Statement of Work
The proposed effort addresses the RCAs of this MURI topic through integration of material characterization (including high temperature), computational modeling, sensor instrumentation, information management, damage detection and benchmark laboratory experiments as depicted in the figure. This overall goal will be pursued using a multidisciplinary approach including the following primary research tasks.
A multidisciplinary approach to prognosis and residual life estimation
Task 1: Physically-based Multiscale Modeling
Prediction of nucleation and growth of defects will be enabled by applying physics-based approaches that support tracking their evolution in the microstructure and predicting structural failure. Treating the microstructure as the evolving state variable in the multistage modeling will eliminate the need to test every conceivable damage scenario for every system. This will result in a robust prognosis procedure capable of assessing system performance under a broad range of future loading conditions. Specific Task 1 objectives are: (i) identify system susceptibility to damage through material characterization and evaluate weak links in the microstructure as potential sites for damage nucleation; (ii) develop multiscale modeling for stochastic simulation of metallic microstructures including: constitutive models to predict the initiation and growth of damage and methodologies to characterize threshold crack growth behavior.
Task 2: Methods for In Situ Interrogation and Detection
Capabilities for in situ interrogation will be advanced by coordinated research in materials physics and fundamental sensor design, sensor electronic systems and their implementation, and sensor management and data processing, and information-level analysis. Methods of multiplexing and communication for practical instrumentation will be developed. Advanced damage processing techniques will be developed to analyze, identify, classify and detect damage in metallic aircraft structures. Recent advances in hierarchical information management and decision processing will be adapted for the real-time SHM application. The overall objectives are: (i) achieve systems-level analysis and networking by considering the implementation of analog or digital strain sensors; (ii) optimally integrate the sensor network with the structure and sensor placement; (iii) investigate multiplexing, communication, and sensor management schemes for practical instrumentation and optimal detection/classification performance; (iv) develop methodologies to analyze, classify and detect structural damage; (v) connect microscopic material and structural damage to methods for macroscopic scale monitoring.
Task 3: Prognosis via State-awareness and Life Models
Prognosis capabilities for predicting failure probability and remaining useful life will be enabled via integration of microstuctural characterization, modeling, in situ defect characterization using sensing networks, probabilistic system identification, and health management. These elements will be used to establish appropriate length scales and physically based models underpinning robust prognosis. Experiments will be used, in conjunction with multiscale models to predict the remaining life of complex airframe components. Stochastic techniques will be used to account for the variability of the associated parameters. The Task 3 objectives are: (i) develop models to provide quantitative measures of damage using simulation and experiments; (ii) formulate stochastic models to account for uncertainties in excitation signals and derive an optimal deterministic strategy to minimize risk; (iii) develop models for estimation of residual life using reduced order models and stochastic surface response techniques.
Task 4: Testing, Validation and Application (Focus Problems)
Validation of all methodology developed will be carried out using sophisticated simulations and real testbed data from a set of representative focus problems. Structural hot spots, such as wing sections near the root will be emphasized. Various types of representative wing sections with edge spar, chord-wise ribs, stiffeners and bolts will be constructed using existing facilities at the participating universities. These testbeds will be instrumented with sensors to validate the proposed methodology and its feasibility and compare it against conventional SHM systems. Standard fatigue crack growth experiments will be performed with fully characterized samples of the materials chosen. Damage will be monitored in situ until failure to provide data for verifying and calibrating the prognosis tools proposed. Experiments will be conducted to: (i) characterize damage evolution and link “damage” parameter to sensor output; (ii) validate and calibrate the modeling methods; (iii) characterize relevant length scales in the microstructure for multiscale modeling; (iv) characterize damage evolution on complex sensor integrated structures; (v) evaluate the length scale affected by defects using mechanical testing. It is envisioned that additional system level experiments will be conducted in collaboration with researchers at AFRL.
Task 5: Extensions of Research into Next-Generation Aircraft and Aerospace Vehicles
Extensions of results from this effort to support health monitoring of next-generation aerospace materials, structures, and vehicles will be identified. Experiments will be conducted for material characterization and damage sensing for high temperature applications. This is not intended as a major focus of the overall program, but primarily as a mechanism for students to envision longer-term problems that they will likely face as future aerospace systems designers and engineers.