Stretching-induced orientation to improve mechanical properties of electrospun pan nanocomposites

Partially aligned and oriented polyacrylonitrile(PAN)-based nanofibers were electrospun from PAN and SWNTs/PAN in the solution of dimethylformamide(DMF) to make the carbon nanofibers. The as-spun nanofibers were hot-stretched in an oven to enhance its orientation and crystallinity. Then it were stabilized at 250 square under a stretched stress, and carbonized at 1000 square in N-2 atmosphere by fixing the length of the stabilized nanofiber to convert them into carbon nanofibers. With this hot-stretched process and with the introduction of SWNTs, the mechanical properties will be enhanced correspondingly. The crystallinity of the stretched fibers confirmed by X-ray diffraction has also increased. For PAN nanofibers, the improved fiber alignment and crystallinity resulted in the increased mechanical properties, such as the modulus and tensile strength of the nanofibers. It was concluded that the hot-stretched nanofiber and the SWNTs/PAN nanofibers can be used as a potential precursor to produce high-performance carbon composites.

Experimental study of the mechanical properties of light gauge cold-formed steels at elevated temperatures

In recent times, light gauge cold-formed steel sections have been used extensively as primary load bearing structural members in many applications in the building industry. Fire safety design of structures using such sections has therefore become more important. Deterioration of mechanical properties of yield stress and elasticity modulus is considered the most important factor affecting the performance of steel structures in fires. Hence there is a need to fully understand the mechanical properties of light gauge cold-formed steels at elevated temperatures. A research project based on experimental studies was therefore undertaken to investigate the deterioration of mechanical properties of light gauge cold-formed steels. Tensile coupon tests were undertaken to determine the mechanical properties of these steels made of both low and high strength steels and thicknesses of 0.60, 0.80 and 0.95 mm at temperatures ranging from 20 to 800ºC. Test results showed that the currently available reduction factors are unsafe to use in the fire safety design of cold-formed steel structures. Therefore new predictive equations were developed for the mechanical properties of yield strength and elasticity modulus at elevated temperatures. This paper presents the details of the experimental study, and the results including the developed equations. It also includes details of a stress-strain model for light gauge cold-formed steels at elevated temperatures.

Mechanical properties of thin sol-gel films

This thesis presents a study of the mechanical properties of thin films. The main aim was to determine the properties of sol-gel derived coatings. These films are used in a range of different applications and are known to be quite porous. Very little work has been carried out in this area and in order to study the mechanical properties of sol-gel films, some of the work was carried out on magnetron sputtered metal coatings in order to validate the techniques developed in this work. The main part of the work has concentrated on the development of various bending techniques to study the elastic modulus of the thin films, including both a small scale three-point bending, as well as a novel bi-axial bending technique based on a disk resting on three supporting balls. The bending techniques involve a load being applied to the sample being tested and the bending response to this force being recorded. These experiments were carried out using an ultra micro indentation system with very sensitive force and depth recording capabilities. By analysing the result of these forces and deflections using existing theories of elasticity, the elastic modulus may be determined. In addition to the bi-axial bending study, a finite element analysis of the stress distribution in a disk during bending was carried out. The results from the bi-axial bending tests of the magnetron sputtered films was confirmed by ultra micro indentation tests, giving information of the hardness and elastic modulus of the films. It was found that while the three point bending method gave acceptable results for uncoated steel substrates, it was very susceptible to slight deformations of the substrate. Improvements were made by more careful preparation of the substrates in order to avoid deformation. However the technique still failed to give reasonable results for coated specimens. In contrast, biaxial bending gave very reliable results even for very thin films and this technique was also found to be useful for determination of the properties of sol-gel coatings. In addition, an ultra micro indentation study of the hardness and elastic modulus of sol-gel films was conducted. This study included conventionally fired films as well as films ion implanted in a range of doses. The indentation tests showed that for implantation of H⁺ ions at doses exceeding 3x10¹⁶ ions/cm², the mechanical properties closely resembled those of films that were conventionally fired to 450°C.

