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Includes bibliographical references. / The abrasive nature of fly ash handled in large coal-fired power stations results in unacceptable material and maintenance costs in ash-water mixing plant. Wear testing has been carried out in situ using a variety of generic materials. A ranking order of wear performance has been established both as a function of material and operating costs, and it has been demonstrated that impressive cost savings can be effected by the use of ceramic-coated steel mixing blades. The performance of such composites has been found to be sensitive to the design and method of application. To optimise materials selection, a family of tungsten carbide-cobalt cermets together with a number of structural ceramics were tested in situ. The modes of wear can be related to material constitution. By ranking the performance of these candidate materials, value based materials selection and design for use can be applied.

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The effect of cooling rate and stacking sequence on fatigue behavior was analyzed for notched quasi-isotropic APC-2 laminates. The fatigue behavior of fast (475° F/min) and slow (1° F/min) cooled specimens of the following two layups was studied: Layup A of (-45/0/45/90), and Layup B of (45/90/-45/0),. All specimens were subjected to a load controlled, Tension - Tension fatigue loading with a stress ratio R = 0.1 at a frequency of 5 Hz. Parameters such as strain, temperature rise across the notch and number of cycles fatigued were continuously monitored during the fatigue tests. Damage was monitored by the reduction in modulus, penetrant enhanced X-ray radiography, and Scanning Acoustic Microscopy (SAM). Post failure analysis of the specimens was carried out by Scanning Electron Microscopy (SEM). A quasi-3D Finite Element Analysis was performed to compare the differences in the interlaminar stresses arising around the notch in the specimens of the two layups under study. The ultimate static strengths did not show any appreciable dependence on either cooling rate or stacking sequence. The maximum load in the fatigue cycle was selected as a fraction of the ultimate notched static tensile strength in each case. The fatigue lives showed appreciable difference between the two cooling rates in layup A when tested at the lower load levels. The fatigue behavior was vastly different between the two cooling rates for specimens of layup B. Also, specimens of layup B, both fast and slow cooled, had longer lives than their counterparts from layup A. A model, based on a constant strain-to-failure criterion, was developed for life prediction and the predicted lives are in good agreement with the experimental values. Fast cooled specimens of layup A showed a gradual degradation in the modulus till failure while slow cooled specimens of the same layup showed a more drastic reduction as they approached failure. No such distinguished behavior was observed in the specimens of layup B. Scanning Electron Micrographs of the fast cooled specimens indicate better fiber/matrix bonding conditions and more matrix plasticity as compared to the slow cooled specimens. A rotated stacking sequence technique was used for the calculation of the interlaminar stresses around the notch. No single stress seems to control failure but it is likely that failure occurs by the interaction of the different stresses since a three-dimensional stress state exists at the notch. Based on this reasoning, effective stresses were calculated at all those interfaces where one of the interlaminar (normal or shear) stresses has a maximum value. A comparison of the effective stresses calculated showed the layup A to be 1.7 times more prone to delamination than layup B. Damage analysis of the fatigued specimens by X-ray radiography and Scanning Acoustic Microscopy shows the specimens of layup A to be dominated by delaminations as compared to those of layup B. The interfaces predicted to be critical by FEA agreed well with the experimental observations, in general. / Master of Science

An analytical model was developed to simulate the stress state of notched tensile specimens. Actual experiments are being carried out by other investigators to study the relaxation of residual stresses in specimens containing stress raisers. In the present work, the stress state developed in notched tensile specimens was assessed by determining the response of the stress state in the form of x-ray line profiles; this is useful in the understanding and measurement of effects due to such stress states obtained in actual experiments. The theoretical relationship between the stress gradient and the depth of penetration of the x-ray beam at the edge of a notch tensile specimen was also studied. In addition, the effect of changes in the radius of curvature of the notch-tip on errors in measured stress values is also considered. Furthermore, a description of the state-of-the-art x-ray system being used in the experimental work is also included. / Master of Science

