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This thesis experimentally investigates the statistics (mean and variance) of the fatigue-cycle-dependent evolutions of both the crack tip front penetration's distribution function and the microscopic growth rate's distribution function, as a fatigue crack propagates to final fracture. A novel technique which facilitates striation counting and striation spacing measurements, is developed and used for extracting and analyzing the relevant statistical data for characterizing the stochastic fatigue crack propagation in polycrystalline metals. Two types of pure copper materials are investigated. / The investigation confirms the existence of the mean and variance of both the crack front penetration and its growth rate. Details of the variations of the mean crack penetration with respect to the dispersion of the crack front distribution and the mean growth rate, respectively, are established. Other contributions include the evaluation of the material characteristic associated with the transition intensity of the growth process. / These results are correlated with the predictions of the "Provan-Ghonem" theory in order to ascertain the validity of the linear Markov birth stochastic process, as a viable description of the fatigue crack propagation process in polycrystalline metals. The trend of the experimental results suggest a spatially correlated Markov process which accounts for both the strong nearest-neighbour-interactions between "points" along the crack front, and the boundary effects as a more viable representation of the fatigue crack propagation process.

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This experiment was initially inspired by the visual imagery studies of R.J. Weber and colleagues (1969, 1970, 1972). The purpose of the present study was to examine various aspects of mental fatigue associated with the forming of mental images as suggested by Weber and Castleman (1970).A total of 45 undergraduates were given 10 trials on the Weber Alphabet Task (Weber and Kelley, 1969) which involves mental imagery, The experimental design compared three inter-trial interval periods, 0 sec, 10 sec, and 30 sec, on speed and accuracy of performance. The results indicated that the more trials a subject completed, the faster he was able to complete later trials, regardless of the intertrial interval. However, on the error data, the effedt of the intertrial interval was significant. It was demonstrated that the 0 sec intertrial interval group made most errors on the imagery task when compared to the other two groups.

Previous studies of cognitive fatigue which have used the pupillary response have centered on long term, time-induced effects. Results have indicated that pupil diameter decreases as a function of cognitive fatigue. This study examined the effects of short term, task-induced fatigue on the pupillary response.The research design involved the use of a pre/post paradigm with an interpolated mental multiplication task designed to produce cognitive fatigue. Pupil diameter and rating scale measures were obtained, as well as a measure of the subjects' numerical aptitude. By analyzing the pre/post differences in pupil diameter over time, the use of the pupillary response for the assessment of short term, task-induced cognitive fatigue was demonstrated to be feasible.

Since a significantly greater percentage of body weight is supported by the non-paretic limb following stroke, a greater amount of fatigue may be present during daily activities. This may affect the ability of these individuals to maintain a stable upright posture. The presence of falls following a stroke has been attributed in part to this asymmetrical stance post-stroke. Therefore the purpose of this study was to assess the effect of quadriceps muscle fatigue on bi-pedal posture in individuals who had a stroke and an age-matched control group. Although individuals after stroke displayed greater postural sway under the paretic limb than the non-paretic limb or control subjects, results of this study show that sustaining an isometric knee extension of the non-paretic limb induces changes in postural control for individuals after stroke, but that these changes do not markedly differ from those of healthy age-matched controls.

Driver fatigue is responsible for up to 30% of fatal car accidents. This issue has been addressed by many scholars in order to save thousands of lives and reduce many costs. The goal of this work is to reduce the number of car accidents caused by mental fatigue or drowsiness. In order to achieve this goal, we propose a personalized Bayesian Network (BN) to detect driver’s fatigue. The detection of driver fatigue is enhanced by combining data that reflects the driver’s performance with context-aware information. The parameters of the system are the angular velocity of the steering wheel, the pressure applied to the gas and brake pedals, the grip force on the steering wheel, weather conditions, current traffic, and time of day. The aforementioned parameters of the network are updated on a regular basis, which makes fatigue detection more reliable. Besides, these parameters allow the system to detect a driver’s fatigue through driving performance which is both individualized and context aware. In our experiment, subjects drove a driving simulator game during six sessions, for a total of one hour. After each session, every subject used the Karolinska Sleepiness Scale (KSS) to rate her fatigue’s level. The system was trained on the data collected separately from each user, allowing BN to be personalized for each subject. The proposed system showed an average accuracy of 96%, and ability to overcome the issue of individual differences and uncertainties which are involved in fatigue detection process.

Composite materials have high in-plane mechanical properties, but are susceptible to out-of-plane impact damage. The use of non-destructive evaluation techniques, combined with mechanical testing, was investigated to characterize the progression of post-impact static and fatigue damage of composite laminates. Quasi-isotropic carbon/epoxy specimens were impacted at energy levels of 35 J and 50 J. The initial damage was characterized using ultrasonic C-scan and thermography. Residual strength testing revealed that the compressive static strength of the test specimens was reduced by over 50%. Digital image correlation was used to characterize the growth of damage and the local strain during compression-compression fatigue tests. Initially, no significant statistical trend could be measured when the fatigue data was plotted as a stress-life curve. When stress concentration factors were used to calculate and plot the local stress amplitude, a correlation with fatigue life was observed. The undamaged fatigue data was altered using damage factors which allowed for post-impact fatigue life predictions.

In 2005, a new, wrought, nickel-base superalloy Haynes 282 was developed. Characteristic of superalloys, Haynes 282 was produced to be used for components, specifically aero and land-based gas turbine engines, which are subjected to high temperatures. The manufacture and/or repair of aero and land-based gas turbine engines require the use of joining processes, an example being welding. However, it was observed that laser beam welding (LBW) of the new superalloy resulted in cracking in the heat affected zone (HAZ) of the alloy. Accordingly, an aspect of this research involves the development of a procedure for preventing or minimizing cracking of the alloy. This is achieved through microstructural modification of the alloy prior to welding through the use of appropriate heat treatment. However, at the core of this research is the main objective of methodically studying the effect of LBW on the high temperature fatigue crack growth (FCG) behaviour of Haynes 282, and how to mitigate any negative effects. Findings from this research show that LBW causes a deleterious effect on the FCG behaviour by increasing the FCG rate. Thus, although this research resulted in a procedure that minimizes cracking during welding of the new alloy, the minimized level of cracking is not tolerable under dynamic loading. Therefore, a procedure to make the minimized level of cracking tolerable during dynamic service is developed. This is achieved by developing two post weld heat treatments, including a new thermal treatment schedule developed in this work, which significantly improved the FCG resistance of the new alloy after LBW. Additionally, Low cycle fatigue (LCF) tests results indicate strong fatigue deformation resistance of Haynes 282. On the other hand, FCG tests results show that, in contrast to common assumption, the loading frequency was observed to have an effect on the FCG behaviour at room temperature. Prior to this research, there was limited publication on the fatigue behaviour of Haynes 282. Hence, the study of the cyclic deformation characteristics and growth behaviour of fatigue crack in Haynes 282 in this research provides valuable information that helps establish design criteria to safeguard against component failure by fatigue.