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This thesis describes the experimental work carried out to further the understanding of the mechanisms involved in the process of fretting fatigue. The design and construction of a rig to enable a controlled fretting action to be applied to a push-pull type of fatigue specimen is discussed. Special consideration is given to the form of the fretting contact geometry with regard to the generated stress field, and the reasons behind the final choice of a circular Hertzian contact are given. The investigation of the effects of various parameters on the fatigue life of the specimen are reported. The parameters chosen for the investigation were the slip amplitude, the bulk stress and the normal load. Examination of the developing fretting damage was carried out ty interrupting the tests at prescribed intervals. The subsequent observations made of the surface and sue-surface damage are illustrated by optical and scanning electron micrographs. Two distinct forms of damage are proposed, these have been termed type I and type II fretting fatigue damage. Type I damage is considered to be by crack nucleation from the conjoint action of the bulk and surface stress fields. These cracks were found to nucleate in the direction of maximum shear at the edge of the fretting contact, and at the position where the alternating tensile stress was largest. The subsequent direction of propagation of these cracks, determines whether or not fatigue failure of the specimen will occur. The type I fretting damage process, is noted to have been responsible for every case of catastrophic fatigue failure of the test specimens. Type II fretting damage was found at the centre of the fretting contact area, where the hydrostatic stress and surface shear traction are maximum. The material in this region was found to have undergone extensive microstructural alteration. White etching layers with a hardness in excess of 1300Hv (more than four times the original hardness) are reported. An extensive literature survey is presented which shows no previous evidence of white etching layers forming under clean fretting conditions. The white layers are shown to have important consequences on the rate of wear, thus it is postulated that under certain conditions fretting wear and fretting fatigue may be linked ty the same mechanism, i.e. white etching layer formation. The white etching layers are also discussed in terms of their potential for nucleating fatigue cracks, this phenomenon is illustrated by a service failure from a diesel engine.

Thirty volunteer subjects were given thirty trials on the pursuit rotor. Twenty trials were given on one day and ten further trials were given approximately twenty-four hours later. The subjects were randomly assigned to one of three groups: 1) the directed stress group: 2) the non-directed stress group: and 3) the control group. Shock and instructions were used to induce the stress in the two stress groups. The instructions to the directed stress group were assumed to have directed the subjects' attention toward the pursuit rotor task, while the instructions to the non-directed stress group were not designed to give direction to their attention. The results showed that there was no difference in performance due to either stress condition when compared to the control group. However, when tested twenty-four hours later, both stress groups showed significant improvements in learning over the control group. There was no significant difference in learning between the two stress groups. / Education, Faculty of / Curriculum and Pedagogy (EDCP), Department of / Graduate

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Thesis (Ph. D.)--University of Kentucky, 2007. / Title from document title page (viewed on March 25, 2008). Document formatted into pages; contains: xii, 216 p. : ill. (some col.). Includes abstract and vita. Includes bibliographical references (p. 171-210).

Thesis (Ph.D.)--Tufts University, 2003. / Adviser: L. Michael Romero. Submitted to the Dept. of Biology. Includes bibliographical references (leaves 158-180). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

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Stress-related functional bowel disease is a common, often incapacitating, problem in humans; the symptomatology of stress-related intestinal dysfunction is: (1) impaired small intestinal transit and motility, and (2) increased large intestinal transit and, commonly, diarrhea. The etiology of stress-induced intestinal dysfunction is completely unresolved, and the lack of an appropriate animal model has hindered studies of causality. We compared a number of stressors and their resultant effects on intestinal transit, a measure of the propulsive motor activity of the gut, in the rat. We found that the response of the intestine to stress, and the neural systems activated by stress, were dependent on the type and duration of stress, as well as the animal strain, and gender. We developed a model, acute wrapping restraint stress, to fully characterize the effects of stress on intestinal transit. Wrap restraint stress is a nonulcerogenic model in which rats are subjected to acute restraint by wrapping them in a harness of paper tape to restrict, but not prevent movement of the upper body and forelimbs. Transit was evaluated by the geometric center method, in which a radiomarker (⁵¹Cr) is instilled directly into the proximal duodenum and proximal colon via a surgically placed intestinal cannula, in fasted, adult female Sprague Dawley rats (150-200g). Subjecting animals to 35 min. of wrap restraint stress resulted in (1) inhibition of small intestinal transit, and (2) increased large intestinal transit and increased fecal output. The effects of stress on intestinal transit in rats resembled symptoms associated with stress in humans, suggesting that wrap restraint stress may be suitable as a model of stress-induced intestinal dysfunction. We found a close correlation between stress-induced intestinal dysfunction and stress-activation of endocrine systems. Stress-induced changes in intestinal function was strongly influenced by circadian variations in endocrine levels, suggesting that stress-induced intestinal dysfunction may be hormonally mediated. However, neither pituitary nor adrenal factors mediated the effects of stress on the gut. To evaluate the role of corticotropin-releasing factor (CRF), the major hypothalamic factor released in response to stress, in stress-induced intestinal dysfunction, we studied the effects of exogenous CRF on intestinal transit. CRF resulted in (1) a potent, dose-dependent inhibition of small intestinal transit, (2) a dose-dependent increase in large intestinal transit, and (3) increased fecal excretion. The effects of exogenously administered CRF closely paralleled the effects of stress on intestinal transit and on ACTH secretion in the rat. Blockade of CRF receptors by means of an antagonist, α helical CRF (9-41), prevented the effects of stress on colonic transit and fecal excretion. These data strongly suggest that endogenous CRF may mediate the effects of wrap restraint stress on intestinal motor activity and coordination in the rat.

A study was done to investigate the contribution of small units of stress, anxiety, and demographic factors in the occurrence of various stress-related diseases. Three groups of thirty subjects each, having three distinct medical diagnoses categories (hypertensives, stress-related disorders, and general medical problems), were compared using a variety of anxiety and stress questionnaires. Subjects were matched in terms of sex, age, race, military history, and place of treatment. Devices administered to the subjects and used to compare the three groups included: A Sociodemographic Questionnaire; The State-Trait Anxiety Inventory; The Cognitive-Somatic Anxiety Questionnaire, and The Hassles and Uplifts Scales. A comparison of groups via demographic factors indicated distinctive similarities between groups on most items except for educational background, obesity ratio, and family history of hypertension. Groups were not found to be significantly different from each other in terms of anxiety styles, and reported stressors and reinforcers on a discriminant function analysis. A multiple regression analysis of the hypertensive group revealed that age; reported stressors and reinforcers; family history of hypertension; cigarette smoking, and obesity contributed to this disease. Measures of actual stressors were proven to be more accurate than anxiety measures in predicting hypertension. Descriptive analysis of the three groups indicated that hypertensives reported a higher number of anxiety type stressors than the other two groups, and the stress-related groups reported more health-related complaints. Theories for the lack of significance between groups are provided as well as suggestions for future studies and applications.

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