Integration of Technologies for Structural Integrity Assessment of Critical Safety-of-Flight Components, 18-R9844

Principal Investigators
Dale Cope
Clint Thwing
Kurt Schrader
Kristopher Kozak

Inclusive Dates:  07/01/08 – 12/31/09

Background - Government agencies and the commercial aviation industry are placing greater emphasis on the need for technology development to help assure the safety and operational readiness of the nation's military and civilian aircraft fleets. This greater emphasis is reflected in the aviation safety goals in the National Plan for Aeronautics Research and Development, which include the development of technologies to reduce accidents and incidents through enhanced vehicle design, structure and subsystems. A near-term objective of this goal is developing vehicle health management systems to determine the state of degradation for aircraft subsystems. This objective is shared by the Air Vehicles Directorate of the U.S. Air Force Research Laboratory (AFRL) and the Aeronautics Research Mission Directorate of the National Aeronautics and Space Administration (NASA). Both directorates have major research programs in integrated vehicle health management.

Approach - In support of the National Plan, SwRI investigated the feasibility of integrating remote sensing technology with probability of failure analyses into a monitoring system capable of assessing the structural integrity of critical airframe components. The project focused on demonstrating the viability of remote sensing to discern structural flaw nucleation and growth along with integrating the sensor data with crack growth analyses based on actual usage to assess the health and integrity of the critical structural component. The research project demonstrated this integration of technologies on a complicated structural component that has limited accessibility with realistic loading. As illustrated in Figure 1, the following technical approach was used for developing the structural health monitoring system.

• Detailed stress analyses using defined flight loads were conducted to characterize failure modes that could result in catastrophic failure of a critical structural component.
• Based on these failure modes, crack growth analyses were conducted to predict the structural component’s fatigue life.
• A damage sensor system was designed and installed on an airframe test article to monitor critical structural components and capture degradation mechanisms during fatigue testing.
• Reasoning algorithms were developed and used to integrate damage sensor data with crack growth analyses to assess the current structural health and integrity of the component.
• Predictions of the component’s structural capability and remaining useful life were periodically updated based on interpretation of sensor data and assumed future operating conditions.

Accomplishments - Drawing on SwRI's extensive knowledge and expertise, researchers chose the T-38 fuselage upper longerons as the critical structural component for the focus of this study. Using previous stress analyses and full-scale testing results along with a probabilistic structural system reliability study, investigators identified four potential failure locations on the upper longeron. Researchers designed a damage sensor system based on the magnetostrictive sensor (MsS^®) technology developed by SwRI and mounted sensors to monitor three of the critical locations. The fourth location was not monitored due to limited accessibility and lack of analytical models. Researchers developed reasoning logic and algorithms to integrate damage sensor data with crack growth analyses to assess the structural health and integrity of the upper longeron on a periodic basis. Researchers conducted a fatigue test of a fuselage article containing both left and right upper longerons, using the MsS damage sensor system and strain gauges installed at critical locations to monitor the longerons during testing. Figure 2 shows a photograph of the test setup and fixture. The fatigue test resulted in failure of the right upper longeron at the location with limited accessibility. Fatigue cracks also developed in splice plates on both longerons. A teardown examination of the right longeron provided crack growth data for correlating sensor readings with flaw sizes in its upper splice plate. The correlation allowed researchers to use Bayesian principles for estimating flaw sizes based on both sensor readings and crack growth analyses to assess the structural health and integrity of the upper longeron. Results validated how fatigue life predictions and probability of failure assessments can be improved with more accurate estimates of actual flaw sizes and continual structural health monitoring.

Figure 1. Technical approach for developing an integrated structural health monitoring system.

Figure 2. Test setup and fixture for T-38 fuselage test article with upper longerons.

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