Other functions:NASA and the FAA are cooperatively pursuing research to understand damage tolerance of composite structures under dynamic (high cycle) fatigue conditions. The use of composites for…

Other functions:NASA and the FAA are cooperatively pursuing research to understand damage tolerance of composite structures under dynamic (high cycle) fatigue conditions. The use of composites for civil aircraft structures has increased dramatically in the last ten years, and IT is expected that they will be used on more structures that are subjected to dynamic fatigue environments. The characteristic responses of dynamically fatigued composite structures are significantly different from those of traditional civil-certified composite structures. The FAA is a research partner in this program and will be participating in formulation of goals and monitoring the research. During the process, key members of the FAA involved in composite civil aviation structures will participate in both focusing the research and assessing results. The FAA will be using the outputs of this research to assess current regulations, policy, and guidance for assuring composites subjected to dynamic fatigue conditions are safe and reliable. The development of damage tolerance certification methodologies for dynamically fatigued composite structures requires understanding of the complex processes involved in structural failure. Recent experiments have shown that the understanding of failure in high cycle fatigue environment is deficient. The ability to predict locations of failure is key to providing an adequate level of safety. NASA and the FAA are interested in identifying the key characteristics of dynamic fatigue of composite structures which affect their ability to resist damage. The understanding of this phenomenon has been hampered, in part, by the inadequate understanding of progressive failure of structural components manufactured from laminated composite materials. The result has been the use of so-called `no growth damage tolerance philosophies, resulting potentially in overly designed structures. The lack of understanding of failure in composite structures is attributed to the fact that failure may often follow a sequence of several different competing failure modes. Furthermore, several factors exist that determine which failure modes ultimately occur, and in which order. These factors include, but are not limited to, manufacturing process conditions, geometry (including stacking sequence), and loading and boundary conditions. The overall effect is that failure often appears to be a stochastic process, which further complicates attempts to adequately capture damage progression in composite structures.