Browsing by Author "Liu, Hao"
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Item Carbon nanotube integrated sensors for structural health monitoring of composite materials via electrical property measurement(University of Delaware, 2018) Liu, HaoNovel composite materials with self-sensing capabilities supplied by nanoscale sensor networks can provide reliable feedback in a real-time and non-invasive way about the structural health condition of composites during their life span. The formation of multi-scale composites ranging from nano to macro scales involves the interactions between at least two components with varying dimension scales. In this research, selective integration of carbon nanotubes has been studied, including macroscopic carbon nanotube sheets/films and carbon nanotube water-based dispersions. Carbon nanotube sheets are especially efficient and useful for laminate composite manufacturing using prepreg fabrics while the water-based carbon nanotube dispersions are highly flexible for patterning onto fabrics followed by infusion of the polymer matrix. ☐ Carbon nanotube integrated damage detection and mapping methods based on piezoresistivity and time domain reflectometry have been implemented on composite laminates. Carbon nanotube sheets have been integrated into glass fiber reinforced composite laminates, carbon fiber reinforced composite laminates, and carbon fiber reinforced stiffeners. Researchers also report composites processing-induced negative piezoresistivity of the carbon nanotube film sensor, which shows negative linear piezoresistivity in compression as well as at low tensile strains. The interlaminated carbon nanotube film sensors’ piezoresistive behavior under mechanical loadings can successfully be utilized for strain measurement and damage detection of the composites, which is highly accurate and efficient as verified by conventional methods such as acoustic emission, ultrasonic C-Scan, X-ray microcomputed tomography and micrographs. In addition to DC resistance measurement, an electrical time domain reflectometry method based on impedance measurement through carbon nanotube transmission lines integrated in composite laminates is found effective for in situ strain sensing and damage detection in composites. Carbon nanotube sheet embedded for prepreg fabrics and water-based carbon nanotube dispersions printed for vacuum assisted resin transfer molding are both proven reliable to integrated transmission lines into glass and carbon fiber reinforced composites. ☐ The integration of carbon nanotubes has been proven as a robust and easy way to increase the strain response and damage detection sensitivities of the composites while maintaining their mechanical properties. This new method offers the potential to revolutionize structural health monitoring methodologies for advanced fiber composites and shows the capability of detecting onset, location, and accumulation of micro-scale damage while being minimally invasive.Item Two essays on the Chinese labor market(University of Delaware, 2018) Liu, HaoMy research includes two essays on Chinese labor markets. In the first essay, I examine how minimum wage increase impacted employment in China in 2011 and 2012, using data from China General Social Survey. I compare employment changes in provinces with large and small minimum wage increases. I find a minimum wage increase has a small and statistically insignificant negative employment effect for two at-risk groups of workers. In Eastern China, minimum wage increase has insignificantly adverse effect on both groups. But in Central and Western China, minimum wage increase positively promotes job opportunities. In the second essay, I analyze the effect of body weight on hourly wages in China by using data from China Health and Nutrition Survey. I find significantly positive effect of body weight on an individual’s hourly wage for male workers. As for females, I find that body weight is negatively related to hourly wages.