This research is concerned with the effects of temperature and frequency on the fatigue crack propagation behaviour. The fatigue behaviour at elevated temperature has been reviewed with emphasis on the mechanisms of propagation proposed to explain the effects of temperature and frequency. The crack propagation characteristics of a single crystal superalloy have been investigated at various temperatures and frequencies. Determination of the fatigue crack propagation rates was combined with the measurement of crack tip plastic zone sizes and observation of the deformation behaviour. At elevated temperature, the fatigue crack propagation rates were determined to be dependent on the frequency of loading. At low and high crack growth rates increased crack propagation rates were associated with higher cycling frequencies. At intermediate crack growth rates, the highest propagation rates were associated with the lowest frequency. Selected area channelling pattern techniques were used to determine the plastic zone sizes and revealed that at high frequencies (>10Hz) the plastic zone sizes at elevated and ambient temperature are similar. With decreasing frequency at elevated temperature creep may lead to increased plastic zone sizes but may also affect the crack tip geometry thereby restricting further effects. At room temperature crack propagation is observed to occur by shear of the γ' precipitate on <110> {111} resulting in {111} crystallographic facets on the fracture surface. At elevated temperature (>6000 C) however, the crack growth mechanism changes and propagation takes place on {100}. Observations of the dislocation structure indicate that deformation is concentrated within the γ matrix. It is proposed that the observed crack propagation in the γ matrix on {100} arises by the constraints imposed by the surrounding γ' precipitate. A model is presented for the frequency dependence of the intermediate crack growth rate in terms of the observed strain gradients within the plastic zone and the achievement of a critical local fracture strain.

published_or_final_version / Pharmacology / Doctoral / Doctor of Philosophy

published_or_final_version / Pharmacology / Doctoral / Doctor of Philosophy

published_or_final_version / Botany / Doctoral / Doctor of Philosophy

published_or_final_version / Physiology / Master / Master of Science

published_or_final_version / Dentistry / Master / Master of Dental Surgery

published_or_final_version / Speech and Hearing Sciences / Doctoral / Doctor of Philosophy

The objective of this research was to indirectly determine the existence of a possible retinal-endocrine pathway activated, or Inhibited, by artificial fluorescent light as evidenced by it subsequent effects on bone metabolism. Forty-eight male Long-Evans post-breeder rats, half of which were bilaterally orbitally enucleated, were used in this experiment. A 21-day pretreatment diet contained calcium and phosphorus, each at a level of 0.2% of the diet, with no dietary vitamin D. During the last seven days of this period the animals were in complete darkness. For the 12-week experimental period, the diet contained 0.6% calcium and 0.4% phosphorus, and oholecalciferol wets administered the first day by intubation. During this time, four groups of 12 animals each, half of which were bilaterally orbitally enucleated, were exposed to either ultraviolet lights, cool green lights, Vita-Lite (full spectrum) lamps, or kept in complete darkness in specially constructed environmental boxes. On the first and last day of the experimental period, the left femurs were x-rayed and blood was sampled. The serum of the animals was analyzed for alkaline phosphatase, total calcium and total phosphorus. Bone density was determined. Femur ash was measured for total calcium and total phosphorus. Under all lighting conditions, for both the sighted and enucleated rats, the parameters measured in the serum declined. For serum alkaline phosphatase, the sighted rats under the cool green lights had the least change; the sighted in darkness had the greatest decline. The least change in serum total calcium occurred for both the sighted and enucleated rats under the cool green lights, whereas the greatest decline was for the sighted rats under the Vita-Lite lamps and the enucleated under ultraviolet lights. The sighted rats under the cool green lights had the highest percent ash per gram of dry fat-free weight; the sighted under the Vita-Lite lamps had the lowest. The highest ash calcium was in the sighted rats under the cool green and the Vita-Lite lamps. By both methods of bone density measurements, specific gravity and densitometry, the sighted rats under the cool green lights had the greatest loss. Overall there appeared to be more significant differences among the sighted animals for the various lighting conditions than for the enucleated. Also the enucleated rats had less extreme variations from the mean than the sighted. The results of this study suggest that the light mediated by the pbotoreceptors of the retina of the sighted rats produced an effect on bone metabolism. / Graduation date: 1982

