Date of Award

2017

Document Type

Open Access Thesis

Department

Nuclear Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Xinyu Huang

Abstract

With growing interest on ceramic fiber reinforced ceramic matrix composites (CMC) for accident tolerant fuel, the need for mechanical characterization of ceramic composite arises. It has been of particular interest to non-destructively evaluate the mechanical performance of these composites. Impulse excitation (IE) is a well-established method for non-destructive mechanical characterization of homogeneous isotropic material of well-defined shapes. In this thesis, impulse excitation technique was applied for non-destructive characterization of composite tube for the first time as far as we know. CMC, when stressed beyond its damage threshold, will experience various forms of structural damage, such as matrix micro-cracking, fiber-matrix debonding, and fiber breakage. The effects of damage on its vibrational frequency and damping were studied using the impulse excitation technique.

Nuclear fuel cladding can experience both tensile and compressive stress, however, most mechanical testing is conducted by putting it under tensile stress: the examples are uniaxial tensile test and internal pressure burst test. Little experimentation involving compression of CMC tubing via external pressure has been performed previously. In this thesis, the mechanical behavior of CMC under compressive stress up to the point of material failure was studied using an adapted rubber plug compression technique and with strain gauge installed on the internal surface of the composite tube.

The experimentation performed in this thesis focuses on: 1) the utilization of impulse excitation as a non-destructive evaluation method to determine mechanical properties and to monitor the damage of silicon carbide fiber reinforced silicon carbide matrix (SiCf-SiCm) composite tubing, and 2) the mechanical characterization of SiCf-SiCm composite tubing under external pressure. In the first focus, novel configuration for enabling the impulse excitation measurement of slender tube was developed. The method was first validated on tubes of well-characterized materials under free-free and clamped-free configurations. Validation testing of all IE setups resulted in less than 6% deviation of mechanical properties when compared to published values. Afterwards, IE was performed on undamaged SiCf-SiCm composite tubular samples to obtain axial elastic modulus and shear modulus. The measured properties fell within 4.1% of the same properties obtained from conventional tension and torsion tests. The IE techniques were found to be relatively simple, quick, and highly accurate for obtaining elastic properties of composite tubes.

In addition, the effectiveness of IE method for detecting damage in CMC tube was studied experimentally. Progressive damage in the ceramic composite tube were gradually induced by subjecting it to internal pressurization cycles. Incremental pressure was applied in these loading cycles to levels over the proportional limit stress (PLS) of the CMC. The occurrence of damage was confirmed by acoustic emission monitoring. IE was performed after each pressurization cycle to detect the changes to its vibrational response under fixed-free boundary conditions. It was noted the presence of micro-cracking and other form of composite damage decrease natural frequency while increasing its vibrational damping. The study indicates that both the natural frequency and the internal damping are very sensitive and change monotonically with material damage. As such they can serve as effective damage indicators for CMC.

The second focus was to study the mechanical response of the composite tube under compressive stress. The experiment involved compressing the outer surface of a CMC tube while measuring hoop strain on the internal surface. The compression test is to simulate the pressurized coolant acting on nuclear fuel during typical Light Water Reactor (LWR) operation. A novel external expanding plug method for applying external pressure to the sample tube was developed and validated on known material. SiCf-SiCm composite tubing was then tested to failure and mechanical compressive stress-strain responses were observed. It was found compressive behavior of CMC was significantly different than the tensile behavior. There is a lack of pronounced “bending” as shown in typical tensile stress-strain curve of CMC. In the course of the study, delicate techniques were developed to install small foil strain gauge on the internal curved surface of small bore tubing. It was found that both curvature and internal pressure affect the strain reading from gauge installed on curved internal surfaces.

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