Introduction:
This study, sponsored by the FAA, develops a comprehensive and systematic risk assessment and management
methodology for rotorcraft damage tolerance (RCDT). This will support FAA's rulemaking regarding the
future implementation of RCDT-based design and maintenance. The outcomes of the proposed study will also
be beneficial to enhance the continued airworthiness and economical operations of rotorcraft. The
traditional damage tolerance (DT) approach to aircraft structures assumes a deterministic damage
accumulation process where deterministic crack growth curves, constant material properties, and specific
initial flaw sizes are used. However, the fatigue damage accumulation process is stochastic in nature.
The uncertainty quantification of fatigue damage and its effect on structural safety needs to be
carefully included in RCDT risk assessment. The probabilistic method is more appropriate for RCDT since
it can properly account for various uncertainties and assist the decision-making process with respect to
design and maintenance scheduling.
Methodology:
The overall objectives of the project are to develop, validate, and implement a general risk assessment
and management methodology for rotorcraft damage tolerance. The project is composed of four major components:
(1) Uncertainty Quantification at the material level, (2) Uncertainty Propagation to the component/structural
level, (3) Risk assessment at system level, and (4) Risk management actions (inspection and maintenance
planning). Task 1 will investigate the uncertainties in the various input quantities to the RCDT methodology.
Uncertainties include but are not limited to equivalent initial flaw shapes, sizes, and locations, fatigue
crack threshold stress intensity factors, loading spectra, structural geometries, manufacturing process
variables, and environmental effects. Task 2: Uncertainty propagation to component/structural level will
analyze analytical and computational methods, fracture mechanics, and random process theory to develop a
suitable methodology to evaluate the effects of uncertainty propagation on fatigue life prediction under
simulated service conditions. Task 3: Risk assessment at system level will examine analytical methods,
response surface methods and efficient Monte Carlo methods to develop an efficient and accurate methodology
for reliability estimation of the rotorcraft system. Task 4 will develop a general methodology for
reliability-based inspection optimization (RBIO) based on the analytical and experimental results described
in tasks 1 to 3.
Application:
The above objectives incorporate implementation and validation with practical rotorcraft material, component
and system, in collaboration with industrial partner, Bell Helicopter Textron Inc. The objectives will help
assist FAA rulemaking and are directly connected to FAR 29.571 (fatigue evaluation). The project will also
help to determine the optimum inspection schedule to ensure the structural reliability level meets or
exceeds industry standards.
Article on FAA Rotorcraft Project