The objective of the current research was to contribute to the area of mechanics of composite polymeric materials. This objective was reached by establishing a quantitative assessment of the fatigue strength and evolution of mechanical property changes during fatigue loading of nanocomposite fibers and films. Both experimental testing and mathematical modeling were used to gain a fundamental understanding of the fatigue behavior and material changes that occurred during fatigue loading. In addition, the objective of the study was to gain a qualitative and fundamental understanding of the failure mechanisms that occurred between the nanoagent and matrix in nanocomposite fibers. This objective was accomplished by examining scanning electron microscopy (SEM) fractographs. The results of this research can be used to better understand the behavior of nanocomposite materials in applications where degradation due to fatigue and instability of the composite under loading conditions may be a concern. These applications are typically encountered in automotive, aerospace, and civil engineering applications where fatigue and/or fracture are primary factors that contribute to failure.

Submitted in partial fulfilment of the requirements for the Degree of Master of Technology: Mechanical Engineering, Durban University of Technology, 2011. / The objective of the present study was to investigate if piezoelectric techniques or sensors can be used in monitoring structural degradation. The study considers experimental results and analytical modelling of a ductile structure under tensile and cyclic loading. Throughout the project the emphasis was placed on the effectiveness of strain measuring sensors. Conventional tensile testing was conducted using a Lloyds testing machine. The testing machine was calibrated to have a lateral movement of 2mm/min (tension force). Rectangular plates were pulled in tension until failure. From that experimental data was produced for a uni-axial loading system. Cyclic testing was carried out using an in-house designed and manufactured fatigue machine. It produced a reciprocating load (force) of 25rad/s on a rectangular plate. Two different sensor measuring instruments (strain gauge and piezoelectric) were used. The strain gauge sensor was attached to a specimen and connected to a Wheatstone bridge. The piezoelectric sensor was attached to the specimen and then linked directly to the capturing system. From these two sensors experimental results were obtained and compared. The mathematical relationships for the rectangular plates were formulated using effective stress-strain behaviour based on the elastic and plastic behaviour of the plates. The analytical and experimental results were compared. Results from this investigation show that piezoelectric sensors can be useful for measuring fatigue failure on a ductile material.

Metallic structures dominated by tensile loads are vulnerable to fatigue and fracture. Fatigue is produced by oscillatory loads. Quasi-static brittle or ductile fracture can result from a "large" load in the random sequence. Moreover, a fatigue or fracture failure in a member of a redundant structure produces impulsive redistributed loads to the intact members. These transient loads could produce a sequence of failures resulting in progressive collapse of the system. Fatigue and fracture design factors are subject to considerable uncertainty. Therefore, a probabilistic approach, which includes a system reliability assessment, is appropriate for design purposes. But system reliability can be improved by a maintenance program of periodic inspection with repair and/or replacement of damaged members. However, a maintenance program can be expensive. The ultimate goal of the engineer is to specify a design, inspection, and repair strategy to minimize life cycle costs. The fatigue/fracture reliability and maintainability (FRM) process for redundant structure can be a complicated random process. The structural model considered series, parallel, and parallel/series systems of elements. Applied to the system are fatigue loads including mean stress, an extreme load, as well as impulsive loads in parallel member systems. The failure modes are fatigue, brittle and ductile fracture. A refined fatigue model is employed which includes both the crack initiation and propagation phases. The FRM process cannot be solved easily using recently developed advanced structural reliability techniques. A "hybrid" simulation method which combines modified importance sampling (MIS) with inflated stress extrapolation (ISE) is proposed. MIS and ISE methods are developed and demonstrated using numerous examples which include series, parallel and series/parallel systems. Not only reasonable estimates of the probability of system failure but also an estimate of the distribution of time to system failure can be obtained. The time to failure distribution can be used to estimate the reliability function, hazard function, conditional reliability given survival at any time, etc. The demonstration cases illustrate how reliability of a system having given material properties is influenced by the number of series and parallel elements, stress level, mean stress, and various inspection/repair policies.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERO. / Includes bibliographical references. / by Olivier Andre Bauchau. / Sc.D.

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