Date of Award

2016

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Xinyu Huang

Abstract

Ceramic Matrix Composites (CMC) have been actively researched for applications requiring high temperature strength and damage tolerance. Examples are turbine engine hot section components, leading edges for hypersonic vehicles, nuclear fusion reactor first wall and blanket, and recently nuclear fuel claddings. Predictable material damage behavior is desirable for these applications so early damage accumulation can be detected before a catastrophic failure occurs. Many ceramic materials can withstand high temperature environments, but due to inherent brittleness of ceramics the high damage tolerance and the predictable damage behavior are difficult to achieve. Ceramic fiber reinforced ceramic composites (CFCC) have demonstrated the above properties through properly designed fiber architecture, carefully engineered fiber-matrix interphase layer, and the advanced manufacturing processes. Because of the number of unknowns associated with the design and manufacturing of CFCCs, mechanical experiments are relied upon to obtain necessary material property data sets for component and system design analysis.

This work focuses on: (1) development and validation of novel mechanical characterization techniques for tubular samples; and (2) characterization of a nuclear grade silicon carbide fiber reinforced silicon carbide matrix (SiCf-SiCm) composite nuclear fuel cladding. A bladder type internal pressure fixture was designed to load the composite cladding tubes to failure in two configurations: tubular sample with both ends open and tubular sample with one closed-end. The test methods were first validated by testing tubular surrogate samples made out of well-known materials, and comparing the test results with finite element (FE) simulation and analytical calculations. The pressure profile measured by Digital Image Correlation (DIC) along sample axis is in good correlation with previously reported analytical solution. Away from the edge, the measured strains were found to be within 3% of calculated values at loading levels of interest. A closed-end validation experiments showed that the novel bladder method can generate a hydrostatic pressure state which closely imitates the operating conditions for nuclear fuel claddings.

Using the above methods, nuclear grade SiCf-SiCm composite tubes had been tested in both configurations to obtain critical stress and strain values at proportional limit and at failure. One group of samples, with only 1.25 mm wall thickness, was able to sustain internal pressures averaging at 99.8 MPa before the final rupture. These samples also demonstrated pronounced progressive damage behavior starting around 33.2 MPa. At the rupture pressure, all samples showed graceful failure modes without excess fragmentation. The surface strain maps measured by DIC revealed highly heterogeneous strain state during loading; the spatial frequency of the strain patterns correlate to the fiber tow braiding architecture. At final failure, the local peak strain was found to range between 115% and 185% of the average failure strain observed on sample surfaces.

Acoustic emission (AE) monitoring method was used to obtain additional information about the progressive damage behavior in this CFCC. From activity and intensity of the captured AE signals, a strong correlation had been observed when stress approaches proportional limit, the first significant damage state in CMC. Contrary to the commonly observed Kaiser and Felicity effects, an interesting new phenomenon of AE signal bursts during unloading process was observed for some SiCf-SiCm composite tubes. It is hypothesized that it is related to the forced closure of mismatched crack surfaces. Frequency content of some AE signals was shown to decrease with the increased amount of damage. Unsupervised Pattern Recognition (UPR) technique was applied to identify and group similar AE signals. Four classes were identified with the clustering algorithms showing consistency with other works. It is believed that the classes are associated with microscopic composite damage mechanisms such as matrix cracking, fiber breaking, fiber-matrix debonding and sliding. Such correlation, with further verification, will help elucidate the progressive damage process of CMCs.

Rights

© 2016, Kirill Shapovalov

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