The Application Of Fracture Mechanics To The Design Of Damage-Tolerant Components For The UTTAS Helicopter

The principles of fracture mechanics have been used extensively in the design of damage-tolerant structural components for the YUH-61A UTTAS helicopter which is being designed, built, and tested by the Boeing Vertol Company under Army contract. The requirements for a high level of structural reliability, considering both combat and noncombat environments, led to establishment of a damage-tolerance criterion. The design of major structural components was often controlled by this criterion which required specific residual fatigue life and static strength after sustaining initial damage of a given magnitude. Implementation of the damage-tolerance criterion relied heavily on the technology of fracture mechanics. This technology was used in conducting material trade-off studies, providing estimates of crack-growth characteristics for sizing components, and establishing procedures for bench tests to check compliance of components with damage-tolerance criteria. Two fracture-mechanics-based techniques, in particular, were widely used to achieve damage tolerance. One technique used the approach of designing a component to prevent damage of a given size from growing under operational loads. This led to the concept of a fatigue-crack-growth threshold, which is a function of stress level and crack size. The other technique consisted of designing a component so that damage growth is limited to non-catastrophic proportions. These techniques entered directly in the design of more than 10 critical dynamic-system components for the YUH-61A helicopter. It is concluded that fracture mechanics provides a valuable tool for the design of damage-tolerant structure.