Thesis (M. Phil.)--University of Hong Kong, 1978.

Thin sheets and membrane structures are widely used in space applications such as solar sails, sunshields and membrane optics. Surface flatness over a large area is one of the key requirements for many applications using the flexible thin structures. However, wrinkles are commonly observed in thin sheets. It is thus important to understand the mechanics of thin sheets for practical applications that require reliable control of surface wrinkles. In this study, a model problem of stretch-induced wrinkling of thin sheets is considered. First, a two-dimensional (2-D) finite element model was developed to determine stretch-induced stress distribution patterns in hyperelastic thin sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction was found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis was performed to predict the critical conditions for buckling of the elastic sheet under the prescribed boundary conditions, followed by a nonlinear post-buckling analysis to simulate evolution of stretch-induced wrinkles. Experiments were conducted to measure stretch-induced wrinkling of polyethylene thin sheets, using the three-dimensional digital image correlation (3D-DIC) technique. It was observed that the wrinkle amplitude first increased and then decreased with increasing nominal strain, in agreement with finite element simulations for a hyperelastic thin sheet. However, unlike the hyperelastic model, the stretch-induced wrinkles in the polyethylene sheet were not fully flattened at high strains (> 30%), with the residual wrinkle amplitude depending on the loading rate. The hyper-viscoelastic and the parallel network nonlinear viscoelastic material models were adopted for finite element simulations to improve the agreement with the experiments, including the wrinkle amplitude, residual wrinkles and rate dependence. Finally it is noted that wrinkling is sensitive to defects and material inhomogeneity in thin sheets. By varying the elastic stiffness in a narrow region, numerical simulations show drastically different wrinkling behavior, including the critical strain and evolution of wrinkle amplitude and wavelength. In conclusion, a comprehensive understanding of stretch-induced wrinkling is established, where geometry, material, and boundary conditions all play important roles. / text

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No description available.

No description available.

No description available.

No description available.

No description available.

Much research effort is focused on the development of reliable empirical and numerical models for the prediction of sand transport. Confidence in these models depends on good agreement between predicted and measured transport rates for controlled conditions and, in the case of process-driven numerical models, good agreement between measured and predicted "sub-processes", such as time-dependent concentration and velocity profiles. The purpose of this project was to conduct experiments that measure these transport "sub-processes" in full-scale sinusoidal and asymmetric oscillatory sheet-flow conditions for well-sorted and graded sands. Detailed measurements have been obtained of concentrations, velocities, total and fractional transport rates and particles sizes in bed samples and in suspended and transported sands. The range and level of detail in the new concentration measurements makes it possible to interrogate concentration behaviour much more rigorously than previously possible. A new equation is presented which characterises time-dependent concentration profiles in the sheet-flow layer. The equation is based on time-dependent erosion depth and reference concentration. Analysis of the dependence of these parameters on flow and bed conditions is presented. The new velocity measurements extend "deeper" into the oscillatory boundary layer than previously possible and the results show classic features of oscillatory boundary layer flow. The product of the measured velocity and concentration data gives time-dependent sediment flux profiles. Analysis of the flux profiles reveals the detailed transport processes. The effects of unsteady behaviour and the effects of interactions between different size fractions in graded beds are evident in the sediment transport results. Unsteady effects act to reduce net transport and result in a strong offshore-directed transport in the case of fine sand. There is strong interaction between size fractions in graded beds. The mobility of the finer fractions is suppressed by the presence of coarser sands whilst the mobility of the coarser fractions is increased by the presence of finer sands.

Sheet metal surfaces with pin-fin features have potential fluid and thermal applications. Extrusion forging process and extrusion rolling process can be used to create such surface features on sheet metals. Extrusion forging process is a metal forming process that combines extrusion and forging into one operation. In extrusion forging, the pin-fin surface feature is created by compressing the work-piece using a punch with designed cavities. Experiments and numerical analysis were conducted to investigate the effects of tooling geometries, material properties, work-piece thickness, thickness reduction ratio and friction on the deformation behavior of sheet metals. It was found that increasing fillet radius of the orifice results in decrease in compressive force and boss height. As the negative draft angle increases, the compressive force and boss height decrease. Higher yield strength and higher friction lead to higher compressive force. The boss height is not significantly affected by the friction between the tooling and the work-piece. Due to limitation in force capacity, it may not be feasible to apply extrusion forging technique to generate surface features on large surface area. As such, the extrusion rolling process is proposed. In extrusion rolling, the pin-fin feature is created by compressing the strip using a pair of rolls. The upper roll is manufactured with surface cavities. Finite element method are employed to investigate the effects of rolling speed, thickness reduction ratio, roll diameter and front tension force on the deformation behavior of metal strip. It was found that the rolling speed has little influence on the roll force and boss height. The front tension force has little effect on the average pressure and boss height. Increase the roll diameter results in in roll force increase. This research investigated the effect of parameters on the deformation behavior during the extrusion forging and extrusion rolling processes. The research generated the knowledge needed for design and manufacture of micro/meso surface features on thin metal sheet. Moreover, this thesis presents a novel bulk forming process on metal sheets, which can have significant impacts in industrial practice.