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  • 1
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    Fiber Contraction Approaches for Improving CMC Proportional Limit (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The fact that the service life of ceramic matrix composites (CMC) decreases dramatically for stresses above the CMC proportional limit has triggered a variety of research activities to develop microstructural approaches that can significantly improve this limit. As discussed in a previous report, both local and global approaches exist for hindering the propagation of cracks through the CMC matrix, the physical source for the proportional limit. Local approaches include: (1) minimizing fiber diameter and matrix modulus; (2) maximizing fiber volume fraction, fiber modulus, and matrix toughness; and (3) optimizing fiber-matrix interfacial shear strength; all of which should reduce the stress concentration at the tip of cracks pre existing or created in the matrix during CMC service. Global approaches, as with pre-stressed concrete, center on seeking mechanisms for utilizing the reinforcing fiber to subject the matrix to in-situ compressive stresses which will remain stable during CMC service. Demonstrated CMC examples for the viability of this residual stress approach are based on strain mismatches between the fiber and matrix in their free states, such as, thermal expansion mismatch and creep mismatch. However, these particular mismatch approaches are application limited in that the residual stresses from expansion mismatch are optimum only at low CMC service temperatures and the residual stresses from creep mismatch are typically unidirectional and difficult to implement in complex-shaped CMC.
    Keywords: Composite Materials
    Type: Paper-18-Vol-1 , Physics and Process Modeling (PPM) and Other Propulsion R and T; 1; NASA-CP-10193-Vol-1
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Thermomechanical Behavior of Advanced SiC Fiber Multifilament Tows (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: In order to relate single fiber behavior to multiple fiber behavior in composites, fast-fracture tensile strength, creep, and stress-rupture studies were conducted on advanced SiC fiber multifilament tows in the temperature range from 20 to 1400 C in air as well as in inert environments. For conditions of small fiber creep (short times and low temperatures), the tow results of this study confirm the ability of limited single fiber data to model the strength behavior of multiple fibers in a bundle. For conditions of high creep (long times and high temperatures), further studies are needed to explain tow rupture behavior being better than average single fiber behavior.
    Keywords: Composite Materials
    Type: NASA-TM-107366 , NAS 1.15:107366 , E-10528 , Annual Conference on Composites, Advanced Ceramics, Materials, and Structures; Jan 07, 1996 - Jan 11, 1996; Cocoa Beach, FL; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Time/Temperature Dependent Tensile Strength of SiC and Al2O3-Based Fibers (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: In order to understand and model the thermomechanical behavior of fiber-reinforced composites, stress-rupture, fast-fracture, and warm-up rupture studies were conducted on various advanced SiC and Al2O3-based fibers in the,temperature range from 20 to 1400 C in air as well as in inert environments. The measured stress-rupture, fast fracture, and warm-up rupture strengths were correlated into a single master time/temperature-dependent strength plot for each fiber type using thermal activation and slow crack growth theories. It is shown that these plots are useful for comparing and selecting fibers for CMC and MMC reinforcement and that, in comparison to stress rupture tests, the fast-fracture and warm-up tests can be used for rapid generation of these plots.
    Keywords: Composite Materials
    Type: NASA-TM-107370 , NAS 1.15:107370 , E-10538 , 1996 Annual Meeting; Apr 14, 1996 - Apr 17, 1996; Indianapolis, IN; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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