Browsing by Author "Hu, Chen"
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Item Development of a Rock Strength Database(Minnesota Department of Transportation, 2018-06) Folta, Brian; Sharpe, Jacob; Hu, Chen; Labuz, JosephRock strength and elastic behavior are important for foundations such as spread footings resting on rock and drilled shafts socketed into rock. In addition to traditional rock quality information, stiffness and failure parameters are helpful for design. MnDOT has previously used a low-capacity load frame for routine rock testing but this apparatus does not generate sufficient force for testing hard rock. The report provides a comprehensive suite of results from 134 specimens tested under uniaxial compression and 33 specimens tested under triaxial compression on a wide variety of rock, including hard rock, which frequently is of interest for high-capacity foundation systems. Thus, an economic benefit is realized if the strength of the rock is measured, as opposed to correlated with an index parameter, due to the potential to reduce foundation size and construction time. Information from the testing was used to expand the MnDOT database of rock properties and allow for improved designs based on accurate measurements of Young’s modulus, uniaxial compressive strength, and friction angle.Item Experimental And Theoretical Investigation Of Sic/Sic Composites Under Multiaxial Loading(2023-09) Hu, ChenOwing to its excellent mechanical properties and stability under high temperature and neutron irradiation conditions, SiC/SiC composites have emerged as promising materials for light water reactors (LWRs) in the development of accident-tolerant fuel (ATF) systems. Structural integrity and retention of hermeticity are two crucial requirements for SiC/SiC claddings during normal operations, and both of them are closely related to the material's proportional limit stress (PLS). Understanding the behavior of SiC/SiC composites under multiaxial stress states and developing a probabilistic approach for evaluating the structural vulnerability is of paramount importance for reliability-based analysis and design of SiC/SiC composite claddings. So far, there has been very limited effort towards experimental and analytical investigations of probabilistic failure of SiC/SiC claddings. This critical knowledge gap motivates the research presented in this dissertation. A probabilistic failure criterion is developed for SiC/SiC composites under multiaxial loading, and this criterion is incorporated into the reliability analysis of the structural integrity of SiC/SiC fuel cladding. The research consists of two parts: 1) experimental investigation of multiaxial failure behavior of SiC/SiC composites, and 2) theoretical modeling of time-dependent probabilistic failure of SiC/SiC cladding. In the experimental investigation, the PLS is determined by examining stress-strain response and acoustic emission measurements. The theoretical framework is derived by combining the finite weakest-link statistical model and the subcritical damage growth model. This theoretical model captures the time-dependent failure mechanism of the material, which has a major consequence in predicting the lifetime distribution of the cladding. Meanwhile, the model also predicts that the failure statistics of the cladding depend strongly on the cladding length. The results of the multiaxial experiments reveal the level of statistical variation of the PLS of SiC/SiC materials under different stress states. The theoretical model provides a robust analytical tool for extrapolation of small-scale laboratory test results to the behavior of full-scale claddings. These findings lay down a scientific foundation for the development of the reliability-based design of SiC/SiC fuel claddings, which will play an essential role in improving the structural safety and integrity of LWRs.Item Mechanical Response of a Composite Steel, Concrete-Filled Pile(Minnesota Department of Transportation, 2018-06) Hu, Chen; Sharpe, Jacob; Labuz, JosephA steel pipe-pile section, filled with concrete, was instrumented and tested under axial load. Two types of strain gages, resistive and vibrating wire, were mounted to the steel-pipe pile and checked by determining the known Young’s modulus of steel E^s. The steel section was filled with concrete and a resistive embedment gage was placed in the concrete during the filling process to measure axial strain of the concrete. The axial load – axial strain responses of the steel (area A^s) and concrete (area A^c) were evaluated. The stiffening of concrete, related to curing, was also studied. Assuming the boundary condition of uniform axial displacement, i.e., equal axial strain in the steel and concrete, εz^s = εz^c = εz, the sum of the forces carried by the two materials, F^s + F^c, where F^s = εz * E^s * A^s and Fc = εz * E^c * A^c, provides a reasonable estimate – within 3% – of the pile force. For the particular specimen studied (12 in. ID, 0.25 in. wall thickness), the stiffness of the composite section of steel and concrete was about three times larger compared to the steel section without concrete. Further, the concrete carried about 70% of the load, but the axial stress in the concrete, at an applied force of 150,000 lb, was less than 20% of the compressive strength of the concrete.