The present study sought to define a possible role of phosphorus-stimulated ammoniagenesis, previously observed in vitro and in experimental animals, in the hypocalciuric effect of phosphorus in young men. It was hypothesized that titrating some of the hydrogen ions destined for excretion with ammonia would be beneficial to kidney calcium reabsorption because ammonium ion unlike hydrogen ion does not increase urinary calcium loss. To test this hypothesis, seven young men (22 - 31 years old and average weight of 70 kg) were fed a single menu providing 800 mg phosphorus, 1200 mg calcium, 11.7 MJ (~ 2800 kcal) and 14.5 g nitrogen for 10 days. In a subsequent 10-day period, dietary phosphorus was doubled to 1600 mg by the addition of cheddar cheese and milk to the menu. Dietary nitrogen, calcium and vitamin D were equalized for the two periods. Three 24-hour urine samples and a fasting blood sample were collected at the end of each experimental period. Comparison of period 2 with 1 showed urinary phosphorus excretion + 82%, urinary calcium excretion - 38%, serum phosphorus + 6.2%, serum calcium - 3.8%, urinary sulfate and creatinine excretion no change. Urinary ammonia nitrogen excretion increased in all seven subjects with an average increase of 13%. Furthermore each individual increase in ammonia nitrogen excretion was found to be directly correlated with urinary phosphorus excretion (r = 0.76, P < 0.05). Urinary urea nitrogen excretion was found to vary inversely with urinary ammonia nitrogen excretion with an average decrease of 12% and urinary total nitrogen excretion decreased 14% for the entire group. Results obtained therefore offer a mechanism by which phosphorus directly improves the non-parathyroid hormone portion of tubular calcium reabsorption. The magnitude of the effects seen however suggest that phosphorus-stimulated ammoniagenesis only plays a small role in the mechanism of the hypocalciuric effect of phosphorus in young men. / Graduation date: 1996

Different concentrations of various browning inhibitors and a firming agent were evaluated to prevent browning and softening of Bartlett, Bosc and Anjou pear slices during cold storage at 2-4°C. Browning inhibition effect of Semperfresh, Snow Fresh and cysteine were compared with that of 4-hexylresorcinol (4-HR). The effect of ripeness on color and texture changes as well as the influence of storage temperature on texture and color retention was investigated. 4-HR residual content under different treatment conditions and its sensory properties were analyzed. 4-HR was an effective browning inhibitor at concentration as low as 0.005% when combined with 0.5% ascorbic acid (AA). Cysteine (0.5%) with 0.5% ascorbic acid (AA) effectively inhibited Anjou and Bosc pear slice browning for 25 days and its sensory properties merit further study. A combination of 1.0% AA and 1.0% calcium lactate prevented Anjou pear slices from discoloration, but also caused tissue leaking and softening. Semperfresh and Snow Fresh did not maintain flesh color effectively. 1.0% calcium lactate maintained slice firmness at an acceptable level for 30 days when the average firmness of whole pears at slicing was 49, 38 and 43 Newton (N) for Bartlett, Bosc and Anjou respectively. A storage temperature of 0°C significantly retained better texture than the control of 2-4°C. Slices with acceptable texture and color were achieved by treatment with a solution of 0.01% 4-HR, 0.5% AA, and 1.0% calcium lactate for 2 min, vacuum packaging, and storage at 2-4°C for 30 days for Bartlett and Bosc, and 15 days for Anjou. 4-HR residual content increased with increasing dipping times and concentrations, with the latter being the major factor. The 4-HR residual decreased with storage time for all the treatments. Sensory tests indicated that panelists detected the overall differences between 0.01% 4-HR treated samples and the controls for both Bartlett and Anjou pears (P<0.01). There were no significant differences in preference between treated sample and control for Anjou pears. / Graduation date: 1999