Built-in diagnostics for sandwich structures under extreme temperatures

In order to maximize the survivability and signal sensitivity of sensors exposed to extreme temperature, piezoelectric ceramics were selected for use as transducers and sensors. An efficient manufacturing technique was developed to install these sensors within a cell of the honeycomb core. This technique doesn't modify the traditional manufacturing technique of the sandwich structures. Instead, the sensor and the wire layer are inserted before closing the sandwich for final curing. The materials used for the sensor and the wire layer were both tested at cryogenic temperature to ensure their survivability.; The active sensing technique uses one built-in piezoelectric transducer to generate propagating waves to be received by the neighboring piezoelectric sensors. The difference in sensor measurements before and after the presence of damage provides information about the location and size of the damage. Due to strong wave attenuation in honeycomb core structures, the signal changes greatly when the damage is located in the direct path between a transducer and a sensor as opposed to very little when off the direct path. Based on these observations, a simple detection algorithm was developed which calculates the relative scatter energy for every path to obtain an image of the damage location and uses the time-domain signals to decipher which paths propagate through the damage and thereby evaluate its extent. This damage detection was validated on a composite sandwich panel with a square array of surface mounted sensors. The effect of sensor density on the size of undetectable damage was also studied for different sensor patterns repeated over a large-scale structure. Using a genetic algorithm called differential evolution, the location of the sensors was optimized to minimize the furthest distance from a diagnostic path. Signal attenuation is a key parameter in the selection of the best sensor pattern for a given structure.