Mechanical properties of mesoscopic objects

This thesis describes measurements of the mechanical properties on the nanoscale. Three different mesoscopic tubular objects were studied: MoS2 nanotubes, carbon nanotubes and microtubules. The main goal was to investigate the interplay between the fine structure of these objects and their mechanical properties. Measurements were performed by elastically deforming tubes deposited on porous substrates with the tip of an atomic force microscope. The first experimental part describes the mechanical characterization of MoS2 nanotube bundles. Elastic deformation of MoS2 nanotube bundles can be modelled, in analogy with carbon nanotube bundles, using two elastic moduli: the Young's modulus and the shear modulus describing the weak intertube coupling. The measured Young's modulus of 120GPa has later been confirmed by theoretical modelling. It is in the range of commonly used engineering materials. The shear modulus corresponding to intertube sliding is an order of magnitude lower than in the case of carbon nanotube bundles. MoS2 nanotubes could therefore prove an interesting model for studying 1D and weakly coupled systems. They could also be interesting as AFM tips, especially for biological applications thanks to their sulphur-based chemistry. In the case of carbon nanotubes, the weak intertube coupling is a serious problem that has to be solved before they could be used as reinforcing fibers or building blocks of macroscopic objects. This problem was addressed in the second part of this thesis. Stable crosslinks were introduced into carbon nanotube bundles by irradiating them with electrons inside a TEM. AFM measurements performed in parallel with TEM observations show that the irradiation process is composed of two competing mechanisms: crosslinking, which is dominant at low exposures, and degradation of the crystalline structure followed by amorphization in the later stages of irradiation. Theoretical modelling shows that the crosslinks are most probably formed by interstitial carbon atoms. The third part of this thesis describes measurements of the mechanical properties of microtubules performed in the liquid environment. The bending modulus shows a pronounced temperature dependence, in good agreement with previously published data on the dynamic instability of microtubules. The shear and the Young's moduli were simultaneously measured, on two different temperatures, using a substrate prepared by electron beam lithography. These measurements have demonstrated that microtubules behave as strongly anisotropic cylinders. This is due to their structure, with large gaps separating neighboring protofilaments. The observed stiffening of microtubules on low temperatures (<15°C) is due to increasing interaction between the protofilaments. This manifests itself as a decrease of disassembly velocity, showing that the dynamic behavior of microtubules is reflected in their mechanical properties

BEHAVIOR OF CONCRETE WITH POLYVINYL ALCOHOL (PVA) FIBERS

Concrete possessed characteristics whereby it has high strength in compression but weak in tension. In order to mend this disadvantage of concrete, Polyvinyl Alcohol (PVA) fibers are infused in concrete mix. It is considered as one of the most suitable polymeric fibers to be used as the reinforcement of concrete due to its advantages such as it will never rust, having high bond strength with concrete or mortar, having high modulus of elasticity etc. Along the project period, properties of PVA fibers were studied as well as some other fibers commonly used in concrete technology. The characteristics of PVA fibers and concrete were also investigated in order to determine the ideality of mixing them together and what the significant impacts are, This project focuses on the properties of hardened PVA fiber reinforced concrete. In order to determine that, all concrete samples w1,1re tested for their compression and flexural strength. From the result obtained, it can be concluded that PVA fibers improved characteristics of concrete in terms of its strength

Control of objects with a high degree of freedom

In this thesis, I present novel strategies for controlling objects with high degrees of freedom for the purpose of robotic control and computer animation, including articulated objects such as human bodies or robots and deformable objects such as ropes and cloth. Such control is required for common daily movements such as folding arms, tying ropes, wrapping objects and putting on clothes. Although there is demand in computer graphics and animation for generating such scenes, little work has targeted these problems. The difficulty of solving such problems are due to the following two factors: (1) The complexity of the planning algorithms: The computational costs of the methods that are currently available increase exponentially with respect to the degrees of freedom of the objects and therefore they cannot be applied for full human body structures, ropes and clothes . (2) Lack of abstract descriptors for complex tasks. Models for quantitatively describing the progress of tasks such as wrapping and knotting are absent for animation generation. In this work, we employ the concept of a task-centric manifold to quantitatively describe complex tasks, and incorporate a bi-mapping scheme to bridge this manifold and the configuration space of the controlled objects, called an object-centric manifold. The control problem is solved by first projecting the controlled object onto the task-centric manifold, then getting the next ideal state of the scenario by local planning, and finally projecting the state back to the object-centric manifold to get the desirable state of the controlled object. Using this scheme, complex movements that previously required global path planning can be synthesised by local path planning. Under this framework, we show the applications in various fields. An interpolation algorithm for arbitrary postures of human character is first proposed. Second, a control scheme is suggested in generating Furoshiki wraps with different styles. Finally, new models and planning methods are given for quantitatively control for wrapping/ unwrapping and dressing/undressing problems

Mechanical optimization of vascular bypass grafts

Synthetic vascular grafts are useful to bypass diseased arteries. The long-term failure of synthetic grafts is primarily due to intimal hyperplasia at the anastomotic sites. The accelerated intimal hyperplasia may stem from a compliance mismatch between the host artery and the graft since commercially available synthetic conduits are much stiffer than an artery. The objective of this thesis is to design a method for fabricating a vascular graft that mechanically matches the patients native artery over the expected physiologic range of pressures. The creation of an optimized mechanical graft will hopefully lead to an improvement in patency rates. The mechanical equivalency between the graft and the host artery is defined locally by several criteria including the diameter upon inflation, the elasticity at mean pressure, and axial force. A single parameter mathematical for a thin-walled tube is used to describe of the final mechanical behavior of a synthetic graft. For the general problem, the objective would be to fabricate a mechanics-matching vascular graft for each host artery. Typically, fabrication parameters are set initially and the properties of the fabricated graft are measured. However, by modeling the entire fabrication process and final mechanical properties, it is possible to invert the situation and let the typical output mechanical values be used to define the fabrication parameters. The resultant fabricated graft will then be mechanically matching. As a proof-of-concept, several prototype synthetic grafts were manufactured and characterized by a single Invariant to match a canine artery. The resultant graft equaled the diameter upon inflation, the elasticity at mean pressure, and axial force of the native canine artery within 6%. An alternative to making an individual graft for each artery is also presented. A surgeon may choose the best graft from a set of pre-manufactured grafts, using a computer program algorithm for best fit using two parameters in a neighborhood. The design optimization problem was solved for both canine carotid and human coronary arteries. In conclusion, the overall process of design, fabrication and selection of a mechanics matching synthetic vascular graft is shown to be reliable and robust.M.S.Committee Chair: David N. Ku; Committee Co-Chair: Alexander Rachev; Committee Member: Elliot L. Chaiko