Innovators at NASA’s Dryden Flight Research Center have developed a software program that predicts the operational flight life of critical aerostructural components. The Half-Cycle Crack Growth Computer Program offers a reliable method for calculating theoretical fatigue crack growths that could lead to catastrophic structural component failures. The program builds upon and integrates Dryden’s proven half-cycle and closed-form aging theories and is especially accurate because it considers every half-cycle of loading spectra for specific structural components. The program works by reading test data files and determining maximum and minimum loads of each half-cycle of random loading spectra in order to calculate theoretical crack growth. The innovation is an improvement on traditional prediction software (and in particular on visual inspections) because it considers mini-amplitude stress loading and half-cycles based on the duty cycle of a particular component or structure. Developed to calculate the number of operational life flights for B-52B pylon hooks at Dryden testing facilities, the program and underlying theories can be applied to estimate the service life of any critical structural component, making it suitable for use in other industries, especially for construction and petrochemical firms.
The software program can be used to monitor the operational life of structural components subjected to cyclical loading, particularly in the aerospace, construction, and petrochemical industries, including:
The Half-Cycle Crack Growth Computer Program is a powerful and practical tool for visualizing crack growth curves associated with critical stress points. It was designed to determine the number of safe operational flights an aircraft can make without structural component failures due to fatigue crack growth.
The computer program was developed after two rear B-52B pylon hooks failed simultaneously during a Dryden test operation. Subsequent examinations revealed that hook failure was caused by rapid crack propagation from existing micro cracks that had been masked by chrome-plated surfaces and thus were undetected during visual inspections.
To obtain baseline data for use in the program, the critical structural components must be proof-load tested to determine the initial theoretical crack size based on fracture mechanics. Next, strain gauges are installed in the vicinity of stress concentration points and are calibrated to record the applied loads. After the failure-critical components are identified, stress analysis is performed for each component to establish the functional relationship between the applied load and the induced tangential stress at the critical stress point (see Figure 1).The program reads the data and selects the maximum and minimum loads of each half-cycle of the random flight loading spectra. Program outputs are used to generate and display crack growth curves, providing a visual warning for preventing catastrophic structural failures. For the B-52B pylon hooks discussed above, crack growth curves were produced for each hook, allowing visual observation of the crack growth behavior during the entire air-launching or captive flight. The crack growth curves provided the visual knowledge that taxiing, takeoff, drop/landing, and sometimes gusts induced a major portion of the total crack growth per operation (see Figure 2).
|Figure 1. Distribution of tangential stress, σt, along the inner boundary of the B-52B pylon front hook |
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|Figure 2. Crack growth curve for the B-52B front hook carrying Hyper-X launching vehicle |
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Written in the C programming language, the program can be adapted for use in other industries by modifying the input model (i.e., data format load spectrum files) for the most expensive and mission critical components.
The program examines test data in a much more detailed fashion than other fatigue crack growth modeling software, counting every half-cycle of each random loading spectrum, so it is able to make better predictions about component life. By improving fatigue and failure predictions, the software provides safer flights and lower maintenance costs. Additionally, these predictions allow engineers to determine the critical points during operation that the majority of stress is placed on a particular component, which could allow for better component design that takes those specific forces into account.
This technology is part of NASA’s Innovative Partnerships Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider opportunities for partnership and usage of the Half-Cycle Crack Growth Computer Program (DRC-010-044).