http://hdl.handle.net/10397/937 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search 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 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 http://hdl.handle.net/10397/5546 Title: Identification of damages in steel structures using guided wave method<br/><br/>Authors: Lu, Mingyu<br/><br/>Abstract: Ultrasonic guided wave (GW) has shown clear superiority and strong potential for identification and real-time monitoring of service-induced damage in structures which need on-demand interrogation, by virtue of its unique capability for forward or inverse analysis. The main challenges in GW-based structural health monitoring (SHM) and nondestructive evaluation (NDE) derive from two aspects of complex wave analysis: first, in applications in real structures, the signals collected are complex due to the large number of overlapping reflections obtained in time-traces because of the multiple boundaries of the structures; second, in application with real damage, the effects of closing and opening of a fatigue crack make the acquired wave signals more complex than in instances of artificial notch or crack. However, relatively little research has been devoted to investigation of the inspection of fatigue cracks, especially in complex structures. In this thesis, a single piezoelectric lead zirconate titanate (PZT) actuator-sensor pair for locating a through-thickness crack within welded zone in thick steel plate is initially investigated through both finite element analysis and experiments to demonstrate GW-based techniques in plates with simple geometries which can be used to construct more complex structures for practical applications. The feature extraction method is applied in a welded tubular steel structure (WTSS). A probability-based damage imaging approach is developed. As validation, the approach is employed to predict the presences and locations of multiple slot-like damages in the welding zones of a WTSS. It can be concluded that the identification results using the extracted signal features are comparable, and accuracy when more damage-impaired sensing paths are involved.; An energy-based damage imaging approach is evaluated by identifying a fatigue crack in a thick steel plate. The propagation of GWs in the plate-like structure is complicated by thick geometry, wave dispersion, boundary reflection, and the existing boundary notch used to initiate the fatigue crack, resulting in diverse forms of interference with fatigue crack identification. Hence, signal features are extracted from the wave energy distribution. Simultaneously, the proposed method is demonstrated by FEM and good agreement is obtained between the numerical and experimental results using a new developed fatigue crack model. The image-based approach is evaluated experimentally by detection and monitoring of a fatigue crack using time reversal method (TRM). Results indicate that several damage-sensitive features extracted in the normalized captured signals and different pattern recognition techniques are effective for monitoring of fatigue crack propagation in the steel plate, such as TRM, transmission coefficient and principal component analysis (PCA). Some of the experimental results are verified by FEM results. PCA is validated by monitoring of the propagation of a surface fatigue crack in a welded steel angle structure (WSAS) using GWs generated by a PZT sensor network which is surface-mounted to classify and distinguish different structural conditions due to fatigue crack initiation and propagation. Instead of directly comparing the changes between a series of specific signal segments, signal statistical parameters extracted from the frequency domain are demonstrated to have the capability of monitoring fatigue crack in welded steel structures. In summary, application of GW-based damage detection techniques using structurally integrated PZT transducers for SHM is still in its formative years, and one of the main challenges is use in complex real-world structures. Different approaches are validated systematically in this thesis via simulations and experiments. Results for typical cases indicate that the proposed methods are applicable and effective for detection and real-time monitoring of non-fatigue damage and fatigue cracks in engineering structures.<br/><br/>Description: xxi, 194 leaves : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P ME 2012 LuM http://hdl.handle.net/10397/5475 Title: Improving the speech intelligibility in classrooms<br/><br/>Authors: Lam, Choi Ling Coriolanus<br/><br/>Abstract: One of the major acoustical concerns in classrooms is the establishment of effective verbal communication between teachers and students. Non-optimal acoustical conditions, resulting in reduced verbal communication, can cause two main problems. First, they can lead to reduce learning efficiency. Second, they can also cause fatigue, stress, vocal strain and health problems, such as headaches and sore throats, among teachers who are forced to compensate for poor acoustical conditions by raising their voices. Besides, inadequate acoustical conditions can induce the usage of public address system. Improper usage of such amplifiers or loudspeakers can lead to impairment of students' hearing systems. The social costs of poor classroom acoustics will be large to impair the learning of children. This invisible problem has far reaching implications for learning, but is easily solved. Many researches have been carried out that they have accurately and concisely summarized the research findings on classrooms acoustics. Though, there is still a number of challenging questions remaining unanswered. Most objective indices for speech intelligibility are essentially based on studies of western languages. Even several studies of tonal languages as Mandarin have been conducted, there is much less on Cantonese. In this research, measurements have been done in unoccupied rooms to investigate the acoustical parameters and characteristics of the classrooms. The speech intelligibility tests, which based on English, Mandarin and Cantonese, and the survey were carried out on students aged from 5 years old to 22 years old. It aims to investigate the differences in intelligibility between English, Mandarin and Cantonese of the classrooms in Hong Kong. The significance on speech transmission index (STI) related to Phonetically Balanced (PB) word scores will further be developed. Together with developed empirical relationship between the speech intelligibility in classrooms with the variations of the reverberation time, the indoor ambient noise (or background noise level), the signal-to-noise ratio, and the speech transmission index, it aims to establish a guideline for improving the speech intelligibility in classrooms for any countries and any environmental conditions.; The study showed that the acoustical conditions of most of the measured classrooms in Hong Kong are unsatisfactory. The selection of materials inside a classroom is important for improving speech intelligibility at design stage, especially the acoustics ceiling, to shorten the reverberation time inside the classroom. The signal-to-noise should be higher than 11dB(A) for over 70% of speech perception, either tonal or non-tonal languages, without the usage of address system. The unexpected results bring out a call to revise the standard design and to devise acceptable standards for classrooms in Hong Kong. It is also demonstrated a method for assessment on the classroom in other cities with similar environmental conditions.<br/><br/>Description: 269 leaves : ill. (some col.); 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P ME 2010 Lam