Mechanical properties of advanced tissue engineering scaffold architectures

In tissue engineering, porous scaffolds are used as a temporal support for tissue regeneration through cell adhesion, proliferation and differentiation. Besides applying a suitable material that is both biocompatible and biodegradable, the architectural design of the porous scaffold can be of essential for successful tissue regeneration. The architecture is of great influence on mechanical properties and transport properties of nutrients and metabolites1.

Spatial and temporal variations of mechanical properties and mineral content of the external callus during bone healing

After bone fracture, various cellular activities lead to the formation of different tissue types, which form the basis for the process of secondary bone healing. Although these tissues have been quantified by histology, their material properties are not well understood. Thus, the aim of this study is to correlate the spatial and temporal variations in the mineral content and the nanoindentation modulus of the callus formed via intramembranous ossification over the course of bone healing. Midshaft tibial samples from a sheep osteotomy model at time points of 2, 3, 6 and 9 weeks were employed. PMMA embedded blocks were used for quantitative back scattered electron imaging and nanoindentation of the newly formed periosteal callus near the cortex. The resulting indentation modulus maps show the heterogeneity in the modulus in the selected regions of the callus. The indentation modulus of the embedded callus is about 6 GPa at the early stage. At later stages of mineralization, the average indentation modulus reaches 14 GPa. There is a slight decrease in average indentation modulus in regions distant to the cortex, probably due to remodelling of the peripheral callus. The spatial and temporal distribution of mineral content in the callus tissue also illustrates the ongoing remodelling process observed from histological analysis. Most interestingly the average indentation modulus, even at 9 weeks, remains as low as 13 GPa, which is roughly 60% of that for cortical sheep bone. The decreased indentation modulus in the callus compared to cortex is due to the lower average mineral content and may be perhaps also due to the properties of the organic matrix which might be different from normal bone.

Investigation of electrophoretic loading and enhanced mechanical properties of hydrogels for delivery of therapeutic proteins

Hydrogels, which are three-dimensional crosslinked hydrophilic polymers, have been used and studied widely as vehicles for drug delivery due to their good biocompatibility. Traditional methods to load therapeutic proteins into hydrogels have some disadvantages. Biological activity of drugs or proteins can be compromised during polymerization process or the process of loading protein can be really timeconsuming. Therefore, different loading methods have been investigated. Based on the theory of electrophoresis, an electrochemical gradient can be used to transport proteins into hydrogels. Therefore, an electrophoretic method was used to load protein in this study. Chemically and radiation crosslinked polyacrylamide was used to set up the model to load protein electrophoretically into hydrogels. Different methods to prepare the polymers have been studied and have shown the effect of the crosslinker (bisacrylamide) concentration on the protein loading and release behaviour. The mechanism of protein release from the hydrogels was anomalous diffusion (i.e. the process was non-Fickian). The UV-Vis spectra of proteins before and after reduction show that the bioactivities of proteins after release from hydrogel were maintained. Due to the concern of cytotoxicity of residual monomer in polyacrylamide, poly(2-hydroxyethyl- methacrylate) (pHEMA) was used as the second tested material. In order to control the pore size, a polyethylene glycol (PEG) porogen was introduced to the pHEMA. The hydrogel disintegrated after immersion in water indicating that the swelling forces exceeded the strength of the material. In order to understand the cause of the disintegration, several different conditions of crosslinker concentration and preparation method were studied. However, the disintegration of the hydrogel still occurred after immersion in water principally due to osmotic forces. A hydrogel suitable for drug delivery needs to be biocompatible and also robust. Therefore, an approach to improving the mechanical properties of the porogen-containing pHEMA hydrogel by introduction of an inter-penetrating network (IPN) into the hydrogel system has been researched. A double network was formed by the introduction of further HEMA solution into the system by both electrophoresis and slow diffusion. Raman spectroscopy was used to observe the diffusion of HEMA into the hydrogel prior to further crosslinking by ã-irradiation. The protein loading and release behaviour from the hydrogel showing enhanced mechanical property was also studied. Biocompatibility is a very important factor for the biomedical application of hydrogels. Different hydrogels have been studied on both a three-dimensional HSE model and a HSE wound model for their biocompatibilities. They did not show any detrimental effect to the keratinocyte cells. From the results reported above, these hydrogels show good biocompatibility in both models. Due to the advantage of the hydrogels such as the ability to absorb and deliver protein or drugs, they have potential to be used as topical materials for wound healing or other biomedical applications.

