The electric resistance of woven SiC fiber reinforced SiC matrix composites were measured under tensile loading conditions. The results show that the electrical resistance is closely related to damage and that real-time information about the damage state can be obtained through monitoring of the resistance. Such self-sensing capability provides the possibility of on-board/in-situ damage detection and accurate life prediction for hightemperature ceramic matrix composites.Woven silicon carbide fiber-reinforced silicon carbide (SiC/SiC) ceramic matrix composites (CMC) possess unique properties such as high thermal conductivity, excellent creep resistance, improved toughness, and good environmental stability (oxidation resistance), making them particularly suitable for hot structure applications. In specific, CMCs could be applied to hot section components of gas turbines [1], aerojet engines [2], thermal protection systems [3], and hot control surfaces [4]. The benefits of implementing these materials include reduced cooling air requirements, lower weight, simpler component design, longer service life, and higher thrust [5]. It has been identified in NASA High Speed Research (HSR) program that the SiC/SiC CMC has the most promise for high temperature, high oxidation applications [6].One of the critical issues in the successful application of CMCs is on-board or insitu assessment of the damage state and an accurate prediction of the remaining service life of a particular component. This is of great concern, since most CMC components envisioned for aerospace applications will be exposed to harsh environments and play a key role in the vehicle's safety. On-line health monitoring can enable prediction of remaining life; thus resulting in improved safety and reliability of structural components. Monitoring can also allow for appropriate corrections to be made in real time, therefore leading to the prevention of catastrophic failures. Most conventional nondestructive evaluation (NDE) techniques such as ultrasonic C-scan, x-ray, thermography, and eddy current are limited since they require structural components of complex geometry to be taken out of service for a substantial length of time for post-damage inspection and assessment. Furthermore, the typical NDE techniques are useful for identifying large interlaminar flaws, but insensitive to CMC materials flaws developed perpendicular to the surface under tensile creep conditions. There are techniques such as piezoelectric sensor [7,8], and optical fiber [9,10] that could be used for on-line health monitoring of CMC structures. However, these systems involve attaching an external sensor or putting special fibers in CMC composites, which would be problematic at high temperature applications.https://ntrs.nasa.gov/search.jsp?R=20080047468 2018-05-11T18:03:58+00:00Z Most composite materials are multifunctional materials in which the damage is coupled with the material electrical resistance, providing the possibility of real-time information about the damage state thro...

SiC/SiC ceramic matrix composites under creep-rupture loading accumulate damage by means of local matrix cracks that typically form near a stress concentration, such as a 901 fiber tow or a large matrix pore, and grow over time. Such damage is difficult to detect through conventional techniques. This study demonstrates that electrical resistance is a viable method of monitoring and inspecting damage in SiC/SiC composites at high temperature. Both interrupted and uninterrupted creeprupture experiments were performed at 13151C and 110 MPa with in situ resistance measurements. A linear relationship was found between resistance and cumulative crack depth.Ã Specimen did not break.w Thermal mechanical fatigue (unloaded and cooled to room temperature at 1 and 5 h).z Inadvertent overload to 160 MPa while cooling.

Light-weight, high-conductivity graphite foams comprised of open cellular structure and dense graphitic matrix are attractive materials for thermal management applications in avionic heat sinks and heat exchangers. Integrating foam in such systems requires robust and thermally conductive joints between the foam and metals such as Ti. Graphite foams with different densities were vacuum brazed to titanium using a Ag-Cu-Ti active braze alloy, Cusil-ABA. Optical microscopy and scanning electron microscopy coupled with energy dispersive spectroscopy were used to evaluate joint integrity, interface microstructure, and elemental composition. The joints were defect-free and well-bonded, and the carbon/braze interfaces were enriched in Ti indicating chemical bonding. The low-density foams exhibited significant braze penetration with the penetration distance increasing with decreasing foam density. The room-temperature tension test on brazed foam/Ti joints revealed that the joints were stronger than the foam, so failure occurred within the foam. The thermal resistance of foam/Ti joints, estimated using 1-D, steady-state heat conduction analysis for a planar configuration, revealed a marginal effect of braze saturated foam on joint conductivity for the practical range of values of foam and metal conductivities, penetration depth, and metal-to-foam thickness ratio. *mrityunjay.singh-1@nasa.gov

Damage mechanism identification has scientific and practical ramifications for the structural health monitoring, design, and application of composite systems. Recent advances in machine learning uncover pathways to identify the waveform-damage mechanism relationship in higher-dimensional spaces for a comprehensive understanding of damage evolution. This review evaluates the state of the field, beginning with a physics-based understanding of acoustic emission waveform feature extraction, followed by a detailed overview of waveform clustering, labeling, and error analysis strategies. Fundamental requirements for damage mechanism identification in any machine learning framework, including those currently in use, under development, and yet to be explored, are discussed.