Tech Briefs

Integral international collaboration advances the understanding of composite materials performance in aerospace applications.

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.

ImageConsiderations 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.