Exploration of the defect’s effect on the mechanical properties of different orientated nanowires

Molecular dynamics (MD) simulations have been carried out to investigate the defect’s effect on the mechanical properties of copper nanowire with different crystallographic orientations, under tensile deformation. Three different crystallographic orientations have been considered. The deformation mechanism has been carefully discussed. It is found that the Young’s modulus is insensitive to the defect, even when the nanowire’s crystallographic orientation is different. However, due to the defect’s effect, the yield strength and yield strain appear a large decrease. The defects have played a role of dislocation sources, the slips or stacking faults are first generated around the locations of the defects. The necking locations have also been affected by different defects. Due to the surface defect, the plastic deformation has received a large influence for the <001>/{110} and <110> orientated nanowires, and a relative small influence is seen for the <111> nanowire.

Mechanical properties of cold-formed steels at elevated temperatures

Mechanical properties have an important role in the fire safety design of cold-formed steel structures due to the rapid reduction in mechanical properties such as yield strength and elastic modulus under fire conditions and associated reduction to the load carrying capacities. Hence there is a need to fully understand the deterioration characteristics of yield strength and elastic modulus of cold-formed steels at elevated temperatures. Although past research has produced useful experimental data on the mechanical properties of cold-formed steels at elevated temperatures, such data do not yet cover different cold-formed steel grades and thicknesses. Therefore, an experimental study was undertaken to investigate the elevated temperature mechanical properties of two low and high strength steels with two thicknesses that are commonly used in Australia. Tensile coupon tests were undertaken using a steady state test method for temperatures in the range 20–700 °C. Test results were compared with the currently available reduction factors for yield strength and elastic modulus, and stress–strain curves, based on which further improvements were made. For this purpose, test results of many other cold-formed steels were also used based on other similar studies undertaken at the Queensland University of Technology. Improved equations were developed to predict the yield strength and elastic modulus reduction factors and stress–strain curves of a range of cold-formed steel grades and thicknesses used in Australia. This paper presents the results of this experimental study, comparisons with the results of past research and steel design standards, and the new predictive equations.

Numerical exploration of the defect’s effect on mechanical properties of nanowires under torsion

Molecular dynamics (MD) simulations have been carried out to investigate the defect’s effect on the mechanical properties of single-crystal copper nanowire with different surface defects, under torsion deformation. The torsional rigidity is found insensitive to the surface defects and the critical angle appears an obvious decrease due to the surface defects, the largest decrease is found for the nanowire with surface horizon defect. The deformation mechanism appears different degrees of influence due to surface defects. The surface defects play a role of dislocation sources. Comparing with single intrinsic stacking faults formation for the perfect nanowire, much affluent deformation processes have been activated because of surface defects, for instance, we find the twins formation for the nanowire with a surface 45o defect.

MD investigations for mechanical properties of copper nanowires with and without surface defects

Based on the molecular dynamics (MD) method, the single-crystalline copper nanowire with different surface defects is investigated through tension simulation. For comparison, the MD tension simulations of perfect nanowire are firstly carried out under different temperatures, strain rates, and sizes. It has concluded that the surface-volume ratio significantly affects the mechanical properties of nanowire. The surface defects on nanowires are then systematically studied in considering different defect orientation and distribution. It is found that the Young’s modulus is insensitive of surface defects. However, the yield strength and yield point show a significant decrease due to the different defects. Different defects are observed to serve as a dislocation source.