No description available.

Thesis (M.S.)--West Virginia University, 1998. / Title from document title page. Document formatted into pages; contains xi, 84 p. : ill. Includes abstract. Includes bibliographical references (p. 64-67).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1956. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves [184]-186).

Thesis (SC. D)--University of Michigan, 1936. / An article, by H.C. Sung, G.G. Brown, and R.R. White, reprinted from Industrial and engineering chemistry, v. 37, Dec. 1945. Bibliography: p. 1161.

No description available.

This research has been concerned with investigating stress cracking phenomena which occur when thermoplastic polyurethanes (TPU's) are immersed in seawater at low temperatures. The investigation was concerned with testing the following hypothesis: a) that cracking of TPU was due to environmental stress cracking (ESC) and/or b) due to structure and morphology changes which occur during the ageing period. The combined effects of polyurethane chemical backbone, domain structure and crystallinity on stress cracking in TPU were studied. Further, the investigation concerned itself with studying, on stress cracking, the effects of storage conditioning the TPU granules prior to processing, the processing conditions and various postcuring treatments of TPU. Characterisation and analytical techniques employed to study the structure of TPU's investigated consisted of thermal analysis and X-ray diffraction. Molecular weight distribution was studied by gel permeation chromatography and solution viscosity techniques. Processing'was evaluated by melt flow index (MFI), injection moulding and extrusion. A new accelerated ageing test for environmental stress cracking has been developed which uses high pressures. Also, a new type of ESC chemical class of reagent which relates the hydrogen bonding parameter of the ESC agent to that of the TPU's has been discovered and a theory developed. A test method has been designed to measure, in active environments, the existence of a critical strain for all the TPU examined whether commercial or laboratory synthesised materials. Results show that the stress cracking in TPU's is due to the following events: (a) a large reduction in molecular weight due to the processing, and (b) the combined effects of applied stresses and the hydrolysis of the polymer. Molecular weight reduction in TPU's and their resistance to ESe was also found dependent on the preconditioning history of unprocessed TPU granules, as well as any postcuring operations applied to the processed materials. It has been established that, the ESe resistance of TPU's can be improved by (i) optimising processing conditions, (ii) by insertion of slight crosslinking or by selecting certain types of diisocyanates, specifically p~phenylene diisocyanate, and trans 1,4-cyclohexane diisocyanate. A mechanism to explain ESe of TPU's inactive and mild environments is also proposed.

The present work is a study of the matrix cracking behaviour of composite laminates which contain plies oriented at an angle to the loading axis. Incremental tensile tests were conducted on a set of glass-epoxy laminates having the [0/θ][formula omitted] geometry where θ takes the values of 45°, 60°, 75° and 90°. At each load increment, the stiffness reduction was measured and the cracking sequence was photographed. A novel technique using image analysis was used to measure the crack length and digitize the crack pattern in each photograph. The results were analysed in two ways - deterministically (using fracture mechanics) and statistically. In the first instance, the relationship between stiffness loss and crack length was used to calculate the strain energy release rate, G from a compliance expression. It was found that the overall stiffness loss for a given crack length increased with increasing θ. As G can also be viewed as the resistance to cracking, the calculated values were used to plot matrix cracking resistance curves (R-curves) for each lay-up. The R-curves showed that the overall resistance to cracking increased with increasing orientation angle, θ. For the [0/45][formula omitted] laminate, where cracking is driven by the highest proportion of G₁₁ component, the least increase in resistance was observed. The differences in crack resistance in these lay-ups could be explained with results from the statistical analysis. A statistical analysis of the changes in distribution of crack length and number indicated that most of cracks in the [0/90][formula omitted] were short even at high loads. In addition, a calculation of the incremental growth with each incremental load showed that the amount of growth in that lay-up was limited. This implied that the process of crack initiation continually dominated crack propagation even late in the loading sequence. The opposite behaviour is seen as θ decreases. In the [0/45][formula omitted], [0/60][formula omitted], and [0/75][formula omitted] lay-ups, the additional Mode II shear loading appeared to have assisted significantly the coalescence and growth of cracks. Hence, the overall crack resistance decreased as the proportion of the GH component increased. Cracking in the off-axis plies is not uniform. In the [0/45][formula omitted], [0/60][formula omitted] and [0/75][formula omitted] laminates, cracking begins in distinct bands and are referred to as shear bands since they occur due to the presence of the Mode II shear loading. This phenomenon, however, has little effect on the stiffness. Although cracking is not uniform, the cracks tend to space themselves to within two ply thickness apart as crack density increases. In the shear band areas, the crack spacing can approach one ply thickness. It was also observed that crack tips stop growing either when they are two ply thickness apart or when they approach a stronger area in the laminate. Generally, the resistance to cracking is not affected when the crack density is low. However, as cracks begin to interact when they are spaced to within two ply thickness, the resistance increases dramatically. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The derivation of the classical equation for flow through an orifice assumes a fixed orifice area. However, pipe materials exhibit expansion behaviour with increasing pressure, which alters the size of orifices and results in greater leakage rates. The purpose of this investigation was to study the behaviour of round holes and cracks in pipes through theoretical and experimental work. The results of the study include equations derived for increased flow through round holes in pressurized cylindrical shells and pipes. The theoretical models explain the increased flow experienced due to the leak area increasing. The models incorporate material properties, shell geometry and fluid properties for both uni-axially and bi-axially stressed pipe sections. Analytical results are compared with previous finite element investigations. In addition, an experimental study into the effects of pressure on a round hole in a class 6 uPVC pipe was conducted. Conclusions are made relating to the influence of material expansion to increased flow rate through openings in pressurised cylinders. The results compared positively with those of the theoretical equations. Conclusions are made relating to the influence of round hole or crack expansion to an increased flow rate through openings in pressurised pipes. Results include the effects of geometrical and material variables on the expansion of round holes. Better explanation of the increased flow through orifices, documented by practical observations, is presented. Results indicate that round hole area is linearly related to pressure. However, testing on longitudinal cracks resulted in a non-linear relationship between crack area and internal pipe pressure. Results indicate the expansion of round hole area is minimal. Leakage however is greatly affected over extended time periods by even the smallest increase in defect area. Critical pressures before brittle fracture obtained from testing on longitudinal cracks were compared to theoretical formulation. Results show a close relationship between current theory and experimentation. / Prof. K. van Zyl

No description available.

The stress corrosion cracking (SCC) behaviour of Mg alloy AZ31B was investigated with respect to surface condition. Salt fog U-bend testing was used to identify changes in SCC as a result of surface conditioning pre-treatments. Six surface conditions were investigated: as-received, mechanically-polished, sulphuric acid (H2SO4)-cleaned, mechanically-polished then H2SO4-cleaned, aged H2SO4-cleaned, and acetic acid (C2H4O2)-cleaned. Results showed that the rate of SCC was accelerated and the SCC mode was intergranular for all surface conditioning treatments involving H2SO4-cleaning. It was found that the accelerated intergranular SCC was a result of three contributing factors: a low pH, the presence of aggressive ions, and a porous film which allowed direct contact between the metal surface and the electrolyte. Characterization of the surfaces using potentiodynamic polarization and cross-sectional images of sample surfaces showed that in the absence of one of these three contributing factors intergranular SCC would not occur. / Thesis / Master of Applied Science (MASc)