9,129 research outputs found
Oral History Interview with David Hung-Chang Du
No full textThis interview was conducted by CBI for CS&E in conjunction with the 50th Anniversary of the University of Minnesota Computer Science Department (now Computer Science and Engineering, CS&E). Professor David Hung-Chang Du begins by discussing his education at National Tsing-Hua University in Taiwan and then his doctoral work and Computer Center work at the University of Washington in Computer Science. The bulk of the interview is his professional career at the University of Minnesota. He discusses his wide-ranging computer science research in integrated circuits (VLSI), disk drives/storage, artificial intelligence, computer networking, security and privacy, and other areas. This includes his work with IBM Rochester, Seagate, Unisys and other companies, and he emphasizes the importance of working with more senior managers at companies so university and company interests can be aligned and can have local buy-in at the company, that resources will come if that there. He relates his leadership with the UMN-led Center for Research and Intelligent Storage: a multi-university partnership with industry supported by National Science Foundation.Du, David Hung-Chang. (2022). Oral History Interview with David Hung-Chang Du. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/226569
On the low strain rate regime of structural superplasticity - an internal variable approach
No full textThe superplastic deformation behavior of a fine-grained 7075 Al has been investigated to clarify the issue of threshold stress. A series of mechanical tests has been conducted at various temperatures for the specimens with various grain sizes. The quantitative constitutive parameters have been determined from load relaxation test by applying the internal variable theory of structural superplaticity (SSP) proposed by Chang et al. The GBS flow could be formulated as a viscosity-type equation, characterized by the Newtonian exponent of 1.0. The unresolved issue of threshold stress is clarified and identified as a critical stress required for the GBS. The micro-mechanical roll of grain size refinement has also been manifested in terms of proposed constitutive parameters.110sciescopu
Yield stress anomalous Behavior in beta-CuZn alloy
No full textseries of tensile and load relaxation behaviors in B2 type beta-CuZn alloy have been examined to clarify the strain rate & the temperature dependence of yield stress anomaly. As strain rate increased, the anomalous peak moved to a higher stress and temperature region. The anomalous yielding region below peak temperature (T-p) revealed slightly positive strain rate dependence, while the region above T-p showed considerably positive strain rate dependence. Based on the different tendency of temperature and strain rate dependence of anomalous yielding, two types of deformation modes were suggested and incorporated into the yield stress vs. temperature curve analysis, which was in accord well with the experimental results. The less sensitive strain rate dependence of yield stress anomaly indicated that the yield stress anomalous behavior in beta-CuZn alloy mainly seemed to be attributed to the vacancy hardening.110sciescopu
Finite element analysis of hot forging with flow softening by dynamic recrystallization
No full textThe change of flow stress due to dynamic recrystallization (DRX) during hot forming process is investigated. A series of mechanical tests had been conducted at various temperatures, and constitutive relations and recrystallization kinetics were formulated from the test results. The effect of DRX on the flow stress was implemented in a commercial FEM code by conditioned remeshing followed by remapping of the state variables. The datum strain of stress compensation was optimized to minimize the overestimation of forming loads. Suggested datum strain were formulated as an exclusive function of critical strain for recrystallization and validated by mechanical tests. (C) 2002 Elsevier Science B.V. All rights reserved.X1111sciescopu
High-temperature deformation behavior of a gamma TiAl alloy-microstructural evolution and mechanisms
No full textThe present investigation was carried out in the context of the internal-variable theory of inelastic deformation and the dynamic-materials model (DMM), to shed light on the high-temperature deformation mechanisms in TiAl. A series of load-relaxation tests and tensile tests were conducted on a fine-grained duplex gamma TiAl alloy at temperatures ranging from 800 degreesC to 1050 degreesC. Results of the load-relaxation tests, in which the deformation took place at an infinitesimal level (epsilon congruent to 0.05), showed that the deformation behavior of the alloy was well described by the sum of dislocation-glide and dislocation-climb processes. To investigate the deformation behavior of the fine-grained duplex gamma TiAl alloy at a finite strain level, processing maps were constructed on the basis of a DMM. For this purpose, compression tests were carried out at temperatures ranging from 800 degreesC to 1250 degreesC using strain rates ranging from 10 to 10(-4)/s. Two domains were identified and characterized in the processing maps obtained at finite strain levels (0.2 and 0.6). One domain was found in the region of 980 degreesC and 10(-3)/s with a peak efficiency (maximum efficiency of power dissipation) of 48 pct and was identified as a domain of dynamic recrystallization (DRx) from microstructural observations. Another domain with a peak efficiency of 64 pct was located in the region of 1250 degreesC and 10(-4)/s and was considered to be a domain of superplasticity.1114sciescopu
OPTICAL RECEIVER DESIGN WITH HIGH-SENSITIVITY AND HIGH DYNAMIC-RANGE USING FEEDBACK AND LOSSY NOISE-MATCHING NETWORK
No full textA new type of optical receiver design providing high sensitivity and high dynamic range is presented that employs a lossy noise-matching network and feedback type amplifier. The design is based on the noise figure concept using the four noise parameters of FET, which makes it easy to design multigigabit optical receivers by combining it with microwave CAD tools. The effects of matching network element value and group delay variation of the preamplifier on receiver sensitivities are addressed. The predicted receiver sensitivity including the effect of group delay is 27.1 dBm, which is the highest sensitivity for a 10 Gbit s-1 PIN-FET optical receiver reported to date
Mechanical and microstructural analysis on the superplastic deformation behavior of two-phase Ti-6Al-4V alloy
No full textPresent investigation has been made to study the superplastic deformation behavior of Ti-6Al-4V alloy based on the theory of inelastic deformation, and to analyze the boundary sliding characteristics by the use of transmission electron microscopy. The flow curves obtained by load relaxation tests at 600 degrees C and 800 degrees C are fitted well with the inelastic deformation theory. It has been found that the deformation mode for the fine grained microstructures (3 mu m) is a type of iso-stress mode,. while that of coarse grained microstructures (11 mu m) is a mixed type of both iso-stress and iso-strain rate modes. The density of dislocations observed near the grain boundaries is higher along alpha/beta phase boundaries than the alpha/beta grain boundaries, which confirms that the sliding resistance of alpha/beta is lower than that of alpha/beta.X118sciescopu
Structural superplasticity of an Al alloy in low strain rate regime - An internal variable approach
No full textMost of the conventional approaches for structural superplasticity have utilized the external variables such as total stress and strain. The effect of grain size and test temperature has not, however, been clarified quantitatively yet from these approaches. A quantitative analysis for structural superplasticity has recently been progressed considerably with the use of the internal variable concept. Nevertheless the precise role of grain size and test temperature on flow characteristics has not been elucidated especially in low strain rate region. Several fundamental characteristics, such as the validity of threshold stresses and the role of grain size, are yet to be resolved or still in controversy in the superplasticity community. In this study, a series of load relaxation and tensile tests has been conducted in the above regard to obtain the flow curves, which were consequently analyzed based on the internal variable theory for structural superplasticity, focusing especially on the low strain rate region. (c) 2005 Elsevier B.V. All rights reserved.1110sciescopu
Role of nitrogen in the cyclic deformation behavior of duplex stainless steels
No full textThe role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It (,an be attributed to the greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with increasing, nitrogen content because austenite dominantly takes part in plastic deformation, being Zn more strain hardened due to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change because ferrite, more dominantly accommodating Zn plastic strain, rarely shows a change of strain-hardening behavior due to the similar nitrogen content in ferrite.X116sciescopu
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