http://hdl.handle.net/10397/36 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search http://hdl.handle.net/10397/5636 Title: Quasi-distributed strain sensing with white-light interferometry: a novel approach<br/><br/>Authors: Yuan, Libo; Zhou, Limin; Jin, Wei<br/><br/>Abstract: An optical fiber ring is used to generate multiple reference waves in a multiplexed fiber-optic Michelson-type sensor array. The array consists of N sensing segments connected in series along a single optical fiber path and is interrogated with a white-light interferometric technique. Experimental results with a two-sensor array are presented.<br/><br/>Description: DOI: 10.1364/OL.25.001074 Tue, 01 Aug 2000 00:00:00 GMT http://hdl.handle.net/10397/5597 Title: Antiferroelectric-like properties and enhanced polarization of Cu-doped K₀.₅Na₀.₅NbO₃ piezoelectric ceramics<br/><br/>Authors: Ke, Shanming; Huang, Haitao; Fan, Huiqing; Lee, H. K.; Zhou, Limin; Mai, Yiu Wing<br/><br/>Abstract: Abnormal evolution of ferroelectric hysteresis (P-E) loops was observed in Cu-doped K₀.₅Na₀.₅NbO₃ (KNN) ceramics. The 1 mol. % Cu-doped fresh sample exhibited double-loop-like characteristics, while the 1.5 and 2 mol. % Cu-doped KNN ceramics showed normal single loops. Electron paramagnetic resonance spectra verified the formation of non-switchable (Cu[sub Nb]‴−V[sub O]••) ′ (DC1) and non-polar (V[sub O]••−Cu [sub Nb]‴−V[sub O]••)• (DC2) defect complexes in these ceramics. According to the experimental results, it is suggested that DC1 would provide the driving force for domain back-switching, leading to the double P-E loops, while DC2 would contribute to the space charges. Dielectric aging behaviors of the samples also supported this mechanism. It is the competition between the DC1 and DC2 defect complexes that induced the observed compositional evolution of P-E loops in the Cu-doped KNN ceramics.<br/><br/>Description: DOI: 10.1063/1.4747212 Mon, 20 Aug 2012 00:00:00 GMT http://hdl.handle.net/10397/5548 Title: Modeling of microstructure evolution induced by Surface Mechanical Attrition Treatment (SMAT) in AISI 316L stainless steel<br/><br/>Authors: Zhang, Xiaochun<br/><br/>Abstract: Surface Mechanical Attrition Treatment (SMAT) has become a promising technique to produce nanostructured surface layers in bulk material. Various experiments have been conducted to prove that SMATed materials show a significant enhancement of the surface mechanical, tribological, chemical and corrosion properties. However, the relationship between the desired surface structures/properties and controlling parameters in SMAT process is still unclear. This work targets to numerically investigate and predict the material behavior under the random high strain rate impacts during SMAT. A number of parameters need to be controlled and regulated during SMAT, such as ball size, impact energy, impact frequency, impact velocity, ball density and incident angle. A computational modeling of SMAT process is thus developed. AISI 316L stainless steel is chosen as the target material. First, the influences of ball parameters on the residual stress are systematically analyzed, and then the numerical solutions of the indent size are proposed and validated by experiments. In addition, the depth of plastic zone which is induced by impact with oblique angle is numerically evaluated. At the same time, a global random impact model and a local impact frequency model are developed to analyze the statistic characteristics of SMAT coverage. A new realistic SMAT model is thus developed which considers the full coverage, random impact location and random impact oblique angle simultaneously. The energy partition during SMAT process is explored by using the new developed model. The components of impinging and rebounding velocities during SMAT are monitored. The stored energy and the fraction of plastic work converted into heat (β) are numerically evaluated. Further investigations focus on the mechanism of highly improved strength of SMATed material. In developing a new material model, not only the varying strain, strain rate, temperature, but also the changing of microstructure such as dislocation density, grain size and/or space between the deformation twinning need to be considered. In this study, the micro-level variable, i.e., grain size, is introduced. A new continuum-based constitutive model involving the evolution of grain size is proposed and implemented by user-defined subroutines. The deformation behavior corresponding to the grain refinement during SMAT process is thus predicted. Finally, a numerical comparison between SMAT and SP is conducted by using the proposed constitutive equation and full coverage random impacts model.<br/><br/>Description: xviii, 184 leaves : ill. (chiefly col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P ME 2012 Zhang Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/10397/5547 Title: Numerical simulation of fluid-structure interaction for elastic cylinders in axial flow<br/><br/>Authors: Liu, Zhengang<br/><br/>Abstract: When elastic cylinders are subjected to an axial flow, they will vibrate due to the loading imposed on the structure by the flow. This vibration is called axial-flow-induced vibration. In general, the amplitude of axial-flow-induced vibration is very small, about 10⁻³ ~ 10⁻² diameter of the cylinder, compared to that of cross-flow-induced vibration. Nevertheless, this so weak vibration can wear the fuel rods in nuclear industry and make the cladding so thin that the radioactive material may be released. Thus it is necessary in engineering to study this vibration. In this project, the numerical simulation is carried out to study the fluid-structure interaction for elastic cylinders subjected to axial flow. The fluid and structure solvers are coupled together by an explicit partitioned scheme. The CFD solver is FLUENT 12.0, in which the ALE N-S equations are numerical solved by finite volume method (FVM). The cylinders are regarded as Euler-Bernoulli beams and the dynamic equation is numerically solved by finite element method (FEM). The structure solver is integrated into the fluid solver by user-defined functions (UDF), which is provided by the fluid solver. The LES model is adopted to model the turbulence for the flow simulation. The fluid solver calculates the loading for the structure solver, while the latter calculates the displacements of the cylinders, which are utilized to update the mesh of fluid domain by the fluid solver.; Firstly the dynamics of a single cylinder, which is subjected to axial flow and is limited to vibrate only in one plane, is studied. When the flow is laminar, the vibration of the cylinder is always damped by the flow for current dimensionless flow velocities and the damping increases with increasing the dimensionless flow velocity. For turbulent flow, when the dimensionless flow velocity is lower, the strong vibration of the cylinder is damped into weak vibration. However, the vibration becomes instable and the cylinder is eventually buckled if the dimensionless flow velocity is large enough. Secondly, the dynamics of a cylinder, however, which can be free to vibrate in any transverse directions in axial turbulent flow, is studied. The dynamics of the cylinder is similar to that when it is constrained to vibrate in one direction. However, the flutter instability is captured as well as the buckling instability. It is more appropriate to explain the simultaneous occurrence of the buckling and flutter instabilities by the nonlinear theory, in which the solution due to the flutter can be a bifurcation based on a buckling solution. The dynamics of two simple clusters consisting of respectively two and four cylinders is also studied. These structures are similar to the fuel assembly in pressurized water reactor (PWR). The cylinders are free to vibrate in any directions. At small dimensionless flow velocity, the damping of the flow on strong vibration is also captured. When the dimensionless flow velocity becomes large enough, the buckling instability can be captured. The flutter instability is not captured, due to the extreme distortion of the mesh of the fluid domain at higher dimensionless flow velocity. The current results are qualitatively consistent with the available experiments and theoretical analyses, and they could capture the features of fluid-structure interaction in detail. The numerical methods applied in this project can be used to predict the vibrations of the rods in nuclear industry, which should be helpful for the design of pressurized water reactor.<br/><br/>Description: xv, 164 leaves : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P ME 2012 Liu Sun, 01 Jan 2012 00:00:00 GMT