fabricated from two or more constituent materials (matrix and reinforcement), that macroscopically remain separate and distinct while forming a single component. The matrix material surrounds and supports the reinforcing materials by maintaining their relative positions, while the reinforcing materials impart special mechanical/physical properties to enhance the matrix properties. The reinforcement materials are often fibers but may also be ground minerals, foam cores or other materials. Thermoset resins such as epoxies and polyesters are readily fashioned into complex shapes replacing metal structural parts on a strength basis.
Advancing Composite Materials in Aerospace & Defense Applications
Understanding the performance of composite materials is essential to advancing their use in aerospace and other applications. Polymer matrix composites with greater specific strength and stiffness on a constant mass basis to many competing materials are finding growing acceptance in the design of modern aircraft structures. The Eurofighter and India's Light Combat Aircraft both contain approximately 50% composites by weight and it is estimated that the Joint Strike Fighter and the Boeing 787 will make use of similar levels of plastics composite materials. For this composite usage in aircraft applications to continue and conceivably increase, engineers and scientists must persuasively display the durability and economic advantages of these materials.
Safety is an essential consideration in the design of aerospace structures. To compensate for uncertainty or insufficient knowledge, 'safety factors' are used in engineering design. A necessary engineering design element, the use of excessively large safety factors is wasteful overdesign that increases cost and can contribute to loss of component functionality. To rationalize safety factors in component design, engineers need accurate information concerning bothapart's specification and finished in service performance requirements as well as the behavior of respective materials. Researchers around the world are expending significant effort to improve design methodology and expand knowledge relative to composite materials, quantify load-stress-strain relations and time-dependent responses, determine the effects of usage and the environment on composites performance, detect/quantify damage and develop composites repair techniques. The U.S. Air Force, a key beneficiary of progress in this area, contributes appreciably to this research effort through the U.S. Air Force Research Laboratory (AFRL).
Modeling Composite Materials
By way of research contracts with the Asian Office of Aerospace Research and Development (AOARD), AFRL has addressed certain key questions regarding the design and durability of composites. Professor Yasushi Miyano, Director of the Materials System Research Laboratory at the Kanazawa Institute of Technology, Japan in collaboration with Drs. Stephen Tsai and Richard Christensen both at Stanford University, California studied 'Accelerated Testing of Durability of Composite Materials and Structures.' As a result these joint studies, Professor Miyano developed a model for long term aircraft use that is said to predict composite failure based on temperature and stress or temperature and fatigue loading history. The model reduces by an order of magnitude the testing needed to certify a composite for use.
Professor Sung Kyu Ha of the Center of Innovative Design Optimization Technology (CIDOT) at Hanyang University, Korea conducted extensive research on modeling stress levels in real composites. Models currently assume fibers in composite materials have cross-sectional distributions that are either hexagonal or square. Professor Ha's makes use of random fiber distributions. This approach is a closer reflection of the fiber distribution that is observed in actual composite materials, thereby permitting scientists to more accurately predict stresses. Stress calculations are 10%-15% more accurate as a result of Professor Ha's work.
Under a separate contract with AOARD and AFRL, Professor Tong-Earn Tay with the National University of Singapore has developed a model to study damage and stress states in fiber composites recognizing that accurate, practical modeling of composite structure damage progression is needed specifically for composite components not just laboratory test specimens, since specimen level results do not always correlate with damage observed at the structural/component level. Professor Tay's unique element failure method (EFM) is more versatile, robust and efficient relative to conventional models based on material property degradation and fracture-mechanics which fail to capture key characteristics of cracks and their propagation. Using EFM, researchers can more precisely evaluate the effects of different types of damage on the composite structure. As no clear crack tip exists for composite materials, nodal forces are assigned in the crack region.
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Nodal Forces Assigned in Crack Region
(Source: Professor Tong-Earn Tay, National University of Singapore
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Work continues to more fully understand composite durability, as the AOARD continues to fund approaches to acquiring this understanding. Under an AOARD contract, Professor Woo Lee at the Seoul National University, Korea is modeling the flow of resin that takes place during the fabrication of a composite. Resin flow is directly related to residual stress development that occurs during fabrication. Composite performance is a function of applied and residual stresses. The results of Professor Lee's work will further provide the tools needed to make intelligent use of composites. The ability of scientists to creatively collaborate across international borders, has been an important factor in advancing the understanding and application of composite materials in aerospace.
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