This project investigates acoustic emission (AE) as a tool for monitoring the degradation of thermal protection systems
(TPS). The AE sensors are part of an array of instrumentation on an inductively coupled plasma (ICP) torch designed for
testing advanced thermal protection aerospace materials used for hypervelocity vehicles. AE are generated by stresses
within the material, propagate as elastic stress waves, and can be detected with sensitive instrumentation. Graphite
(POCO DFP-2) is used to study gas-surface interaction during degradation of thermal protection materials. The plasma is
produced by a RF magnetic field driven by a 30kW power supply at 3.5 MHz, which creates a noisy environment with
large spikes when powered on or off. AE are waveguided from source to sensor by a liquid-cooled copper probe used to
position the graphite sample in the plasma stream. Preliminary testing was used to set filters and thresholds on the AE
detection system (Physical Acoustics PCI-2) to minimize the impact of considerable operating noise. Testing results
show good correlation between AE data and testing environment, which dictates the physics and chemistry of the
thermal breakdown of the sample. Current efforts for the project are expanding the dataset and developing statistical
analysis tools. This study shows the potential of AE as a powerful tool for analysis of thermal protection material
thermal degradations with the unique capability of real-time, in-situ monitoring.
Self-repairing structural systems can potentially improve performance ranges and lifetimes compared to those of
conventional systems without self-healing capability. Self-healing materials have been used in automotive and
aeronautical applications for over a century. The bulk of these systems operate by using the damage to directly initiate
the repair response without any supervisory coordination. Integrating sensing and supervisory control technologies with
self-healing may improve the safety and reliability of critical components and structures. This project used laboratory
scale test beds to illustrate the benefit of an integrated sensing, control and self-healing system. A thermal healing
polymer embedded with resistive heating wires acted as the sensing-healing material. Sensing duties were performed
using an impedance, capacitance, and resistance testing device and a PC acted as the controller. As damage occurs to the
polymer it is detected, located, and characterized. Based on the sensor signal, a decision is made as to whether to
execute a repair and then to subsequently monitor the repair process to ensure completeness. The second demonstration
was a self-sealing pressure vessel with integrated sensing and healing capability. These proof-of-concept prototypes can
likely be expanded and improved with alternative sensor options, sensing-healing materials, and system architecture.
A series of experiments have been conducted that microscopically image thermal diffusion and surface acoustic phonon
propagation within a single crystallite of a polycrystalline Si sample. The experimental approach employs ultrashort
optical pulses to generate an electron-hole plasma and a second probe pulse is used to image the evolution of the plasma.
By decomposing the signal into a component that varies with delay time and a steady state component that varies with
pump modulation frequency, the respective influence of carrier recombination and thermal diffusion are identified.
Additionally, the coherent surface acoustic phonon component to the signal is imaged using a Sagnac interferometer to
monitor optical phase.