The development of rapid, on-line monitoring techniques to evaluate the current state of critical elements in complex structures has been the objective of many recent research studies. " Smart " materials offer the promise of being able to detect and identify potential material performance problems in the early stages of their development, before they reach criticality and substantially degrade material performance capabilities.

Technical Approach

We propose to develop a system for automatically evaluating the current state of the shuttle’s thermal protection system through the use of specialized sensors, imbedded within tile or the adhesive bonding the tile to the substrate. In order to be useful, any proposed sensor must meet several stringent criteria. In particular, prospective sensors must provide the basic inspection capability of the current procedure as well as being:

without adversely affecting the thermal insulation protection afforded by the tiles or the structural integrity of the spacecraft. Two imbedded possibilities are suggested here. One sensor is proposed to monitor and record the temperature extremes experienced . The second is an optical fiber used as the basis for a laser inspection system for bondine integrity.

Passive Temperature Sensors

The proposed peak temperature sensor is a passive device which can be remotely interrogated after each flight to determine the maximum temperature seen at its location. Active devices, like thermocouples, measure current temperature and require elaborate electronics or telemetry systems to monitor current conditions as continuous monitoring is required to insure that relevant data are not lost. These devices are based on a temperature dependent phase transformation from a ferromagnetic state to a non-ferromagnetic state and can be used to track the peak temperatures. Altering the composition of the sensor material changes the transformation temperature. Therefore, a series of sensors with different chemical compositions can be fabricated to provide a means of determining peak temperature. Electromagnetic properties ( here simply whether or not a given sensor is ferromagnetic at a particular point )can be rapidly measured using SQUID sensors, GMR circuits or eddy current probes.

Optical Fibers for Laser UT

The second sensor is proposed to monitor bondline property degradation, either via defect initiation ( cracks, voids, porosity, delamination ) or property reduction due to deterioration in the adhesive due to mechanical loading and/or environmental exposure. With this approach, imbedded optical fibers are use to introduce laser signals into the structures. Optical fibers can be used either to generate or to sense. As a generator, the fiber is coupled to a laser source to locally heat a point the test region. The resultant thermal expansion launches an acoustic wave into the material which can be used to probe structural integrity. As a sensor, the fiber is coupled to an interferometer to sense the waves generated by the laser source after they have interacted with the structure. Analysis of these wave patterns can be used to identify structural defects or property degradation which can adversely affect the tile.